Creep behaviour of components containing cracks—a critical review

Abstract

The prediction of crack propagation rates at elevated temperatures is important and this paper provides a critical review of available information and models for behaviour. For simplicity, behaviour is divided into three situations. At one extreme, a brittle situation may exist in which the material is brittle and the degree of constraint high, so that substantially plane strain conditions exist and stress redistribution at the crack tip is small; in this situation, the fracture is a local crack tip event and the stress intensity may be of use in correlaiting creep crack propagation data. At the other extreme, ductile behaviour may result if the material is ductile and the constraint is low with plane stress conditions prevailing; in this case, stresses at and near the crack tip will redistribute quickly down to more even values and conventional creep analysis techniques using, say, the reference stress will be most useful, particularly for estimating times to rupture. It is postulated that there is also a large intermediate régime, termed quasi-brittle, in which ductile materials under high constraint exhibit brittle characteristics. A new parameter, C∗, appears to be of more general use in this case and even be extended to other situations. Finally, an attempt is made to survey the information available on the effects of environment on crack propagation at high temperature, though conclusions are necessarily tentative at this time.