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It has been reported in the literature that high-temperature fatigue exposure initially leads to an increase in the subsequently measured creep ductility, whereas prolonged fatigue exposure lowers the creep ductility in low-alloy Cr-Mo-V-steel. In the work now being initially reported an aim has been to observe microstructural changes due to high-temperature strain-controlled fatigue exposure with or without hold times. The controlled strain was the principal (axial) strain. For test response variables, the load and diametral strain were measured. While the cycle-to-cycle flow stress was observed to decrease monotonically—that is, the material cyclically softened—, the diametral strain range initially increased and subsequently decreased. It is concluded from the similarity in behavior between the changes in post-fatigue creep ductility (lateral contraction at fracture, RA) and in situ diametral strain range that the in situ diametral strain range is a powerful means of observing ductility changes due to fatigue. The effect of hold time at the tensile limit of the cycle is to reduce the diametral strain range as a whole and to speed up the drop in diametral strain range after it passes its maximum value. The maximum is generally reached during the few tens of initial cycles. The subsequent exhaustion of ductility develops quite rapidly at first but then slows down until the onset of cracking eventually speeds up the ductility exhaustion rate. In all cases the diametral strain amplitude behaved in a more strongly changing manner than the flow stress, indicating more sensitivity to the microstructural damage than the drop in the flow stress.
fatigue, high temperature, hold time, creep ductility, creep-fatigue, damage
Technical Research Center of Finland (VTT/MET), Espoo,
University of Pennsylvania, Philadelphia, Pa.