Abstract
In order to make clear the effect of residual stress on fatigue crack growth behavior, fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on SM50A steel in the near threshold region by using the specimens with or without longitudinal residual stress. The crack length and crack opening point were measured by the unloading elastic compliance method.The results obtained are as follows.(1) Under varying loadings, a fatigue crack was found to grow in the residual stress field even when the low level effective stress intensity range (ΔKeff)L was less than (ΔKeff)th.(2) However, the fatigue crack growth rate under low level load (da/dn)*L was influenced by the number of cycles of low level load NL. When NL was rather large, the crack growth increment ΔaL became saturated and the averaged growth rate decreased.(3) The modified Miner type linear accumulation law of crack growth in terms of ΔKeff was confirmed to give a conservative estimation of near threshold fatigue crack growth rate under varying loadings even in the residual stress field.
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