Fatigue crack closure over the range of stress ratios from −1 to 0.8 down to stress intensity threshold level in HT80 steel and SUS304 stainless steel

The closure phenomena of fatigue cracks were investigated with a 1 mm gage length extensometer over the range of stress ratio, R, from −1 to 0.8. Plate specimens with a center slot of HT80 steel and SUS304 stainless steel were fatigued under push-pull loading, and the crack propagation rate, da/dn, was measured. The stress ratio, R, was found to influence da/dn in both materials. The crack opening stress intensity factor, Kop, was determined from the relationship between the crack tip extensometer displacement and the load. The effective stress intensity range ratio, U(=ΔKeff/ΔK), decreases with the decrease of the stress intensity amplitude, ΔK/2. As for the data which show the crack closure phenomena (R≦0.4), the relationship between log(da/dn) and log(ΔKeff/2) falls on a straight line near the stress intensity threshold level. For R=0.8 where the crack tip is fully open over the whole range of loading, the data show a discrepancy from the same line. The strain at the crack tip was also measured with the Moire fringe multiplication method. A large amount of plastic strain at the crack tip was observed even below the crack opening load for R=−1 in HT80 steel. These phenomena show that fatigue damage still exists when the crack is closed. These also show that the crack closure cannot fully account for the effect of R on da/dn.