Colloquiam: ECCOMAS Congress 2022 - 8th European Congress on Computational Methods in Applied Sciences and Engineering

A coupled multiphysics approach for modelling in-stent restenosis

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:27:40 +0100

Drug-eluting stents were developed to counteract the severe restenosis observed after bare-metal stent implantation. The risk of restenosis still prevailed due to the inhibitory effect of the drug on endothelial healing. The current work focuses on extending a multiphysics-based modeling framework to include the effect of anti-inflammatory drugs embedded in the drugeluting stents. An additional advection-reaction-diffusion equation governing the drug transport is introduced herein. Additionally, the effect of drugs, specifically rapamycin-based ones, on the proliferation of smooth muscle cells is captured. An optimal level of drug embedment is realized through the modeling framework.

A gradient-extended anisotropic damage-plasticity model in the logarithmic strain space

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:23:29 +0100

Within this contribution, we discuss additional theoretical as well as numerical aspects of the material model developed in [1, 2], where a `two-surface' damage-plasticity model is proposed accounting for induced damage anisotropy by means of a second order damage tensor. The constitutive framework is stated in terms of logarithmic strain measures, while the total strain is additively decomposed into elastic and plastic parts. Moreover, a novel gradientextension based on the damage tensor's invariants is presented using the micromorphic approach introduced in [3]. Finally, going beyond the numerical examples presented in [1, 2], we study the model's ability to cure mesh-dependency in a three-dimensional setup.

A hybrid-Trefftz finite element for the postbuckling analysis of composite shell structures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:25:15 +0100

A corotational mixed flat shell element for the geometrically nonlinear analysis of laminated composite structures is presented. The stress interpolation is derived from the linear elastic solution for symmetric composite materials.Displacement and rotation fields are only assumed along the contour of the element. As such, all the operators are efficiently obtained through analytical contour integration. The geometrical nonlinearity is introduced by means of a corotational formulation. The proposed finite element, named MISS-4c, proves to be locking free and shows no rank defectiveness. A multimodal Koiter's algorithm is used to obtain the initial postbuckling response. Results show good accuracy and high convergence rate in the geometrically nonlinear analysis of composite shell structures.

A machine learning based approach to predict the stress intensity factors in 2D linear elastic fracture mechanics

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:02:32 +0100

Within the framework of linear elastic fracture mechanics, the stress intensity factors (SIFs) are the mostly applied crack-tip characterizing parameters. To obtain the SIFs, approximate formulae are widely used because exact analytical solutions are available only for very simple geometrical and loading configurations [1]. However, even approximate solutions for SIFs are also rather limited to very simple geometrical and loading conditions. In this work, an accurate and efficient SIF prediction model based on Physics-informed neural network (PINN) [4] is developed, where we incorporate the equilibrium equations and constitutive relations into the PINN. In order to capture the singular behavior of the stress and displacement fields around the crack tip, we extend the standard PINN structure by adding two more trainable parameters

A material interpolation technique using the simplex polytope

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:06:20 +0100

The Discrete Material Optimization (DMO) and the Shape Function with Penalization (SFP) constitute the state-of-the-art material interpolation techniques for identifying from a list of pre-defined candidate materials the most suitable one(s) for the structural domain. The candidate materials are represented on this list through their mechanical properties, and are interpolated within the domain of interest (DOI), whether that is the finite element (FE) domain or groups of FEs, so-called patches. Depending on the technique preferred to interpolate the mechanical properties within the DOI, a different type of weights is selected. Goal of the discrete material optimization problem (MOP) is to solve for these weights and determine for each FE/patch a unique material from the list. The current work extends the concept of the SFP technique by employing as weights the shape functions of the hyper-tetrahedral FE, the dimension of which is dynamically adapted depending on the number of candidate materials considered for the structural domain. This generalized hyper-tetrahedral FE constitutes what is defined as a simplex, and similar to the SFP technique each of its nodes is tied to a specific candidate material. In the context of discrete optimization and utilizing the shape functions of an abstract high-dimensional FE as weights for the candidate materials, the proposed interpolation technique secures the continuity between the number of candidate materials that can be considered for the structure, a feature lacking in the SFP technique. Additionally, given that the number of nodes forming the simplex FE is always one unit greater than the dimension of the space it is defined within, the dimension of the resulting MOP drops by one per DOI. The developed material interpolation technique is combined with the topology optimization problem (TOP) to formulate the concurrent material and topology optimization problem for compliance minimization of the structure. Finally, the latter is examined on the academic case study of the 3D Messerchmitt-B¨olkow-Blohm (MBB) beam for the case of the concurrent topology and discrete fiber orientation optimization problem.

A method for communication between user materials during runtime in ABAQUS®

Jesús Sánchez Pinedo — Wed, 07 Dec 2022 09:33:42 +0100

The application of the Finite Element Method (FEM) has developed considerably in recent decades. While in the early days of the FEM only linear-elastic material models were available in commercial Finite Element (FE) programs, today non-linear and also damage-considering material models are offered. But even these are often not capable of correctly representing the complex material behaviour of modern composite materials. Therefore, FE programs often provide the users with the option of integrating their own material models into the simulation. These programs, often called "user subroutines" or "user materials", represent an intensively researched area in the simulation of material behaviour, which is reflected in the large number of publications on such developments (exemplarily see [1-14]). These material models often require a large amount of data values if they are introduced within the simulation model by finite elements [2]. Especially in the field of composites there is a need to use a separate "user material" for each constituent material. Since there is a mutual interaction between the components in real composites, it is obvious that this interaction must also be represented The 8th European Congress on Computational Methods in Applied Sciences and Engineering ECCOMAS Congress 2022 5 – 9 June 2022, Oslo, Norway PHILIP F. ROSE, LUKAS MÜNCH, MARKUS LINKE AND PETER MIDDENDORF 2 between the material models within a simulation in order to make accurate predictions about the material behaviour. To enable such an interaction between different material models in a simulation, a communication during runtime is required in which additionally needed data from the surrounding elements is exchanged between the user materials. In this paper, a method is presented to enable such information exchange during simulation runtime in the commercial FE software ABAQUS/CAE 2019 (Dassault Systèmes, Vélizy-Villacoublay, France) using external databases as well as structured global arrays and some built-in functions of the software. This allows the simultaneous application of several advanced user material models within one simulation and enable them to communicate during runtime, resulting in the possibility of making high accurate simulations of composite materials.

A modified cohesive zone model for the simulation of mixed-mode fracture of co-consolidated thermoplastic laminates considering fiber bridging

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:46:57 +0100

Fiber bridging is a mechanism that may significantly alter the fracture behavior of composite laminates, adhesively bonded laminates, welded laminates, and co-consolidated laminates. It is therefore quite important for the finite element to take that mechanism into consideration. Such models have been developed for thermosetting laminates; however, this is not the case for thermoplastic laminates and thermoplastic joints. In the present work, a numerical model based on the cohesive zone modelling (CZM) approach has been developed to simulate mixed-mode fracture of co-consolidated thermoplastic laminates by considering fiber bridging. A modified traction separation law of tri-linear form has been developed by superimposing the bi-linear behaviors of the matrix and fibers. Initially, the data from mode I (DCB) and mode II (ENF) fracture toughness tests were used to construct the R-curves of the joints in the opening and sliding directions. The aforementioned curves were embedded into the numerical models through a user-defined material subroutine developed in the LS-Dyna FE code, in order to extract the fiber bridging law directly from the simulation results. The model was used to simulate fracture of a Single-Lap-Shear (SLS) specimen in which a considerable amount of fiber bridging was observed on the fracture area. The numerical results show that the developed model presented improved accuracy in comparison to the CZM employing the bilinear traction-separation law.

A thermo-coupled constitutive model for semi-crystalline polymers at finite strains: Application to varying degrees of crystallinity and temperatures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:45:16 +0100

Thermoplastic materials are widely used for thermoforming and injection moulding processes, since their low density in combination with a high strength to mass ratio are interesting for various industrial applications. Semi-crystalline polymers make up a subcategory of thermoplastics, which partly crystallize after cool-down from the molten state. During the thermoforming process, residual stresses can arise, due to complex material behavior under different temperatures and strain rates. Therefore, computational models are needed to predict the material response and minimize production errors. This work presents a thermomechanically consistent phenomenological material formulation at finite strains, based on [1]. In order to account for the highly nonlinear material behavior, elasto-plastic and visco-elastic contributions are combined in the model formulation. To account for the crystalline regions, a hyperelastic-plastic framework is chosen, based on [2, 3]. Kinematic hardening of Arruda-Boyce form is incorporated in the formulation, as well as associated plastic flow. The material parameters depend on both, the temperature as well as the degree of crystallinity. A comparison to experiments with varying degrees of crystallinity and temperatures is presented, where a special blending technique ensures stable crystallinity conditions.

A Validation Program for Dynamic High-Lift System Aerodynamics

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:49:39 +0100

The feasibility of laminar flow control technology for future wing is bound to the development of a leading edge high-lift system that complies with the requirements on smooth surfaces to enable maintaining the laminar boundary layer flow, such as a Krueger flap. Although in principle the aerodynamic performance of a Krueger flap is known, the unsteady behaviour of the flow during deployment and retraction is completely unknown. This is as even more important as during deployment the Krueger flap is exposed to highly unfavourable positions perpendicular to the flow. To mitigate the risk of unfavourable aircraft behaviour, it is therefore expected that a Krueger flap has to be deflected significantly fast and may trigger unsteady aerodynamic effects. The European H2020 project UHURA, running from September 2018 to August 2022, has been focusing on the unsteady flow behaviour around such high-lift system and will first time deliver a deeper understanding of critical flow features at this type of high-lift device during their deployment and retraction together with a validated numerical procedure for its simulation. UHURA performed detailed experimental measurements in several wind tunnels to obtain a unique data set for validation purposes of Computational Fluid Dynamics (CFD) software, including detailed flow measurements by Particle Image Velocimetry (PIV) and other optical measurement technologies.

A variational-based mixed finite element formulation for liquid crystal elastomers

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:22:00 +0100

Liquid crystal elastomers (LCEs) are soft materials, which are capable of large deformations induced by temperature changes and ultraviolet irradiation [1]. Since many years, these materials are under investigation in experimental researches as actuator materials. LCEs arise from a nematic polymer melt, consisting of long and flexible polymer chains as well as oriented and rigid rod-like molecules, the so-called mesogens, by crosslinking. In order to numerically simulate LCE materials by using the finite element method, a continuum model is necessary, including in a thermo-viscoelastic material formulation of the polymer chains the orientation effects of the mesogens. This can be performed by introducing a normalized direction vector as an independent field, and deriving from additional (orientational) balance laws independent differential equations [2]. These differential equations describe the independent rotation of the rigid mesogens connected with the flexible polymer chains. The orientation-dependent stress law of LCEs arises from an anisotropic free energy, comparable with fibre-reinforced materials. But, in contrast to fibre-reinforced materials, the direction vector of a LCE model has to be independent. In contrast to [2], we apply a variational principle for deriving a new mixed finite element formulation, which is based on drilling degrees of freedom for describing the mesogens rotation [3]. This principle leads to an extended set of balance laws.

Advanced deep learning comparisons for non-invasive tunnel lining assessment from ground penetrating radar profiles

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:03:07 +0100

Innovative, automated, and non-invasive techniques have been developed by scientific community to indirectly assess structural conditions and support the decision-making process for a worthwhile maintenance schedule. Nowadays, machine learning tools are in the spotlight because of their outstanding capabilities to deal with data coming from even heterogeneous sources and their ability to extract information from the structural systems, providing highly effective, reliable, and efficient damage classification tools. In the current study, a supervised multi-level damage classification strategy has been developed regarding Ground Penetrating Radar (GPR) profiles for the assessment of tunnel lining conditions. In previous research, the authors firstly considered a convolutional neural network (CNN), adopting the quite popular ResNet-50, initialized through transfer learning. In the present work, further enhancements have been attempted by adopting two configurations of the newest state-of-art advanced neural architectures: the neural transformers. The foremost is the original Vision Transformer (ViT), whose core is an encoder entirely based on the innovative self-attention mechanism and does not rely on convolution at all. The second is an improvement of ViT which merges convolution and self-attention, the Compact Convolution Transformer (CCT). In conclusion, a critical discussion of the different pros and cons of adopting the above-mentioned different architectures is finally provided, highlighting the actual powerfulness of these technologies in the future civil engineering paradigm nevertheless.

