Colloquiam: 9th ECCOMAS Thematic Conference on the Mechanical Response of Composites (COMPOSITES 2023)

Progressive Damage in thin 2D Woven CFRP Laminates due to Stress Concentrations at Free Edges and Notches

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:30:36 +0100

Stress concentrations are present at cut-outs, notches, and generally at free edges in woven CFRP structures. Under cyclic loading, damage initiates from these stress raisers and progresses into the laminate, leading to strength reduction and structural failure. The present contribution provides a literature review summarizing analytical, experimental and numerical investigations regarding damage initiation and propagation in the presence of free edges and at notches in thin plain-woven 2D CFRP laminates. For free edges, initiation of damage is given as interlaminar matrix cracking. Modelling approaches for the progression by cohesive zone models or linear-elastic fracture mechanics are summarized. Recent advances using image correlation and numerical modelling are presented. In terms of notches, a brief survey of relevant literature is given, followed by a more detailed treatment of the damage progression originating from a circular hole. Additionally, the shortcomings of standard specimens with holes for fatigue damage progression investigations are addressed, since both mechanisms, damage from the free edge and the hole, interact. Latest research to uniquely identify the damage emanating from the hole is presented

Investigation into the Manufacture of Hybrid Wear Resistant Forging Tools using Tailored Forming Technology

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:30:21 +0100

Forging tools that are subject to high thermo-mechanical loads require a correspondingly high heat resistance, hardness and ductility to prevent undesirable failure patterns due to wear, plastic deformation and crack formation. These properties are mainly required in the layer of the tool engraving close to the surface, as this area is exposed to the highest loads due to the contact with the hot workpiece. With the heat transfer from the workpiece to the tool, a cyclic tempering process of the tool material often occurs in this area, resulting in a decrease in wear resistance. Diffusion treatments of the tool surface, such as additional nitriding, cannot provide sufficient protection in thermally highly stressed tool areas, as the heat-affected zone in these areas extends beyond the surface modification. As a result, a tempered layer forms under the nitrided layer, on which it can slide off or break as a result of the high process loads and the wear protection is lost. The use of nickel-based alloys promises an improvement in service life due to their high specific heat. However, these alloys are much more expensive than hot-work tool steels and are more difficult to machine, which has a negative effect on the economic use as a die material. Furthermore, nickel-based alloys do not have the high strength of steel that is often required in the base material that is subject to low thermal loads. To reduce the material usage of nickel-based alloys, but to fully take advantage of their positive properties, the suitability of the Tailored Forming concept in thermo-mechanically highly stressed areas were investigated within the scope of this research. For this reason, hybrid forging tool inserts with a base material of hot-work tool steel and a protective nickel-based alloy surface layer were produced. The hybrid tools are manufactured through a process combination of rotatory friction welding and die forging. The surface enlargement as a result of the forming process is to be used specifically to protect the relevant tool areas with a layer of nickel-based alloy and at the same time minimize the use of the expensive material. The effects of the thermo mechanical treatments occurring in the joining zone were examined and the potential of the technology was investigated

Evaluation of Bend-Twist Coupling in Shape Memory Alloy Integrated Fiber Rubber Composites

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:30:01 +0100

Advancements in textile technologies such as the integration of wire shaped Shape Memory Alloys (SMAs) on to the fabric with the help of Tailored-Fiber-Placement (TFP) method, and weft insertion of SMAs during manufacturing of textiles using knitting machines are helping to create composites capable of bending deformations without any external loads. These advancements laid the foundations for versatile applications especially in soft robotics. One such application is Interactive Fiber Rubber Composites (IFRC). The aim of this project is to evaluate the bend-twist coupling in the IFRC. The SMA reinforced composite is made of polydimethylsiloxane (PDMS) and has two layers of glass fibers stacked upon one another and joined with the help of TFP machine. This work focuses on the simulation of this approach in ANSYS with the Woodworth & Kaliske material model for SMA. The important feature of this model is that the shape memory effect can be achieved for different profiles of SMA, thus eliminating the necessity for a pre-stretch in contrast to the built-in model. The experimental values are evaluated from Multi-DIC technique, which is capable of determining deformations with respect to all directions. A comparative study with simulation and experimental results of the deformation and twisting angles is carried out. The derived conclusions will be helpful in obtaining and evaluating 3D spatial movements in IFRC structures with multiple joints in the future projects

