Modelling the effect of surface roughness on the localization and failure behaviour of AA6111 sheet.

摘要

Localization and failure of sheet metals are known to be sensitive to the presence of surface asperities. In this thesis, characteristic frequencies and amplitudes of surface roughness have been obtained from undeformed and deformed AA6111. These measures are then incorporated into two-dimensional finite element method (FEM) models to study the relationship between surface roughness and microstructural damage in sheet forming failures. The FEM models are capable of simulating the coupled effect of shear bands and voids on sheet localization behaviour by employing a mixed isotropic/kinematic hardening form of the Gurson-Tvergaard-Needleman (GTN) constitutive softening equations. The effect of the relative positions of top and bottom surface defects was studied using three different model geometries.; The FEM models predict that the defect amplitude plays a strong role in causing earlier onset of localization and failure. Furthermore, the relative positions of defects on the top and bottom surfaces plays a significant role in the determination of failure mode, specifically necking failure versus through-thickness macroscopic shear banding. The FEM models also show that, under certain geometric conditions (i.e. the combination of defect amplitude, wavelength, and phase), macroscopic shear banding can cause failure at lower values of strain than necking.