Analytical modeling of shock wave stresses and spall caused by laser plasma in a material interface – application to paint stripping on aluminum substrates

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:06:04 +0100

In the present work, analytical and numerical models have been developed to calculate shock wave stresses caused by laser plasma in a material interface. The input pressure causing the shock wave has been derived from a laser-matter interaction model. The material velocity, stresses, and strains versus time in the two materials and the interface have been computed by solving the jump equations for conservation of mass and balance of momentum. The material interface has been considered as an immovable boundary while the back free surface as an unrestrained boundary. Spall fracture strength of the interface was evaluated and was used as stripping criterion. The model has been used for the fast computation of interfacial stresses causing paint stripping on aluminium substrates and the subsequent fast assessment of stripping initiation. An explicit finite element model combined with the cohesive zone modelling method and a spall fracture model have been developed. These models have been compared to each other in terms of time, accuracy and input properties demand and to the experimental results.

ANN-based surrogate model for predicting the lateral load capacity of RC shear walls

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:07:45 +0100

Reinforced concrete (RC) shear walls are often used as the main lateral-resisting component in the seismic design of buildings. They provide a large percentage of the lateral stiffness of the structure, and therefore, they may experience large shear stresses at some point under earthquake loading. Consequentially, to accurately predict their behavior, it is recommended to use detailed finite element (FE) modeling with appropriate non-linear constitutive models for concrete and steel. However, such types of simulations are challenging and could significantly increase the computational time required to obtain the analysis results. In this paper, we study the viability of creating an artificial neural-network-based surrogate model of the RC shear wall that is able to capture its nonlinear behavior and predict the results obtained with a detailed FE model, offering a much lower computational effort. For this purpose, we develop a detailed parametric non-linear FE model based on well-established practices and validated studies using the OpenSees finite element software framework. The FE model consists of multi-layer shell elements with the vertical and transverse reinforcement included as smeared rebar layers. The concrete layers implement a damage mechanism with smeared crack constitutive model, whereas the rebar layers consider a uniaxial plasticity material law. The parametric FE model is used to build a large database of RC walls of different sizes and characteristics, with their corresponding lateral load capacity that is obtained through the detailed non-linear pushover analysis. Finally, the obtained database is used to train and validate the ANN-surrogate model. The developed model is able to accurately predict the lateral load capacity of RC shear walls without the need of detailed FE modeling, thus drastically reducing the complexity and the computational time required for the numerical solution and providing a reliable and robust analysis alternative, with only small compromise of accuracy.

Application of an adaptive stable GFEM for fracture propagation in plain concrete

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:41:40 +0100

Generalized finite element method (GFEM) has proven itself as a tool of choice over the conventional FEM in fracture analysis due to enhanced computational efficiency as well as allowing cracks to propagate independently of the domain mesh Thanks to the use of enrichments chosen based on the a priori knowledge of the solution behavior. With the many versions of the method's formulations in the literature, their stability issues, compared to the standard FEM, are often unresolved. This paper presents the use of an adaptive stable GFEM to plain concrete fracture propagation. Having verified the formulation's accuracy and stability based on the Linear Elastic Fracture Mechanics in previous studies and its two-scale (global-local) version on concrete fracture, the present work seeks to verify its capabilities in capturing the size effect behavior in concrete. A set of fracture simulations in geometrically similar experimental concrete beams, under a 3-point bending regime, is presented based on a bilinear cohesive model. In addition to the GFEM's agreement with the experimental load-displacement response and the effect of the initial notch-to-depth ratio, the simulation successfully captures the size effect behavior when presented on the popular Type II Size Effect plot the so-called Bazant's law.

ARCHITECTURAL AND ENVIRONMENTAL IMPACT OF RETROFITTING TECHNIQUES TO PREVENT IN-PLANE ≪DOMINO≫ FAILURE MODES OF UNREINFORCED MASONRY BUILDINGS

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:48:30 +0100

The preservation of heritage buildings is not just about the structural safety, but it is necessarily related to the central themes of restoration, fruition and reuse of ancient buildings. Such topic requires an interdisciplinary design approach that involves, among the others, structural engineering, numerical modelling and architecture to address the challenges of contemporaneity in heritage management also in terms of interests of the stakeholders. In this regard, the opportunities offered by natural F.R.C.M. (Fibre Reinforced Cementitious Matrix) composites, made of basaltic fibres and lime mortar, are analysed.

Architectured and additively manufactured double-negative index metamaterials

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:23:52 +0100

Developing a new generation of multifunctional metamaterials with unusual thermoelastic properties enables a wide range of industrial applications, particularly in the aerospace industry. However, obtaining metamaterials with target properties by the systematic design of their microstructure and architecture remains a major challenge to this day. Topology Optimization (TO) is a powerful tool that can be used to develop the so-called anepectic metamaterials that combine both negative Poisson's Ratio (NPR) and negative thermal expansion (NTE). Here, an overview of the existing contributions in the literature regarding such metamaterials is presented. A Finite Element (FE) model for an anepectic microstructure is presented here for the purpose of simulating in silico the experimental results obtained in previous works. It is noted that scarce contributions resort to TO to design such metamaterials and even fewer present experimental validation. The present work presents a state of the art of anepectic metamaterials and emphasizes thus the importance of the engineering-cycle completion, i.e., starting with the systematic and optimal design of metamaterials and ending up in prototype fabrication and its verification.

Attenuation and localization of waves in taut cables with suspended masses

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:45:39 +0100

This work analyzes structural waves that propagate freely along taut cables, characterized by a discrete array of scatter elements. The outcomes underline the role played by the periodic distribution of such elements, whose presence alters the response of the system when subjected to propagating waves. Namely, when the domain is perfectly periodic, band gaps are found in the spectrum of the problem. It is also shown that the introduction of a defect of periodicity can lead to the appearance of eigenvalues inside band gaps, corresponding to a motion localized around the defect.

Classification of compromised DOFS data with LSTM neural networks

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:47:28 +0100

Distributed optical fiber sensors (DOFS) are gaining momentum for in-situ condition monitoring and damage detection purposes. Although DOFS are a versatile sensing method enabling high-resolution strain and temperature mapping, they are also sensitive to mechanical vibrations. Vibrations are typically created by the ambient environment (e.g acoustic background, rotating equipment) which can produce high levels of measurement noise. With physical access to DOFS installations, the principle of acoustic or mechanical vibrations can also be utilized for malicious sensor tampering. The current lack of anomaly-detection systems suggests that practical DOFS applications would benefit from an automated analysis to detect and classify compromised measurements. Noise classification makes it possible to identify its source and potentially remove its effects from the measurement in the future. This would expand the commercial applications of DOFS systems significantly. Neural networks have been used for error detection in cyber-physical applications in numerous studies with high-accuracy results. Specifically, long short-term memory (LSTM) neural network models have become popular in recent years to classify anomalies in sequential e.g time-series data. Our investigation conducted a series of physical experiments using magnitude-controlled mechanical disturbances on bare free-hanging DOFS. Both random low-frequency vibrations at large displacement amplitudes and a constant high-frequency acoustic source at a low amplitude were employed. Experiments revealed that strain patterns are visually different with varying types and levels of disturbances. For the numerical analysis, statistics and machine learningbased approaches were applied for DOFS vibration noise classification, and their accuracy is discussed in detail. Results from the post-processing of compromised DOFS data suggest that it is possible to develop a vibration detection or classification system based on off-the-shelf DOFS interrogation equipment coupled with LSTM numerical tools.

Closed - form expressions for the optimum winding angles of fibres in laminated pressure vessels subjected to internal pressure and axial force

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:05:09 +0100

The present paper aims at deriving closed-form expressions for the optimum winding angles of fibres in laminated cylindrical pressure vessels subjected to internal pressure, axial load and torque. To achieve this goal, the state of the stress on the surface of the vessel is represented by a composite layered element subjected to general in-plane loading. The symbolic software Mathematica is used to formulate, solve the optimization problem and to generate the data of optimum fibre angles for different loading combinations. The generated data is fitted with simple polynomial expressions capable of accurately predicting the optimum winding angles. The optimization is performed for three candidate composite materials: carbon/epoxy, glass/epoxy, and aramid/epoxy laminates.

Comparing three calculation methods of load distribution in radial bearings

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:47:10 +0100

There are two basic methods for radial external load distribution calculation on rolling elements in a rolling element bearing: the discrete method and the integral method. Solving the discrete equilibrium equation using the Newton-Raphson scheme, more accurate results are derived than those based on the integral method, with small theoretical and computational efforts. The Sjövall's radial integral factors, as well as some approximations proposed in the literature, for lineand point-contacts, are given. Numerical approximations for the Sjövall's radial integrals are proposed. The approximations' errors with respect to the Sjövall's radial integral's numerical integration are shown.

Constitutive model for the analysis of the behavior and mechanics of wood damage

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:26:47 +0100

Wood is a heterogeneous material, whose morphology and topology make the prediction of its mechanical behavior complex under different boundary conditions [1-3]. In this way, different constitutive models with different scale lengths have been developed since the middle of the 20th century to predict the behavior and mechanics of wood damage [3-8]. However, there is still no agreement on which constitutive model or scale length allows for a more consistent representation of behavior and damage mechanics mentioned. This article seeks to contribute to the discussion a new constitutive model for the analysis of the behavior and mechanics of wood damage. To do this, we present the implementation in a user subroutine for Abaqus [9], of a viscoelastic constitutive model based on the generalized well criterion for the elastic regime with a macroscale model [10] [11]; the onset and the damage evolution law are analyzed under a mesoscale model based on the progressive degradation of cracking parallel and perpendicular to the fiber [12-14]

Coupled thermomechanical analysis of thermoplastic composite pipe by FEM simulations

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:05:46 +0100

Spoolable thermoplastic composite pipe (TCP) is an ideal alternative to traditional, heavier metallic counterparts for deepwater riser applications. During operation the pipe is subjected to mechanical loads simultaneously with through-wall thermal gradients arising from the mismatch between temperatures of hot pipe contents and cool surrounding ocean. In this work, structural analysis of TCP under coupled thermomechanical loads is performed using the finite element method (FEM). Temperature-dependent material properties are considered. Material safety factors for different laminate stacking sequences are compared and multi-angle stacking is shown to be effective for both pressure- and tensiondominated scenarios. Safety factors are also generated for TCP bent at reduced and elevated temperatures illustrative of spooling in different environments. It is clear that optimising the laminate for operation will adversely affect spooling capacity and vice-versa, i.e. TCP intended for extreme in-service conditions will require large spools.

Data-led Mechanical and Thermal Analysis of Layered Structures Based on Parametric Finite Element Analysis and Neural Network

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:50:18 +0100

The mechanical and thermal behaviours of layered structures is of great importance for many advanced material systems and loading conditions. The responses of layered structures are controlled by the constitutive properties of each layer as well as the thicknesses. A comprehensive data-based approach is essential for both material analysis and design in direct or inverse problems. In this work parametric numerical modelling and Artificial Neural Network (ANN) are jointly used to develop data for layered structures. Mechanical and thermal finite element (FE) models are used to produce data for different material property and thickness domains. The use of ANN program is established and evaluated for different loading conditions. Using indentation as a typical case for mechanical loading and localized heating as typical example for thermal loading, ANN program was used to predict the behaviour of layered structures with different properties and layer thicknesses. Use of the data system in establishing dominating factors, synergetic effect on mechanical-thermal performance in advanced materials design is discussed.

Deep neural networks for unsupervised damage detection on the Z24 bridge

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:04:48 +0100

During their life-cycle, civil infrastructures are continuously prone to significant functionality losses, primarily due to material's degradation and exposure to several natural hazards. Following these concerns, many researchers have attempted to develop reliable monitoring strategies, as integration to visual inspections, to efficiently ensure bridge maintenance and early-stage damage detection. In this framework, recent improvements in sensor technologies and data science have stimulated the use of Machine Learning (ML) algorithms for Structural Health Monitoring (SHM). Among unsupervised learning techniques, the potential of autoencoder networks has been attracting notable interest in the context of anomaly detection. In this light, the present paper proposes two different autoencoder-based damage detection techniques, focused on the Multi-Layer Perceptron (MLP) and the Convolutional Autoencoder (CAE) networks, respectively. During the training, the selected ML models learn how reconstructing raw acceleration sequences acquired from sound conditions. Unknown data, including both healthy and damaged bridge responses, are afterwards used to test the implemented networks and to detect damage occurrence. To this aim, a specific index of reconstruction loss is selected as a damage sensitive feature with the aim to quantify the errors between the original and reconstructed sequences. The performance exhibited by the two approaches is compared and evaluated by application to the Z24 benchmark bridge. Results demonstrate the effectiveness of the proposed methodology to perform feature classification and real time damage detection at the level of macro-sequences as new sensor data is collected, resulting suitable for continuous assessment of full-scale monitored bridges.

