| Initiation and early growth of fatigue microcracks in a ferritic (62 v/o) austenitic (38 v/o) duplex stainless steel (Ferralium Alloy 255) were studied as a function of cyclic stress amplitude. Replicas of specimen surfaces (etched prior to testing) were examined in the scanning electron microscope to determine (i) crack initiation site(s), and (ii) interactions between microcracks and microstructural features, particularly during early growth of microcracks. Optical microscopy with Nomarski interference contrast was employed for in situ measurements of crack lengths at different stages of fatigue life. Crack length data were used to determine crack growth rates. Cracks initiated in the slip bands about half a grain diameter away from interphase boundaries both in the ferrite and the austenite. Cracks, however, initiated in the ferrite more often than the austenite. Number of cycles to initiation increased as the cyclic stress amplitude decreased. Early growth of microcracks can be divided into three consecutive stages, (i) stage 1 when a {dollar}{dollar} a{dollar}sb{lcub}rm cri{rcub}{dollar}: once a crack penetrated the first interphase boundary it encountered, its subsequent growth was continuous.; Separate models were proposed to characterize crack growth behavior when a {dollar}leq{dollar} a{dollar}sb{lcub}rm cri{rcub}{dollar} and a {dollar}>{dollar} a{dollar}sb{lcub}rm cri{rcub}{dollar}, and verified. |
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