Stress Redistribution Effects on Creep Crack Growth

Abstract

Under steady loading, the stress and strain rate fields near the tip of a stationary crack relax from high initial values, describe by the parameter C(t), to steady state values described by the parameter C* For creep ductile materials, the transitional phase before attainment of the steady state is usually neglected and crack initiation and subsequent creep crack growth rates can be determined from calculations of C*. However, for creep brittle materials it is important to estimate the additional strains accumulated near the crack tip during the period of stress redistribution.

The steady state amplitude C* may be estimated for cracks in components using approximate reference stress methods. In this paper an estimation formula for C(t) is developed and expressed in these reference stress terms. The formula is particularly convenient for integration to obtain strains near the crack tip. This integration is performed and used to assess the effect of the initial period of stress redistribution on creep crack initiation and growth. It is shown that the transitional effects may be approximately described by the factor [1+ elastic strain at the reference stress/creep strain at the reference stress]. For creep ductile materials this factor will often be close to unity, but for creep brittle materials where the accumulated creep strains may be low, it can be significantly greater.