Thibaut Chaise

Fully Coupled Resolution of Heterogeneous Elastic–Plastic Contact Problem

The recent development of semi-analytical methods (SAM) has led to numerous improvements in their capabilities in terms of phenomena that can be accounted for and numerical efficiency. They now allow to perform fast and robust simulations of contact between inelastic—with either elastic–plastic or viscoelastic behavior—and anisotropic or heterogeneous materials. All effects may be combined, with either coating, inclusions, cavities, or fibers as inhomogeneities. The coupling between local and global scales remains numerically difficult. A framework is proposed here for contact problems considering the effect of elastic heterogeneities within an elastic–plastic matrix. The mutual interactions among heterogeneities and their surrounding plastic zone as well as the interactions between them and the contact surface through which the load is transmitted should be accounted for. These couplings are outside the validity domain of the Eshelby’s equivalent inclusion method (EIM) that assumes a uniform stress field in an infinite space far from the inhomogeneity. In the presence of heterogeneities close to the surface or located at the Hertzian depth, the yield stress can be reached locally due to the additional stress it generates, whereas the stress and strain state would remain purely elastic for a matrix without inclusion. It is well known that for rolling element bearing and gear applications, the ruin of components is often linked to cracks initiated in the vicinity of large or hard inclusions that act as stress raisers. It turned out that plastic strains tend to reduce the stress generated by the contact pressure while hard heterogeneities will increase it. As plastic strain accumulation can provide the basis for fatigue damage criteria, the second half of the paper will illustrate how the method can be used to identify and rank geometrical and material parameters that influence the location and magnitude of the maximal plastic strain.

3D modelling of tyre-pavement contact pressure

Pavement design assumes uniform contact loading because of bottom-up traditional cracking (far from loading), but nowadays more and more surface damage (top-down cracking and rut) due to new technology in pavement and tyre design is observed. Then, a more realistic tyre–pavement contact modelling has to be considered. In order to go towards more realistic tyre–pavement contact, finite element method consumes a lot of calculation time; so in this article, we propose a semi-analytical model developed for the calculation of ball bearings. The contact is considered as a Hertzian contact, and the surface of the road is flat and that the tyre is smooth. Calculation of contact pressure is compared and validated with experimental data using a device called Stress-In-Motion, which allows contact pressure measurement. Different parameters are assumed in the modelling such as tyre pressure, axle loading and tyre shape. We have now a successful tool to analyse the tyre–pavement contact, more accurate and realistic and especially faster than those used at the moment.

Modelling of Ultrasonic Shot Peening

Component manufacturing may induce cold work, residual stresses, microstructure changes and even surface defects. This initial condition is usually ignored in component integrity assessments, but can strongly affect its lifetime. For instance, it is well-known that a rough surface finish associated to the presence of tensile residual stresses may favor fatigue damage. In the same manner, cold work and tensile residual stress will assist initiation of Stress Corrosion Cracking (SCC) for susceptible materials. As the manufacturing process can affect the lifetime of the structure, mitigation treatments such as precompressive loadings, chemical treatments, film deposits or coatings may be applied to sensitive areas. The objectives of these complementary operations are to avoid or compensate negative effects of manufacturing consequences. In the industry surface mechanical treatments such as Ultrasonic Shot Peening (USP) are then used in order to improve surface integrity. Even if these mitigation treatments are well known to increase component lifetime regarding corrosion and fatigue damages, a good understanding of their consequences is required to assess their efficiency and perpetuity under operating conditions. Numerical modelling of USP is one solution to simulate the motion of beads in the peening chamber and to predict the level of stresses in the peened part as shown in this paper. This model which gives a better understanding of the effect on surfaces should help the manufacturers to select the best process parameters.Copyright

Prediction of the Rolling Contact Fatigue Behavior of Pre-Indented Hybrid Bearings

The objective of this work is to study the Rolling Contact Fatigue (RCF) behavior of hybrid bearings. The studied bearings are composed of Si3N4 balls rolling on steel raceways. The raceways are made out of nitrided 32CrMoV13 steel. The nitriding treatment aims at reinforcing the surface mechanical properties. As the presence of an indent on the raceway surface will dramatically decrease the fatigue life of the rolling element [1], this study focuses on the RCF behavior of pre-indented rolling element bearings.It is thus necessary to study the fatigue behavior of both the steel and the ceramic material under fatigue loading. The study presented here focuses on the fatigue behavior of nitrided 32CrMoV13 steel under rolling contact and aims at proposing a crack initiation criterion based on experimental results.Fatigue tests are performed on a bi-disks machine with indented 32CrMoV13 samples to observe the damage evolution and crack initiation stages under various indent dimensions and test conditions.In parallel simulations are performed with a semi-analytical method to accurately determine the stress history under elastic-plastic rolling contact. Semi analytical methods, classically used for the simulation of elastic contacts, have recently been extended to the consideration of plasticity [2], allowing to simulate the ball-raceway interaction in ball bearings [3] and wear or running in [4]. The main advantage of these methods is their ability to simulate the coupling between the contact conditions and the plastic behavior in reasonable computational time.Based on the experimental and simulation results, a crack initiation criterion based on the dislocation theory proposed by Tanaka and Mura [5] is proposed allowing to predict the number of cycles for crack initiation for the given material.Copyright

Characterization and Modelling of Tensile Flow Behavior of Ni Base Alloy 690 at Various Temperatures and Strain Rates

Pressurized Water Reactor components are welded by Gas Tungsten Arc Welding (GTAW). To achieve good corrosion resistance and mechanical properties, Ni base alloy 690 is used to manufacture these components. The understanding of physical phenomena involved during welding and the prediction of induced residual stresses are crucial to guarantee high quality of these components. Welding induces drastic changes in the microstructure of the molten zone and heat-affected zone of metallic alloys especially for multi-pass welding. These changes may deteriorate the mechanical properties of the assembly. In order to reproduce the complex thermo-mechanical loading occurring within the heat affect zone, experiments on a thermo-mechanical simulator Gleeble 3500 have been carried out. In order to characterize the base alloy, isothermal tensile tests have been performed at various strain rates and temperatures (from 25 to 1100°C). A constitutive law has been proposed to predict the mechanical properties under different strain rates and temperatures. Tensile tests have also been performed after several thermal cycles to understand the effect of welding on mechanical properties of Ni alloy 690. In parallel, grain size evolution and carbide precipitation have been characterized and correlated to measured mechanical properties.Copyright

Evolution of Mechanical Behavior of 6XXX Aluminium Alloy due to the Precipitation State During a Thermo-Mechanical Process

The aim of this research is to link the microstructural state and the mechanical properties of an age hardening alloy during a fast heat treatment such as encountered during welding.A coupled model between precipitation state and mechanical properties is used to predict the yield strength and hardening behavior that can be observed experimentally. The method permits the identification of the kinematic and isotropic contributions in the hardening model. The methodology is applied to a 6061-T6 aluminium alloy which is used in the Jules Horowitz reactor vessel.The general idea of this methodology is to couple an efficient microstructural model to a mechanical one based on the dislocation theory and ad’hoc experiments. The theoretical background is based on the work of Kampmann and Wagner, known as the KWN model, to account for nucleation, growth/dissolution and coarsening of precipitates. This analysis requires transient thermo-mechanical experimental data. The efficiency of these models and their coupling are shown for a serie 6XXX aluminium alloy which contains β″ and β′ precipitates. Ultimately these models are coupled to a FEA model and allows to predict the distribution of precipitates within each element of the mesh, and subsequently its mechanical behavior.Copyright