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The main motivation for the present work is to provide an improved description of the response of Functionally Graded (FG) structures under a spherical indenter, considering material nonlinearities. This is achieved through the implementation of elastoplastic material behavior using integration points to avoid the division of the structure into multiple layers. The current paper proposes a numerical investigation into the mechanical response of functionally graded materials (FGMs) in contact with a rigid hemispherical head indenter. The numerical model considers both the Mori–Tanaka model and self-consistent formulas of Suquet to accurately model the smooth variation of material properties through the thickness of the elastoplastic FG material. The model execution involves a UMAT user material subroutine to implement the material behavior into ABAQUS/Standard. The user material UMAT subroutine is employed to introduce material properties based on the integration points, allowing for an accurate representation and analysis of the material's behavior within the simulation. The developed numerical model is validated through a comparison with experimental results from the literature, showing a good correlation that proves the efficiency of the proposed model. Then, a parametric study is conducted to analyze the effect of the indenter dimension, the indentation depth and the gradient index on the indentation force, the contact pressure evolution, von Mises equivalent stress and equivalent plastic strain distributions located on the vicinity of the contact zone. The results showed that the elastoplastic response of TiB/Ti FG plates is significantly influenced by the gradient index, which determines the properties of the FG composite through the thickness. These results may help development engineers choose the optimal gradation for each industrial application in order to avoid contact damage. [ABSTRACT FROM AUTHOR]

The aim of the present paper is to study and model the fatigue behavior of short glass fibers reinforced polyamide-66. The effect of fiber content on the fatigue and static behavior of this composite is investigated. In such composites fatigue damage growth exhibits three stages. A continuum damage based model is presented to predict damage evolution during these three stages. Experimental results show that increasing the fiber content increases the elastic modulus and the tensile strength of the studied materials under tensile tests. However, the rupture behavior changes from ductile to brittle. Moreover increasing the fiber percentage changes the S-N curves slope and decreases the fatigue life. Analytical results predicted by the proposed model, compared to experimental ones shows good agreement and the developed model predicted fatigue damage growth in its three stages of evolution with good performance. [ABSTRACT FROM AUTHOR]

In the present work, an elastoplastic-damage model fully coupled with the constitutive equations of the non-associated plasticity is developed, to enrich the existing research results on the non-associated plasticity finite element models. The yield function and the plastic potential are represented independently by two different (Hill, 1948) quadratic functions to describe the anisotropic behavior of 5083-aluminium alloy. The current model develops a first attempt to establish a general formulation with two ductile degradation functions, one for elasticity and the other for plasticity, with variable degradation coefficients. Further, the elastoplastic behavior of 5083-aluminium alloy is determined through uniaxial tensile tests, so that the anisotropic, the isotropic hardening and the damage parameters are acquired. Experimental-numerical confrontation proves the reliability of the current formulation to describe the behavior of 5083-aluminium alloy with good accuracy. Furthermore, a novel numerical optimization procedure is firstly used to analytically identify Lemaître damage parameters in order to limit the non-unicity of these parameters. The adopted optimization procedure offers a compromise between good accuracy and low computational time to determine the damage parameters. [ABSTRACT FROM AUTHOR]

Highlights • Nonlinear evolution of flax fabric properties with respect to water content are observed. • Two-linear region behaviour is pointed out for samples with specific water content range. • Water uptake in fabric are not uniform leading to local behaviour heterogeneities. Abstract In order to analyse the influence of conditioning at relative humidity on the behaviour of flax fibre reinforced composites, a preliminary study on flax fabrics needs to be conducted. Therefore, this paper investigates the effect of conditioning on the tensile properties of non-impregnated flax fabrics at different humidities. These fabrics are used as reinforcement materials for semi-structural composite materials. The major drawback of natural fibre reinforced composites is moisture regain. Understanding how the moisture affects the behaviour of non-impregnated flax reinforcement is then crucial. The conditioning of the fabrics in different humid atmospheres made it possible to study the influence of the water content on the hygro-mechanical behaviour of flax reinforcement through tensile tests. The water content of samples is linked to the relative humidity of conditioning atmosphere through the isothermal sorption curves. This paper brings up the significant influence of water content on the stiffness and elongation at break of the fabric, while water content does not seem to have an influence on the breaking load which remains practically constant. Non-linear decrease of flax fabric rigidity and increase of its elongation at maximal force with the increase of water content are observed. It is concluded that water molecules act as a softening agent on flax fabrics. Two kinds of typical tensile curves are observed. Most of the tensile curves of samples show one unique linear region where the behaviour appears to be linear prior to the first yarn failure. For some samples in which water content is between 2.7% and 3.7%, the emergence of a two-linear region behaviour is pointed out. This two-linear region behaviour could be attributed to local heterogeneities. Indeed, this study shows that, at saturation, the amount of absorbed water is not uniform through flax fabric samples. The moisture uptake heterogeneities did not resorb with time after saturation. These local heterogeneities would induce a premature local plasticity in some regions of the flax fabric sample. These regions would present local weakness areas that could induce an exhibition of the two-linear region behaviour. [ABSTRACT FROM AUTHOR]