The effect of mean stress on the fatigue behavior of 316 LN stainless steel in air and mercury

Design of the mercury target system components for the spallation neutron source (SNS) requires data on high- and low-cycle fatigue behavior. The research and development program in progress includes determining the effects of mercury on the fatigue behavior of type 316 LN stainless steel, the primary material of choice for the target vessel. Uniaxial, load-controlled, fully reversed tension–compression R=−1 (minimum stress/maximum stress) fatigue tests have been conducted in air and mercury at room temperature employing constant amplitude sinusoidal loading at frequencies from 0.2 to 10 Hz. Stress amplitude versus fatigue life data (S–N curves) for both air and mercury show a sharp knee at approximately 1 million cycles indicating a fatigue endurance limit in either air or mercury around 240 MPa. Tensile mean stress (R=0.1) lowers the endurance limit to 160 MPa. Lower frequency and mercury environment had some impact (degradation) on fatigue life of type 316 LN stainless steel at high stress levels (i.e., stresses considerably above the apparent fatigue limit). Test results for high mean stress conditions (R=0.3, 0.5, and 0.75) at a cyclic frequency of 10 Hz exhibited further reductions in the endurance limit.