Effect of Stress Concentration on Fatigue-Crack Initiation in HY-130 Steel

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Abstract
The useful fatigue life of structural components is determined by the sum of the elapsed load cycles required to initiate a fatigue crack and to propagate the crack from subcritical size to critical dimensions. Thus, to predict the service life of many steel structures and to establish safe inspection intervals, an understanding of the fatigue-crack-initiation and fatigue-crack-propagation behavior in steel is required. In the present study, the fatigue-crack-initiation life in HY-130 steel was investigated by testing specimens having widely varying notch acuities. The variation in notch acuity covered the range from fatigue-cracked specimens to polished, unnotched specimens, and fatigue-crack-initiation data were obtained in the range 103 to 106 cycles. The data were analyzed by using linear-elastic fracture-mechanics concepts and the theory of stress concentration in notched specimens. The results showed that the number of elapsed load cycles required to initiate a fatigue crack in a notched specimen is related to the ratio of the fluctuation of the stress-intensity facor, ΔKI, to the square root of the notch-tip radius, ρ. Fatigue-crack-initiation life can also be expressed in terms of the fluctuation of the maximum stress at the notch tip, Δσmax, because ΔKI/√ρ can be related to Δσmax. Moreover, preliminary observations suggested that the fatigue-crack-initiation threshold in martensitic steels of various yield strengths tested under zero-to-tension load fluctuation can be predicted by the equation ΔKσysρ=0.6 is the 0.2 percent offset yield strength. By using a weakest-link statistical model, it was shown that surface irregularities can appreciably decrease the fatigue-initiation life of structural components.

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