Papers by Author: Fabrice Pierron

Abstract: Welding is one of the most popular joining technologies in industry. Depending on the materials to be joined, the geometry of the parts and the number of parts to be joined, there is a wide variety of methods that can be used. These joining techniques share a common feature: the material in the weld zone experiences different thermo-mechanical history, resulting in significant variations in material microstructure and spatial heterogeneity in mechanical properties. To optimize the joining process, or to refine the design of welded structures, it is necessary to identify the local mechanical properties within the different regions of the weld. The development of full-field kinematic measurements (digital image correlation, speckle interferometry, etc.) helps to shed a new light on this problem. The large amount of experimental information attained with these methods makes it possible to visualize the spatial distribution of strain on the specimen surface. Full-field kinematic measurements provide more information regarding the spatial variations in material behaviour. As a consequence, it is now possible to quantify the spatial variations in mechanical properties within the weld region through a properly constructed inverse analysis procedure. High speed tensile tests have been performed on FSW aluminium welds. The test was performed on an MTS machine at a cross-head speed of up to 76 mm/s. Displacement fields were measured across the specimen by coupling digital image correlation with a high-speed camera (Phantom V7.1) taking 1000 frames per second. Then, through the use of the virtual fields method it is possible to retrieve the mechanical parameters of the different areas of the weld from the strain field and the loading. The elastic parameters (Young’s modulus and Poisson’s ratio) are supposed to be constant through the weld. Their identification was carried out using the virtual fields method in elasticity using the data of the early stage of the experiment. Assuming that the mechanical properties (elastic and plastic) of the weld are constant through the thickness, the plastic parameters were identified on small sections through the specimen, using a simple linear hardening model. This method leads to a discrete identification of the evolution of the mechanical properties through the weld. It allows the understanding of the slight variations of yield stress and hardening due to the complexity of the welding process.

Abstract: This paper studies the effect of delaminations on strain maps for a simple cantilever beam. The aim is to build an experimental set-up which allows detecting very slight modifications in the strain maps. The case studied is a single delamination on the mid-plane. The measurement method is the deflectometry technique which enables direct slope measurements on a reflective specimen. The comparison with finite element models clearly indicated that the surface strains bear the information of the extent of the delamination. The second step is to use these surface strains to identify a stiffness reduction map for real impact damages.

Abstract: This paper presents a short overview of the state of the art and future challenges of the use of full-field measurements and inverse procedures to identify the constitutive mechanical parameters of a wide range of materials. It concentrates on the so-called Virtual Fields Method (VFM) which is a tool fully dedicated to the processing of full-field measurements. Some of the future challenges are briefly covered here, namely the design of test configurations and the application to damage assessment, high strain rate testing and biomaterials. Some examples are given and the main scientific issues briefly discussed.

Abstract: This work deals with the development of a full-field extensometric method at a micrometric scale in order to precisely identify the local features of a metallic alloy at the scale of the grains. The full-field method that has been chosen is the grid method that applies a spatial phase-shifting algorithm to a periodic pattern. To mark the sample, direct interferometric photolithography was used. The paper presents the basic features of the technique and first mechanical test results are commented.

Abstract: The virtual fields method is a tool dedicated to the identification of the mechanical properties of materials from full-field deformation measurements. It is now validated in elasticity and plasticity but one of the remaining problems is the fact that researchers wanting to use the method must invest significant time in order to programme the routines. To help them, a software called CamFit has been developed. The purpose of this paper is to present this software. It is based on MATLAB® and uses a graphical pre-processing interface to produce the geometry, the conditions on the virtual fields, to choose the type of behaviour etc... Then, series of displacement maps are uploaded and the identification is launched. Since no iterative solution of the direct problem is required, computation times are very small compared to updating techniques. An important step in the procedure is the smoothing of the displacement measurements to produce strains. FE based approximations are presently available in the software. The final purpose is to introduce the software onto the market. This will be done in the very near future.

Abstract: In order to fully understand the thermomechanical phenomena involved in high-cycle fatigue, a method for determining the dissipative thermal sources in a loaded metallic specimen from the spatio-temporal integration of the heat diffusion equation is proposed. Temperature fields obtained through a focal array infra-red camera are processed with this technique. After a refined analysis of the sensitivity of the method, preliminary tests have shown that it is possible to detect a burst of heat sources within the first couple of cycles for the specimen tested above a certain level of tensile stress. This behaviour is thought to be related to the microplasticity level.