EFFECT OF CYCLIC OVERLOAD ON THE CRACK-GROWTH BEHAVIOR DURING HOLD PERIOD AT ELEVATED-TEMPERATURE

Effect of tensile overload on elevated temperature crack growth behavi or during the subsequent load hold period has been studied by numerica l and experimental methods. Finite element analysis of compact specime ns shows that when the tensile overload precedes the load hold period, C(t) during the hold period is significantly smaller (i.e. retarded) compared to the case without the overload. This is due to crack tip st ress relaxation associated with large crack tip plasticity generated b y the overload. A modified C(t) estimation scheme is proposed by intro ducing a new equation for t(pl). Using this scheme, the retardation be havior of C(t) due to the overload is successfully modeled. Creep-fati gue crack growth data for an ex-service 1.25Cr-0.5Mo steel at 538-degr ees-C (1000-degrees-F) were generated in air. The hold times are 10 se conds, 98 seconds and 10 minutes. Time-dependent crack growth rate dur ing the load hold period, (da/dt)avg, is correlated with (C(t))avg est imated by the new estimation scheme. (da/dt)avg data from all the test s with overload are higher than those from the tests without overload. The peak stress associated with the overload seems to have enhanced v oid nucleation and to increase the time-dependent crack growth rate du e to creep. This argument is supported by microscopic observations.