Influence of precipitates on ductile fracture of aluminium alloy AA7075 at high strain rates

Under dynamic loading, the stabilising effect of the material's increased strain rate sensitivity leads to an increase in ductility. At the starting point of necking, the local strain rate increases resulting in a corresponding increase of local flow stress within the neck zone. Therefore, other regions of the specimen undergo additional plastic deformation and the necking process is shifted to high strain values. On the other. hand, an increased strain rate reduces the local fracture strain, leading to an increased dynamic notch sensitivity. Notches are often the starting point for ductile fracture caused by impact. This kind of failure occurs due to nucleation, growth and coalescence of voids around inhomogeneities, such as inclusions and precipitations. Therefore, dynamic and quasi-static tension tests were carried out on smooth and notched specimens of age-hardenable aluminium alloy AA7075. Using different heat treatment conditions, the influence of size, volume fraction and distribution of particles on the fracture strain were investigated. Tests with various notch geometries allow the influence of the degree of multiaxiality to be analyzed. By combining experiment and numerical simulation, the location of void initiation has been estimated and the influence of strain rate on the local fracture strain has been determined. (C) 2001 Elsevier Science B.V. All rights reserved.