Effect of Crack Closure on the Fatigue Limit of Material Containing Small Defect

It has been recognized that the threshold stress intensity factor range Delta;K_th of short crack is dependent on many factors. Fatigue tests were performed on materials containing small defects under a wide range of mean stress for three grades of steels with different hardness. Strong dependencies of ΔK_th on material hardness, mean stress and defect size were observed. Major tendencies such that softer material had lower ΔK _th and smaller defect had lower ΔK_th were obtained, which were quite different from those of long cracks. The causes of these effects were investigated based on the crack closure behavior of short crack. The crack closure measurement on a very shallow crack was done. Test results showed that softer material and shorter crack had lower crack closure stresses. It was shown that the threshold effective stress intensity factor range (ΔK_eff)_th was a unique value for defects deeper than 0.1mm. The crack closure was the major cause of the abovementioned effects of material hardness and mean stress for defects deeper than 0.1mm. On the contrary, (ΔK_eff)_th significantly decreased compared with that for long cracks when the defect was shallower than 0.1mm. This is another short crack effect which could not be explained only by the crack closure. It was shown that (ΔK_eff)_th was no more a unique value when the defect depth was shallower than 0.1mm.