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Investigating the mechanical behaviour of silica bodies in oil palm empty fruit bunches (OPEFB) is important to improve the process of silica body removal. This study will assist in providing an understanding of the role of OPEFB as a bioresource material for the bioconversion process. The microstructure of silica bodies/protrusions on the OPEFB fibre surface was modelled using the finite element method, based on the information obtained from scanning electron microscopy (SEM). The effects of silica body geometry, possible anisotropy/orthotropy, and debonding between the interface of the silica body and OPEFB fibre were investigated. Agreements were observed between the results using both circular and spiked silica body models with different geometries and volume fractions. In addition, the cohesive debonding modelling results showed that once critical stress was activated, the stress-strain curve deviated from the no-debond model. The results also suggested that the value of cohesive energy should be between 0.5 kN/m and 4 kN/m.

Submitted by Ana Rebelo (amsr@isep.ipp.pt) on 2018-06-05T13:24:11Z No. of bitstreams: 1 DM_JoaoValente_2017_MEM.pdf: 8939251 bytes, checksum: 2175c62253ce5025813bd470993e3e6b (MD5) Approved for entry into archive by Ana Rebelo (amsr@isep.ipp.pt) on 2018-06-05T13:24:33Z (GMT) No. of bitstreams: 1 DM_JoaoValente_2017_MEM.pdf: 8939251 bytes, checksum: 2175c62253ce5025813bd470993e3e6b (MD5) Made available in DSpace on 2018-06-05T13:24:33Z (GMT). No. of bitstreams: 1 DM_JoaoValente_2017_MEM.pdf: 8939251 bytes, checksum: 2175c62253ce5025813bd470993e3e6b (MD5) Previous issue date: 2017

To face challenges like damping effects or weight reduction in the automotive sector, new hybrid material combinations are developed. One possibility is the combination of several symmetric material layers with varying material characteristics to achieve in the component production less weight and appropriate stiffness in comparison to components produced with sheet metal. This article deals with the characterization of deep drawing behaviour of layered sandwich structures. The behaviour of the several layers and the layer interaction have been taken into account for the technical design of a deep drawing process. A material layer characterization is performed. Instabilities as interlaminar failures, ruptures or wrinkling of the structure have been investigated as part of additional experimental characterization tests on the basis of various deep drawing process parameters. Finally, the experimental data is used as input for the numerical modelling and simulation of layered structures. The FE simulation includes the material behaviour of the layers and layer interactions with cohesive zone modelling. Based on the results an important contribution for prediction accuracy in the numerical simulation has been provided.

This thesis deals with numerical simulation of a peel test with an Aluminium foil and Low Density Poly-Ethylene (LDPE) laminate. This work investigates the effects of the substrate thickness and studies the influences of interfacial strength and fracture energy of the cohesive zone between the Aluminium and LDPE. This study evaluates the proper guidelines for defining cohesive properties. A numerical cohesive zone model was created in ABAQUS. Continuum tensile tests were performed to extract LDPE material properties. The aluminium properties were found in literature. After acquiring material parameters, the simulation continued with studying the effects of changing interfacial strength, geometric parameters and fracture energy. The results were obtained in the form of root rotations and the force displacement response was studied carefully. It was validated by comparison to the traction separation curve.

Simulation of hydrogen embrittlement requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth. The present study presents a review of coupled diffusion and cohesive zone modelling as a method for numerically assessing hydrogen embrittlement of a steel structure. While the model is able to reproduce single experimental results by appropriate fitting of the cohesive parameters, there appears to be limitations in transferring these results to other hydrogen systems. Agreement may be improved by appropriately identifying the required input parameters for the particular system under study. © 2017. This is the authors’ accepted and refereed manuscript to the article. Locked until 23.3.2019 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

This paper presents an approach to the modelling of quasi-static interface fracture of epoxy adhesive reinforced bolted lap-connections. The full-scale experiments have been performed in order to validate simulation results. Shear fracture of the pre-defined annular interface areas in the vicinity of the fasteners was modelled using decohesion finite elements involving a parametric cohesive zone model. Non-local friction was also incorporated into the interaction between the contacting elements. Based on the principle of superposition, the fracture potential and frictional dissipation of the interface were independently characterized. The measured peak shear loads show a good correlation with the computed results.

