SHORT CRACK COALESCENCE AND GROWTH IN 316-STAINLESS-STEEL SUBJECTED TO CYCLIC AND TIME-DEPENDENT DEFORMATION

A study has been undertaken into short crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550 degrees C within the high strain range of 0.9 to 2.5% and including a 60 min hold-time. During the high-temperature, reverse-bending tests, surface crack initiation and growth on both the tensile-hold and the compress ive-hold sides of circular-section specimens were monitored by means o f a plastic replication technique. Detailed analysis revealed that und er creep-fatigue conditions, the initiation and growth behaviour of ma ny individual cracks and their subsequent coalescence to form a major Stage II (tensile) crack was the dominant feature in the failure proce ss. A life prediction model is proposed which incorporates the process of short crack coalescence. Satisfactory predictions of creep-fatigue lifetimes are derived from the model.