Evaluation of Low Cycle Creep-Fatigue Degradation for Notched Components

Abstract

Four approaches have been examined to predict the endurance of circumferentially notched, uniaxially loaded test pieces subjected to fully reversed strain-controlled cyclic loading at elevated temperatures. Results are compared with uniaxial data derived from smooth test pieces. The strain-time waveforms include both trapezoidal and triangular shapes to distinguish between fatigue and fatigue-creep damage. The four materials investigated are HP-45 (1144 K), Incoloy 800H (1144 K), Incoloy 800 (922 K), and 1¼Cr-½Mo (811 K). The analysis has reflected the various degrees of cyclic hardening and creep deformation exhibited by these materials. The predictive approaches include Neuber notch strain analysis and finite element analyses to identify the maximum plastic strain range, the equivalent plastic strain range, and the total plastic work per cycle. The study provides a simple, pragmatic approach to predicting whether cyclic plant operation is lifetime limiting. The primary conclusion is that the total plastic work-per-cycle approach is the preferred method for evaluation of fatigue-limited plant lifetimes.