Influence of hold times on the low cycle fatigue behaviour of the material in 792

The LCF- (low cycle fatigue) tests performed on the alloy IN 792 showed the influence of temperature and the tension hold time which is already known from ferritic steels. An additional effect of the compression hold time was found. A shift of the hysteresis loop in the tension range during tests including compression hold times takes place. Thus the effective mean stress is also in the tensile range and as a consequence influences crack initiation and propagation. A significant reduction of fatigue endurance in tests with compression hold times compared with pure tension hold time tests could be observed. This specific behaviour of Ni-based alloys could be explained on the basis of a different cyclic hardening/softening behaviour or relaxation behaviour within the tension and compression range. In contrast to this, however, the effect of compression hold times can be described by means of the Smith-Watson-Topper parameter.