The effects of microstructural inhomogeneity on damage accumulation and fracture

Abstract

<p>A theoretical and experimental study has been made which indicates the importance of the spatial distribution of damage and a complete physical description of the operative damage processes in providing a fracture condition. A continuum damage model is presented which investigates the influence of void coalescence in accelerating damage levels, for a random array of voids growing via a ductile hole growth mechanism. A simple geometric coalescence condition was assumed. The dependence of fracture strain upon initial void volume fraction and stress state history is predicted. This approach is extended in a simulation to predict the void size distribution which develops. The Dirichlet tessellation was evaluated as a method for characterizing dispersions of points or particles. Point dispersions were generated, ranging from strongly periodic to strongly clustered, and properties of their associated tessellations were evaluated. This suggested parameters which were sensitive indicators of periodicity or clustering in a dispersion. this approach was extended to the characterization of inclusion and particle distributions in steels and several other alloys. The influence of temperature and stress on damage and fracture was studied in several continuously-cast HSLA steels. Fracture was controlled by the inclusions, the centre-line transformation products, or the interaction between these two sources of damage. Observations also suggested that the distribution of the banded transformation products could influence damage levels in the ferritic regions. This study concludes with an investigation of the creep fracture in a Ni-Sn alloy, which contained an inhomogeneous tin distribution. The regions with high tin content accumulated damage rapidly, however, ductility was promoted by the material low in tin which did not cavitate as readily. The influence of stress level on the strain dependence of damage accumulation rate, and the shift to surface crack-controlled fracture was interpreted as an effect of the enhanced relative contribution of grain boundary sliding to total strain at lower stresses.</p>