Search

Highlights • Strain-softening followed by re-hardening impacts the adhesive bond toughness. • Decreasing the peak hardening stress increases bond toughness. • Decreasing the minimum softening stress increases bond toughness. • Strain-softening and elastic mismatch act in a coupled manner. Abstract The mode I fracture toughness of an interface between an adhesive material exhibiting strain-softening behavior and an elastic substrate is investigated by finite element modeling. The model is based on an asymptotic K -field formulation relying on cohesive zone elements to simulate the fracture process. A parametric phenomenological hardening law is used to cover the plastic response behavior of a wide range of amorphous glassy polymers, involving a first hardening peak followed by a softening and a re-hardening response. The effect of the peak hardening stress and of the softening stress on the joint toughness is investigated for different elastic mismatch and interface strength. The joint toughness is found to significantly increase with increasing amplitude of the softening, and, this, at constant yield stress and peak hardening stress. This is related to larger plastic zone sizes and larger plastic strains, hence to more energy dissipation. Increasing the interface work of fracture magnifies this effect. There is an interesting coupling between the elastic mismatch and softening, related to their effect on the fracture process zone length. One outcome of these findings is the potential impact of ageing which is known to modify the amplitude of the peak hardening and softening, hence leading to an evolution of the joint toughness with time. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Lime treatment of soils is a complex process which combines chemical and mechanical aspects of the soil behavior. The investigation presented here aims at understanding the effect of lime treatment of clayey soils by characterizing their microstructure evolution, along curing time, using X-Ray Micro-Computed Tomography (XRμCT) and Mercury Intrusion Porosimetry (MIP). Binary sand–bentonite mixtures are considered as a model material to simplify the soil microstructure and the diversity of phenomena involved in lime treatment. Samples containing 10%, 15% and 20% of bentonite and, respectively 90%, 85% and 80% of sand have been treated with 1% lime and compacted. Results in XRμCT show first that porosity is present at two scales: micropores within the bentonite aggregates and macropores between sand particles and bentonite aggregates. Micropores are shown to be exclusively saturated with water, while macropores are only full of air. Second, XRμCT images on the same sample at different curing times show the migration of lime enriched aggregates diffusing into bentonite during the first weeks of curing. Third, bentonite is shown to shrink progressively and to form clusters around the sand grains. Consequently, the fraction of macropores increases while the micropore size decreases. On the other hand, through MIP, three pore size categories have been determined: micropores, mesopores and macropores. The evolution in time of the three pore size categories seen in MIP confirms the behavior observed by XRμCT. [ABSTRACT FROM AUTHOR]

Paper is a material exhibiting a complex microstructure that is composed of a network of fibres at the micro-level. When subjected to external loading or variations in moisture conditions over different time scales, changes in strain that are non-linear with respect to time are observed at the sheet level (macro-scale). In order to investigate this time-dependent behaviour of paper, a creep power law model is implemented within a finite element approach at the level of single fibres. This rate-dependent model is found to capture experimental results available in literature for single fibres with a good agreement (both quantitatively and qualitatively). Based on the identified model at the level of single fibres, the time-dependent hygro-mechanical response is upscaled towards the network scale. To this end, random model networks of ribbon shaped fibres are generated and their response is simulated. The network-scale response, emerging from the rate-dependent fibre model, demonstrates the ability to predict the response of networks subjected to relaxation at a constant moisture level. The developed numerical model predicts lower values of overall stress response in single fibres as compared to networks. Also, stress relaxation predicted by the rate-dependent model in the cross-direction of the networks is in agreement with the experimental observations by Johanson and Kubát (1967). Therefore, one of the remarkable findings of the present work is that the developed rate-dependent model is robust enough to capture the sheet scale response also qualitatively. Based on the study of these computational results, a better understanding is achieved regarding the influence of mechanical and rate-dependent properties of single fibres on the hygro-expansion of complete fibre networks, and in particular of paper sheets. • A rate-dependent non-linear model is developed to study creep in paper networks. • It is calibrated with experiments on the single paper fibres with a good match. • It also captures the experimental response of the networks qualitatively. • One of the remarkable achievement of our work is the robustness of the model. [ABSTRACT FROM AUTHOR]

