Evaluation of Actual Fatigue Stress by Using Striation Spacing and Hardness

The relation between the range of stress intensity factor ΔK and striation spacing S was obtained by using CT specimens of various kinds of structural steels with different hardness, and the actual fatigue stress of square or round bars under repeated 3-point bending load, which was assumed to be an actual machine part, was evaluated from the Vickers hardness HV and the striation spacing measured in the bars. The material constants, Cs and ms, obtained from the relation between S(mm) and ΔK (MPa√m) for the CT specimen, S=Cs(ΔK)ms, were able to be approximately expressed as Cs=1.68×10-10(HV)1.88 and ms=19.1(HV)-0.53, respectively. The actual fatigue stress of the square bar under 3-point bending agreed relatively well with the stress calculated from the striation spacing measured on the bar and the values of Cs and ms, which were obtained from the above equation by substituting the hardness. For the round bar, however, the calculated stress was larger or smaller than the actual fatigue stress when the crack length is short or long, respectively.