Abstract
The effects of loading history on threshold stress intensity range (ΔKth) were investigated in a low alloy steel SFVQ1A (A508-3) and a low carbon steel S10C. A single overload and multiple overloads were chosen as loading history. Crack growth and crack closure following the loading histories were measured at load ratios of 0.05 and 0.07. Threshold values were determined as a fatigue limit of preloaded specimens. The ΔKth values increased with increasing overload stress intensity factor (Kh). For a given Kh value, multiple overloads produced much larger increase in ΔKth than a single overload and threshold values expressed by maximum stress intensity factor (Kmax, th) were almost constant, independent of stress ratio. The results obtained were discussed in terms of crack closure behavior, and a method was proposed to evaluate the threshold value based on plasticity-induced crack closure.
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