Documents published in Scipedia

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

  I. Sioutis, K. Tserpes

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  Fiber bridging is a mechanism that may significantly alter the fracture behavior of composite laminates, adhesively bonded laminates, welded laminates, and co-consolidated [...]

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• Phase field model for simulating fracture of ice

  R. Sondershaus, R. Müller

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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

  K. Martin, J. Waimann, S. Reese

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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• Design of piezoelectric lattice metamaterials

  N. Nodargi, C. Intrigila, P. Bisegna

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  Piezoelectric lattice metamaterials are considered. A computationally-effective homogenisation method is developed based on the recent solution to the Saint-Venant problem [...]

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• Attenuation and localization of waves in taut cables with suspended masses

  M. Moscatelli, C. Comi, J. Marigo

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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

  M. Reuvers, B. Boes, S. Felder, T. Brepols, S. Reese

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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

  T. Laschuetza, T. Seelig

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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

  J. Barrera Cruz, K. Maute

  eccomas2022.

  
  

  Abstract

  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).

  Abstract
  Research on Soft Active Materials (SAMs) has flourished in recent years driven mainly by potential applications to actuation systems, tissue engineering, and soft robotics. [...]

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• Simple modeling for stiffness evaluation of bolted joints using interfacial element

  Y. Kishimoto, Y. Kobayashi

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  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 [...]

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

  A. Fanara, L. Courard, F. Collin

  eccomas2022.

  
  

  Abstract

  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.

  Abstract
  Decreasing CO2 emissions and preserving natural resources are necessary to the well-being of our civilisations. In the construction industry, recycling old concrete members [...]

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