Ductile fracture theories for pressurised pipes and containers

Abstract The probable mechanisms of fracture which may be encountered in various components may generally be classified in two main groups. In the first the fracture is of ‘plane strain’ type which may occur in components (e.g. thick walled pressure vessels and other heavy section components) where prior to and during a possible fracture propagation the material is not expected to undergo large scale plastic deformations. In this case the underlying fracture theory is rather well-understood, and a criterion based on KIC usually provides a highly reliable tool to deal with the problem. The second type of fracture failure falls into the general category of ‘plane stress’ or ‘high energy’ fracture. In a great variety of tubings and containers, due to relatively small wall thickness, large defect size, high material toughness, and high temperature, prior to and during a possible rupture process, around the defect region the material would be expected to undergo large scale plastic deformations. In this case the standard theories of fracture based on the concept of plane strain fracture toughness are not applicable. This type of fracture which is generally accompanied by large inelastic deformations is (somewhat loosely) termed the plane stress fracture for which currently there does not seem to be a universally accepted criterion. In some components an additional complicating factor arises where one is dealing essentially with a shell of given curvature rather than a flat plate. The theories which are currently in use in practice to analyse plane stress type of fracture are those which are based on the concepts of critical crack opening stretch, K R - characterisation , J- integral , and the recently proposed plastic instability. In this paper the application of the fracture criteria based on these concepts to the fracture of shells will be discussed and the concept of plastic instability will be developed in some detail. Since there is no widely accepted standard criterion to deal with this type of fracture, one of the aims of the paper will be to provide an up-to-date critical appraisal of the current theories.