ABSTRACT
A main part of the paper is the treatment of the micro-mechanistic models which have been used to address the problem of brittle, transgranular cleavage fracture in structural steels. A particular area of concern is the behaviour of these steels when used in applications such as nuclear pressure vessels. Sir Alan Cottrell played a major part in setting-up the original analysis and analysing the effects of flow characteristics on behaviour. More recent developments have shown how to include also the features of the heat-treated micro-structure of a steel, with particular emphasis on the role of brittle second-phase particles, such as carbides in wrought steel or oxide/silicate inclusions in weld metals. It is possible to express the local fracture criterion for cleavage in a notched or pre-cracked testpiece as a critical tensile stress, essentially satisfying a variant of the Griffith energy balance on the micro-scale. The paper shows how the values of effective work-of-fracture may be interpreted in terms of the crack-tip separation processes, but describes also the way in which the local tensile stress can be used to predict the temperature dependence of the macroscopic fracture toughness throughout the cleavage range. Similar principles apply to brittle, low-temperature intergranular fracture.
It is shown that the modelling gives rise to the phenomenon of the ductile/bnttle transition and both ‘microvoid coalescence’ and ‘fast shear’ modes of ductile fracture are described. There is discussion of the implications of the effect of mixity of loading on the transition, and of the effects of prior plastic strain in warm prestressing or during the growth of a ductile crack on subsequent cleavage fracture. The final issue addressed is that of extrapolation of data to very low failure probabilities.
