The cumulative damage concept in low-cycle fatigue with intermittent hold times was assessed for Udimet 700 at 1400 F. A 20 or 60 min period of tensile stressing in each strain cycle reduced the cyclic life in conservative agreement with the linear cumulative damage rule. However, the effect of holding compressive strain was more damaging than holding tensile strain. Similarly, the effect of creep prestrain on the subsequent fatigue behavior was more deleterious when applied in compression than in tension. Furthermore, the creep rate in compression was less than half that in tension and continuously decreased with strain.
Observations of the Bauschinger effect indicated that high internal stresses accompanied both creep and plastic deformation; these stresses are attributed to the pileup of dislocations at grain boundaries. It is postulated that the pileup is relaxed by cavitation and that the rate of deformation is governed by the growth of these cavities as they annihilate dislocations. These cavities were observed to be a source of intergranular cracking in creep and low-cycle fatigue. The rate of growth of the cavities, in turn, is affected by the hydrostatic component of stress, accounting for the decelerating creep in compression. In addition the shape of the cavities influences the rate of crack propagation and fracture toughness: round cavities promote toughness, while cavities flattened by compressive creep provide easy propagation paths.