Biaxial Creep-Fatigue Failure Characteristics in Two FCC Materials

This paper describes the failure mode observed in two types of FCC structural materials: waspaloy and type 316 stainless steel as a result of biaxial low cycle fatigue at elevated temperatures. Torsional cycling was applied at high as well as low strain ranges. Creep effect was assessed by introducing hold periods of 90 seconds in the waspaloy tests and 30 minutes in the stainless steel tests. Data obtained from fatigue and creep-fatigue tests have shown that the failure process in the two materials was controlled by two failure mechanisms which depended, to different degrees, on the state of stress, dwell time, and temperature. The failure mechanisms were assessed by observing crack growth in each material under strain level and temperature. In the waspaloy, the mode of crack growth was more temperature than stress dependent, while in the stainless steel, it was stress dependent. The microstructure analysis showed that each type of crack growth was caused by variations in slip band formation, stages of crack initiation and propagation, secondary cracking and cracking of grain boundaries. Creep showed more interaction with fatigue in the stainless steel material than in the waspaloy.