Fracture mechanics in design and service: ‘living with defects’ - Subcritical crack growth: fatigue, creep and stress corrosion cracking

Abstract

Subcritical crack growth can occur under steady or varying loads. In the former it is precipitated by specific environmental conditions that encourage the operation of time-dependent processes controlling crack advance. These include aggressive environments leading to stress corrosion cracking, or elevated temperature conditions leading to creep cavitation. The result is a time-dependent maintenance of a sharp crack profile during crack extension. Under varying loads such a sharp profile is readily achieved by plastic deformation on load reduction. Net crack advance in fatigue therefore occurs in each load cycle by this blunting-resharpening process, and empirical crack growth laws reflect this physical basis. Parameters such as K and J, which define crack tip deformation, are useful for correlating fatigue crack growth. In that they define crack tip stress-strain fields under load, they also partly describe crack advance for steady load creep and stress corrosion cracking. In particular they can define a threshold state for crack extension by all three processes. Under varying loads, if fatigue conditions are combined with an aggressive or high-temperature environment the description of crack growth can be complex. These areas of corrosion fatigue and creep fatigue are of considerable current practical interest.