Abstract: The loss of uniform elongation with decreasing grain size in polycrystals is analysed using a strain gradient plasticity-based finite-element model involving an enhanced description of the grain boundaries. The grain interiors are modelled by the finite strain version of the isotropic Fleck–Hutchinson theory with one internal length parameter. The grain boundaries are modelled using cohesive zone elements imposing higher-order boundary conditions at the interface between the grain interior and grain boundary layer. The plastic strain rate is initially set to zero at these interfaces to account for their impenetrability to dislocations. With increasing stress levels, the higher-order constraint can be suppressed in order to mimic grain boundary relaxation mechanisms. The model is validated towards experimental data on ferritic steels with grain sizes varying between 100nm and 10μm. The model reproduces not only the yield stress evolution, but also the drop of ductility taking place around 1μm grain size while using a single constant internal length. Relaxation of the grain boundary constraint is needed to correctly predict the response at the smallest grain sizes. The back stress increases with decreasing grain sizes. Additional analysis of bimodal grain size distributions is provided showing a large potential for ductility enhancement. [ABSTRACT FROM AUTHOR]

Abstract: The generalised plane state assumption is examined in order to assess its ability to represent in a simplified fashion three-dimensional effects in the behaviour of planar masonry structures subjected to in-plane loads. Realistic failure mode and load-bearing capacity predictions for various loading directions are verified using homogenisation techniques by building failure loci under proportional loading. When introduced in a homogenisation scheme, the resulting macroscopic continuum presents itself in a two-dimensional format, suggesting the use of generalised plane state in multi-scale approaches where three-dimensional effects have to be taken into account. [Copyright &y& Elsevier]

Masonry may be considered macroscopically as a periodic two-phase material. The possible occurrence of cracking in each of the phases leads to a complex mechanical behaviour. Most existing macroscopic models defined for such materials are phenomenological and either isotropic or orthotropic. In this paper, a scalar damage model is used in a mesoscopic study to assess the need for incorporating non-orthotropic induced anisotropy in macrocopic models. Based on unit cell computations and homogenization techniques under a plane stress assumption, it is shown that scalar damage meso-models allow to obtain realistic in-plane damage patterns encountered in experiments. Results suggest that at the meso-scale, it is possible to use a scalar damage model for the individual phases which naturally leads to an overall anisotropy evolution. This evolving macroscopic anisotropy is illustrated using a numerical homogenization procedure to identify the degraded stiffness associated to the obtained damage patterns. It is shown that the characteristic anisotropic shape of experimental failure envelopes for masonry may be reproduced by unit cell computations, as far as in-plane failure mechanisms are concerned. [Copyright &y& Elsevier]

A new gradient damage model is derived starting from thermodynamic statements. Nonstandard terms are added to the free energy potential for this purpose. In contrast with existing approaches, these terms are related to the kinematics of the problem and therefore come into play already in the elastic regime. Following the argument that the second law of thermodynamics need only be satisfied globally for a nonlocal continuum, an energy balance equation is derived which must be solved together with the usual equilibrium equations. Numerical solutions of this coupled set of equations have been generated for a one-dimensional problem in order to study the localisation behaviour of the new model. In the limit where the elastic modulus related to the energy balance goes to 0, an earlier gradient damage model is retrieved. This simplified model is used to study cracking of brick masonry under the influence of mechanical loading. [Copyright &y& Elsevier]

