Search

An advanced numerical model is developed to investigate the influence of heat transfer and fluid flow on crack propagation in multi-layered porous materials. The fluid flow, governed by the Navier–Stokes and Darcy’s law, is discretized with the nonconforming Crouzeix–Raviart (CR) finite element method. A combination of Discontinuous Galerkin (DG) and Multi-Point Flux Approximation (MPFA) methods is used to solve the advection–diffusion heat transfer equation in the flow channel and in the fluid phase within the porous material. The crack is assumed to affect only the heat diffusion within the porous layer, therefore a time splitting technique is used to solve the heat transfer in the fluid and the solid phases separately. Thus, within the porous material, the crack induces a discontinuity of the temperature at the crack surfaces and a singularity of the flux at the crack tip. Conduction in the solid phase is solved using the eXtended Finite Element Method (XFEM) to better handle the discontinuities and singularities caused by the cracks. The XFEM is also used to solve the thermo-mechanical problem and to track the crack propagation. The multi-physics model is implemented then validated for the transient regime, this necessitated a post processing treatment in which, the stress intensity factors (SIF) are computed for each time step. The SIFs are then used in the crack propagation criterion and the crack orientation angle. The methodology seems to be robust accurate and the computational cost is reduced thanks to the XFEM. [ABSTRACT FROM AUTHOR]

Highlights: [•] The auxeticity is introduced through the viscoelastic layers of sandwich structures. [•] The Poisson’s ratio of the viscoelastic layers remains the design parameter. [•] An analytical formulation is based on the principle of virtual displacements. [•] An increase of the natural frequency and loss factor is noticed for auxetic layers. [•] No significant impact of the shear function is reported for the free vibration response. [Copyright &y& Elsevier]

Highlights: [•] Fatigue resistance of cardiovascular stent is sensitive to the strut dimensions. [•] The fatigue lifetime may be determined in one day using a piezoelectric testing machine. [•] The percentage of stent’s radial expansion has an impact on its fatigue behavior. [ABSTRACT FROM AUTHOR]

Highlights: [•] Incremental Micromechanics Scheme based on the Eshelby inclusion concept. [•] Elasto-plastic tangent operators obtained from the J 2 flow rule. [•] Derivation of the strain concentration tensor A I after S injection steps. [•] Computation of the macro-stress versus macro-strain response. [ABSTRACT FROM AUTHOR]

Highlights: [•] The stent over-sizing and strut dimensions have an effect on the stent fatigue life. [•] Strains amplitude and mean strains increase with increasing stent over-sizing. [•] Fatigue resistance of the stent was improved after optimization of the strut dimensions. [ABSTRACT FROM AUTHOR]

Abstract: This work aims to analyze the damping response of viscoelastic composite reinforced by elastic auxetic heterogeneities by means of micromechanical modeling. The linear viscoelastic problem can be transformed into the associated elastic one via the Carson-Laplace transform (C-LT). Loss factors are taken into account by the introduction of the frequency-dependent complex stiffness tensors of the viscoelastic phases. The micromechanical formalism, based on the kinematic integral equation, leads to the computation of effective storage modulus and its associated loss factor in the quasi-static domain. The possibility to enhance viscoelastic (VE) properties of a polymeric material such as PVB is examined through several mixing configurations. Thus, the use of elastic auxetic heterogeneities is analyzed in comparison with classical elastic and viscoelastic reinforcements. The model predictions for VE phases, confirm the possibility to improve the global material stiffness. Also, it is shown in the particular case of elastic and spherical heterogeneities, by a proper choice of phases’ stiffness ratio , that auxetic reinforcements represent a good compromise to have simultaneously enhanced stiffness and loss factor response in composite materials. [Copyright &y& Elsevier]

