Effects of R-ratio on high-temperature fatigue crack growth behavior of a precipitation-hardening stainless steel

The high-temperature fatigue crack growth (FCG) behavior was investigated for a precipitation-hardening stainless steel, 17-4 PH, in three differently aged conditions. FCG tests were conducted at 400 and 500 °C under various load ratios (R-ratios) ranging from R = 0.1 to 0.7. Results indicated the fatigue crack growth rates (FCGRs) increased with R-ratio at the lower ΔK regime and merged together at the higher ΔK regime for all the given heat-treated conditions tested at 400 °C, except for Condition H900, in which the FCGRs increased with R-ratio for the entire ΔK range. At 500 °C, the FCGRs for a given heat-treated condition increased with R-ratio at lower R-ratios and became comparable at higher R-ratios for the entire ΔK range. The variations of FCGR with R-ratio could be described as an intrinsic behavior of FCG based on a Unified Approach. Accordingly, the dominating crack growth mechanism was similar to a general plastic blunting process except for Conditions A and H900 at 400 °C at the high R-ratio regime, in which the primary crack growth mechanism was associated with a crystallographic-faceted fracture mode. The variations of FCGR with R-ratio for all the given testing conditions were well correlated with a modified FCG driving force parameter, M∗ΔK.