Trends in high temperature structural integrity assessment

Failure in components that may contain defects and that operate at elevated temperatures can occur by creep, fatigue, or fast fracture, or some combination of these processes. Frequently, these components include welds and repairs to welds. Codes and standards are available for assessing the structural integrity of these types of components. This paper addresses the main issues of concern in making safety assessments and reliable lifetime predictions for situations where creep processes dominate. Emphasis is placed on the creep component of cracking. Initially, models of crack initiation and growth are reviewed. The creep fracture mechanics parameter C* is used to make predictions. The influence of constraint on the mechanisms responsible for void nucleation and growth is considered and creep crack growth rate laws derived. Methods of calculating C* in components from the plastic fracture mechanics parameter J or, approximately, from stress intensity factor K and limit analysis using reference stress concepts are described. A procedure for making remaining life assessments for cracking into progressively deteriorating material is considered. Techniques for dealing with an initial transient phase, or incubation period, during which damage builds up at a crack tip are presented. An indication of the scatter to be expected in creep crack growth data, for use in sensitivity studies, is provided. The role of residual stress on the early stages of cracking and subsequent crack propagation is discussed.