Interpretation of material hardness, stress ratio, and crack effects on the ΔKth, of small cracks based on crack closure measurement

Fatigue tests were performed on materials containing small cracks to investigate the effects of material hardness, mean stress, and crack size on the threshold stress intensity factor of small cracks. The crack closure measurement on a very small crack was done. Although most of those effects could be explained by the characteristic behavior of crack closure in small cracks, it was also shown that (ΔK_eff)_th was also affected by crack size. The combination of (ΔK_eff)_th and the crack closure behaviors caused the peculiar characteristics of ΔK _th in small cracks. At an extremely high R region, an unusual decrease in ΔK_th was found to occur. The large reduction occurred.under the conjunction of three factors: extremely high stress ratio higher than 0.8, shallow crack less than a few tenths of a millimeter, and hard material whose HV is higher than 300. The near-threshold fatigue crack propagation rate could be uniquely evaluated even in such a short crack regime using the effective stress intensity factor with the following modification {ΔK_eff-(ΔK_eff)_th,a}. This expression could be applicable in the short crack regime. ΔK_eff still plays a role as the governing parameter for fatigue crack propagation of short crack as short as 0.04mm.