Abstract: Crack-tip enrichment functions are determined for a crack terminating at a bi-material interface. The stress and displacement components at the crack-tip are expressed considering plane elasticity solution based on Airy functions. The crack-tip characteristic exponents and the enrichment functions are determined using the problem boundary conditions. Single real, double real or complex characteristic exponents are found depending on the crack-angle and the relative mechanical properties of the considered bi-material. Crack-tip enrichment functions are determined for cases where the characteristic exponent is real and complex. To validate the efficiency of the enrichment functions at the crack-tip singularity, the eXtended Finite Element Method (XFEM) is used to solve a mode-I crack terminating at the bi-material interface. Thanks to two Level-set functions, the crack and the bi-material interface are located and the nodes belonging to elements cut by these discontinuities are enriched. The Stress Intensity Factors (SIF) are calculated for a crack perpendicular and terminating at the bi-material interface using the body force method (BFM). The implemented method was tested and good agreement is found when compared to other investigations in the literature. [Copyright &y& Elsevier]

Abstract: This paper presents a free vibration and a stability analysis of three-dimensional sandwich beams. Several higher-order displacements-based theories as well as classical models (Euler–Bernoulli’s and Timoshenko’s ones) are derived assuming a unified formulation by a priori approximating the displacement field along the cross-section in a compact form. The governing differential equations and the boundary conditions are derived in a nucleal form that corresponds to a generic term in the displacement field approximation. The resulting fundamental nucleo does not depend upon the approximation order N that is a free parameter of the formulation. A Navier-type, closed form solution is used. Simply supported beams are, therefore, investigated. Slender up to very short beams are considered. As far as free vibrations are concerned, the fundamental natural frequency as well as natural frequencies associated to torsional and higher modes such as sheet face bending and twisting (typical of sandwich structures) are investigated. The stability analysis is carried out in terms of critical buckling stress in the framework of a linearised elastic approach. Results are assessed towards three-dimensional FEM solutions. It is shown that upon an appropriate choice of the approximation order, the proposed models are able to match the three-dimensional reference solutions. [Copyright &y& Elsevier]

Abstract: The rule for cumulative damage and the model for predicting the fatigue life under two amplitudes of plastic strain are studied and summarized in the present paper. Based on the length of the principal crack, obtained by a series of microscopic observations in SEM, an evolution law of this principal crack was established. It was found that, under (L–H) level loading sequence, Miner’s linear damage rule and the propagation law of the main crack are completely fitted to predict the residual lifetime. While a big discrepancy was found between experimental results and Miner’s rule prediction in the (H–L) loading sequence. Thus, an approach based on both Grover’s formulation and the transition inter/transgranular propagation was proposed. The last was found to give relatively interesting results in terms of residual lifetime. [ABSTRACT FROM AUTHOR]

Over the last few years, several studies have shown the potential of multi-stability in engineering applications such as shape-adaptive structures and energy harvesting devices. Due to their highly non-linear mechanics, multi-stable structures are inherently more complex to model than conventional engineering structures. Especially when composite materials are involved, two-dimensional or three-dimensional finite elements solutions are often required in order to have a thorough understanding of the stress field evolution within the structure throughout the snap-through phenomenon. Nevertheless, such modelling strategies yield time-consuming design and optimisation procedures. In order to exploit multi-stability in industrial applications, an enhancement of the existing modelling capabilities is needed. Starting from the state-of-the-art methodologies available in the literature, a novel computational framework is proposed in this study for the analysis of bistable composite beam structures. An advanced structural modelling approach based on Carrera's Unified Formulation (CUF) is adopted. By harnessing the potential of CUF, a full in-plane stress field (including axial, transverse shear and transverse normal stress) can be accurately yet efficiently predicted with no need to resort to two-dimensional or three-dimensional FEM solutions. Furthermore, the equilibrium paths of slender as well as thicker bistable beams are investigated, showing that refined predictions of the stable geometries and snap-through loads in bistability analyses can be obtained via the proposed advanced one-dimensional formulation. Validation of the proposed modelling approach towards reference solutions and commercial software finite elements is provided. [ABSTRACT FROM AUTHOR]

