The effect of mean stress on the high-cycle fatigue behavior of a wrought nickel-base superalloy, Udimet 710, was studied in air at 1000 F. Under the conditions of testing, which varied from a ratio of the alternating to mean stress, A, of 0.25 to ∞, the endurance limit was almost independent of mean stress. At all values of mean stress, the alloy deformed by superlattice dislocation pairs, shearing both the gamma and gamma-prime phases, resulting in dense planar bands of dislocations. The number of planar bands of dislocations, however, was found to decrease with decreasing mean stress. Independent of the mean stress, fatigue crack initiation took place along the dense bands of dislocations by slip plane decohesion.
Once initiated, the fatigue crack continued to propagate for one or two grain diameters in a Stage I mode before changing to a Stage II mode of propagation. The amount of the remaining fracture surface covered by Stage II crack growth prior to overload decreased with increasing mean stress. However, in the Stage II region of crack growth, the striation spacing, 0.5 µm, was found to be independent of mean stress.
The independence of the fatigue strength on mean stress was concluded to result from the interaction of two competing factors. As the mean stress is increased, the time spent in crack propagation is decreased due to the fact that the Stage I crack growth rate is accelerated, and less Stage II crack growth is required to exceed the fracture strength of the remaining specimen. In addition, at the higher mean stresses, the increased slip dispersal resulted in a greater amount of time being required to build up the dislocation density within a deformation band to the density required for decohesion. The independence of the endurance limit on mean stress, therefore, is related to the shorter time spent in crack propagation at the high mean stress being compensated for by the longer time spent in initiating a crack due to the dispersal of slip onto a greater number of slip planes.