Dependence of threshold stress intensity factor range .DELTA.Kth on crack size and geometry, and material properties.

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Abstract
The dependence of ΔKth on crack size and geometry, and Vickers hardness Hv under stress ratio R=-1 was studied. The effects of crack size and geometry are unified with a geometrical parameter √(area) which is the square root of the area occupied by projecting defects or cracks onto the plane normal to the maximum tensile stress. The dependence of ΔKth on √(area) is expressed by ΔKth ∝ (√(area))1/3 and that of ΔKth on Hv is expressed by ΔKth∝(Hv+C). For small cracks and defects with √(area)≤1000 μm, the following equation for predicting ΔKth and the fatigue limit σω are available : ΔKth=3.3×10-3(Hv+120)(√(area))1/3 σω=1.43(Hv+120)/(√(area))1/6 where the units in these equations are ΔKth : MPa·m1/2, σω : MPa, √(area) : μm. For cracks and defects with √(area) >1000 μm, the dependence of ΔKth on crack size gradually changes from (√(area))1/3 to (√(area))0 and this causes the difference in the exponent n in the equation of the type σωnl=C which was first obtained by N.E. Frost, and was confirmed later by other researchers. Although the tendency of many data indicates that there may be a linear correlation between ΔKth for a large crack and Hv, more systematic studies are necessary to establish the exact relationship between ΔKth and Hv.

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