Improved Method for Δ<i>K</i> Decreasing Fatigue Crack Growth Test and Crack Closure Measurement—toward Establishment of a JIS Standard

The effects of the normalized stress intensity factor gradient, C, on the relationship between crack propagation rate, da/dN, and stress intensity factor range, ΔK, in the near-threshold regime were investigated. When the force shedding rate at each crack extension step was less than 2%, and the force shedding interval for crack extension was 50 µm, the value of C exceeded −0.4 mm−1 and a unique da/dN–ΔK relationship independent of the value of C was observed at the same force ratio.