Abstract
Low-cycle-fatigue tests under creep- and ratcheting-conditions were conducted at 800 and 900°C on a heat-resisting cast alloy, HP-Nb. The fatigue life in the cp-test (slow-fast test) at 800°C was shorter than at 900°C, which was attributed to the lower creep ductility at 800°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°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; [{nf·∑(1/Nij)}α]β+∑(δ/εc)=1, where nf and Nij are the number of cycles to failure and the partitioned life respectively, δ is the racheting strain, εc is the creep ductility and α and β are material constants. The severe life reduction in racheting-condition at 800°C was thought to be caused by accelerated grain-boundary void formation with an increase in tensile peak stress.