Analysis of Cure Behaviour Uncertainties in Thermoset Composite Parts using Particle Filter

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:29:39 +0100

Process-induced deformations result from internal residual stress caused by the anisotropic properties of thermoset composite parts. The study’s focus is diagnosing the polymerization process, or curing, and considering how uncertainties in boundary conditions affect cure kinetics. This is achieved through a Particle Filter approach, utilizing a Bayesian framework. This framework recursively estimates the evolving cure state’s posterior distribution based on observed measurements from Differential Calorimetry Scanning tests and thermocouples. The algorithm simultaneously estimates the cure state parameters and predicts part temperature and process-induced deformations, which are closely tied to cure behaviour. This is accomplished using diffusion cure kinetics and analytical deformation models. Furthermore, it introduces an augmented cure state formulation to address uncertainties in cure boundary conditions, which conventional models overlook. The developed stochastic approach adeptly captures uncertainties related to cure evolution while providing comparable deformation predictions with minimal computational costs and memory usage. Experimental measurements of process-induced deformations in C-shaped thermosetting parts, made of unidirectional 8552/AS4 fibres and cured following the Manufacturing Recommended Curing Cycle, are validated using the developed algorithm. After validation, the proposed model is employed to predict outcomes, which are then utilized to determine the optimal curing cycle using a Genetic Algorithm.

Numerical Simulation of the Fatigue Damage Growth in Unidirectional Composites based on Fibre-Matrix Debonding

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:29:04 +0100

A methodology for investigating the micromechanical fatigue behaviour of unidirectional composites based on fibre-matrix debonding is developed. The fatigue damage mechanism is based on the progressive failure of fibres caused by debond crack tip stress fields resulting from fibre breaks in previous load cycles. The methodology combines an analytical model to describe the debond crack initiation and growth with a numerical finite element model to calculate resulting stresses. The methodology is applied on a two-fibre model composite. It can qualitatively predict the stress development within the simulation domain as well as the mechanism of a debond crack tip stress field triggering a break in a neighbouring fibre. Both is consistent with microscale observations in the literature.

Micromechanical Homogenization Methods of Short Glass Fiber-reinforced Injection-molded Bio-based Composite Material

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:28:44 +0100

Herein, microscale approaches were explored to determine the homogenized properties of short fibre reinforced polymer material. The analytical homogenization follows the shear lag principle to approximate elastic modulus in the case of longitudinally oriented short fibres. For the finite element-based homogenization, a periodic 3D representative volume element of the composite is constructed to apply forward numerical homogenization. This unit cell is discretized by tetrahedral 3D finite elements resulting in a periodic mesh. An effective spring element method was further developed to homogenize the properties of short fibre-reinforced material. The reduced order spring method predicted the elastic properties almost equally to the finite element-based homogenization. A novel bio-based polyamide matrix with 40% glass fibre content and a traditional polyamide with 30% glass fibre reinforcement serve for the application and validation of the developed micromechanical methods. An additional effectivity parameter must be considered to capture the manufacturing imperfections of the injection molding process. This parameter can be calibrated based on experimental data from tensile testing. The developed numerical frameworks show good potential for extensions to more advanced modelling of the composite, such as nonlinear behaviour or failure mechanism.

Impact damage modelling in composite laminates – numerical implementation of a strain rate dependent damage model

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:28:29 +0100

In this work, a numerical methodology for simulation of impact damage in laminated CFRP structures has been developed and implemented into the Abaqus/Explicit software. The methodology is based on the recent insights into the mechanical behaviour of CFRP materials at various strain rates. Failure initiation is modelled using the failure theory that was introduced in Coles et al. (2019). This approach has been modified to include the strain rate effects according to work presented in Raimondo et al. (2012) and to account for the mesh objectivity of the damage process in this work. The model has been implemented into Abaqus/Explicit using the user material subroutine VUMAT and details of the implementation are discussed in the work. The model has been applied to the simulation of two impact simulations demonstrating that the damage modes of the composite plate, as well as damage scope and displacement fields, have been simulated accurately. The methodology has been previously developed for application in unidirectional composite plates, whereas this work and current research phase focus on woven composites. Additionally, only the in-plane failure modes are currently considered whereas the out-of-plane damage modes will be investigated in the future research.

