Papers by Author: A.G. Crocker

Abstract: This study aims to develop computer models, with a microstructure representative of the PGA graphite, to contribute to the understanding of the relationship between the amount of porosity, the load-displacement behaviour and crack propagation. The project is in two linked parts, the first provides a model of the porous graphite which is then introduced into a lattice type finite element model to provide the load-displacement and crack propagation predictions. Microstructures consisting of matrix and pores with added aligned filler particles, typical of needle coke, were studied. The purpose was to isolate the effect of filler particles on fracture strength and the fracture path. In the paper crack paths and fracture mechanisms are discussed for different amounts of porosity and various filler particle arrangements.

Abstract: It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.

Abstract: In polycrystalline materials grain boundaries provide an important contribution to the resistance to the propagation of both brittle and ductile cracks. In this paper we describe experimental measurements of brittle cracks developed within both small punch and matchstick test specimens of polycrystalline hcp zinc. These specimens were tested over the temperature range 77 to 423K. Fractography undertaken using focussed ion beam imaging provides detail of the propagation from grain to grain and across {10-12} twins of (0001) basal and {10-10} prismatic cleavage cracks. The results are discussed by comparison with the predictions from previously described 3-D geometric modelling applied to this hcp polycrystalline material.