Design of integrated fluidic actuators for muti-axial loaded structural elements

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:48:53 +0100

The construction sector is responsible for high grey energy consumption and high greenhouse gas emissions. Adaptive structures can be a suitable solution to counteract this. Actuation of a beam to reduce the mass by counteracting the deflection with integrated fluidic actuators has been proven in previous studies. New challenges are brought about with the actuation of slabs due to the multi-axial load transfer. Many actuator principles are conceivable for this application. A combination of uniaxially acting actuators and complex designs that generate forces in different spatial directions in a targeted manner are possible. This paper presents various principles for the development of actuators integrated into the cross-section of a slab. These are able to manipulate the multi-axial load transfer behaviour directly. For this purpose, the actuator principles are classified according to various aspects. In a second step, numerical investigations are used to prove the effectiveness of the actuator principles.

Design of piezoelectric lattice metamaterials

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:46:04 +0100

Piezoelectric lattice metamaterials are considered. A computationally-effective homogenisation method is developed based on the recent solution to the Saint-Venant problem for general anisotropic piezoelectric cylinders. A publicly available repository of unit cell topologies is used to identify piezoelectric metamaterials with optimal figures of merit.

Development of a 3D visco-elastic model for wood under large deformations

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:24:33 +0100

Some of the most frequently observed phenomena in structural materials are creep and relaxation. Both are associated with time-dependent behavior and dissipative rheological variables. In the case of wood, long-term creep can produce excessive deformation and instability problems by magnifying short-term deflections. Also, wood presents changes in its mechanical properties due to its hygroscopy, so that moisture appears as an important parameter to be considered. A priori, it is known that the moisture content in wood cells and the angle of microfibrils are parameters that directly affect the overall cell stiffness. With this information, material physics hypotheses can be elaborated to develop a constitutive model in large deformations to predict phenomena such as creep and relaxation in the medium and/or long term. The microstructure of the cell wall can be represented through a model of fiber-reinforced composite material originally developed for biomaterials such as arteries and fibrous tissues. Where the anisotropic character is conferred by the distribution of the fibers within the isotropic matrix of the material. This work aims to adapt these models to represent the mechanical behavior of the wood cell with faithful representation of its microstructure. FEniCS is used for the numerical implementation of the material. In this paper the validations, current status, conclusions, and perspectives of this research are presented.

Development of a FE² Multiscale Model of Chloride Ions Transport in Recycled Aggregates Concrete

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:44:05 +0100

Decreasing CO2 emissions and preserving natural resources are necessary to the well-being of our civilisations. In the construction industry, recycling old concrete members could be part of the solution to reach theses objectives. Recycled Concrete Aggregates (RCA), obtained by crushing of demolished concrete structures, can substitute the Natural Aggregates (NA) inside the so-called Recycled Aggregates Concrete (RAC). The durability of RAC is not guaranteed in the current state of research. RCA are indeed composed of natural aggregates partially embedded in an adherent mortar paste, increasing the porosity and water absorption of RAC.

This research aims to better predict the influence of RCA on chloride ions ingress inside concrete. It started with an experimental phase where multiple experiments have been performed to determine the transfer properties and the chloride ions diffusion coefficients of a mortar paste and concretes made from NA or 100% RCA. In this context, the microstructure of the RCA influences deeply the permeability, water content distribution and chloride ingress. Therefore, these properties must be included into a numerical model that integrates the microstructural information in a proper way. A numerical homogenization technique, based on the Finite Element square (FE2 ) method [5, 13], is implemented into a coupled multiscale model of water flows and advection/diffusion of chlorides in saturated concrete, in order to model the complex flow behaviour encountered.

Director-based IGA beam elements for sliding contact problems

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:37:59 +0100

The geometrically exact beam theory is one of the most prominent non-linear beam models. It can be used to simulate aerial runways or pantograph-catenaries, where a sliding contact condition between two or more beams is used. A smooth discretization of at least C1continuity is needed to not introduce any unphysical kinks. This can be achieved using the isogeometric analysis, which we apply to a director-based formulation of the geometrically exact beam. For a stable time integration scheme we use an energy-momentum conserving scheme. Using the notion of the discrete gradient, an energy-momentum conserving algorithm is constructed, including the case of sliding contact between beams.

Distinct element modelling of the seismic response of historical masonry constructions: insight on the out-of-plane collapse of façades

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:51:45 +0100

Façades belonging to historical masonry constructions typically fail by out-of-plane mechanisms. The estimate of their out-of-plane capacity is not a trivial task, due to the different possible collapse modes (overturning, bending, disaggregation, leaf separation, sliding) and to the discontinuous nature of masonry, influencing the non-linear seismic behaviour of walls. Simplified approaches, proposed by building codes, mainly based on the mechanics of the rigid block, may not always be suitable for the purpose. Indeed, they disregard the real morphology of masonry, which instead influences weaker failure mechanisms (such as disaggregation and leaf separation). Furthermore, they neglect the interaction of the façade with the rest of the building and its interlocking with transversal walls. These shortcomings can be overcome resorting to distinct element method (DEM), in which masonry is modelled as an aggregation of discrete units and no-thickness interfaces and the actual morphology of constructions is considered. In this paper, DEM is adopted to investigate the out-of-plane seismic behaviour of façades through non-linear analyses, by focusing on vertical bending and overturning failure mechanisms. The former is studied by comparing results of shake table tests on both single-leaf and double-leaf masonry walls to dynamic simulations in which real accelerograms are applied. The latter is analysed by performing non-linear static analyses on the Romanesque church of St. Maria Maggiore in Tuscania, Italy, by focusing on its façade. Distinct element method provided a realistic description of the behaviour of façades under earthquake loadings, in terms of both seismic capacity, crack pattern and failure mode.

Dynamic characterization of offshore wind turbines supported on a jacket using Artificial Neural Networks

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:02:13 +0100

The dynamic characterization of an offshore wind turbine (OWT) and its foundation is an important task within the design stage of the support structure. The system fundamental frequency should not coincide with that of the loads that affect it, otherwise resonance phenomena can conclude with the collapse of the structure or its deterioration due to fatigue. Assuming a linear behaviour, the system fundamental frequency can be computed by solving the eingenvalue problem after defining the stiffness and mass global matrices. This procedure can become computationally expensive if soil-structure interaction (SSI) phenomena is taken into account. For this reason, a surrogate model based on Artificial Neural Networks (ANN) is proposed for estimating the fundamental frequency of the wind turbine assembly, jacket support structure and pile foundation considering SSI effects. A dataset is generated to train the ANN. This synthetic data collects the characteristics of the OWT-jacket-foundation and its fundamental frequency, which is obtained by a finite element substructuring model. The SSI is reproduced through impedance functions computed through a previously developed continuum model. Comparing the predictions of the ANN with the results obtained by the structural model, it is observed that this type of regression allows to reproduce in a sufficiently precise way the dependence of the fundamental frequency with respect to the variables that define the system. Thus the use of Machine Learning techniques, such as ANNs, makes it possible to take into account the system behaviour obtained by rigorous models in large-scale calculations that it would be unfeasible otherwise.

Dynamic Response of Offshore Wind Turbines under Nonlinear Irregular Ocean Waves

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:23:11 +0100

Offshore wind turbines (OWT) are exposed to different categories of ocean waves during their lifetime. Most ocean waves are categorized in the second-order nonlinear theory in normal and severe sea states, and their spectra combine different heights and frequencies. In the present study, a model for the dynamic response of OWTs to the wave load obtained irregular second-order nonlinear ocean waves is proposed. The foundation-wave-structure interaction and the effect of the nacelle-rotor assembly are simulated. Numerical results are provided and discussed.

Effect of graphite-particle morphology on thermomechanical performance of compacted graphite iron: Numerical modelling

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:45:43 +0100

Compacted graphite iron (CGI) is used in numerous applications, e.g., in the automotive industry, in tools and pipes, thanks to its excellent thermomechanical properties. Despite its extensive industrial use, its fracture at the microscale was not thoroughly investigated. Interfacial debonding is the main mechanism of fracture of CGI at the microscale and is based on the deformational incompatibility between graphite and the surrounding matrix. To study this decohesion, a two-dimensional micromechanical model of elliptical inclusions embedded in a metallic matrix is generated. An elastoplastic response is assumed for both constituents, and cohesive finite elements are applied to the graphite-matrix interface to simulate decohesion. A parametric analysis of interfacial features such as thickness and stiffness is performed to identify the effect of these parameters on debonding. The obtained results provide insights into the thermomechanical behaviour of CGI at the microscale and enable more effective modelling of this engineering alloy.

Efficient assessment of noise transmission through highly flexible slender structures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:39:55 +0100

In this work, we propose an efficient methodology for the assessment of noise transmission through cables and hoses. An interactive simulation with a geometrically exact Cosserat rod enables simple and fast modelling of various configurations. Subsequently, we linearise the equations of motion at the static equilibrium for given boundary conditions and, using the resulting system matrices, compute the mechanical impedance matrix. The computation result, i.e. the impedance matrix, is available within seconds. The impedance matrix either can be used to compute reaction forces for given excitation or, if the excitation is unknown, allows to analyse the transmission of noise by looking at single matrix elements. The latter is especially useful in early, purely virtual development phases.

Efficient dispersion curve computations for periodic vibro-acoustic structures using the (generalized) Bloch mode synthesis

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:25:34 +0100

Periodic structures such as metamaterials and phononic crystals hold potential as promising compact and lightweight solutions for noise and/or vibration attenuation in targeted frequency ranges. The performance of these structures is usually investigated by means of dispersion curves. The input for dispersion curve computations is often a finite element model of the corresponding unit cell. Nowadays, the vibration and noise attenuation of the periodic structures are generally tackled as separate problems and their performance is investigated with either structural or acoustic dispersion curves, respectively. Recently, vibro-acoustic unit cell models have come to the fore which can exhibit simultaneous structural and acoustic stopbands. However, the vibro-acoustic coupling inside the unit cell is usually not taken into account during the dispersion curve computations. To consider this coupling during their performance assessment, the computation of vibro-acoustic dispersion curves is required. Although these dispersion curves provide valuable information, the associated computational cost rapidly increases with unit cell model size. Model order reduction techniques are important enablers to overcome this high cost. In this work, the Bloch mode synthesis (BMS) and generalized BMS (GBMS) unit cell model order reduction techniques are extended to be applicable for 2D and 3D periodic vibro-acoustic systems. Through a verification case, the methodologies are shown to enable a strongly reduced dispersion curve calculation time while maintaining accurate predictions.

Parallelization of a finite element solver for chemo-mechanical coupled anode and cathode particles in lithium-ion batteries

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:05:24 +0100

Two chemo-mechanical coupled models for electrode particles of lithium-ion batteries are compared. On the one hand a CahnHilliard-type phase-field approach models lithium intercalation, phase separation and large deformations in phase transforming cathode materials like lithium iron phosphate. On the other hand a chemo-mechanical particle model for lithium intercalation and large deformations for an anode material such as silicon is studied. The comparison of two different ways to define the deformation gradient for the large deformation approach and the two different material properties lead to differences in the resulting quantities and equations for the coupling of the chemo-mechanical model. The usage of an adaptive solution algorithm as well as the parallelization of the finite element solver via the message passing interface concept results in a more reasonable computation time to perform two-dimensional simulations. Both materials are numerically investigated and the results are compared from a physical point of view. When fast charging a battery, higher stress values are reached, which can cause a shorter cycle life. A strong scalability analysis shows good performance for the assembling, however a saturation occurs in the performance of the solver used.