Lightweight bistable deployable structures can be designed to be transportable and reusable. They instantaneously achieve some structural stability when transformed from the compact to the deployed state through a controlled snap-through, as a result of intended geometric incompatibilities between the beams. Due to their transformable bistable nature their design requires assessing both their non-linear transformation behaviour, as well as their service state in the deployed configuration. The requirement of a low peak force during transformation can be shown to oppose the high stiffness requirement in the deployed state; their design can therefore be formulated as a multi-objective non-linear optimisation problem. In this contribution, a size and shape optimisation method is elaborated by choosing the best material combinations, the optimal geometry of the structure and beam cross-sections. The originality of this contribution is the use of a multi-objective evolutionary algorithm to structurally optimise bistable scissor structures taking into account the deployed state as well as the transformation phase. First, the method is applied to optimise a single bistable scissor module. Next, a multi-module bistable scissor structure is optimised and the single module and full structure based approaches are critically compared. • A multi-objective optimization approach is proposed for bistable scissor structures; • The optimization incorporates the nonlinear transformation as well as the service state; • The optimization problem is solved using NSGA-II fed by nonlinear FE results; • Pareto fronts are derived for single bistable modules with different materials; • 5x5 modular bistable structures are investigated with different boundary conditions; [ABSTRACT FROM AUTHOR]

Pre-assembled scissor structures can be transformed from a compact bundle of elements to a fully deployed configuration, offering a considerable volume expansion. Intended geometrical incompatibilities during transformation can be introduced as a design strategy to obtain bistability, which allows instantaneously achieving some structural stability in the deployed state. Because of these incompatibilities, some specific members bend during transformation, resulting in a controlled potentially tunable snap-through behaviour. Geometric design methodologies were proposed in the literature to obtain a compatible geometry (i.e. with all of the beams straight) in the folded and the deployed configurations. However, most of these approaches do not consider finite hub sizes or introduce extra incompatibilities in the geometry by adding hub legs. In this contribution, deployability conditions are derived taking the finite hub size, i.e. the spacing between the connections of the different beams to the hub, into account to make triangulated bistable scissor modules fully geometrically compatible in the folded and the deployed configuration. [ABSTRACT FROM AUTHOR]

In fibrous paper materials, an exposure to a variation in moisture content causes changes in the geometrical and mechanical properties. Such changes are strongly affected by the inter-fibre bonds, which are responsible for the transfer of the hygro-mechanical response from one fibre to its neighbours in the network, resulting in sheet-scale deformation. Most models developed in literature assume perfect bonding between fibres. In the 3D reality, there is some flexibility in the bond region, even for the perfectly bonded fibres, because of the possibility of deformation gradients through the fibre thickness. In earlier 2D idealizations, perfectly bonded fibres were assumed, implying full kinematic constraint through the entire thickness of the sheet. The purpose of the present study is to assess the effect of this assumption. Using a homogenization approach, a random network of fibres is generated with different coverages and modelled using finite elements. In order to understand the role of bonding between fibres on the hygro-expansive behaviour of a network, a bond model is developed. In this model, the fibres are modelled using 2D regular bulk finite elements and the bonds are represented by interfacial elements of finite stiffness, which are introduced between each pair of fibres bonded in the network. These embedded interfacial elements form a connection between two respective fibres, allowing relative displacements between their mid-planes. The hygro-elastic response of networks obtained with this bond model is investigated by varying the bond stiffness and the network coverage under the application of mechanical loading and changes in moisture content. Furthermore, the bond model is used to analyse the influence of inter-fibre bonds on the anisotropic response of the paper fibre network. [ABSTRACT FROM AUTHOR]

Abstract: In the frame of the multi-scale computational analysis of complex materials, the generation of Representative Volume Elements (RVE) is often a crucial step. Various microstructure generation tools may be used, depending on the material to be considered, such as Discrete Element Methods (DEM), Random Sequential Addition (RSA) based methods for particulate media requiring important computation times; or Voronoï tessellation methods for polycrystalline materials. Besides being material specific, some of these methods may become unaffordable when considering complex microstructures, large inclusions numbers or high volume fractions. The present contribution presents a unified level set based methodology for complex, periodic (or not) and random RVE generations. The presented methodology allows RVE generation for particulate granular media, polycrystalline aggregates with large size distribution and arbitrary shapes, as well as for complex three-phase or poly-phase microstructures. A level set controlled Random Sequential Addition algorithm is used for particle distribution generation, allowing increasing the RSA algorithm efficiency, generating large and dense populations of arbitrary shaped inclusions with precise control on neighboring distances. Starting from this, several methods are presented to add specific realistic features to the generated RVEs. Modifications and densifications allow the distribution pattern to fit observed real samples or to present a specific spatial organization. The addition of one (or more) phase(s) obtained from the growth of the initial inclusions allows reproducing some typical microstructural patterns such as grain bridging in clayey soils, interfacial transition zones in concrete or hydrated gel in cement paste. The versatility of the proposed RVE generation method is illustrated by means of various examples, reproducing realistic microstructural arrangements of clayey soils, irregular masonry and polycrystalline aggregates with bimodal size distributions. [Copyright &y& Elsevier]

