Project 1: Identification of constitutive equations of composite materials from whole field measurements: the Virtual Fields Method (VFM)

(PhD thesis of M. Derbel, Supervisors: F. Pierron and L. Fouilland)

Composite materials are generally anisotropic and therefore, more difficult to characterize than isotropic materials. Moreover, the anisotropy tends to induce parasitic effects that make "simple" tests on coupons more difficult. The recent breakthrough of optical whole-field measurements (moiré, ESPI, digital image correlation, grid methods…) opens new perspective in mechanical testing since displacement or strain fields are now measured. Nevertheless, the usual mechanical tests designed for scarce and local strain gauge or extensometers measurements are still used and one feels that there is a need for novel mechanical testing procedures to take advantage of all these field data. F. Pierron and Prof. M. Grédiac from Blaise Pascal University in Clermont-Ferrand, have been developing for a few years a method for extracting parameters governing linear and non-linear constitutive equations from displacement/strain fields. The method, known as the "Virtual Fields Method" is completely novel and relies on the writing of the specimen equilibrium with the principle of virtual work. Compared to alternative methods such as parameter updating from FE models, this method is direct, i.e. the parameters are retrieved directly from the strain maps without any iterative FE scheme.

There are many aspects of this method presently under developments for different types of constitutive equations and test specimens.

Project 2: Fatigue behaviour of lightweight materials from thermo-mechanical data (PhD thesis of F. Maquin, Supervisors: F. Pierron et F. Meraghni)

Since the early 90's, a method has been developed to identify the fatigue limit of metallic materials from local temperature rise. This is of great interest to industry since related costs are cut dramatically, reducing the testing time from a few weeks at least to one day to obtain a fatigue limit. Nevertheless, the mechanisms underlying this particular thermo-mechanical behaviour are not well understood yet. In particular, the method works for a certain class of materials but not for others. The idea of this project is to examine the thermo-mechanical aspects at the microscopic scale and relate the differences of behaviour observed between different materials to thermo-mechanical constitutive equations. Applying the technique to a wide range of materials (steels…) and in particular lightweight materials (aluminium alloys, polymer matrix composites…) will give more insight in the fundamental thermo-mechanical aspects of the method.

Information provided by: http://www.chalons.ensam.fr