The influence of test duration and geometry on the creep crack initiation and growth behaviour of 316H steel

Short-term creep crack growth (CCG) tests have often been performed so that they may be incorporated into research projects with a relatively narrow time frame. Therefore tests are generally performed at relatively high loads, leading to high values of the crack-tip driving-force parameter C*, to reduce the test duration. For the case of ductile materials with relatively low yield strengths, this can lead to significant plastic strains being generated in the specimen on loading. Recently, long-term CCG tests have been performed on the low constraint double edge notch tension and high constraint compact tension specimen geometries of 316H stainless steel at 550 °C. The CCG test data is examined in terms of the experimentally determined C* parameter and compared to data available from a wide range of specimen geometries over a range of C* values. At high C* values similar CCG behaviour is observed for the various specimen geometries. A difference in CCG behaviour is observed between long and short-term tests on the compact tension geometry which may be explained by the effects of constraint loss due to plasticity effects. However, similar trends are observed in the CCG rate data of the alternative, relatively lower constrained geometries, at both high and low C* values. The influence of test duration and specimen geometry on creep crack initiation times is also studied and experimental results are compared with predictions from analytical models.