LMT-Cachan

This approach consists of representing the movement in the vicinity of a contact front by using a set of degrees of freedom and reference spatial fields. The latter are built, once and for all, by a principal component analysis of the velocity field (Karhunen-Loeve transform) and are hence tailored to provide the best possible approximation of the velocity field close to the contact front. The intensity factors of these reference spatial fields are hence a set of non-local variables which constitute the degrees of freedom of the problem. It is shown that a very small number of them is required to accurately describe the mechanical problem. The so-called "linear intensity factors" I characterize the elastic part of the field while the "complementary intensity factors" I c characterize partial slip within the contact area. 3D finite element analyses were conducted, first to build the framework of this approximation, second to qualify its accuracy and finally to determine the non-linear response of a contact in multiaxial fretting-fatigue conditions. Finally an incremental constitutive model was developed to predict this non-linear response and was compared to the results of the finite elements analyses. This non-local representation has the advantage of being independent of the geometry of the contacting bodies. So, intensity factors can be used to predict the behavior of real-scale industrial assembly using data obtained on laboratory test geometry.

Les disciplines représentées dans la section 09 sont à la fois des disciplines de savoir, et des disciplines de l'ingénierie. On les retrouve aussi bien dans les activités de génération de connaissances de base, que dans la démarche de mise en oeuvre de ces connaissances dans des réalisations concrètes.

Various types of interactions between overloads were studied in a 0.38% C low carbon steel. The retarding effect due to consecutive overloads is found to increase with the number of overloads, until it reaches a maximum. Similarly, it is found that a critical distance between overloads ensures the highest retarding effect, while shorter or longer spacing are less efficient for retarding crack growth. These effects are successfully explained using FEM calculations of the effective stress intensity factor. The kinematic hardening of the alloy, which is very efficient in ferritic±pearlitic steels, is shown to be mostly responsible for those effects. Taking into account the amplitude of kinematic hardening allows qualitative explanation of the observed effects. The order of application of the cycles during variable amplitude fatigue is thus important and should be taken into account for predicting fatigue lives.

Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (a p) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary a p grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples.

The effect of overloads, underloads and stress ratio on plasticity-induced crack opening level is examined for different 'model' materials. This study is focused on the consequences of the Bauschinger effect on the crack opening level. Various finite element analyses were conducted using ABAQUS to test these effects, involving the Chaboche constitutive equations that take into account both the Bauschinger effect of the material and its cyclic hardening or softening. The cyclic plastic behaviour of the material is found to strongly affect the crack behaviour after an overload or an underload. The experimental data obtained on a 0.4% carbon mild steel confirm the numerical results.

The paper deals with a study of heterogeneous stress and strain distribution in polycrystals in relation with elastic anisotropy of grains. A similitude with the arching effect widely studied in granular materials is proposed and this concept is extended to heterogeneous polycrystals in which the load transfer is not binary in the way it is in granular media but may vary significantly and suddenly from one grain to another according to the crystal orientation to the load direction. Experiments and 3D finite element analyses show that though the individual orientation of grains is random, the strain and stress distribution is not. A network is formed inside the polycrystal whose scale is larger than the grain size. The load percolation network consists in heavily loaded links whose direction is coincident with the direction of the principal stresses. So, the typical scale for the variability of the local stresses is not the grain size but the size of the load percolation network. Since this scale is found to be rather large in particular for iron, zinc and copper, this effect should contribute significantly to the variability of the fatigue lives of notched vs. smooth components.