On void coalescence in ductile fracture of aluminum sheet.

摘要

The current research seeks to investigate localized void coalescence in ductile fracture of aluminum alloy sheet. The effect of stress state, measured by stress triaxiality, is exploited on coalescence. A series of notched sheet samples were designed to enable a stress state variation in the sheet metal. A relationship between stress triaxiality and notch geometry for the notched tensile samples of AA5182 sheet has been obtained. Finite element simulations have been used to verify the experimental results of the stress triaxiality for the notched tensile sheet samples. Relations between stress triaxiality and notch geometry have been proposed according to the experimental and simulation results. The fracture modes observed in the fractographs of the notched samples using Scanning Electron Microscope (SEM) are related to the stress state variation across the fracture surface. Basically, higher triaxiality promotes void growth and ductile fracture, whilst lower triaxiality retards void growth leading to shear failure through void sheeting. To further explore the effect of triaxiality and void distribution on ductile fracture, a 3D unit cell model is developed. The cell model contains two aligned voids in a damage-free matrix. A coalescence criterion is proposed in which the critical void volume fraction is expressed in terms of stress triaxiality. Then a modified plastic limit-load criterion (Zhang et al., 2000) is used as coalescence criterion and the modified void clusters (Thomson et al., 1999) are used to simulate ductile fracture in a three-dimensional framework. The effects of void cluster density, cluster type, primary void volume fraction and void aspect ratio on coalescence have been investigated under different stress triaxialities.