In this work a cohesive zone model is developed to simulate fatigue crack propagation along bonded interfaces. The model is implemented by means of the USDFLD subroutine in the commercial software Abaqus. In particular, the crack growth increment is translated into a damage increment per cycle in the cohesive elements ahead of the crack tip. The computation of the strain energy release rate, not available in output for cohesive elements in Abaqus, was specifically implemented.

Thin laminates of Aluminum (Al) foil and Low Density Polyethylene (LDPE) film are essential constituents of food packages where these two substrates are bonded together with a thin layer of LDPE acting as adhesive. Noticeably, Al is a low ductile/quasi brittle material, whereas LDPE is highly ductile. The mechanism of delamination and strength of bond between the interfaces dictates the continuum and damage behavior of this composite. However, measuring the shear delamination properties is challenging as conventional test methods have limitations when the substrates are very thin and flexible. This study explains a tentative method that uses uniaxial tensile testing on the pre-cracked specimen of this composite to find energy dissipation due to shear delamination and successfully uses it in Finite Element Simulation in Abaqus. The delamination was observed in a narrow strip-like region close to fracture surfaces and measured with special visualization aid. A similar response was found in FEM simulation. Scanning Electron Microscopic (SEM) study of delaminated interface confirms the delamination to be shear in nature. In a cohesive zone modeling in Abaqus, the measured shear delamination energy was used as input parameter along with an arbitrary bi-linear cohesive law for validation of the experimental measurement. Tunna laminat av aluminium (Al) folie och lågdensitetspolyeten (LDPE) film är väsentliga beståndsdelar i livsmedelsförpackningar, där dessa två substrat är sammanbundna med ett tunt skikt av LDPE som fungerar som bindemedel. Märkbart är Al en låg segt / kvasi sprött material, medan LDPE är mycket formbart. Mekanismen för delaminering och styrka bindningen mellan gränssnitten dikterar kontinuum och skador beteende denna komposit. Men mäta skjuvning delamineringsegenskaper är utmanande som konventionella testmetoder har begränsningar när substraten är mycket tunn och flexibel. Denna studie förklarar en preliminär metod som använder enaxlig dragprovning på förhand spruckna exemplar av denna komposit för att hitta energiupptagning på grund av skjuvning delaminering och framgångsrikt använder den i finita elementsimulering i Abaqus. Delamineringen observerades i en smal remsliknande region nära brottytor och mättes med särskild visualisering stöd. Ett liknande svar återfanns i FEM simulering. Svepelektronmikroskop (SEM) studie av delaminerat gränssnitt bekräftar delaminering att vara skjuvning i naturen. I en sammanhängande zon modellering i Abaqus, var den uppmätta skärdelaminering energi som används som inparameter tillsammans med en godtycklig bilinjär sammanhängande lag för validering av experimentella mätningen. Open access

The applications of adhesive joints are increasing in various industrial applications because they offer several advantages over traditional methods. The combination of adhesive bonding with spot-welding enables some advantages over adhesive joints such as increased stiffness, and higher static and fatigue strength. This work relates to the adhesive selection for single-lap adhesive joints by the bonding and hybrid (bonded and welded) techniques with different overlap lengths ( L O ). The adhesives are the brittle Araldite AV138 ® , and the ductile Araldite ® 2015 and Sikaforce ® 7752. The experimental results were compared against a Finite Element (FE) study coupled with Cohesive Zone Modelling (CZM). The results validated the numerical technique and also showed varying strength improvements of the hybrid joints over bonded joints depending of the adhesive.

Adhesive bonding is a widely used joining method in industries such as aerospace, aeronautical and automotive because of specific advantages compared to the traditional fastening methods. Numerical approaches for the damage simulation of bonded joints based on fracture mechanics usually rely on Cohesive Zone Models (CZM). CZM suppose the characterization of the CZM laws in tension and shear, which are combined in mixed-mode criteria to predict the strength of bonded joints. This work evaluated the tensile fracture toughness ( G IC ) and CZM laws of bonded joints for two adhesives with distinct ductility. The Double-Cantilever Beam (DCB) test was used. The experimental work consisted of the tensile fracture characterization by the J -integral technique. A digital image correlation method was used for the evaluation of the tensile relative displacement (δ n ) of the adhesive layer at the crack tip. Finite Element (FE) simulations were carried out to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the DCB tests. As output of this work, information regarding the applicability of these CZM laws to each type of adhesive is provided, allowing the subsequent strength prediction of bonded joints.

