Abstract
In our previous paper, an analytical crack growth law to predict crack growth rate during the retardation period was established through the detailed evaluation of fatigue crack growth behavior under two-step varying load conditions with the stress ratio R(=σmin/σmax)=0. The main purpose of this study is to develop the above crack growth law to make applicable to various types of two-step varying load conditions. For this purpose, the effects of the tensile mean stress and the stress ratio of the lower stress level on the crack growth behavior during the retardation period induced by load change were investigated by carrying out fatigue crack growth tests on aluminum alloy (A2017-T3) compact tension specimens under four types of two-step varying load conditions with the lower stress level of positive stress ratio. Detailed observations for the crack length grown by the application of lower stress cycles were made by an optical microscope and/or on SEM micro-fractographs. The results indicated that crack growth behavior in the deceleration stage during the retardation process was governed only by the lower stress intensity range, ΔKL, and that the crack growth ratio in the acceleration stage was presented as a function of the maximum values of the higher and the lower stress intensity ranges together with the value of ΔKL. A new crack growth law to estimate the fatigue crack growth rate during the whole retardation process including the deceleration and acceleration stages was derived by using the empirically determined parameters.
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