Life prediction rule under creep-fatigue ratcheting conditions of heat-resisting cast alloy, HP-Nb
Yokomaku Toshinori; Kinefuchi Masao; Terada Susumu; Sugimura Hideki; Minokata Yasuro
Abstract:Low-cycle-fatigue tests under creep- and ratcheting-conditions were conducted at 800 and 900 deg. On a heat resisting cast alloy, HP-Nb. The fatigue life in the cp-test (slow-fast test) at 800 deg. Was shorter than at 900 deg., which was attributed to the lower creep ductility at 800 deg.. 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.. 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; [{nfS(1/Nij)}a]b + S(d/ec) = 1, where nf and Nij are the number of cycles to failure and the partitioned life respectively, delta is the ratcheting strain, ec is the creep ductility and a and b are material constants. The severe life reduction in ratcheting condition at 800 deg. Was thought to be caused by accelerated grain boundary void formation with an increase in tensile peak stress. Key Words:creep-fatigue, ratcheting, HP-Nb cast steel, life prediction, strain range partitioning