The Fracture Properties of High Strength Aluminium Casting Alloys

Abstract

The fracture properties of a range of high strength aluminium casting alloys, in various heat treated conditions, have been investigated in three point bending. Sharp cracks, necessary for fracture toughness testing, were produced by both fatigue cracking and spark machining and the compatibility of the two methods has been discussed. Stress intensity factors were determined for the curve crack fronts from experimental compliance data and a practical method of dealing with crack front curvature, has been proposed. The five available methods for determining the critical stress intensity factor (K1c) have been evaluated and their accuracies and limitations discussed. In order to measure the load-point displacements of the specimens, a rig, considered to be an improvement on existing rigs, has been developed. Initiation of the cracks was detected using an electrical potential techniaue, which could also be used to determine crack growth rates and predict crack lengths. Geometrical effects on the stress intensity factor, such as blunt notches and short cracks emanating from the roots of blunt notches, have been assessed and are compared with the theoretical solutions. Structural influences on the stress intensity factor, such as material condition and porosity, have also been investigated. The data collected from the majority of the specimens was tabulated, punched and fed into a computer programme designed to calculate the fracture toughness parameters. The results have been presented graphically and analysed statistically using standard analysis of variance packages. Optical and electron microscopy has been used to examine the fracture surfaces and an explanation of the mechanism of fracture has been suggested for each alloy.