Behavior of Fatigue Crack Growth Threshold of SUS304 Steel in Low .DELTA.K Region at an Elevated Temperature by the Increase of Kmax

Fatigue crack growth tests using center-cracked [M (T)] specimens and single-edge-cracked (SEC) specimens of SUS304 steel were conducted to examine the behavior of fatigue crack growth in the near-threshold regime at 623 K. Crack closure was eliminated by keeping Kmax constant in ΔK decreasing test in order to obtain easily the threshold value expressed in terms of effective stress intensity factor range, ΔKett. By increasing the value of Kmax and/or ΔK for a crack in a threshold condition, the fatigue crack growth behavior was investigated even in the region below the threshold value obtained by using the conventional ΔK-decreasing test. When the value of Kmax was increased in the conventional threshold condition, crack growth was observed in the lower ΔK region. Crack growth was further observed repeatedly after successive increase in Kmax in the newly established lower region below the conventional threshold value. In the region below a certain value of ΔK, however, crack no longer grew even after the change of Kmax. Discussion was made on the characteristics of threshold.