Energy-momentum conserving dynamic variational modeling of fiber-bending stiffness in composites

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:26:09 +0100

We propose a new variational formulation for large deformations in dynamical systems made of 3D-fiber-reinforced composites. The formulation emanates from the dynamic variational approach based on the principle of virtual power. The use of higher-order gradient theory along with multi-field mixed-finite element method enables us to model the fiber-bending stiffness in fiber reinforced composites for numerical simulations accurately. Our proposed model capture higher-order energy contributions exhibited by fibers that influence the fiber-bending curvature, and consequently the fiber-bending stiffness behaviour. For this, we introduce a higher-order gradient of the deformation mapping as an independent field in the internal energy functional formulation. Along with the energy-momentum scheme, our new time integrator makes possible to perform long-term dynamic simulations with larger time steps and efficient CPU-time. We demonstrate our model using transient dynamical simulations on thre geometrical examples that exhibit hyperelastic, transversely isotropic, polyconvex gradient material behaviour. In the first example, a cantilever beam is self-excited due to its body weight, in the second, a L-shaped block tumbles free in the ambient space after an initial loading phase and in the third a turbine rotor is rotated due to hydrodynamic pressure. It is observed that our model conserves total momenta and total energy and preserves their time evolution in all these examples along with spatial and temporal convergence.

Estimating stress fluctuations in polycrystals with an improved maximum entropy method

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:38:53 +0100

The prediction of local field statistics from effective properties is an open problem in the field of micromechanics. Partial information on the local field statistics is accessible from homogenization assumptions. In particular, exact phase-wise second moments of stresses can be calculated analytically from the effective strain energy density. In recent years, full-field calculations have become efficient enough to sample large ensembles of microstructures in the plastic regime (e.g. Gehrig et. al [4]). In the present work, the maximum entropy method known from statistical thermodynamics is used to estimate first and second moments of local stresses from known eigenstrain distributions. The simple and refined formulations of the maximum entropy method proposed by Kreher and Pompe [9] are considered. While the simple method yields satisfactory results for a large amount of material classes (cf. Krause and Böhlke [7]), we prove that it does not respect the linearity of the eigenstrain problem. We further show that neither method corresponds to the exact second moments of stresses known from the effective strain energy density. By incorporating additional information, we find an improved maximum entropy method. As an example, we analyze stress fluctuations in polycristalline titanium.For the exact analytical solution and the maximum entropy methods, we use the singular approximation and the Hashin-Shtrikman bounds. For comparison, we numerically approximate full-field statistics using an FFT approach. In all methods, the stress fluctuations caused by the anisotropy of the single crystal strongly influence the elastic-plastic transition.

Fast and simple creation of powder beds for selective laser melting

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:40:14 +0100

In selective laser melting, components are produced by layer-by-layer melting of a powder bed. To investigate the interaction between the powder bed and the laser energy in the process, it is necessary to generate different powder bed configurations with a defined particle size distribution. For this purpose, based on different particle contact approaches, a simple algorithm for planar particle bed configurations was developed in the Julia programming language. Based on the so called 0and 1-particle-contact approaches, a monodisperse sphere packing with a filling ratio of up to 64%, and with a normally distributed particle size with a filling ratio of up to 67% were generated. With the 0-particle-contact approach, the individual powder beds could be generated more quickly, but showed an insufficient degree of filling. In contrast, the 1-particle-contact approach can produce powder beds realistically. An extension for spatial problems, as well as variations in the contact approaches, is given by the simple algorithm design and shall be implemented and further investigated in simulations of selective laser melting.

FEM analysis of FRCM strengthened RC columns exposed to fire

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:36:14 +0100

Reinforced concrete (RC) structures deteriorate over time and therefore, need to be strengthened. Despite the fact that RC structurtes have a decent fire rating, the performance of the strengthening system under fire exposure needs to be evaluated. One of the main restrictions associated with Fibre Reinforced Polymer (FRP) systems is their poor resistance to high temperatures, which originates from the combustible polymer matrix. Therefore, Fibre Reinforced Cementitious Matrix (FRCM) systems have been introduced due to their improved performance during fire exposure. The potential of Poly-paraphenylene-ben-zobisoxazole (PBO) FRCM to strengthen circular RC columns is evaluated using a nonlinear finite element analysis (FEA). The commericial software ABAQUS is used to develop 3D FE models to investigate the axial performance of the strengthening system. The modeling approach is performed by first conducting a thermal analysis to generate the nodal tempertaures. The second step of the model includes a displacement-controlled loading condition with imported nodal temperatures from the first model. The temperature dependent material properties are incorporated in both models. The modeling approach is validated against published literature and a parametric study is conducted on PBO FRCM strengthened and unstrengthened columns heated at durations of 1, 2, and 4 hours following the ASTM standard fire temperature-time curve. Results indicated a decrease in the axial capacity and stiffness of the strengthened and unstrengthened columns upon heating. Moreover, an increase in the columns' ductility due to the increase in temperature was observed. A decrease in ultimate strength of up to 78 and 68% was observed for the unstrengthened and strengthened columns respectively.

Finite element analyses of shear yielding and crazing in glassy polymers under cyclic mode I loading

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:44:56 +0100

A simple and yet physically motivated continuum-micromechanical model for crazing is developed, focussing on cyclic loading. The model features fibril drawing and fibril creep deformation, loose hanging fibrils upon unloading and the morphology change fibrils undergo between craze initiation up to a fully developed craze. The crazing model is implemented in a user material subroutine in the commercial finite element programme ABAQUS. The performance is investigated on a mode I crack growth boundary value problem under cyclic loading. Experimentally measured craze/crack opening profiles from the literature are reasonably-well captured by the model. The results exhibit further interesting model characteristics, such as a variation of the craze length in the course of a load cycle.

Finite element model of kurpsai dam in kyrgyzstan based on actual response measured by extensive network of various sensors

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:48:26 +0100

The paper presents recent results of an ongoing collaborative research project focused on modelling the Kurpsai water dam in Kyrgyzstan. The research team includes scientists and engineers from the USA, Kyrgyzstan, and Uzbekistan. This water dam was selected for modelling because of the recent installation of an extensive network of various sensors aimed at monitoring its performance under seasonal changes, ambient vibration, and seismic excitation. The installed instrumentation network includes the following sensors: (1) a set of fiber-optic strainmeters and temperature meters, (2) a set of velocimeters for seismic monitoring, and (3) a set of GNSS receivers to measure absolute static displacements. A 3D model of the water dam was generated based on a utilization of the finite element approach. As a starting point the water dam’s concrete was assumed to be elastic material. The latter assumption is considered acceptable, because (as of today) only responses to relatively small excitations were measured by the sensors. The actual responses of the dam were compared to that of the finite element model to achieve a close correlation with each other. Resonant frequencies of the water dam and its vibrational modes were estimated from the model. In the next phase of the project, the research team is planning to update the geometry of the model based on laser scanning that will be conducted this year. Local anomalies (bulging areas, cracks and so on) of the water dam will be studied via an analysis of point clouds collected by the laser scanner. The fully developed model will be used in an extensive numerical study to predict the dam’s performance and its response to strong seismic events and other hazards.

Forming effects on high-cycle fatigue in an aluminum sheet structure using the Ottosen-Stenström-Ristinmaa model

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:04:13 +0100

In this paper a multi-stage numerical analysis is presented with the aim to investigate effects of forming on the high-cycle fatigue performance of a deep-drawn aluminum sheet structure for use in a floating photovoltaic system. Forming simulations of a subsection of the full geometry are performed in a realistic two-step drawing-springback cycle. A simplified global analysis of service load response is performed to obtain displacements at the submodel boundary, that are used to generate boundary conditions for a local service load analysis. The local analysis is then performed on three different models of the subsection: (I) excluding forming effects, (II) including effects of thinning, and (III) including effects of thinning and residual stresses. The critical location with respect to the fatigue limit criterion in the Ottosen-Stenström-Ristinmaa high-cycle fatigue model was identified for case (I), and this location was used to compare the different models to assess effects of forming on high-cycle fatigue performance. Furthermore, the dynamic friction coefficient , and the isotropickinematic mixing coefficient were varied in order to investigate their respective effects.

From freezing-induced to injection-induced non-isothermal saturated porous media fracture

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:25:55 +0100

The focus of this contribution is laid on different aspects and instances related to porous media fracture under non-isothermal conditions. This includes the extreme case of fracturing due to pore-fluid freezing, where the micro-cryo-suction plays an important role in generating the required stresses for crack onset. This also includes studying the instances related to hydraulic fracturing and heat transfer under non-isothermal conditions. In all cases, the continuum mechanical modeling of the induced fractures is based on macroscopic porous media mechanics together with the phase-field method (PFM) for fracture modeling. For the micro-cryo-suction in saturated porous media, the water freezing is treated as a phase-change process. This is modeled using a different phase-field approach, in which the thermal energy derives the phase change and, thus, leads to the occurrence of micro-cryo-suction. Two numerical examples are presented to show the effectiveness of the proposed modeling frameworks.

Homogenization of the constitutive properties of composite beam cross-sections

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:39:36 +0100

When modelling slender bodies made of composite materials as beams, homogenized stiffness coefficients must be obtained. In [2, 3], analytic expressions for these are obtained by comparing the solutions of some Saint-Venant extension, bending and torsion 3D linear elasticity problems with their corresponding beam theory counterparts. In [2], the authors provide general expressions for the determination of these coefficients for multilayered beams. The present work consists in the study of a homogenization procedure of the stiffness coefficients for circular cross-sections with two layers. This will help in the study of the constitutive behavior of unloaded shafts of endoscopes since their cross-section could be studied as a simplified model of a three-layers hollow circular cross-section. In preparation of this geometry, results of an experimental campaign carried out at KARL STORZ GmbH & Co. KG (Tallinn, Estonia) are presented in a second part of this paper. The purpose of the testing was the experimental characterization of the torsional stiffness of such devices.

Immersed Boundary Analysis of Models with Internal State Variables: Applications to Hydrogels

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:44:39 +0100

Research on Soft Active Materials (SAMs) has flourished in recent years driven mainly by potential applications to actuation systems, tissue engineering, and soft robotics. These applications benefit from the unique properties of SAMs such as large deformations, a wide range of stimulants, and high motion complexities. Hydrogels are among the dominant members of SAMs. Their highly nonlinear chemo-mechanical transient behavior is described by equations that include rates of internal state variables representing the local swelling state of the gel. Hence, the simulation of hydrogels requires intricate numerical approaches with stabilization schemes. This paper presents an immersed boundary analysis technique to simulate models with internal state variables. A hydrogel model is used as an example to describe the components of the proposed technique. Level sets define the material layout on a fixed background mesh and a generalized version of the extended finite element method predicts the response. The influence of the internal state variables on the stability of the physical analysis is examined. While focusing on an XFEM approach for hydrogels, the presented theory can be extrapolated to similar applications using models with internal state variables (e.g., shape memory polymers) and other immersed boundary analysis technique (e.g., CutFEM).

Influence of Cooling Lubricants on the Interaction Between Indenter and Material Surface During Scratch Tests

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:41:09 +0100

Striving for the optimization and the increase of efficiency of various systems demands further developments of the classic manufacturing methods. Especially grinding processes, which are characterized by undefined cutting-edge geometries, reveal many fields where there still is a lack of understanding. In particular, the processes at and their effects on the individual abrasive grit are insufficiently researched and, therefore, do not allow sufficiently accurate behavior predictions. In order to optimize grinding processes and, ultimately, the resulting quality of the workpiece surface, it is necessary to look at the entire process in a holistic way. Due to the large number of influences to which the grinding process is subject, it is initially advisable to break down the process as far as possible into individual scratch tests and then gradually return to the overall process. One approach is the development and expansion of an FEM-based physical force model, which allows for the simulation and prediction of a scratch tests and, subsequently, also the entire grinding process with all relevant influencing factors. One of these influencing factors, which are essential but mostly unconsidered, are cooling lubricants, especially their tribologically favorable influence on the interaction between workpiece and indenter. Therefore, it is important to identify and investigate the different aspects, such as the friction phenomena of scratch tests that are influenced by the use of cooling lubricants. In addition to temperature and force characteristics, which have been found to differ with and without cooling lubricant, differences in the scratch geometry on the material surface have also been observed in recent tests. Based on these findings, this work examines the relationship between scratch geometry and cooling lubricant. It turned out that scratch tests conducted with cooling lubricants have an influence on the topography of the scratch on the workpiece surface in addition to the influence on the tangential and normal forces. The ratio of scratch width to scratch depth is used for evaluation. A reduction of this ratio is observerd in the scratches with cooling lubricants and is, therefore, interpreted as a reduction of the scratch width as a result of the use of cooling lubricants.

