Large reduction in the (ΔK_eff)_th of short crack at extremely high stress ratio

It has been recognized that the threshold stress intensity factor Delta;K_th of short crack is dependent on many factors. Dependencies of Delta;K_th on material hardness, mean stress and crack size could be successfully evaluated on the basis of crack closure phenomenon. As a consequence, the (Delta;K_eff)_th was almost constant for a wide range of conditions. In this study, however, extraordinary decrease in the (Delta;K_eff)_th was found to occur in some limited condition. It was shown that the reduction was caused by the conjunction of three conditions, namely, high stress ratio (R) larger than 0.8, short crack and hard material whose Vickers hardness was higher than 300. The lowest value of (Delta;K_eff)_th obtained in this study was 0.7MPam ^1/2 for steel. The fracture surface morphology was trans-granular and no indication of environmental effect was found. The fatigue crack propagation rate near the threshold condition could be uniquely evaluated even in such a short crack regime using the effective stress intensity factor with the following modification {Delta;K_eff - (Delta;K_eff) _th.a} Here, (Delta;K_eff)_th.a is the (Delta;K_eff)_th for crack length a. This evaluation could be applicable in short crack regime as short as 0.04mm. Delta;K_eff still plays a role as the governing parameter for fatigue crack propagation of short crack.