COMPARISON OF LOW CYCLE (NOTCH) FATIGUE BEHAVIOUR AT TEMPERATURE INSINGLE CRYSTAL TURBINE BLADE MATERIALS

摘要

The scatter in notch fatigue lifetimes of CMSX4 at 650°C and725°C in air and vacuum and with Rene N5 in air at 650°C iscompared under the same (plastic) notch strain range levels. ReneN5 shows shorter lifetimes under equivalent conditions andalways exhibits multiple initiation sites. The role of inter interdendritic dendriticporosity in initiating fatigue in both alloys is identified, and thenumber of initiation sites is found to directly affect fatigue life. Inair in CMSX4 and Rene N5, subsurface pores initiate fatigue, andthis is believed to be due to the repea repeated in ted in-filling of surface poresby oxidation product, reducing their associated stressconcentration and effectively deactivating them as a fatigueinitiation site. Tests in vacuum support this hypothesis as cracksdo initiate at surface porosity under va vacuum conditions. Ancuum attempt to evaluate initiating porosity distributions, has indicateda correlation between total area of initiating porosity and fatiguelifetime, which to some extent may allow for crack coalescencebehaviour. A deterministic fractur fracture mechanics based model hase been proposed to allow for the effect of pore shape, size andposition in determining subsequent fatigue life (and hencescatter). The predictions of the model have been assessed usingfull factorial design of experiments, asse assessing the effects ofssing variability in pore shape, size and distance below the notch root,as well as the materials parameters (crack growth laws and Kmax)used in the lifing calculation. The model successfully explainssome, but not all of the observed scat scatter in lifetimes.