Influences of the geometrical nonlinearity on the complex band structures of periodic lattice frame structures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:22:18 +0100

In this study, the complex band structures of geometrically nonlinear periodic frame structures are calculated by using the Spectral Element Method (SEM). For this purpose, the spectral element matrix for a geometrically nonlinear beam element is derived. By solving the inverse eigenvalue problem for computing the complex dispersion curves k() instead of the conventional eigenvalue problem for calculating the real dispersion curves (k), the complex wave vector can be obtained, whose imaginary parts describes the evanescent behavior of the Bloch waves. Subsequently, the geometrically nonlinear effects on the evanescent behavior of the Bloch waves are investigated by evaluating the dispersion curves and the transmission spectra.

Intelligent Initiatives to Reduce CO2 Emissions in Construction

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:47:56 +0100

Global warming is one of the most important environmental issues that threatens the living on this globe so far.

Interval field methods with local gradient control

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:47:13 +0100

This paper introduces a novel method to create an interval field based on measurement data. Such interval fields are typically used to describe a spatially distributed non-deterministic quantity, e.g., Young's modulus. The interval field is based on a number of measurement points, i.e., control points, expended throughout the domain by a set of basis functions. At the control point the non-deterministic quantity is known and bounded by an interval. However, at these measurement points information about the gradients might also be available. In addition, the non-deterministic quantity might be described better by estimating the gradients based on the other measurements. Hence, the proposed interval field method allows to incorporate this gradient information. The method is based on Inverse Distance Weighing (IDW) with an additional set of basis functions: one set of basis functions interpolates the value, and the second set of basis functions controls the gradient at the control points. The additional basis functions can be determined in two distinct ways: first, the gradients are available or can directly be measured at the control point, and second, a weighted average is taken with respect to all control points within the domain. In general, the proposed interval field provides a more versatile definition of an interval field compared to the standard implementation of inverse distance weighting. The application of the interval field is shown in a number of one-dimensional cases where a comparison with standard inverse distance weighting is made. In addition, a case study with a set of measurement data is used to illustrate the method and how different realisations are obtained.

Inverse Dynamics of Geometrically Exact Beams

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:41:25 +0100

This paper is concerned with the inverse dynamics of flexible mechanical systems whose motion is governed by quasi-linear hyperbolic partial differential equations. Problems that appear by applying classical solution strategies to the problem at hand, e.g. integrating the problem at hand sequentially in space and time will be adressed in this work. Motivated by the hyperbolic structure of the underlying initial boundary value problem, two methods that are based on a simultaneous space-time integration will be presented. Special emphasize will be given to the phenomena of wave propagation within geometrically exact beams and its relevance regarding the inverse dynamics problem.

Investigation of higher harmonic Lamb waves for facilitating delamination characterization

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:04:32 +0100

We advance this field by systematically exploring nonlinear interactions of A0 Lamb mode signals with delamination defects of various sizes and interlaminar locations, to facilitate the characterization of delaminations. The interrogation signal, in this regard, is a modulated sinusoid whose frequency varies in steps of 20 kHz between 40 kHz and 100 kHz. Commercial FEM software is used for modelling the contact at delamination interfaces and for simulating the Lamb wave propagation through the waveguide with delamination defect. It is demonstrated that the intermittently acting contact pressure between the two surfaces of delamination acts as a source of nonlinearity, resulting in generation of higher order harmonics of interrogation frequency. A metric for measuring nonlinearity, the nonlinearity index (NI), is used to quantify the strength of wave-damage interactions over a range of interrogation frequencies. The NI index is observed to vary with both the interlaminar location as well as the width of the delamination. The maximum value of NI is further influenced by the frequency of excitation signal. To infer the effect of delamination parameters on the NI, a concept of a concept of contact energy intensity is introduced, which is largely dependent on the size and the interlaminar position of the delamination. The nonlinearity index patterns are explained by combining the intensity of the contact energy with the phase difference between waves traveling through the sub-laminations and the flexural rigidity of the two sub-laminates at the delamination location. The inferences provided can potentially be used for determining the interlaminar location and width of delamination employing higher harmonic Lamb wave signals generated by the breathing delamination.

Investigation of performance metrics in regression analysis and machine learning-based prediction models

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:08:43 +0100

Performance metrics (Evaluation metrics or error metrics) are crucial components of regression analysis and machine learning-based prediction models. A performance metric can be defined as a logical and mathematical construct designed to measure how close the predicted outcome is to the actual result. A variety of performance metrics have been described and proposed in the literature. Knowledge about the metrics' properties needs to be systematized to simplify their design and use. In this work, we examine various regression related metrics (14 in total) for continuous variables, including the most widely used ones, such as the (root) mean squared error, the mean absolute error, the Pearson correlation coefficient, and the coefficient of determination, among many others. We provide their mathematical formulations, as well as a discussion on their use, their characteristics, advantages, disadvantages, and limitations, through theoretical analysis and a detailed numerical example. The 10 unitless metrics are further investigated through a numerical analysis with Monte Carlo Simulation based on (i) random guessing and (ii) the addition of random noise with various noise ratios to the predicted values. Some of the metrics show a poor or inconsistent performance, while others exhibit good performance as evaluation measures of the 'goodness of fit'. We highlight the importance of the usage of the right metrics to obtain good predictions in machine learning and regression models in general.

Krueger High-Lift System Design Optimization

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:49:20 +0100

This work describes the cooperative/competitive design process that led to the definition of the Krueger flap to be used in the numerical and experimental tests of the European project UHURA. The project requirements are particularly challenging because it is necessary to develop a device with good aerodynamic high-lift characteristics, but it is necessary to consider many constraints of structural and kinematic nature. Indeed, the kinematics for its deployment is quite complex and imposes hard constraints on the Krueger shape, and the structural characteristics must allow it to withstand considerable structural stresses in the deployment phase which is studied in the wind tunnel.

Laminar Flow Control along the Attachment Line of a Swept Wing

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:09:16 +0100

Reducing aircraft fuel consumption by maximising the extent of laminar flow on wings assumes that the initial flow, along the wing's attachment line, is laminar. However, if the wing is attached to a solid wall, the wing's attachment line can be contaminated by the turbulent boundary layer developing over the solid wall for flow conditions summarised in a critical Reynolds number (R) greater than 250. Since typical R values encountered in flight can be well above 400, techniques, such as wall suction along the wing's leading edge were developed to further delay the threshold R at which contamination occurs. The present paper presents the results from an experimental investigation performed on the ONERA DTP-A model fitted with leading edge suction capabilities. The experiment was performed in the ONERA F2 wind tunnel in the framework of the EU-funded Clean Sky 2 HLFC-WIN project (LPA-IADP platform), while the suction panels were manufactured by Aernnova, an aero-component manufacturing company. Hot film measurements and infra-red thermography showed that attachment line contamination could effectively be delayed up to threshold R values of 1000 for large suction flow rates. Although panels from different manufacturing processes and with different geometric characteristics were tested, no significant difference from these parameters were observed.

Lattice Boltzmann simulation of a deploying Krueger device

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:49:05 +0100

In this paper, we describe the numerical simulations carried out within the H2020 UHURA project of the turbulent unsteady flow generated during the motion of a Krueger device for laminar wings using a commercial lattice Boltzmann solver based on a Wall-Modelled LES approach. The simulations are focused on reproducing one of the experimental test cases carried out in the ONERA-L1 wind tunnel during the UHURA project. The numerical method and the simulation setup are described. The simulation results are compared with the high-quality experimental data obtained in the ONERA-L1 wind tunnel in order to assess the accuracy of the predictions.

Lessons Learnt from Chimera Method Application to a Deploying Krueger Device

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:18:58 +0100

The Chimera method [1] is an established method for simulation of overlapping grids. Meshing parts independently has made this method popular for complex geometries as well as moving bodies like propellers and rotors (e.g. [2]) or control surfaces (e.g. [3]). It is thus a promising method to simulate deflecting high-lift systems. The motion of the Krueger flap – as the most promising leading edge high-lift system device for laminar wing technology – is characterized by a relatively large movement (about 140 deg deflection) at a relatively high deflection speed (up to 200 deg/s) compared to classical leading edge devices. In terms of simulation, the grid properties of the overlapping mesh regions vary throughout the motion from a nearly sealed retracted position to a gapped flow in fully deflected position comparable to a slat device. This expects dynamic effects may get dominant and a valid simulation of this flow is needed for proper design and analysis. In the frame of the UHURA project1 , several partners applied their CFD capabilities based on Chimera in order to validate the method for this specific application in comparison to wind tunnel tests. The presentation outlines the different Chimera approaches ranging from structured/2D to hybrid/3D in steady and unsteady simulations for the different type of setups investigated, namely straight and swept wing with full-span and part-span Krueger flap. It summarizes common challenges and best practice for application of the Chimera approach for such a device.

Limit analysis of non-periodic masonry by means of Discontinuity Layout Optimization

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:51:22 +0100

Masonry structures forming part of our historical heritage were often constructed using nonperiodic textures. In this case, unlike the situation for masonry with periodic textures, few methods are available to estimate wall strength and, moreover, available methods are often difficult to apply. In this work, Discontinuity Layout Optimization (DLO) is proposed as a method of estimating the failure load and associated mechanism of masonry walls constructed with non-periodic textures. In the first part three different textures are considered (periodic, quasi-periodic and chaotic) with a simplified scheme and a parametric analysis is undertaken, considering the variation of the height of the panel. A further classification for quasi-periodic textures is then provided and a DLO rigid block analysis is carried for square panels to show the influence of such textures. The results highlight the importance of the parameters considered in the analysis, and that DLO is a suitable method to investigate their influence.

Mathematical modeling and numerical results on the propagation of solitary waves on tensegrity lattices

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:48:04 +0100

This work discusses the mathematical properties of the interaction potential that characterizes tensegrity mass-spring chains, and its implications in terms of the propagation of compact compression waves in such systems when impacted by a striker. Numerical simulations show evidence of the dependence of the wave form on the speed of the propagating compression pulses, which change shape when passing from the sonic to the super-sonic wave propagation regime.

Microscale numerical simulation of yarn tensile behavior using a high-fidelity geometrical fiber model extracted from micro-CT imaging

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:36:36 +0100

Air jet weaving, where the weft yarn is transported through the machine using air as propelling medium, is a popular weaving method due to its superior productivity, however at the cost of a high energy demand. The interactions between the weft yarn and the air jets are complex and not yet fully understood. Moreover, state-of-the-art techniques to simulate these interactions, are far from mature since the yarn is often simplified as a smooth and solid cylinder. Therefore, a novel multi-scale and multi-physics approach is proposed to simulate the interaction between weft yarns and air jets. Starting from microcomputed tomography (µCT) scans of a yarn used in air jet weaving, a high-fidelity microscale geometrical model is constructed, representing the yarn by its fibers. This geometrical model is used as input for microstructural simulations and will be used for flow simulations on microscale, where the aim is to extract local coefficients and as such characterize the yarn. These coefficients are then used as input for computationally cheap macroscale models, where the yarn is represented by its centerline containing the microscale properties. In a final stage, the macroscale structural and flow models will be coupled as to obtain a full FSI simulation of a weft insertion in an air jet loom. Current paper highlights the microscale geometry extraction of a fine wool fiber yarn of 28.8 tex. Consecutively, a computational framework is proposed to simulate the tensile behavior of this yarn, using the previously obtained microscale geometrical model. The resulting stress-strain curve of the yarn is compared to experiments and shows good correspondence.

Modeling imperfect interfaces in layered beams through multi- and single-variable zigzag kinematics

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:41:56 +0100

Multiscale structural models based on the coupling of a zigzag kinematics and a cohesive crack approach have been recently formulated to analyze the response of shear deformable layered structures with imperfect interfaces and describe progressive delamination fracture (Massabò, in Handbook of Damage Mechanics, Springer, 2022, pp.665-698). The zigzag kinematics accounts for zigzag effects associated to the elastic mismatch of the layers and displacement jumps due to interfacial imperfections, using a limited number of variables, which is independent of the number of layers. The effects of imperfect interfaces on the response of structures subjected to thermo-mechanical loading and on wave propagation and dispersion have been studied analytically and the advantages of this approach over discrete layer models and layerwise theories have been highlighted and discussed. In the presentation we review and discuss these models and present preliminary results on novel single-variable formulations, which have been inspired by a technique developed for homogeneous Timoshenko beams in (Kiendl et al.