SHM system for bonding line monitoring of composite wing box skin-spar cap during manufacturing

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:28:10 +0100

The CFRP wing box section under investigation is a stringer less wing-box (develop within the OPTICOMS research project) and consists of two portions: an upper part, made of co-cured spars and a top skin panel, and a bottom cured skin panel. The two portions are joined with a bonding process, giving rise to the final wing-box. During that final assembly step, distributed fibre optics were embedded between the spar caps and the bottom skin panel along the bonding lines. The embedded FO consists of six distributed fibres running within the bonding layer for about 1 m along the span direction. An irregular damage map was defined, by simulating the presence of manufacturing bonding defects by the intersection of teflon patches, different for width and length, to check an SHM system capabilities in detecting such flaws. The SHM system was tested after the final bonding process, by exploring the info contained within the “residual strains” data of the unloaded structure. Results obtained by post processing data for each fibre optic, are reported. The damage index associated to the eligible sensors is provided. Based on the available data, the SHM algorithm appeared to be sensible enough to hundreds of microstrain signals. Concerning faults detection, sensor density seems a key. Errors in the estimate of the damage extension can be assessed to be around 25%

Application of Model-based Design Approach on Dynamic Tensile Testing of Carbon/Epoxy Composites at Intermediate Strain Rates

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:27:52 +0100

The reliable strain rate-dependent material properties at intermediate strain rate levels (1-200 s-1 ) are crucial for an accurate crashworthy design of fibre-reinforced polymer (FRP) composite structures. However, the presence of unacceptable oscillations in measured force signals hinders the precise identification of the dynamic mechanical response of materials. The current work reports the results of gained in an initial study using a novel numerical model developed through a Model-based Design (MBD) approach. A multi-degree of-freedom (MDOF) mass-spring-damper model is employed to investigate the dynamic characteristics of a whole experimental test setup to gain insights into the dynamic interaction between the test machine and the test specimen. The developed model was calibrated by the results from dynamic tension testing of Aluminium Alloy 2024-T3. Then, the model parameters were optimised using a genetic algorithm (GA). Subsequently, the adaptability of the developed model to carbon/epoxy composites, IM7/8552, was examined. The proposed model is promising to identify the influence of the test setup on the measurements and effectively distinguish excessive oscillations caused by its inertial effect at intermediate strain rate levels. The model will offer a robust solution to identify oscillations and, therefore, expand the testing capabilities to a broader range of strain rates.

Tensile Testing of Pinned Hybrid CFRP/Titanium Joints and Damage Monitoring with Electrical Resistance Measurements

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:05:09 +0100

This contribution presents a new evaluation approach for Structural Health Monitoring of a pinned hybrid CFRP/titanium single-lap shear joint with the help of direct current electrical resistance measurements. The result is a dimensionless, load-independent damage indicator that is similar to an already developed evaluation approach by the authors but is simpler and more robust in comparison. Readily published test data is re-evaluated with the new evaluation approach and compared with the existing structural as well as the electrical resistance results. Finally, further test setup improvements for future tests are discussed.

Effect of Ply Thickness on Dynamic Damage Progression in Cross-ply Laminates Under Low-velocity Impact

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:03:59 +0100

: In this study, we have conducted in-situ LVI experiments on cross-ply CFRP beams having stacking layups [04/904/02]s and [02/902/02/902/02]s. The progression of damage is observed through high-speed photography. In addition to LVI, quasi-static indentation (QSI) experiments are performed to reduce challenges in monitoring damage progression during the short impact loading interval. QSI experiments provide magnified in-situ observations on the free edge of the beam using a traveling digital microscope. Numerical simulations of these experiments are carried out using the finite element method in ABAQUS/Explicit. The orthotropic constitutive material model, predicting fiber and matrix damage initiation and evolution, is implemented through a VUMAT subroutine. The comparison between numerical simulations and experimental observations allowed us to investigate the influence of ply clustering on the LVI-induced damage mechanisms.

Investigation of Several Impact Angles for Predicting Bird-Strike Damage in a Riveted eVTOL Composite Wing

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:02:36 +0100

A 3D Damage Model for Simulating Damage Modes in Fibre Metal Laminates

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:02:19 +0100

This paper presents a 3D damage model utilized for studying the failure characteristics of GLARE. The current damage model adopted the 3D forms of Hashin’s and Puck’s failure criteria for predicting the onset of failure of fibres and matrix in the composite plies. Whilst the damage evolution is modelled based on the dissipation of fracture energy. In addition, a ductile damage model was employed to study the failure of metal layers and the delamination was assessed via a cohesive interface model. The current damage model was adopted to predict the failure modes and the blunt notch strength of GLARE; where various failure modes were observed, such as Fibre breakage, matrix cracking, delamination and plastic damage of aluminium layers. The model showed strong agreement with experimental results.