The surface of oil palm empty fruit bunch fibres contains embedded silica bodies or protrusions. The mechanical contribution of the protrusions towards the integrity of the fibres is still not clearly investigated. In this work, 2D and 3D finite element simulations on the surface and cross section of the fibres, respectively, were performed. The information for the models was obtained from scanning electron microscopy analysis and mechanical tests for the silica body characteristics and elastic modulus, respectively. Different silica bodies arrangements and the effect of spiked geometry of the silica bodies was investigated using 2D models. Cohesive zone modelling was introduced to simulate damage or debonding between the interface of silica bodies and fibre. A 3D finite element model was later developed consisting of a silica body (sphere) embedded halfway in the matrix. The numerical results showed that the 2D model was sensitive to critical stress compared to silica bodies spiked geometry, arrangement of silica bodies on the fibre surface, and cohesive energy. On the other hand, the results showed that for 3D models with thicknesses larger than 0.2 mm, the effect of the silica bodies on the elasticity of the fibre was not significant.

Ceramic‐Matrix Composites (CMCs) are high‐potential materials for light‐weight structures in high temperature applications. CMCs show high temperature resistance like monolithic ceramics. But in contrast to monolithic ceramics CMCs do also show a high damage tolerance which makes the design of large light‐weight CMC‐structures possible. One limiting factor for CMCs in industrial application is the precise design of CMC components. Due to the complex, high temperature manufacturing of CMC components it is not always possible to prevent the formation of processing defects completely. Therefore, manufacturing defects are leading to high rejection rates, costs and to an oversizing of CMC components. In this way the full light‐weight potential is not tapped and new fields of application are lost. With the help of modern non‐destructive testing methods, like micro‐Computer Tomography (CT), most of the macroscopic defects can be detected with high resolution and detection reliability. Nevertheless, the impact of defects on stiffness and strength of CMC materials and structures is still not clear. Thus the effect of interlaminar defects is approached by integrating the defect information from micro‐CT into Finite Element (FE) models. With the help of the adequate material and fracture mechanical models, it should be possible to predict the stiffness and strength of samples and structures with manufacturing defects. The ability to assess the impact of defects on the performance of CMC structures can help to minimize rejection rates and accelerate material and process optimization by focusing on design critical defects only.

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International audience; Purpose - The purpose of this paper is to describe finite element modelling for fracture and fatigue behaviour of zirconia toughened alumina microstructures. // Design/methodology/approach - A two-dimensional finite element model is developed with an actual Al2O3-10 vol% ZrO2 microstructure. A bilinear, time-independent cohesive zone law is implemented for describing fracture behaviour of grain boundaries. Simulation conditions are similar to those found at contact between a head and a cup of hip prosthesis. Residual stresses arisen from the mismatch of thermal coefficient between grains are determined. Then, effects of a micro-void and contact stress magnitude are investigated with models containing residual stresses. For the purpose of simulating fatigue behaviour, cyclic loadings are applied to the models. // Findings - Results show that crack density is gradually increased with increasing magnitude of contact stress or number of fatigue cycles. It is also identified that a micro-void brings about the increase of crack density rate. // Social implications - This paper is the first step for predicting the lifetime of ceramic implants. The social implications would appear in the next few years about health issues. // Originality/value - This proposed finite element method allows describing fracture and fatigue behaviours of alumina-zirconia microstructures for hip prosthesis, provided that a microstructure image is available.

Micro electromechanical systems (MEMS) are nowadays used in many applications, such as airbag accelerometers and inkjet printer heads. With the number of applications growing, the need for advanced numerical tools to aid in the design of MEMS increases. The development of such tools is far from trivial, since numerical modelling of MEMS poses various challenges, such as the simulation of multiphysical effects, multiscale effects and effects of microstructural randomness. In this dissertation, computational techniques have been developed to asses the reliability of MEMS. More specifically, a cohesive zone formulation has been used to mimic crack growth in piezoelectric continua. A multiphysical finite element model has been developed to correctly simulate the electromechanical coupling in such materials. A computational homogenisation method has been proposed to efficiently incorporate microstructural effects in a macroscale analysis. Stochastic finite element methods have been employed to include the effects of microstructural imperfections on the behaviour of MEMS.