Modeling the Effective Inelastic Behavior of Multi-Wire Cables Under Mechanical Load Using Finite Elements

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:39:12 +0100

In the development and manufacturing process of modern cars, cables and hoses are important system components. In automotive industry, virtual assembly planning and digital validation of system layouts require a fast and physically correct simulation of the mechanical behavior of cables and hoses. The mechanical response of cable systems and hoses under load is typically non-linear and inelastic due to their multi-component structure. However, those effects can hardly be observed and investigated separately in experiments. Thus, the authors recently presented simplified cable models using the commercial finite element tool ANSYS which take wire interactions into account. As cables in automotive applications are often subject to large deformations, finite beam elements with quadratic shape functions were used to discretize the single helix wires. The comparison of simulation results obtained for helix wire strands under bending with analytical results based on wire rope theory showed good agreement for the case of frictionless interactions. Furthermore, the modeling approach serves as a versatile toolbox for the investigation of material and structural inelastic effects which commonly occur when cables are deformed. A significant influence of structural parameters, such as the helix angle of the wires or the choice of friction model parameters, on the mechanical response could be found. In this work, this modeling approach is applied to the simulation of multi-wire strands consisting of parallel elastic wires under bending and torsion. The results of these mesoscopic simulations will be compared to experimental results.

Modelling of two-scale contact - Investigation of leakage in polymer seals at cryogenic temperatures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:46:23 +0100

The prediction of leakage in polymer seals is still a particular challenge due to many dependencies: manufacturing inaccuracy, microparticles on the contact surface and surface asperity. Polymer seals, which are operated at cryogenic temperatures, undergo a material behaviour change at the so-called glass transition temperature. At this temperature, its behaviour changes from viscous/rubbery to glassy. There is a significant stiffening of the polymer material, which leads to a worse compensation of roughness in the contact surfaces. As a consequence the tightness of the valve may no longer be sufficiently given. The leakage through the valve is numerically investigated by a two-scale contact simulation, which is based on the concept of Representative Volume Elements, which are known in homogenization of microstructures. The deformations on the microstructure are prescribed by the macroscopic kinematics at the contact area. The mean microscopic friction coefficient is determined in Representative Contact Elements (RCE), which are node-wise linked to the macroscopic contact area. The RCEs surface texture is parameterized based on optical measurement data. As the polymer seal is operated over a wide temperature range, a fully coupled thermo-viscoelastic material model at finite strains is used to simulate the material behaviour at both scales. Due to the change from entropy to energy dominated behaviour over the glass transition temperature the model is extended to account for the transition from viscous/rubbery to glassy as the temperature is decreased. The surface asperity needs to be represented explicitly as the gap and volume between both contact surfaces and the fluid path through the seal are used to determine the leakage through the seal.

Molecular dynamics investigation of the effect of interlayer cavities of the structure of calcium silicate hydrate at the atomistic scale

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:37:09 +0100

Calcium silicate hydrate (CSH) gel, as the most important component of hydration products, has the most significant effect on the properties of hardened cement paste. One of the most critical factors affecting the mechanical properties of CSH is the interlayer cavities in the gel. In this study, the effect of these cavities on Young's modulus of CSH has been investigated. For this purpose, first, the atomic structure of CSH is created, and then interlayer cavities with different dimensions are created inside the structure. For modelling, first, a super cell with dimensions of 3 × 6 × 1 times the unit cell of Tobermorite is prepared, and then each of these layers are placed on both sides of the new cell, and a space is created between these two layers. This distance is basically the cavity between the layers.

Multi-objective optimization for understanding tree design rules with finite element modelling.

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:27:04 +0100

This research presents an optimization problem, which aims to understand the inherent design rules of the shape of a tree. It makes use of a classical optimization problem, the Nowacki beam, which consists of a cantilever beam with a point load at the end which seeks to obtain the lowest cross-sectional area and bending stress under a set of constraints [1]. Forrester et al. developed an algorithm to address this problem [2] by means of machine learning techniques. They began defining the beam properties and building a distributed random sampling plan and then computed the objective function and constraint functions. The process starts from an initially computed dataset that is used to train Kriging models, which are later used as a filling strategy, and genetic algorithms, as an optimization strategy. The result of each iteration is added to the dataset, and the process is repeated until convergence is found. In this way, the Pareto front with the optimal solutions is obtained.

Multi-physical modelling and analysis of lubricated transmissions using a coupled finite element approach

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:03:36 +0100

Lubrication is vital to improve performance, efficiency and durability in transmissions; it serves to minimize wear, noise, vibration and friction. The proper functioning of lubricated transmissions relies on interactions amongst a wide range of physical phenomena (contact dynamics, fluid-structure interaction and heat transfer) operating at different spatial and temporal scales. A lubricated transmission model is proposed in this work to obtain accurate information regarding all physical domains and the coupling thereof to quantify performance, efficiency and durability. The model consists of a thermo-elastically coupled flexible multibody model for the gear pair and a Thermo-Elasto-Hydrodynamic Lubrication (TEHL) model for the lubricant, both based on first principle modelling to ensure high fidelity.

Multiscale finite element modeling linking shell elements to 3D continuum

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:47:10 +0100

The present paper investigates the response of masonry structural elements with periodic texture adopting an advanced multiscale finite element model, coupling different formulations at the two selected scales of analysis. At the macroscopic structural level, a homogeneous thick shell is considered and its constitutive response is derived by the detailed analysis of the masonry repetitive Unit Cell (UC), analyzed at the microlevel in the framework of the threedimensional (3D) Cauchy continuum. The UC is formed by the assembly of elastic bricks and nonlinear mortar joints, modeled as zero-thickness interfaces. The Transformation Field Analysis procedure is invoked to address the nonlinear homogenization problem of the regular masonry. The performance of the model in reproducing various masonry textures is explored by referring to an experimentally tested pointed vault under different profiles of prescribed differential settlements. The structural behavior of the vault is studied in terms of global load-displacement curves and damaging patterns and the numerical results are compared with those recovered by detailed micromechanical analyses and experimental evidences.

New predictive models for ballistic limit of spacecraft honeycomb-core sandwich panels subjected to hypervelocity impact

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:01:39 +0100

Parameters of the honeycomb core (such as cell size and foil thickness), as well as the material of the core, influence the ballistic performance of honeycomb-core sandwich panels in cases of hypervelocity impact (HVI) by orbital debris. Two predictive models capable of accounting for this influence have been developed in this study: one utilized a conventional approach based on a dedicated ballistic limit equation, while the other employed an artificial neural network trained to predict the outcomes of HVI on HCSP. BLE fitting and ANN training were conducted using a database composed of 46 numerical experiments, performed with a validated numerical model and ten physical tests derived from the literature. The new ballistic limit equation is based on the Whipple shield BLE, in which the standoff distance between the facesheets was replaced by a function of the honeycomb cell size, foil thickness, and yield strength of the HC material. The corresponding fit factors were determined by minimizing the sum of squared errors between the BLE predictions and the results of HVI tests listed in the database. The BLE was then tested against a new set of simulation data and demonstrated an excellent predictive accuracy, with the discrepancy ranging from 1.13% to 5.58% only. The artificial neural network was developed using MATLAB's Deep Learning Toolbox framework and was trained utilizing the same HCSP HVI database as was employed for the BLE fitting. The ANN demonstrated a very good predictive accuracy, when tested against a set of simulation data not previously used in the training of the network, with the discrepancy ranging from 0.67% to 7.27%. Both of the developed predictive models (the BLE and the ANN) are recommended for use in the design of orbital debris shielding for spacecraft, involving honeycomb-core sandwich panels.

Nonlinear macroelement based on Bouc-Wen formulation with degradation for the equivalent frame modelling of masonry walls

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:51:03 +0100

The equivalent frame model takes into account the shear and bending mechanisms that take place in piers and spandrels through plastic hinges. It represents a good compromise between accuracy and computational burden in the analysis of complex masonry walls. For the purposes of dynamic analysis, in addition to the hysteretic behaviour of the plastic hinges, it is also necessary to introduce their degradation of strength and stiffness. This study presents a macroelement model for piers and spandrels in which the bending mechanism is described by two hinges at the ends of the macroelement, and the shear mechanism by a shear link. They are characterized by a hysteretic behaviour with progressive plasticity, described by the Bouc-Wen model, and by degradation of strength and stiffness. Degradation is described through a damage parameter, which governs both strength and stiffness decay, and a flexibility increase parameter, which only governs stiffness reduction. In this way it is possible to independently control both strength and stiffness degradation. The model is applied to simulate experimental tests on panels, highlighting a good agreement with the experimental results.

Nonlinear seismic analysis of reinforced concrete structures using POD reduced order method

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:37:42 +0100

This paper presents an extension of the Proper Orthogonal Decomposition method (POD) to nonlinear dynamic analysis of reinforced concrete multistory frame structure where the material nonlinearity is modeled by the multi-fiber section. To test the effectiveness of this approach, we first perform a nonlinear dynamic analysis under a seismic excitation using a direct implicit time integration scheme. Then, based on structural response observations, POD modes were extracted and used to reduce the structural system subjected to different earthquakes. A comparison was made between full model and reduced model analysis in order to assess the effectiveness of this technique.

Numerical investigation of Hydrogen self-ignition and deflagration-to-detonation phenomena using automated meshing approach and detailed chemistry

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:48:46 +0100

Computational fluid dynamics (CFD) plays a critical role in designing safe storage and transport systems for hydrogen. Fine mesh resolution and detailed chemistry are essential for the accurate prediction of self-ignition and deflagration-to-detonation (DDT) in hydrogenair mixtures. However, simulating H2 venting and explosion in real-life scenarios (e.g., with complex obstacle shapes and a large computational domain) involves tedious meshing effort and several mesh iterations to capture flame and shock locations. This paper addresses these challenges by assessing the capability of a detailed-chemistry approach combined with automated meshing based on a cut-cell technique and Adaptive Mesh Refinement (AMR). Furthermore, three different turbulence-chemistry interaction modelling approaches are compared for self-ignition and DDT scenarios: a homogeneous reactor model, an eddy dissipation model, and a flame thickening approach.

On the existence of compression-only discrete force networks that support assigned sets of nodal forces

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:48:14 +0100

We formulate a linear programming approach to the limit analysis of discrete element models of masonry structures. Numerical results show the ability of the given procedure to predict the limit value of the multiplier of variable horizontal forces, which are applied to masonry walls in association with a fixed vertical loading.

Phase field model for simulating fracture of ice

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:46:42 +0100

In the last decade, the phase field model has been established to simulate crack nucleation as well as crack propagation. In this variational approach the physically discontinuity of a crack is modeled by a continuous field variable that distinguishes between intact and broken material. The phase field model has been recently extended to viscoelastic materials in various ways, in which the rate dependent response of viscoelastic materials are taken into account. We propose a viscoelastic fracture phase field model and apply it to simulate the fracture in ice shelves. Thereby we consider the viscoelastic rheology of ice, which can be represented by a Maxwell model. The elastic response is often neglected in ice dynamic simulations but crucial for fracture mechanical studies. The numerical examples of this contribution are implemented and conducted in the finite element software FEniCS and data mimic typical situations in Antarctic and Greenland ice shelves.

Photoelasticity of YVO4 through first-principle calculation

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:46:01 +0100

The laser host materials undergo relatively small changes in their intrinsic properties due to various sources during an experiment. These sources are often related to temperature change and vibrations due to crystal mounting, which may cause stress-induced birefringence in laser host materials[1]. Birefringence is a phenomenon that causes optical anisotropy due to external load or residual stress. As a result, the final outcome of the experiments can be affected. One way to reduce probable noises is to predict the change in optical properties with respect to load. In the case of laser host materials, refractive index is one of the most prominent properties that undergoes change. The change in refractive index due to external load in the linear response regime is known as photoelasticity. Therefore, to predict the change pattern of refractive index in a crystal, one needs to know the elastic and photoelastic constants of the material.

Postbuckling failure mechanism of square aluminum plates under shear loading

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:01:55 +0100

Buckling is very important for structural design, especially when the structure is thin-walled. Slender plates are widely used as structural members in civil, naval and aerospace engineering and they are able to carry considerable additional loads in the postbuckling range. Engineers take advantage of this postbuckling reserve and design structures that are allowed to buckle and operate below their ultimate loads for further weight and cost reduction. Consequently, both buckling and postbuckling behavior of slender plates are of critical importance during the design of lightweight thin-walled engineering structures.

