Stress concentrations in non-convex elastic particles embedded in a ductile matrix

Abstract

Aluminium sheet is currently used for body panels on a number of mass-produced vehicles, in particular for closure panels. AA5xxx alloys always contain coarse inter-metallic particles (Al x(Fe,Mn) ySi, Mg 2Si) after casting, undesirable for the final sheet forming and stamping operations. In the present work inter-metallic particle break-up during hot reversible rolling of AA5182 alloy sheets has been analysed by both experiments and by a micromechanical model. The sizes and shapes of inter-metallic particles in as cast and industrially hot rolled AA5182 alloys sheets were characterised by 3D X-ray tomography observations. The evolution of inter-metallic particle size and shape vs. hot rolling deformation was studied. The stresses and strains in inter-metallic particles, embedded in an elasto-viscoplastic aluminium matrix submitted to plane strain compression, were analysed by a finite element (FE) model. First, the mechanisms leading to stress and strain concentrations were analysed on several model particle shapes. A new algorithm for FE-analysis of stress concentrations around hard second phase particles in Al alloys was developed. A new failure criterion for brittle pre-cracked solids embedded in a soft matrix submitted to large straining was proposed. Secondly, this new algorithm and the new failure criterion were used to analyse the stress and strain concentrations in the real particles observed by X-ray tomography. The ability to break-up of the particles was analysed. The essential outcomes of the present work are as follows: a precise description of stress concentration mechanisms in non-convex particles, a new failure criterion based on the comparison between an active and a critical volume, a close description of the parameters controlling particle break-up, and finally a classification of the failure behaviour of real inter-metallic particles.