Linkage of intermediate damage in metallics to material and fracture models

Abstract

The main objective of this work was to identify the effect parameters in the method of material testing have on three constitutive models that are commonly used to predict high strain rate, large strain material behaviour. These are the Goldthorpe path-dependent model, the Goldthorpe-modified Armstrong-Zerilli model and the path-dependent fracture model, also developed by Goldthorpe. To investigate the material models, numerous interrupted tensile, torsion and compression tests were performed. This led to key improvements to the image analysis and acquisition steps of the interrupted tensile test methodology used to quantify necking. Whilst the models used to predict dynamic deformation and damage to very high strains for ballistic impact applications, such as the Goldthorpe path-dependent failure model, incorporate temperature and strain-rate dependence, they do not consider the effect of specimen size or the microstructural length scale. To investigate this, geometrically similar specimens spanning a scale range of 100:1 were tested quasi-statically to failure. Images of neck evolution were acquired using optical techniques for large specimens, and in-situ scanning electron microscope testing for small specimens, to examine the dependence of neck geometry on a broad range of specimen sizes. Size effects typically arise when the smallest specimen dimension is on the order of a microstructural length (e.g. grain size, dislocation mean free path, etc.), or in the presence of significant plastic strain gradients, which increase the density of geometrically necessary dislocations. An assumption also made to develop the models is that material deformation is isotropic. This encouraged a study where calibrated cameras, multiple view geometry and edge detection tools were used to measure the anisotropic deformation of uniaxially loaded cylindrical specimens. For this investigation tension tests were performed at quasi-static strain rates on a variety of materials using screw-driven test machines. The elliptically evolving specimens were then measured at multiple interruptions to determine the true direct stress and strain at the neck. It is hoped the anisotropic data presented in this thesis will aid model development.