Multiscale modelling of 3D multi-axial knitted 3D spacer composites is proposed. The developed modeling approach is based on a full-field finite element homogenization and conducted at each microstructural level. The relevant scales and their related material features have been identified by means of micro-tomography imaging. The homogenized mechanical response at each level has been identified using appropriate (isotropic or anisotropic) hyperelastic material model and used in the upper level. Thus, the multiscale modeling is carried out through the transfer of information between different length scales rather than by coupling different simulation techniques. The multiscale modeling presented in this work sheds light on the complex mechanics of the textile-rubber microstructure. Moreover, our study may support the design of this class of 3D-fabric-rubber materials by enabling the optimization of the different constituents to obtain specific mechanical properties. The correlation between models and experiments is good, both in terms of details in the architecture and mechanical properties. There are however some deviations that could be explained by the models being more regular than the real material microstructure. [ABSTRACT FROM AUTHOR]

In this study, we developed a constitutive law for wood useful as a user material subroutine (Umat) in Abaqus software. To calibrate the model, compression/tensile tests were carried out on simple geometry samples and to check the validity, we subsequently considered two simulation cases; namely the push out double shear and the four-point bending tests. Moreover, we developed a generic Python script for each test to be used repeatedly for parametric studies. All the numerical aspects are controlled by a user-friendly graphical interface that can display the relevant results and performs simultaneous comparisons with the experimental results. Firstly, the properties of the compressed and uncompressed wood were identified in the longitudinal, radial and tangential directions using a quasi-inverse problem. Then these properties are used in the simulation of the two numerical tests. Consequently, the obtained numerical joint-slip versus load curve from the push out double shear simulation agrees well with the experimental curve. Furthermore, dowel laminated timber tested in four-point bending were correctly simulated and provided numerical results very close to experimental results. [ABSTRACT FROM AUTHOR]

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformations of fibrous architecture. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. In Part I, material twin geometric models of dry fibrous reinforcements were created at different levels of compaction to study the evolution of the morphological features and displacements of fiber tows when compressed during processing. Part II proposes a new approach to track the motion of contour points on cross-sections of fiber tows during compaction. This will allow investigating more precisely the behavior of fiber tows by calculating their mesoscopic deformations. In particular, it will also be possible to study in more details the effect of contacts between tows and of nesting between fabric plies on the mesoscopic deformations of textile preforms during compression. [ABSTRACT FROM AUTHOR]

In Liquid Composite Molding (LCM), compaction of the reinforcement occurs during several stages of the entire process, including before and during resin injection, which leads to significant deformation of the fibrous architecture. This affects not only the manufacturing process, but also the mechanical properties of final parts. This article aims to study by X-ray microtomography the mesoscopic deformations of 2D glass woven fabrics under transverse compaction for a range of fiber volume fractions encountered in high performance composite applications. The analysis is based on the recently proposed Micro-CT Aided Geometric Modeling (AGM) technique Huang et al. (2019) [1], which is used to create, from the three-dimensional images of dry textile preforms obtained by microtomography, "material twin" geometric models representative of fiber architecture. Three "material twin" mesostructural geometric models of a stack of multiple layer 2D woven fabrics are generated from microtomographic images at different compaction levels. Since all the fiber tows are reconstructed from real fabrics and are labeled separately, the deformation and displacement of fiber tows during compaction can be subsequently studied, which is not necessarily possible with other reconstruction methods. The results show that contacts between fiber tows have an effect on the evolution of their morphological features, which is related to the material variability of fiber tows in real fabrics. Tracking of the fiber tows positions also reveals that the vertical displacement of fiber tows are significant, while their horizontal movements are negligible. [ABSTRACT FROM AUTHOR]

