Combined Low-Cycle Fatigue and Stress Relaxation of Alloy 800 and Type 304 Stainless Steel at Elevated Temperatures

Abstract

A detailed analysis was made of data from low-cycle fatigue tests of solution-annealed, nickel-iron-chromium Alloy 800 at 1000, 1200, and 1400 F and of Type 304 austenitic stainless steel at 1000 and 1200 F with holdtimes at maximum tensile strain. A single equation was found to approximate the cyclically stable stress relaxation curves for both alloys at these temperatures. This equation was then used in making a linear time fraction creep damage analysis of the stable stress relaxation curves, and a linear life fraction rule was used to compute fatigue damage. Creep-fatigue damage interaction was evaluated for both alloys using the results of these damage computations. Strain range was found to affect the damage interaction for Type 304 stainless steel but not for the Alloy 800. With increasing holdtime, both creep and total damage increased for the Alloy 800 and decreased for the Type 304 stainless steel, and fatigue damage decreased for both alloys. A method was developed to relate length of holdtime and fatigue life to total strain range. This method provides a simple and reasonable way of predicting fatigue life when tensile holdtimes are present.