PhD defense : Finite Element implementation of a finite strain constitutive model for amorphous polymers: applications over a wide range of strain rates and forming processes

PhD defense : Chrystelle BERNARD

Team : MMB

Title : Finite Element implementation of a finite strain constitutive  model for amorphous polymers: applications over a wide range of strain rates and forming processes

Abstract : Nowadays, numerous structural parts are made in polymeric materials. These materials exhibit a complex behavior strongly sensitive to strain rate and temperature. Numerous constitutive equations have been developped during the last decades in order to describe the elatic-viscoplastic behavior of polymers in finite strain for a wide range of strain rates and temperature. To provide for the holding of mechanical parts polymer subject to a complex set of loads, the use of numerical methods, such as Finite Element (FE) method, are unavoidable. However, the quality of the numerical prediction is strongly dependant to the used constitutive equations. Thus, we proposed to study two models of mechanical behavior implemented in two FE softwares: a simple
phenomenological model, introduced in CAST3M, and a micromechanical model, introduced in ABAQUS/Explicit. The phenomenological model allow to simulate the mechanical behavior of glassy polymers in small strains over a reduced range of strain rates and temperatures. A compressive test reproducing the Split Hopkinson Pressure Bar device is simulate for several strain rates and temperatures. A good correlation is found between experimental results and numerical predictions. Moreover, an estimation of Taylor-Quinney coefficient and the interfacial stress, due to the friction between the polymer sample and the compressive bars, have been found. The micromechanical model describes the mechanical behavior of amorphous polymers in finite strain over a wide range of strain rates and temperatures. It has been developped in our research team by Richeton et al. [Int. J. Solids Struct. 44 (2007) 7938] and proposes to take into account the strain rate and temperature dependance of various material properties (elastic modulus, yield stress, orientational hardening). In order to simulate the mechanical behaviour of polymeric structures under dynamic loadings or during forming processes, a VUMAT subroutine is written. After validation of the numerical implementation of the VUMAT subroutine for simple compressive/tensile tests, two applications were simulated. The first application is a normal impact test of a polymeric plate by a hemispherical projectile. The second application is a cold forging test. In both cases, numerical predictions are in agreement with the experimental results from the litterature.

This PhD defence was supervised by Nadia Bahlouli and Saïd Ahzi and Joao Pedro de Magalhaes Correia was the co-supervisor. The jury is comprised of Noëlle Billon (CEMEF, Mines ParisTech), Loïc Daridon (University of Montpellier), Laurent Gornet (GeM, Centrale Nantes School), Joao Pedro de Magalhaes Correia, Nadia Bahlouli, Saïd Ahzi, Christophe Fond and Yves Rémond (University of Strasbourg).

The presentation will take place on tuesday 16th june 2015 at 2.00pm in the seminar room, 4 rue de Boussingault in Strasbourg.