Micro electromechanical systems (MEMS) are nowadays used in many applications, such as airbag accelerometers and inkjet printer heads. With the number of applications growing, the need for advanced numerical tools to aid in the design of MEMS increases. The development of such tools is far from trivial, since numerical modelling of MEMS poses various challenges, such as the simulation of multiphysical effects, multiscale effects and effects of microstructural randomness. In this dissertation, computational techniques have been developed to asses the reliability of MEMS. More specifically, a cohesive zone formulation has been used to mimic crack growth in piezoelectric continua. A multiphysical finite element model has been developed to correctly simulate the electromechanical coupling in such materials. A computational homogenisation method has been proposed to efficiently incorporate microstructural effects in a macroscale analysis. Stochastic finite element methods have been employed to include the effects of microstructural imperfections on the behaviour of MEMS.

A detailed numerical analysis of the Edge Crack Torsion test was performed to verify its adequacy for the mode III interlaminar fracture characterization of composite laminates. A new data reduction scheme based on specimen compliance was proposed. Three-dimensional finite element analyses including a cohesive damage model were performed in order to evaluate the accuracy of perceived G IIIc values obtained with the proposed method. Despite some degree of crack length dependency of perceived G IIIc , acceptable errors could be achieved within a certain crack length range, which is thus recommended for experimental tests.

This paper focuses on the effect of fiber orientation and stacking sequence on the progressive mixed mode delamination failure in composite laminates using fracture experiments and finite element (FE) simulations. Every laminate is modelled numerically combining damageable layers with defined fiber orientations and cohesive zone interface elements, subjected to mixed mode bending. The numerical simulations are then calibrated and validated through experiments, conducted following standardized mixed mode delamination tests. The numerical model is able to successfully capture the experimentally observed effects of fiber angle orientations and variable stacking sequences on the global load–displacement response and mixed mode inter-laminar fracture toughness of the various laminates. For better understanding of the failure mechanism, fracture surfaces of laminates with different stacking sequences are also studied using scanning electron microscopy (SEM).

Diplôme : Dr. d'Université; This work concerns the mechanical testing, numerical analysis and modelling of cohesive fracture (Mode I) on the purpose to study the Resistance-curve behaviour and the size effect in wooden notched structures. The mechanical testing is combined with the numerical analysis to evaluate fracture properties by means of an equivalent LEFM approach based on the structure compliance. The Resistance-curve being revealed from the experiments, by means of a self-weight compensation method, correction puts into evidence that a non-negligible damaged domain (Fracture Process Zone) is under development in the crack front during the loading process. This being the case, among other fracture parameters issued from the Resistance-curve, the critical (asymptotic) energy release rate is determined, turning possible to use it in combination with other cohesive crack properties in the crack modelling (in Mode I). Thus, for a given geometry it is possible to monitor the critical dimension being revealed by the Fracture Process Zone (FPZ) during the crack propagation. The well known Bažant’s size effect law provides the scaling of the nominal strength through the asymptotic matching performed both on the small (Strength Theory) and on the large (LEFM) structure sizes. An analytical procedure is proposed to determine an additional asymptotic regime in the intermediate size range through a more accurate manner. Numerical validations of the proposed procedure are made and experimental data is presented revealing the scaling of the nominal strength through an envelop of values.; Ce travail concerne des expériences mécaniques, des analyses numériques et des modélisations analytiques de la rupture cohésives (Mode I), vis-à-vis de l’étude du comportement mis en évidence par la courbe de Résistance (Courbe-R) et l’effet d’échelle de structures entaillées en bois massif. Des expériences de fissuration sont combinées à des analyses numériques pour déterminer les propriétés de rupture au moyen d’une procédure appelée Théorie de la Mécanique de la Rupture Linéaire Élastique équivalente (TMRLE), basée sur la complaisance de la structure. La courbe-R, obtenue à partir des expériences, selon une méthode de correction du poids propre, montre l’existence d’un domaine endommagé (Zone de Processus de Rupture) de taille non négligeable se développant en fond de fissure. Dans des conditions de fissuration stationnaire, ce domaine atteint une taille critique, et l’énergie nécessaire pour faire propager la fissure avec ce domaine endommagé (par unité de surface de rupture), reste constante. Le taux de libération de l’énergie de fissuration ainsi attendu, joue un rôle important en Mécanique de la Rupture, car il est possible simuler le comportement quasi-fragile du matériau en combinaison avec les autres propriétés de cohésion. La loi d’effet d’échelle de Bažant, utilisée pour prévoir l’influence de la taille sur la contrainte nominale, est estimée à partir de la réunion de deux comportements asymptotiques réalisés sur de petites tailles (Analyse limite ou RdM) et des grandes tailles. Une procédure analytique est présentée pour évaluer le comportement asymptotique additionnel exhibé par la contrainte nominale dans le régime intermédiaire, de façon plus exacte. Une validation numérique est présentée, et l’information expérimentale vient confirmer ce comportement asymptotique.

