Guillaume Kermouche

Experimental and numerical study of the ploughing part of abrasive wear

Understanding and quantification of ploughing resistance and associated deformation is of primary importance for materials undergoing abrasive wear. The aim of this study is to understand mechanical phenomena induced by a ploughing process. A comparison of numerical simulation with experimental results has been performed in order to validate our numerical model. The process to be studied is the ploughing of a soft flat surface with a rigid sphere. A numerical study with a finite element code has been performed in order to understand the role of different parameters on strain and stress fields. First of all, we have studied the process in terms of applied loads and strained material but the material characteristics seemed to have a larger influence on the process than the boundary conditions themselves. A profilometric study on numerical simulations has shown us that the elastic strain was not totally recovered and gave rise to residual stresses. Moreover, a study of the contact surface and the ploughing friction coefficient has been developed and a comparison to Bowden and Tabor model has been made. It has shown us that strain hardening modeling has an essential influence on the ploughing friction coefficient. Thus, we have performed scratch test on an AISI316L stainless steel in order to validate our numerical results.

Viscoelastic-Viscoplastic Modelling of the Scratch Response of PMMA

This paper aims at understanding how to model the time-dependent behavior of PMMA during a scratch loading at a constant speed and at middle strain levels. A brief experimental study is first presented, consisting of the analysis of microscratches carried out at various scratching velocities and normal loads. The loading conditions have been chosen in such a way that neither (visco)elasticity nor (visco)plasticity of the PMMA may be neglected a priori. The main analyzed parameter is the tip penetration depth measured during the steady state. Then, a finite element model is used to investigate the potential of classical elastic-viscoplastic constitutive models to reproduce these experimental results. It is mainly shown that these models lead to unsatisfying results. More specifically, it is pointed out here that the time-independent Young modulus used in such models is not suitable. To take into account this feature, a viscoelastic-viscoplastic model based on the connection in series of a viscoelastic part with a viscoplastic part is proposed. It is shown that it leads to more acceptable results, which points out the importance of viscoelasticity in the scratch behavior of solid polymers.

A new index to estimate the strain rate sensitivity of glassy polymers using conical/pyramidal indentation

The increasing use of polymeric materials as thin coatings requires appropriate mechanical characterization methods taking account of time-dependent effects. A well-known result is that a constant indentation strain rate allows us to determine the mechanical properties of such solids from indentation tests. To do that, most people assume a power-law creep behaviour and use the viscoplasticity index to determine the strain rate sensitivity. In the first part of this paper, we investigate the influence of the indentation strain rate on the hardness of power-law creep solids and elastic-viscoplastic solids. It is shown that the assumption of a power-law creep solid is not sufficient to describe the behaviour of polymeric materials in indentation. In the second part, we propose a new index, the EVP index, which gives a better approximation of the strain rate sensitivity than the viscoplasticity index. In the last part, we apply this method to nanoindentation tests on PMMA and polycarbonate samples.

Simulation numérique de la rayure des matériaux

Les essais de rayure sont souvent utilisés pour étudier les phénomènes de frottement et d’usure ainsi que pour identifier localement les propriétés mécaniques des matériaux. La complexité du comportement des matériaux et des interfaces rend difficile l’analyse expérimentale de ce type de tests. La simulation numérique par éléments finis permet d’améliorer la comprehension de ces essais. Dans cet article, une méthode de remaillage dédiée spécifiquement à la modélisation numérique de l’essai de rayure est décrite. Une application est ensuite propose concernant l’étude des effets de la vitesse de rayure sur des solides viscoélastiques et élastiques-viscoplastiques. Les effets sur la déformée en rayure et le coefficient de frottement sont étudiés.