Abstract: The mechanical response of thin metallic films is simulated using a two-dimensional strain gradient plasticity finite-element model involving grain boundaries in order to investigate the effect of the thickness, grain shape and surface constraint on the strength, ductility and back-stress. The grain boundaries and surface layers are modeled as initially impenetrable to dislocations while allowing for relaxation at a critical stress level. The model captures the variation of the strength with grain size, film thickness, and with the presence or not of constraining surface layers, in agreement with experimental results on Al and Cu films. A decrease in the uniform elongation is predicted with decreasing film thickness due to a loss of strain-hardening capacity and the possible presence of imperfections. These two effects dominate over the stabilizing contribution of the plastic strain gradients. Accounting for the relaxation of the interface constraint affects the magnitude of the back-stress as well as the drop in ductility. [ABSTRACT FROM AUTHOR]

Abstract: The size dependent strengthening resulting from the transformation strain in Transformation Induced Plasticity (TRIP) steels is investigated using a two-dimensional embedded cell model of a simplified microstructure composed of small cylindrical metastable austenitic inclusions within a ferritic matrix. Earlier studies have shown that within the framework of classical plasticity or of the single length parameter Fleck–Hutchinson strain gradient plasticity theory, the transformation strain has no significant impact on the overall strengthening. The strengthening is essentially coming from the composite effect with a marked inclusion size effect resulting from the appearance during deformation of new boundaries constraining the plastic flow. The three parameters version of the Fleck–Hutchinson strain gradient plasticity theory is used here in order to better capture the effect of the plastic strain gradients resulting from the transformation strain. The three parameters theory incorporates separately the rotational and extensional gradients in the formulation, which leads to a significant influence of the shear component of the transformation strain, not captured by the single-parameter theory. When the size of the austenitic inclusions decreases, the overall strengthening increases due to a combined size dependent effect of the transformation strain and of the evolving composite structure. A parametric study is proposed and discussed in the light of experimental evidences giving indications on the optimization of the microstructure of TRIP-assisted multi-phase steels. [Copyright &y& Elsevier]

Abstract: When nanoindentation is used for thin film characterization, the penetration of the indenter is usually limited to shallow indents to avoid the spurious effect of the substrate. However, the surface effects stemming from the sample surface topography and friction are the most pronounced in shallow indentation depths, potentially resulting in variations of the evaluated material parameters. A numerical study is conducted to understand the coupled influence of friction and sample surface roughness in nanoindentation of pure nickel. Results show a strong interaction between these two contributions of surface effects, and their cumulative effect leads to significant variations in the load–displacement curves. Two experimentally used post-treatment methods are applied to obtain the elastic properties from the raw numerical data and a large variation in the evaluated elastic modulus is observed comparable to experimental dispersion levels, independently of the post-treatment method. This suggests that the coupled effect of roughness and friction may at least partially explain the dispersion otherwise attributed to the tested material behavior. Surface effects are found to contribute significantly to the energy balance of shallow nanoindentations, raising the question of the interpretation of the evaluated plastic material properties with work of indentation based methods. [Copyright &y& Elsevier]

Abstract: This paper presents a computational homogenisation-based technique for localisation detection in planar masonry shells. A computational homogenisation procedure is used for the in-plane and the out-of-plane behaviour of masonry walls taking the periodicity of the material into account. The quasi-brittle nature of the masonry constituents results in initial and damage-induced (evolving) anisotropy properties with localisation of damage at both the structural and fine scales. Using a closed-form damage model at the mesoscopic scale, it is shown that a structural scale localisation criterion based on the acoustic tensor adapted to shell kinematics allows to detect the structural scale localisation. This detection identifies average preferential cracking orientations consistent with the stacking mode of masonry for both in-plane and out-of-plane failure. This approach is illustrated by examples of bed joint and stair-case failure, and its subsequent integration in multi-scale nested computational schemes is discussed. [Copyright &y& Elsevier]