Probabilistic modeling of LCF failure times using a epidemiological crack percolation model

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:43:44 +0100

The analysis of standardized low cycle fatigue (LCF) experiments shows that the failure times widely scatter. Furthermore, mechanical components often fail before the deterministic failure time is reached. A possibility to overcome these problems is to consider probabilistic failure times. Our approach for probabilistic life prediction is based on the microstructure of the metal. Since we focus on nickel-base alloys we consider a coarse grained microstructure, with random oriented FCC grains. This leads to random distributed Schmid factors and different anisotropic stress in each grain. To gain crack initiation times, we use Coffin-MansonBasquin and Ramberg-Osgood equation on stresses corrected with probabilistic Schmid factors. Using these single grain crack initiation times, we have developed an epidemiological crack growth model over multiple grains. In this mesoscopic crack percolation model, cracked grains induce a stress increase in neighboring grains. This stress increase is realized using a machine learning model trained on data generated from finite element simulations. The resulting crack clusters are evaluated with a failure criterion based on a multimodal stress intensity factor. From the generated failure times, we calculate surface dependent hazard rates using a Monte Carlo framework. We compare the obtained failure time distributions to data from LCF experiments and find good coincidence of predicted and measured scatter bands.

Quantifying errors due to the Hertzian contact model in multi-sphere Discrete Element Modelling simulations

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:38:35 +0100

In Discrete Element Modelling (DEM), non-spherical particles are often emulated using clusters of rigidly connected spheres that can be either overlapping or not. Within this multi-spherical approach, two sources of error directly affecting the normal contact forces present can be identified. One is due to the difference between the true particle shape and the multispherical approximation; the other arises from the contact model used in the DEM simulations. The potential for inaccuracy in multi-sphere DEM simulations is well known. However, for a DEM simulator it remains unclear what error might be expected when multi-sphere particles are adopted. This contribution focuses on the role of the contact model as a source of error for the special case of two-sphere particles. Considering a single multi-spherical rod consisting of two identical spheres and quasi-statically compressing it until a total of 5% strain has been applied, the force response obtained through Finite Element Analysis (FEA) was compared against the Hertzian contact model. The process was repeated for spheres with varying degrees of overlap, ranging from 0 to 100%, and the relative errors of the FEA models against the Hertzian contact model were calculated. Up to a 60% sphere overlap, Hertz underpredicts the normal contact forces beyond 0.5% strain, where the disparity between the FEA model and Hertz forces is increasing monotonically with strain. However, beyond an overlap of 70%, Hertz overpredicts the normal contact forces with the sphere overlap being the main driver of this deviation. Future research will involve comparing these errors with the `shape' source of error by compressing perfect spherocylinders, considering rods composed of more spheres, and investigating how the error is affected by the relative orientation of two contacting particles.

REDIM based model reduction of the decomposition of urea-water-solutions in films and droplets

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:47:31 +0100

Selective catalytic reduction (SCR) process with urea-water solution (UWS) is often used in automotive industry to decrease emissions of nitric oxides (NO x) in the exhaust gas. In this process the urea from UWS decomposes to isocyanic acid and ammonia, where the latter is needed to increase the efficiency of the NO xreduction on the catalyst surface. Along with the advantages of using UWS several drawbacks reduce the performance of a SCR system. Incomplete decomposition of urea leads to a formation of residuals affecting the efficiency of the exhaust gas systems. Therefore, the complete decomposition of urea and homogeneous distribution of the resulting ammonia in front of the SCR catalyst represent main challenges in improving the SCR technology. In order to investigate the process of the urea decomposition a detailed chemical kinetic mechanism in the liquid phase is employed. The results are compared with a commonly used approach to model urea decomposition as an evaporation with a following decomposition reaction in the gas phase. It is shown that by using such a mechanism, the decomposition of urea and the gas phase composition with the urea decomposition products can be described more accurately. However, implementing these mechanisms in computations (in CFD approaches) requires a large amount of computational (CPU) time and memory. The method of Reaction Diffusion Manifolds (REDIMs) is implemented for the reduction of the detailed chemical kinetics in the stage of urea decomposition such that the distribution of products of the urea decomposition can be captured accurately in the gas phase with only two reduced variables instead of the 7 gas phase species of the original model.

Rocking Analysis for the bell tower of Sant’anna in Cervino

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:50:33 +0100

In seismic prone areas ecclesiastical masonry complexes have shown a very high vulnerability, as detected after the last Italian earthquakes, such as those occurred in L'Aquila (2009), Emilia-Romagna (2012), Central Italy (2016), and Ischia (2017). These are particular types of aggregate buildings subjected often to partial collapses, due to the presence of highly vulnerable elements, like the bell towers. Preliminary analyses should including straightforward and quick methods are necessary. In this paper the bell tower vulnerability is analyzed taking into account the rocking behaviour of the tower only and considering the contribute of the entire ecclesiastical complex as a rigid body sliding with a fixed friction coefficient with respect to the foundations. It is shown that suitable values of maximum oscillations and horizontal displacements are obtained. The case study is the ecclesiastical complex of S. Anna in Cervino (Caserta, Italy).

Seismic performance of an innovative dissipative replaceable components bracing steel frame (DRBrC)

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:47:26 +0100

The structural performance of a steel Concentrically Braced Frame (CBF) equipped with replaceable dissipative seismic components, called DRBrC, is presented. X-diagonal CBFs are an efficient structural solution for buildings in seismic prone areas, being conceived to dissipate the energy stored during the earthquake through plastic deformation of bracing elements; all the other components remain in the elastic field thanks to opportune design criteria. Of course, structural damages, even if voluntarily located in specific regions, need to be repaired after the seismic event to restore the functionality of the building, leading to relevant economic (and time) effort since the full replacement of damaged dissipative components is necessary after irreversible plastic deformations. Recently, research activities have been widely carried out to provide repairability of steel buildings by means of easily replaceable dissipative components. The Research Fund for Coal and Steel (RFCS) of European Commission, for instance, promoted and funded the research project DISSIPABLE Fully dissipative and easily reparable device for resilient buildings with composite steel-concrete structure', with the aim of designing, producing, optimizing and testing several dissipative components for steel structures having, as fundamental feature, the full repairability after the earthquake without impacting on other components. In the present paper, the seismic performance of a steel braced frame equipped with a specific typology of dissipative replaceable device at the ends of braces is presented by means of nonlinear analyses.

Seismic performance of dissipative automated rack supported warehouses

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 12:47:42 +0100

In this paper, performance of dissipative reduced-section diagonals implemented in Automated Rack Supported Warehouses (ARSWs) is analysed. ARSWs are storage systems where the steel racks traditionally used to store goods only, constitute also the primary system of the building. Given the lack of a reference design code to design these structures as seismicresistant, and starting from a critical analysis of the current design approaches, a proper design strategy for these structures has been developed, which is based on the possibility to dissipate seismic energy in the bracings. However, the design of an over-resistant connection that would allow the yielding of the bracings is quite tricky due to the very low thickness of the elements usually adopted for these structures and is only possible by reducing the cross section of the profile. A wide experimental campaign is preformed to validate the behaviour of these diagonals, to be later implemented in the global numerical model of the structure to measure the effective performances of the case study ARSWs, both at local and global level.

Simple modeling for stiffness evaluation of bolted joints using interfacial element

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:44:22 +0100

This study has developed a novel finite element named interfacial element which simulates the contact between microscale asperities at contact surfaces of bolted joints. In this element, the contact is assumed to be the Hertzian contact of elastic asperities whose peak heights obey the Gaussian distribution. Based on this assumption, the stiffness of the interfacial element is derived from the compressive stress and the surface texture of the interfaces. On the other hand, it is necessary for large-scale simulations that target the entire vehicle body to reduce the number of nodes and elements in the finite element models. This study has further proposed simple modeling for stiffness evaluation of bolted joints using the interfacial element. Finite element simulations by simplified models in which heads, axes and holes of bolts were ignored were conducted and compared with detailed models and hammering tests. The results revealed that the mean value of the natural frequency of the simplified models had good agreement with that of the detailed models and the hammering tests though the calculation accuracy of the simplified models were lower than the detailed models. The bolt heads and the nuts could be ignored by increasing the density of the bolt axes to be equal to the total weight.

Simulating Nonlinear Elastic Behaviour of Cables Using an Iterative Method

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:38:16 +0100

This contribution introduces a novel approach to simulate the nonlinear elastic bending behaviour of cables. Bending tests on real cables with complex structures clearly show the existence of nonlinear constitutive bending behaviour. In our current framework, only constant effective stiffness parameters are used. In order to enable nonlinear bending behaviour within the current framework, we propose an iterative method where, at each step, constant stiffness parameters are used and are updated according to the current cable state. The presented method is demonstrated by means of numerical experiments. Moreover, the inverse problem, i.e. the determination of state-dependent bending stiffness characteristic, is considered.

Simulation and experimental investigation of tire tread block wear in three-body contact

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:36:54 +0100

On gravel roads, tires are not in direct contact with the surface. Particles of e.g. sand or stone contact directly with the tire and cause the tire to wear. During the sliding process, an interaction between particles and the tire occurs. On simplified conditions investigation of the movement of particles, friction and wear of a tire tread sample are done experimentally for different settings. Additionally, a finite element simulation is built up to simulate the wear of the tire tread sample under varying conditions. In this paper results from the experimental and numerical investigation of the wear of the tire tread sample are shown.

Simulation of non-round particles in tribological three-body systems

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:37:25 +0100

Particles between the contact interfaces of two components in relative motion are present in many technical applications and can strongly influence the system behavior. In this context, the focus is often on the investigation of wear and damage. In addition to such undesirable phenomena, however, there is also the targeted use of hard particles, for example in the lapping process. In lapping, hard particles are intentionally inserted between a lapping disc and the workpiece surface to be processed in order to cause material removal with the help of the particles and to improve the morphology of the workpiece surface for certain applications. Many simulations of such tribological systems are based on the assumption of spherical particles. However, both, size and shape of the particles have an essential effect on the system behavior. Here, an approach is presented in which hard, arbitrarily shaped particles in tribological contacts can be studied a priori using the finite element method by performing indentation simulations for various particle orientations. Based on the results, an orientation-dependent particle model is created for simulations of the overall system, which includes particles in narrow gaps. This modular design allows direct control in the implementation of phenomenological effects and new insights into the behavior of such systems, as well as the estimation of the resulting surface topography.

Simulation of phase transformations in polycrystalline shape memory alloys using fast Fourier transforms

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:40:50 +0100

The properties of shape memory alloys (SMA) are mainly influenced by phase transformations between austenite and martensite. The complex material behavior is described by a variational method which describes the evolution of the phase fractions. We combined the method with a microstructural analysis based on fast Fourier transformations. Such a highly resolved microstructural analysis comes along with a high computational effort.To reduce the later one, we propose a model order reduction technique that uses just a reduced set of Fourier modes, which is adapted to the underlying microstructure. The presentation of the theoretical background as well as of the implemented algorithm is followed by numerical results that underline the performance of our method.

Simulation of soft robots with nonlinear material behavior using the cosserat rod theory

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:24:15 +0100

For the simulation and model-based control of soft robots accurate models are required. In this contribution the simulation of a simple soft robot with the cosserat rod theory is examined for both linear and nonlinear material models. The soft robot is fabricated out of silicone. Thereby stiffness and damping properties are investigated. In addition to the achievable accuracy, the computation time is also examined.

S-N-based Fatigue Damage Modelling of Offshore Structures

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:22:38 +0100

Recently proposed damage models are presented and compared. In addition, a deterministic algorithm for nonlinear fatigue damage monitoring is presented and discussed. Furthermore, the commonly adopted functional forms for damage modelling is proposed and the adequacy of their functional form is directly investigated by the help of experimental data. Thereafter, the accuracy and whether the presented models give conservative estimates in comparison to Miner’s rule are checked with experimental data. It is found that the proposed models generally perform better for various materials commonly subjected to fatigue. Finally, it is discussed how both the theory and deterministic algorithms now exist for both adopting nonlinear functions in design if the expected loading sequence can be determined, whereas it can always be adopted for fatigue based structural health monitoring.