Virtual manufacturing of thick composite beams, investigating cure cycle and shrinkage induced stress

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:01:14 +0100

Investigating cure shrinkage-induced stress in thick composite beams by virtual manufacturing is the focus of this study. The research aims to understand the behaviour of thick-walled composite structures, particularly in relation to curing shrinkage-induced damages. The curing process of resin is simulated thermally and mechanically to investigate the residual cure-induced stress. The study utilizes a finite element model in Abaqus, considering material properties, mesh, boundary conditions, and user subroutines. Ten different cure cycles are investigated, showing improvements in reducing internal stresses after curing compared to the manufacturer's cycle of about 20%. However, during curing, the investigated cycles provide marginal improvements. This study demonstrates the potential for optimizing cure cycles to reduce internal stresses in thick-walled applications. It is important to note that the proposed method is not experimentally validated and requires accurate measurements for validation.

Free Vibration Analysis of Thermally Prestressed Constant and Variable Stiffness Laminated Beams using Strong Unified Formulation

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:00:51 +0100

Free vibration analysis is an essential requirement to capture the behaviour of composite structures subject to dynamic loading environment. To enhance the vibratory behaviour of composite structures, variable stiffness (VS) concept offers increased design flexibilities to tailor the structural response to meet a wide range of applications. Mechanically, the increased design space created by VS techniques leads to complexities of non-classical stiffness couplings which necessitate robust computational frameworks with enriched kinematics to predict the dynamic response accurately and efficiently. In this regard, this study proposes an enhanced differential quadrature based Strong Unified Formulation (SUF) to investigate the free vibration behaviour of thermally prestressed constant and variable stiffness composite beams. The proposed SUF model exploits the flexible kinematical description of the Theory of Unified Formulation to combine a hierarchical serendipity Lagrange-based 2D finite element (FE) with 1D differential quadrature method beam element for efficient free vibration characterisation of composite beams induced with prestress at different temperatures. The proposed SUF free vibration solutions of constant stiffness and VS beams demonstrate satisfactory accuracy and achieved improved efficiency with up to 99.9% computational savings when benchmarked against ABAQUS 3D FE solutions. Finally, a numerical study reveals that the effects of thermal prestress significantly contribute to the free vibration response of constant stiffness and VS laminated beams underscoring the importance of the study.

On the Importance of Fibre Direction Mesh Alignment for Artificial Lightning Strike Simulations

Jesús Sánchez Pinedo — Thu, 09 Nov 2023 14:00:36 +0100

Composite materials, used in primary aircraft structures, produce weight reduction and improved fuel efficiency over legacy metal airframes but are more susceptible to lightning strike damage. Therefore, research into lightning strike damage and protection systems, through experiments and simulations, is an important research topic. For any FE simulation appropriate representation of the material behaviour, the loading and boundary conditions are key to accurate predictions. In addition, an aspect which has been under reported in many studies is the meshing strategy. Fibre direction mesh alignment has been reported to yield more accurate results in the modelling of mechanical damage (intralaminar damage initiation and propagation) in unidirectional fibre reinforced composite structures. However, this model meshing strategy has not found wide application and has not been used for the modelling of thermal damage events, e.g. lightning strike direct effect simulation. Instead, authors have typically refined the mesh around the arc attachment area. This paper, for the first time, examines the influence of fibre direction mesh alignment for artificial lightning strike simulations and the prediction of thermal damage. Initially, the mesh alignment is introduced partially in the central region of the specimen. The paper uses a mature modelling approach with a transient, fully coupled, thermal-electric step in ABAQUS with a lightning test Waveform A (40 kA, 4/20 µs) applied to the specimen. Specimen boundary conditions match those typically used in experiments and a mesh convergence study is undertaken to ensure no element size influence on the results. The use of this meshing strategy has been shown to significantly improve the prediction of both moderate and severe thermal damage profiles, when compared with the standard meshes used in previous research. The predicted moderate (2659 mm2 vs 2833 mm2 ) and severe (1059 mm2 vs 1061 mm2 ) damage areas were improved to within 4% and 1% of experimental results, respectively, using this meshing strategy.