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

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.