Near-threshold fatigue crack growth behaviour of a ferritic stainless steel at elevated temperatures

Abstract

Near-threshold fatigue crack propagation (FCP) behaviour has been studied in an 18%Cr---Nb stabilized ferritic stainless steel at temperatures ranging from room temperature to 700 [deg]C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 [deg]C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a [delta]K level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 [deg]C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of [Delta]K to thresholds lower than the room temperature value. Crack closure measurements suggest that asperity-induced closure dominates at room temperature but transitions to plasticity-induced closure dominate at 500 [deg]C. A constant-Kmax, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 [deg]C in an attempt to identify near-threshold FCP data in the absence of crack closure. However, the type of crossover behaviour identified with constant R-ratio tests at room temperature and 500 [deg]C was also observed in the CKIR tests. This is attributed to a change in the closure mechanism from a roughness-induced one to one involving crack tip plasticity.