Fatigue life estimation under cumulative cyclic loading conditions

The cumulative fatigue behavior of a cobalt-base superalloy, Haynes 188, was investigated at 760°C in air. Initially, strain-controlled tests were conducted on solid cylindrical gage section specimens of Haynes 188 under fully reversed, tensile and compressive mean strain-controlled fatigue tests. Fatigue data from these tests were used to establish the baseline fatigue behavior of the alloy with (1) a total strain range type fatigue life relation and (2) the Smith-Watson-Topper (SWT) parameter. Subsequently, two load-level multi-block fatigue tests were conducted on similar specimens of Haynes 188 at the same temperature. Fatigue lives of the multi-block tests were estimated with (1) the linear damage rule (LDR) and (2) the nonlinear damage curve approach (DCA) both with and without the consideration of mean stresses generated during the cumulative fatigue tests. Fatigue life predictions by the nonlinear DCA were much closer to the experimentally observed lives than those obtained by the LDR. In the presence of mean stresses, the SWT parameter estimated the fatigue lives more accurately under tensile conditions than under compressive conditions.