Abstract: “To what extent do plastic strain gradients affect the strengthening resulting from the transformation of small metastable inclusions into hard inclusions within a plastically deforming matrix?” is the central question addressed here. Though general in the approach, the focus is on the behavior of TRIP-assisted multiphase steels. A two-dimensional embedded cell model of a simplified microstructure composed of a single metastable austenitic inclusion surrounded by a soft ferritic matrix is considered. The cell is inserted in a large homogenized medium. The transformation of a fraction of the austenite into a hard martensite plate is simulated, accounting for a transformation strain, and leading to complex elastic and plastic accommodation. The size of a transforming plate in real multiphase steels is typically between 0.1 and 2μm, a range of size in which plastic strain gradient effects are expected to play a major role. The single parameter version of the Fleck–Hutchinson strain gradient plasticity theory is used to describe the plasticity in the austenite, ferrite and martensite phases. The higher order boundary conditions imposed on the plastic flow have a large impact on the predicted strengthening. Using realistic values of the intrinsic length parameter setting the scale at which the gradients effects have an influence leads to a noticeable increase of the strengthening on top of the increase due to the transformation of a volume fraction of the retained austenite. The geometrical parameters such as the volume fraction of retained austenite and of the transforming zone also bring significant strengthening. Strain gradient effects also significantly affect the stress state inside the martensite plate during and after transformation with a potential impact on the damage resistance of these steels. [Copyright &y& Elsevier]

Paper is a complex material consisting of a network of cellulose fibres at the micro-level. During manufacturing, the network is dried under restraint conditions due to tension in the paper web in machine direction. This gives rise to internal strains that are stored in the produced sheet. Upon exposure to a moisture cycle, these strains may be released. This results in permanent shrinkage that may cause instabilities such as curl or waviness of the sheet. The prime objective of this paper is to model this irreversible shrinkage and to link its magnitude to the properties of the fibres and of the network. For this purpose, randomly generated fibrous networks of different coverages (i.e. ratio of the area occupied by fibres and that of the sheet) are modeled by means of a periodic representative volume element (RVE). Within such RVEs, a finite element method combined with a kinematic hardening plasticity model at the scale of the fibres is used to capture the irreversible response. The computational results obtained demonstrate that the magnitude of the irreversible strains increases with coverage until a certain coverage and beyond that coverage decreases in magnitude. This phenomenon is explained by considering the area fraction of free-standing fibre segments relative to bonded fibre segments in the network. A structure–property dependency of irreversible strains at the sheet-level on the micro-structural parameters of the network is thereby established. [ABSTRACT FROM AUTHOR]

• A nonlinear computational model for bistable scissor structures is set up using a tailor-made joint finite element (FE). • The effects of joint size, inter-beam spacing, finite joint stiffness and friction are assessed on a single bistable scissor module case. • The inter-beam spacing is shown to lead to a different snap-through mechanism with significantly lower peak force. • The main equations of the proposed joint FE are presented, details necessary for the implementation of the formulations are given in the appendices. • Numerical benchmarks are used to verify the correct functioning of each implemented numerical ingredient. • The performance of single built and modular design approaches is compared computationally for large and complex bistable deployable structures. The 3D deployable frames studied in this work are structures composed of elastic beam elements connected by complex joints. During transformation a controlled snap-through allows the instantaneous stabilization of the structure in an open and in a closed, compact configuration. The mechanics of the transformation is highly nonlinear, since it relies on finite rotations of the structural elements. It is also strongly influenced by geometrical features required for a manufacturing-ready design, such as the finite size of structural elements and sufficient spacing between the beams. These features are generally disregarded in the usual wireframe-based design, but they are taken into account in this work by applying a tailor-made corotational 3D joint finite element, developed to incorporate naturally finite joint size, finite nonlinear joint stiffness and friction effects. The formulation of the proposed joint FE is presented and the performance of the numerical implementation is verified using computational benchmarks. The joint FE is then applied to the numerical investigation of the transformation response of bistable deployable structures from a single module case to large, complex structures. Among other findings, it is shown that the incorporation of finite joint size and beam spacing in the numerical model leads to a different snap-through mechanism that significantly reduces the peak force required for transformation, which could be a basis for future design strategies. Additionally the performance of structures applying the bistable structural pattern on the whole structure or following an entirely modular design (interconnected single modules) is also compared, as a function of the structural size. [ABSTRACT FROM AUTHOR]

