Influence of reference stress formulae on creep and creep-fatigue crack initiation and growth prediction in plate components

Creep and creep-fatigue crack growth in pre-cracked plates of 316L(N) austenitic stainless steel, containing a semi-elliptical surface defect and tested at 650 °C under combined axial and bending loading, are investigated. The results have been interpreted in terms of the creep fracture mechanics parameter C∗ and compared with data obtained on standard compact tension (CT) specimens of the same material and batch. In making the assessments, the reference stress method has been used to determine C∗. Several formulae exist for calculating the reference stress depending on whether it is based on a ‘global’ or a ‘local’ collapse mechanism and the assessment procedure adopted. When using this approach, it has been found that the most satisfactory comparison of crack growth rates with standard CT specimen data is obtained when the ‘global’ reference stress solution is used in conjunction with mean uniaxial creep properties. It has been found that the main effect of changing the fatigue cycle range from 0.1 to −1.0 is to cause an acceleration in the early stage of cracking.