NEAR-THRESHOLD FATIGUE CRACK GROWTH IN SUS304 STEEL AT ELEVATED TEMPERATURES

摘要

Fatigue crack growth behavior in the type 304 austenitic stainless steel was investigated at elevated temperatures, i.e. 350, 450, 550, 650 and 700 (°C). Crack growth rates for stress ratio R=-0.l plotted against stress intensity factor range, ΔK, deviated from those for R=0.5. Using the effective stress intensity factor range, ΔK_eff and effective J-integral range, ΔJf,eff, which were calculated using crack opening ratio, the stress ratio dependence in crack growth behaviors for R = -0.1 and 0.5 was reconciled. The value of fatigue crack growth threshold at elevated temperatures expressed in terms of ΔKeff or A7f,eff was much higher than that at room temperature. It was found that the threshold value reached a maximum at a temperature between 450 and 550 (°C). The behavior of fatigue crack growth was investigated using the Kmax-constant ΔK-decreasing test method. By increasing the values of Kmax and/or ΔK for a crack in a threshold condition, the behaviors of fatigue crack growth were investigated. It was found that the crack grew even in the region below the threshold value obtained by using the conventional ΔK-decreasing test. Crack growth was observed by successively increasing the Kmax value in newly established threshold regions. In the region below a certain value of ΔK, however, the crack no longer grew.