Highlights: [•] The behaviour of masonry is deduced using Transformation Field Analysis (TFA). [•] The homogenised material tangent stiffness is derived computationally. [•] Localisation analyses show that TFA captures average masonry cracking directions. [•] A masonry failure locus can be determined qualitatively using TFA. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a computational homogenisation-based technique for flexural effects in textile reinforced composite planar shells. An homogenisation procedure is used for the in-plane and the out-of-plane behaviour of three-dimensional woven composite shells, taking the in-plane periodicity of the material into account while relaxing any periodicity tying in the thickness direction. Several types of damage (matrix or reinforcement cracking, delamination, …) can appear in a composite material. In this paper, material non-linear computations are used to assess the importance of bending on the risk for delamination at the reinforcement/matrix interface. The normal and tangential stresses at the interface are computed and a simplified criterion for delamination is used for this purpose. The effect of flexural loading on the stress components responsible for a potential delamination failure mode at the interface is analysed. The values of interface stresses obtained by means of flexural homogenisation are compared with 3D homogenisation results using periodicity constraints along the thickness direction, and compared qualitatively with experimental facts available from the literature. The importance for taking flexural effects into account properly is emphasised. [Copyright &y& Elsevier]

Abstract: This contribution deals with the modelling of reinforced concrete (RC) structures in the context of progressive collapse simulations. One-dimensional nonlinear constitutive laws are used to model the material response of concrete and steel. These constitutive equations are introduced in a layered beam approach, in order to derive physically motivated relationships between generalised stresses and strains at the sectional level. This formulation is used in dynamic progressive collapse simulations to study the structural response of a multi-storey planar frame subjected to a sudden column loss (in the impulsive loading range). Thanks to the versatility of the proposed methodology, various analyses are conducted for varying structural design options and material parameters, as well as progressive collapse modelling options. In particular, the effect of the reinforcement ratio on the structural behaviour is investigated. Regarding the material modelling aspects, the influence of distinct behavioural parameters can be evaluated, such as the ultimate strain in steel and concrete or the potential material strain rate effects on the structural response. Finally, the influence of the column removal time in the sudden column loss approach can also be assessed. Significant differences are observed in terms of progressive failure patterns for the considered parametric variations. [Copyright &y& Elsevier]

Abstract: The use of specific metallic materials in micro-mechanical systems is promising for surgical applications due to their bio-compatibility and interesting mechanical and wear properties compared to the widely used material, silicon. Nanoindentation is one of the methods used to investigate the mechanical properties of materials at the nano-scale. Numerical simulation tools are useful in the understanding of the nanoindentation experiment. In this paper, the results of numerical simulations coupled to nanoindentation experiments conducted at various indentation depths and at different load rates on pure nickel are presented. The study parameters are carefully chosen to ensure the closest possible conditions between the experiments and the numerical simulations. It is shown that a good agreement between the experimental and the numerical results can be obtained for both the load levels and the creep indentation phase (displacement–time curves in the holding period) when taking into account a simple model with rate-dependent material behavior, and using a material parameter set that is in the acceptable domain for metals. [Copyright &y& Elsevier]