This article introduces an innovative approach that utilizes machine learning (ML) to address the computational challenges of accurate atomistic simulations in materials science. Focusing on the field of molecular dynamics (MD), which offers insight into material behavior at the atomic level, the study demonstrates the potential of trained artificial neural networks (tANNs) as surrogate models. These tANNs capture complex patterns from built datasets, enabling fast and accurate predictions of material properties. The article highlights the application of 3D convolutional neural networks (CNNs) to incorporate atomistic details and defects in predictions, a significant advancement compared to current 2D image-based, or descriptor-based methods. Through a dataset of atomistic structures and MD simulations, the trained 3D CNN achieves impressive accuracy, predicting material properties with a root-mean-square error below 0.65 GPa for the prediction of elastic constants and a speed-up of approximately 185 to 2100 times compared to traditional MD simulations. This breakthrough promises to expedite materials design processes and facilitate scale-bridging in materials science, offering a new perspective on addressing computational demands in atomistic simulations., (© 2024. The Author(s).)

The standard Yee algorithm is widely used in computational electromagnetics because of its simplicity and divergence free nature. A generalization of the classical Yee scheme to 3D unstructured meshes is adopted, based on the use of a Delaunay primal mesh and its high quality Voronoi dual. This allows the problem of accuracy losses, which are normally associated with the use of the standard Yee scheme and a staircased representation of curved material interfaces, to be circumvented. The 3D dual mesh leapfrog-scheme which is presented has the ability to model both electric and magnetic anisotropic lossy materials. This approach enables the modelling of problems, of current practical interest, involving structured composites and metamaterials., (© The Author(s) 2016.)

This paper deals with fatigue life prediction of 316L stainless steel cardiac stents. Stents are biomedical devices used to reopen narrowed vessels. Fatigue life is dominated by the cyclic loading due to the systolic and diastolic pressure and the design against premature mechanical failure is of extreme importance. Here, a life assessment approach based on the Dang Van high cycle fatigue criterion and on finite element analysis is applied to explore the fatigue reliability of 316L stents subjected to multiaxial fatigue loading. A finite element analysis of the stent vessel subjected to cyclic pressure is performed to carry out fluctuating stresses and strain at some critical elements of the stent where cracks or complete fracture may occur. The obtained results show that the loading path of the analysed stent subjected to a pulsatile load pressure is located in the safe region concerning infinite lifetime.

Cosserat models of cancellous bone are constructed, relying on micromechanical approaches in order to investigate microstructure-related scale effects on the macroscopic properties of bone. The derivation of the effective mechanical properties of cancellous bone considered as a cellular solid modeled as two-dimensional lattices of articulated beams is presently investigated. The cell walls of the bone microstructure are modeled as Timoshenko thick beams. The asymptotic homogenization technique is involved to get closed form expressions of the equivalent properties versus the geometrical and mechanical microparameters, accounting for the effects of bending, axial, and transverse shear deformations. Considering lattice microrotations as additional degrees of freedom at both the microscopic and macroscopic scales, an anisotropic micropolar equivalent continuum model is constructed, the effective mechanical properties of which are identified. The effective elastic moduli of various periodic cell structures are computed in situations of low and high effective densities to assess the impact of the transverse shear deformation. The stress distribution in a cracked bone sample is computed based on the effective micropolar model, highlighting the regularizing effect of the Cosserat continuum in comparison to a classical elasticity continuum model., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Molecular dynamics (MD) simulations of liquid tin between its melting point and 1600 °C have been performed in order to interpret and discuss the ionic structure. The interactions between ions are described by a new accurate pair potential built within the pseudopotential formalism and the linear response theory. The calculated structure factor that reflects the main information on the local atomic order in liquids is compared to diffraction measurements. Having some confidence in the ability of this pair potential to give a good representation of the atomic structure, we then focused our attention on the investigation of the atomic transport properties through the MD computations of the velocity autocorrelation function and stress autocorrelation function. Using the Green-Kubo formula (for the first time to our knowledge for liquid tin) we determine the macroscopic transport properties from the corresponding microscopic time autocorrelation functions. The selfdiffusion coefficient and the shear viscosity as functions of temperature are found to be in good agreement with the experimental data., (© 2012 American Institute of Physics)