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 °C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 °C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a δK level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 °C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of ΔK 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 °C. A constant- K max, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 °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 °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.