This study presents an imaged-based shell modelling strategy for closed cell metallic foams exploiting X-ray Computed Tomography scans, with its illustration on ALCORAS® foams. Based on an in situ X-ray CT compression test, the 3D segmentation is complemented by a watershed method and a geodesic reconstruction technique to isolate cells and to identify missing walls. An implicit 3D geometry is reconstructed for each cell based on a distance field computation technique. An automated procedure extracts a shell geometry from this implicit 3D geometry, followed by a finite element meshing step. The resulting FE model is solved using ABAQUS/Explicit with an elasto-plastic-damage constitutive model for the cell walls behaviour, and with consideration of contacts between cell walls. A comparison of the simulation results is conducted with experimental force-displacement curves and scanned deformed configurations. The deformation and failure mechanisms under quasi-static compression are investigated numerically and compared with the result of the in-situ experimental measurement. The simulation is shown to predict the yield zones on both the macroscopic and cell levels. A combination of correlated microstructural morphological features (cell size, cell wall thickness, surrounding cells) is identified as critical in deformation and failure mechanisms. Unlabelled Image • An automated methodology extracts a shell FE model from CT images for closed cell foams. • A characterization of the cell scale structure of ALCORAS® foam is performed. • Simulation and experimental results are compared for a foam compression test. • CT scans and simulations are used to compare mesoscale deformation and failure mechanisms. [ABSTRACT FROM AUTHOR]

• A shell-based RVE generation methodology is proposed for closed cell metallic foams. • The surface geometry is extracted from an implicitly defined 3D geometry. • A parametric study using shell FE models is performed and compared to 3D FE results. • The accuracy and efficiency of the shell description is compared to the 3D model. • The shell model is used to reproduce a foam compression test up to densification. This contribution addresses the finite element modelling of closed cell metallic foams using Representative Volume Elements (RVEs) based on shell geometries directly extracted from implicitly defined 3D geometries. 3D RVEs of closed cell foam materials are produced by means of a generation strategy allowing a close morphological control reproducing fine scale geometrical features incorporating cell size, cell wall thickness and cell wall curvature distributions. The strategy is built on three computational ingredients: (i) a random packing algorithm based on random sequential addition assisted by neighbour distance control, (ii) a distance field-based shape tessellation (morphing) that allows incorporating cell wall curvatures and varying cell wall thicknesses and (iii) a close control on the shape of the cells. In order to decrease the computational cost of a full 3D finite element model, an original approach is proposed to produce a shell-based geometry directly from 3D information. Extracting the shell geometry from the implicitly defined 3D geometry based on the zero level of distance fields that would represent the cell walls is computationally impossible. Therefore, a novel robust procedure is proposed using careful cutting operations on distance fields for this purpose. The effect of the different microstructural geometrical features of interest on the average mechanical behaviour of the foam is investigated using shell-based finite element analyses. The computational cost and the accuracy of the proposed shell models are then assessed by comparing their results to full 3D simulations. The macroscopic behaviour of the generated shell-based model under compressive loading is then assessed up to the densification stage (including contact), and compared qualitatively with experiments from the literature. The macroscopic behaviour of the shell-based model is explained by linking it to cell/wall level deformation mechanisms. [ABSTRACT FROM AUTHOR]

This paper presents an automated approach to build computationally Representative Volume Elements (RVE) of open-foam cellular materials, enabling the study of the effects of the microstructural features on their macroscopic behavior. The approach strongly relies on the use of distance and level set functions. The methodology is based on the extraction of random tessellations from inclusion packings following predetermined statistical packing distribution criteria. With the help of simple recombination operations on the distance fields, the tessellations are made to degenerate in Laguerre tessellations. Predetermined morphological characteristics like strut cross-section variation based on commercially available materials are applied on the RVE to ensure the extraction of closely matching models using simple surface extraction tools, and a detailed morphology quantification of the resulting RVEs is provided by comparing them with experimental observations. The extracted RVE surface is then treated with smoothening criteria before obtaining a 3D tetrahedralized model. This model can then be exported for multi-scale simulations to assess the effects of microstructural features by an upscaling methodology. The approach is illustrated by the simulation of a compression test on an RVE incorporating plasticity with geometrically non-linear behavior. • An automated approach to build open-foam representative volume elements is presented. • The use of distance and level set functions makes it versatile and time effcient • Strut cross-section variation and other morphological characteristics are represented. • A detailed morphology comparison with experimental data is provided. • A 3D tetrahedralized model is generated allowing performing simulations. [ABSTRACT FROM AUTHOR]