Failure mechanisms in materials and structures can be studied on different length scales. On a structural level, failure can be observed as the propagation of a single crack. However, when zooming into the apparent crack tip, it is revealed that the actual failure process is set by the nucleation and growth of multiple micro-cracks that together form the dominant crack. In crystalline materials, the nucleation of cracks can be traced back to even smaller levels of observation. The collective glide of dislocations in the atomistic structure of these materials appears to be the driving mechanism for the nucleation of micro-cracks near free edges. From a mathematical point of view the creation and propagation of cracks and the glide of dislocations can be considered as evolving discontinuities in the material. This thesis focuses on the numerical representation of these discontinuities on different length scales, varying from the simulation of delamination growth in fibre-metal laminates to a model that incorporates slip due to the motion of discrete dislocations.

Failure mechanisms in materials and structures can be studied on different length scales. On a structural level, failure can be observed as the propagation of a single crack. However, when zooming into the apparent crack tip, it is revealed that the actual failure process is set by the nucleation and growth of multiple micro-cracks that together form the dominant crack. In crystalline materials, the nucleation of cracks can be traced back to even smaller levels of observation. The collective glide of dislocations in the atomistic structure of these materials appears to be the driving mechanism for the nucleation of micro-cracks near free edges. From a mathematical point of view the creation and propagation of cracks and the glide of dislocations can be considered as evolving discontinuities in the material. This thesis focuses on the numerical representation of these discontinuities on different length scales, varying from the simulation of delamination growth in fibre-metal laminates to a model that incorporates slip due to the motion of discrete dislocations.

This paper proposes a new methodology for the finite element (FE) modelling of failure in adhesively bonded joints. Cohesive and adhesive failure are treated separately which allows accurate failure predictions for adhesive joints of different thicknesses using a single set of material parameters. In a companion paper (part I), a new smeared-crack model for adhesive joint cohesive failure was proposed and validated. The present contribution gives an in depth investigation into the interaction among plasticity, cohesive failure and adhesive failure, with application to structural joints. Quasi-static FE analyses of double lap-joint specimens with different thicknesses and under different levels of hydrostatic pressure were performed and compared to experimental results. In all the cases studied, the numerical analysis correctly predicts the driving mechanisms and the specimens’ final failure. Accurate fatigue life predictions are made with the addition of a Paris based damage law to the interface elements used to model the adhesive failure.

The mode I delamination behavior of a unidirectional glass-fiber reinforced polymer with a prototype elasto-plastic polyurethane matrix (GF/PU) was investigated and compared with that of glass/epoxy (GF/EP) using double cantilever beam specimens. Fracture resistance was assessed using experimental data based on the strain energy release rate (ERR), G, calculated by means of the modified compliance calibration, and the J-integral, calculated using the applied force and arm rotations measured by digital image correlation. The fracture energies given by the two methods differed by ∼5%. In both systems large scale bridging was observed and fracture resistance at initiation and steady state was about 4 times greater in GF/PU and correlated to adhesive failure in GF/EP and cohesive in GF/PU. Since the fiber sizing was the same, the higher ERR of GF/PU was attributed to strong glass/PU interface allowing higher local matrix strains and larger fiber bundles to develop. Traction-separation relations were determined by the direct and an indirect method using Fiber Bragg Grating sensors to construct numerical models to simulate the delamination process. The simulated force-displacement and R-curve data were in good agreement with experiments.

Cohesive Element (CE) is a well-established finite element for fracture, widely used for the modelling of delamination in composites. However, the computational time of CE-based method is prohibitive. This is because the steep and non-smooth stress gradient in the cohesive zone requires a very fine mesh. In this context, a new type of CE is here proposed, aiming to loosen the mesh constraint and reduce the computational time. It uses a higher-order interpolation of the displacement field with rotational degree of freedom and an adaptive integration scheme based on the status of the element. The proposed CE has been validated through comparison with benchmark solutions of delamination in Mode I, Mode II and Mixed-Mode cases, and has demonstrated superior performance than standard CE in computational efficiency while retaining a high level of accuracy.