Stochastic Simulation of the Head Impact on Windscreens

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:06:41 +0100

In accidents involving cars with pedestrians, the impact of the head on structural parts of the vehicle takes a significant risk of injury. If the head hits the windscreen, the injury is highly influenced by glass fracture. In pedestrian protection tests, a head impactor is shot on the windscreen while the resultant acceleration at the COG of the head is measured. To assess the risk of fatal or serious injury, a head injury criterion (HIC) as an explicit function of the measured acceleration can be determined. The braking strength of glass which has a major impact on the head acceleration, however, is not deterministic but depends on production-related micro-cracks on the glass surface as well as on the loading rate. The aim of the present paper is to show a pragmatic method, how to include the stochastic failure of glass in crash and impact simulations. The methodology includes a fracture mechanical model for the strain rate-dependent failure of glass, the experimental determination of the glass strength for the different areas of a windscreen (surface, edge, and screen-printing area), the statistical evaluation of the experimental data and the computation of a HIC probability distribution by stochastic simulation.

Tangential differential calculus for curved, linear Kirchhoff beams with systematic convergence studies

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:46:15 +0100

We propose a reformulation of linear Kirchhoff beams in two dimensions based on the tangential differential calculus (TDC). The rotation-free formulation of the Kirchhoff beam is classically based on curvilinear coordinates. However, for general applications in engineering and sciences that take place on curved geometries embedded in a higher-dimensional space, the tangential differential calculus enables a formulation independent of curvilinear coordinates and, hence, is suitable also for implicitly defined geometries. The geometry and differential operators are formulated in global Cartesian coordinates related to the embedding space.

Taut domain analysis of transversely isotropic dielectric elastomer membranes

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:26:28 +0100

Actuation devices made of dielectric elastomers are prone to compression induced wrinkling instabilities, which can adversely affect their performance and may lead to device failure. On the other hand, wrinkles can be used constructively in certain applications demanding a controlled alternation of the surface morphology. The idea of taut states and the natural width under simple tension plays an important role in the analysis of compression generated instability (wrinkling). In case of electrically driven DE membranes, the domain of taut states in the plane of principal stretches is influenced substantially by the applied voltage and the film's constitutive properties. In the recent past, there has been an increasing interest in exploiting anisotropy in the material behavior of dielectric elastomers for improving their actuation performance. Spurred with these ongoing efforts, this paper presents a continuum mechanics based electromechanical model for predicting the thresholds on the domain of taut states of transversely isotropic planar dielectric elastomers. The developed analytical framework uses an amended energy function that accounts for the electromechanical coupling for a class of incompressible transversely isotropic dielectric membranes. The required expressions for the total Cauchy stress tensor and the associated principal stress components are evaluated utilizing the amended energy function. Finally, the concept of natural width under simple tension is implemented to obtained the nonlinear coupled electromechanical equation that evaluates the associated taut states domain of the transversely isotropic planar dielectric elastomers. Our results indicate that the extent of taut domain can be controlled by modifying the level and the principal direction of the transverse isotropy. The taut states domain for a particular level of applied electric field increases with increase in the anisotropy parameter, while the taut domains depleted with the increase in fiber orientations from 00to 900for an applied level of electrical loading. The fiber-reinforced wrinkle-tunable surfaces can be effectively designed and developed using the underlying analytical framework and the trends obtained in this study.

Theoretical procedure to predict the local buckling resistance of aluminium members in elastic-plastic range

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:27:59 +0100

In the present research work, a theoretical approach to evaluate the ultimate resistance of aluminium alloy members subjected to local buckling under uniform compression is provided.

Thermal and structural modelling of thermoset composite repairs towards optimization of the cure cycle for minimum distortion

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:02:45 +0100

During manufacturing of composite materials residual stresses can result in distortion of the final part. This distortion is even more critical when building on existing parts, such as for a repair, as the added material has to conform to the original structure. To predict this distortion due to curing of thermoset carbon-matrix composite repairs, a numerical modelling method is employed. The temperature cycle applied for the cure of thermoset composites can significantly influence the amount of residual stress and resulting deformation after manufacturing. Therefore, a method is devised to parametrise and subsequently tune this temperature cycle for minimum distortion after manufacturing. Numerical tests with the optimised temperature cycle resulted in a 36% reduction in process induced strain for a repair of a flat laminate plate. The same methodology is applied to a wing box consisting of two composite skins connected by two C-spars where a scarf repair is applied in the skin. The repair patch is locally heated by a heating blanket to cure the repair patch. A subsequent thermalmechanical model is used to investigate the amount of residual stress and strain after cure and the influence of underlying structural elements on the repair. The developed framework can support the patch fabrication with accurate design and analysis for repairs with minimum distortion. Which in turn will result in development of cost-effective composite repairs.

Two-scale asymptotic homogenization in a MEMS auxetic structure for over etch identification

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:24:55 +0100

The development and optimization of Micro electro-mechanical systems (MEMS) devices, due to their small size scale, require testing and precise characterization. As an example, over etch, which is the deviation between the designed masks and the effective dimensions of the suspended parts, strongly influences the performances of MEMS; therefore, to predict the correct functioning of the device its actual value must be carefully identified. In this work, we propose an efficient, time-saving tool to identify fabrication imperfections in MEMS devices. In particular, we replace the complex geometry of a MEMS mechanical filter with an equivalent homogeneous medium, whose linear-elastic effective properties are evaluated employing twoscale asymptotic homogenization and we identify the over etch by minimizing the relative error between experimental data and corresponding predictions obtained for different combinations of over etch.

Uncertainties of Parameters Quantification in SHM

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:27:23 +0100

The uncertainties of parameters quantification due to various known and unknown conditions are crucial to understand structural health monitoring (SHM) systems. For instance, the amplitudes and the variation of loading conditions play a vital rule how the structural parameters are going to be changed. Hence, the aforementioned issue leads to an additional challenge in the area of SHM that requires attention. This study observed the behaviour of a steel bridge experimentally by employing multi-sensors scenarios e.g. accelerometers and laser triangulation sensor. The dynamical properties such as the peak (e.g. maximum-minimum) accelerations and displacements are evaluated. Additionally, the frequencies and damping ratio from the measured data of the tested bridge has been estimated by utilizing the fast Fourier transform (FFT) estimation. The outcome shows that the variation of input excitations (i.e., random, free-decay, extra-loading) effects the investigated properties as well as on their magnitudes considerably. Therefore, the findings suggest that before making a final judgement based on the identified/estimated properties from measured data, the underlying uncertainties need to be considered to avoid sub-optimal assessment strategy.

Using a reduced set of Fourier modes in terms of a FFT-based microstructure simulation

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:40:32 +0100

Given a heterogeneous material, the mechanical behavior of its microstructure can be investigated by an algorithm that uses the Fourier representation of the Lippmann-Schwinger equation. Incorporating a model order reduction technique based on calculations with a reduced set of Fourier modes, the computational cost of this algorithm can be decreased. It was shown that the accuracy of this model order reduction technique strongly depends on the choice of Fourier modes by considering a geometrically adapted rather than a fixed sampling pattern to define the reduced set of Fourier modes. Since it is difficult to define a geometrically adapted sampling pattern for complex microstructures, additionally a strain-based sampling pattern was introduced. The accuracy and adaptability of this strain-based reduced set of Fourier modes is shown by incorporating a polycrystalline microstructure.

Using artificial intelligence techniques for the accurate estimation of the ultimate pure bending of steel circular tubes

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:03:56 +0100

In this paper, the potential of building more accurate and robust models for the prediction of the ultimate pure bending capacity of steel circular tubes using artificial intelligence techniques is investigated. Therefore, a database consisting of 104 tests for fabricated and cold-formed steel circular tubes are collected from the open literature and used to train and validate the proposed data-driven approaches which include the Random Forest methodology in two variants: the original version in which the control parameters are manually updated, and an enhanced RF-PSO variant, where Particle Swarm Optimization is used for optimizing these parameters. The data set has four input parameters, namely the tube thickness, tube diameter, yield strength of steel and steel elasticity modulus, while the ultimate pure bending capacity is considered as the target output variable. The obtained results are compared to the real test values through various statistical indicators such as the root mean square error and the coefficient of determination. The results indicate that the proposed enhanced model can provide an accurate solution for modelling the complex behavior of steel circular tubes under pure bending conditions.

Validation of the temperature history during extrusion based additive manufacturing

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:46:52 +0100

Additive manufacturing (AM) is considered as a key technology for the efficient production of individualized components. The technique enables the tool-less production of complex geometries and designs that could not be realized cost-effectively with conventional manufacturing methods. The focus of this work is on the Fused Filament Fabrication (FFF), where a thermoplastic filament is extruded trough a nozzle. The material is deposited layer by layer until the final part is build. Thereby, high temperature gradients occur within the part when the hot material is deposited on the lower layers. During the printing process the lower layers are reheated several times so that the material properties are influenced even after the deposition has been made. The thermal history has major effects on crucial material properties such as the degree of crystallization or thermal and mechanical properties. An inadequate degree of crystallization influences both the structural properties such as the stiffness or the degree of bonding and the dimensional accuracy due to subsequent shrinkage effects. Additionally, the temperature gradients cause residual stresses which are partly relaxed to varying part deformations. The remaining stresses can lead to premature failures. To achieve a high and repeatable part quality as well as a low scatter in the final part dimension an in-depth process understanding is required considering the underlying material-process-partinteractions. In order to analyse these complex multiphysical processes, manufacturing process simulations are a suitable method. A main requirement for the numerical analysis of AM processes is the calculation of the thermal history as accurately as possible. The objective of this work is, therefore, to evaluate the prediction accuracy of currently available AM process simulation tools. For this purpose, a gcode-based AM process simulation of a cuboid is performed in order to calculate the transient temperature fields using the Abaqus AM plug-in from Dassault Syst`emes. The cuboid made of PETG is printed with a Prusa i3 MK3.

Voxel-based density registration of trabecular bone: a longitudinal HR-pQCT study of postmenopausal women

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 09:46:33 +0100

Bone mineral density (BMD) is one of the important parameters used to characterise bone quality. Clinically, the only recommended method - dual X-ray absorptiometry - can only evaluate a two-dimensional areal BMD. Currently, three-dimensional localised BMD information is absent. HR-pQCT enables the assessments of 3D microstructure down to trabecular bone. Therefore, in this study, a voxel-based density registration (VDR) method is proposed to analyse the longitudinal changes of trabecular-bone density distribution. The VDR techniques were evaluated based on a six-month longitudinal study of five postmenopausal women. The time effect on localised changes of trabecular-bone mineral density was visualized and variations between different anatomical regions were quantified for the first time. Different distributions between anatomical regions were found in bone mineral density of trabecular bone (vBMDtrab), with a change of vBMDtrab at medial region (-0.56%) significantly higher than anterior (-1.58%) (p = 0.032). This study indicates that localised density changes might be used as a prior indicator for the effect of aging or other interventions.

Wave Based Method for 2D Unsaturated Elastodynamic Soil under Harmonic Loading

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 10:22:56 +0100

The Wave Based Method (WBM), which describes field variables of a boundary value problem with weighted wave functions, is used to model a layered halfspace under harmonic loading. This 2D soil structure is investigated for partially saturated layers. According to the Berryman-Thigpen-Chin model (BTC model) the capillary effects between the water and air phases enclosed by pores are negligibly small for elastic waves in the low frequency range. The mixture of water and air is treated as one fluid, for which the material parameters used in Biot's theory for fully saturated poroelastic structures have to be computed. Within this work, the BTC model is applied to describe soil layers within a halfspace, which are not perfectly dry or fully saturated. It is presented how the degree of saturation within a partially saturated soil layer affects the system's response.

Wear modelling in elasto-plastic wheel-rail contact problems

Jesús Sánchez Pinedo — Wed, 23 Nov 2022 11:47:50 +0100

The paper is concerned with the development of the numerical procedure to solve the wheel-rail contact problem and the computation of the distribution of surface flash temperatures, stresses as well as the wear evolution due to friction. The two-dimensional wheel-rail contact problem between a rigid wheel and an elasto-plastic rail lying on a rigid foundation is considered. The contact phenomenon includes Coulomb friction, frictional heat generation as well as the wear of the contacting surfaces. The displacement and stress of the rail in contact are governed by the coupled elasto-plastic and heat conductive equations. The wear depth function appears as an internal variable in the non-penetration condition updating the gap between the worn surfaces of the bodies. Moreover the dissipated energy due to friction is calculated to evaluate the loss of rail material and to determine the shape of the contacting surfaces during the wear evolution process. This contact problem is solved numerically using the finite element method as well as the operator splitting approach. The plastic flow and friction inequality conditions are reformulated as equality conditions using the nonlinear complementarity functions. The distribution of surface temperatures and stresses as well as the evolution of the shape of the contact surfaces and the wear depth are reported and discussed.