Life Prediction Rule under Creep - Fatigue - Ratcheting Conditions of Heat-Resisting Cast Alloy, HP-Nb

摘要

Low-cycle-fatigue tests under creep- and ratcheting-conditions were conducted at 800 and 900 deg C on a heat-resisting cast alloy, HP-Nb. The fatigue life in the cp-test (slow-fast test) at 800 deg C was shorter than at 900 deg C, which was attributed to the lower creep ductility at 800 deg C. By adding tensile ratcheting-strain to each cp-strain cycle, the tensile peak stress increased, which led to life reduction. The life reduction by adding ratcheting-strain was remarkable in the small strain range region and especially at 800 deg C. The strain range partitioning method proposed by Manson et al. was appropriate for the creep-fatigue life prediction of this material. In the case of creep plus ratcheting-condition, a reasonable and conservative life prediction was obtained by combining the strain range partitioning and the ductility exhaustion as follows; ({n_f centre dot #SIGMA#(1/N_(ij)}~(#alpha#))~(#beta#)+#SIGMA#(#delta#/#epsilon#_c)=1, where n_f and N_(ij) are the number of cycles to failure and the partitioned life respectively, #delta# is the racheting strain, #epsilon#_c is the creep ductility and #alpha# and #beta# are material constants. The severe life reduction in racheting-condition at 800 deg C was thought to be caused by accelerated grain-boundary void formation with an increase in tensile peak stress.