Papers by Author: Saïd Ahzi

Abstract: In this work, we propose a comparison between two different approaches for the simulation of the large deformation response and crystallographic texture evolution in polycrystals. The first approach is the well-know self-consistent scheme. For this, we used the Visco-Plastic- Self-Consistent (VPSC) approach. The second approach is based on a recently developed intermediate modeling. In a first part of this paper, we present the VPSC model. In a second part, we define the intermediate linear modeling which is based on a linear combination of Taylor and Sachs models using a weight parameter. For the comparison of these two approaches, we present different results in the case of uniaxial tests for an FCC polycrystal for different values of the weight parameter for the intermediate modeling and for different formulations of the macroscopic moduli in the self-consistent model.

Abstract: In the selective laser sintering (SLS) manufacturing technique a pre-heated layer of material powder undergoes a laser radiation in a selective way to produce three dimensional metallic or polymeric solid parts. Here, we consider sintering of polymer powder. The phase transformation in this process involves the material heat transfer which is strongly affected by the material sintering phenomena. A transient three dimensional finite element model is developed to simulate the phase transformation during the selective laser sintering process. This model takes into account the heat transfer in the material (powder and solid), the sintering and the transient nature of this process. The numerical simulation of the set of equations, describing the problem, is made possible by means of the commercial finite element software Abaqus. A bi-level structure integration procedure is chosen, in which the density is integrated at the outer level and the heat equation is integrated in the inner level. After successfully computing the integration of the density, a material Jacobian representing the thermal phenomena is computed and supplemented the Abaqus Code via an implicit user subroutine material. Results for temperature and density distribution, using a polycarbonate powder, are presented and discussed.

Abstract: The arrangement of ceramic layers in laminated structures is an interesting way to enhance the flaw tolerance of brittle ceramic materials. The interfaces are expected to deflect cracks, increasing the fracture energy of the laminate compared to a monolithic material and thus raising the toughness. The target of this study is to predict the volume fraction of pores, in porous layers, required to cause crack deflection. Formulation of the fracture toughness and fracture energy as function of the material porosity is presented for random and ordered pores distribution. The effect of crack tip-flaws interaction is considered to estimate the pores volume fraction needed for crack deflection. In this work, dense and porous layers of NiO-YSZ material similar to the one used in the fuel cells technology are considered. The fracture energy of a porous material with an ordered distribution of pores shows a possibility of crack deflection at a porosity of 22.5%. However for a system with randomly distributed pores this possibility can be seen at 36% of porosity.

Abstract: The aim of this work is to discuss a new non linear intermediate model for large viscoplastic deformations that could predict the texture transition and stress-strain behavior in a range that spans from the upper bound (Taylor) to the lower bound (Sachs or static) estimates. In this model, we introduced a single parameter, f , as a weight function to formulate the proposed intermediate approach which combines the Taylor and Sachs estimates. This formulation leads to an interaction law by the minimization of a tensorial function which depends on the parameter f . For the applications, we focus on the uniaxial tension test. The results for texture evolution in an FCC polycrystal show that a transition between the copper type and brass type textures can be obtained by the proposed non-linear intermediate model.

Abstract: The aim of this work is to propose the use of an intermediate model for large viscoplastic deformations that could predict the texture transition and stress-strain behavior in a range that spans from the upper bound (Taylor) to the lower bound (Sachs) estimates. In this model, we introduced a single parameter as a weight function to formulate the intermediate model which combines the Taylor and Sachs estimates. For the applications, we focus on the three main tests: plane strain compression, uniaxial tension and compression. An FCC polycristal represented by 100 crystals with an initially random orientation is used. The results for texture evolution show that a transition between the copper and brass type textures can be obtained by the proposed intermediate model.