Crack growth in brittle materials exhibiting primary creep

Abstract

Engineering materials such as ferritic steels for turbine rotors and in weld heat affected zones exhibit substantial primary creep, and low failure strains (less than 1%), at low operating stresses. In components containing initial defects, the possibility of the growth of the defect by creep must be considered. This paper initially examines time and strain hardening primary creep under conditions of stress relaxation in uniaxial specimens. Uniaxial failure by damage accumulation during stress relaxation is also considered for a brittle ferritic steel representative of weld heat affected zone material. An approximate analysis is presented to obtain near crack tip stress and strain rate amplitudes for time and strain hardening primary creep. The amplitudes are found for the transient state from the initial elastic to the fully primary creep stresses. Characteristic times for the transition from small scale to large scale primary creep are determined for time and strain hardening creep. It is found that the strain hardening transient amplitude and characteristic times are greater than for the time hardening case. The derived transient amplitudes and characteristic times are used to determine crack initiation times assuming that near crack tip damage accumulation takes place. Crack initiation times are shorter in strain hardening than in time hardening primary creep. Subsequent crack growth using experimental crack growth rate data is explored. The growth of a crack relative to the growth of the small scale creep zone in both time and strain hardening primary creep is determined. Results show that for strain hardening primary creep in a creep brittle weld heat affected zone steel, the crack grows rapidly out of the transient creep stress field into the surrounding elastic field.