Models for creep crack initiation and growth

Abstract

Methods have been presented for performing creep analyses of cracked and uncracked bodies. Fracture mechanics concepts have been developed for characterizing stress distributions ahead of a crack in a component deforming by creep. In this chapter models are derived for describing the onset of cracking and crack propagation in terms of creep damage accumulation in a process zone ahead of the crack tip. Initially, models are developed for dealing with crack growth when a steady state distribution of damage has been generated at the crack tip in materials which deform in secondary creep. In addition the importance of state of stress at the crack tip is examined. Afterwards, extensions to primary, secondary and tertiary creep are considered. Emphasis is then placed on the early stages of cracking. Criteria for the onset of cracking are introduced that are based on the attainment of a critical crack opening displacement or a specific amount of damage at the crack tip. Procedures for dealing with a gradual build-up of damage at the crack tip are also included. Finally the situation of a crack growing into progressively deteriorating material is considered for making remanent life assessments of plant that has been in service for a period of time.