Papers by Author: Yvan Chastel

Abstract: Strain field measurement with non intrusive techniques is needed in order to characterize the behaviour of steels at high temperatures subjected to small displacements. In this work we present a technique based on laser-produced speckles coupled with a cross-correlation cross-spectrum method. This method proves more accurate than cross-correlation for small displacements. The laser wave length used (532 nm) allows to perform strain measurements, even with heat radiation.

Abstract: According to various studies, Grain Boundary Engineering (GBE) is likely to enhance mechanical properties of polycrystalline materials. The present investigation highlights some relationships between thermomechanical process (TMP) parameters of a commercial nickel-base superalloy PER72, supplied by Aubert & Duval (equivalent to Udimet®720™) and the resulting microstructure. The long-term goal is to develop TMPs that modify the Grain Boundary Character Distributions (GBCD) in order to improve high temperature properties. In this context, Grain Boundary Engineering (GBE) techniques are considered, thinking of replacing standard forming processes by optimised thermomechanical treatments. Mechanical testing at high temperature (compression and torsion tests) has been carried out and it is shown that multi-step treatments promote twinning. Some clues are then presented in an attempt to explain when and how twins are created.

Abstract: A 3-D solid finite element simulation of sheet forming processes is briefly discussed. Examples of cold or warm deep-drawing and sheet hydro forming are presented. Sheet work-pieces can be assembled to produce complex components by using different techniques: such as welding or mechanical fastening. They must also be simulated in order to evaluate and optimise the quality of the parts; examples of hemming and of self piercing riveting are described. Structural computation allows us to evaluate the strength of a component and especially the strength of the joining. In the future, more precise optimization of the components will be possible by the transfer of data from the previous stages of sheet forming and joining, to the structural computation code. This input data will be firstly the distribution of residual stresses, the evolution of local properties such as elastic limit, damage and anisotropy. An example of structural computation on a system of two sheets assembled by a single rivet is presented.

Abstract: This paper presents a means of reducing the computational cost of finite element (FE) simulations coupled to polycrystal plasticity theory. One typically assumes that a polycrystal with a large number of grains underlies every integration point of the FE mesh. Instead, it is suggested here using reduced samplings of grains which differ from one integration point to another. On average, every set of 5 to 25 finite elements contains a variety of lattice orientations that is representative of the macroscopic texture. The model is applied to deep-drawing of a cylindrical cup made of steel. In a first set of simulations, grains are assigned orientations representative of a cold rolling texture and the “earing” profile is compared to experiment. In a second set of simulations, lattice orientations are random and an isotropic deep-drawing result is expected. It is demonstrated that using a minimum of 20 grains per integration point allows properly predicting the final shape of the cup and the texture development.