Fatigue limit and Hysteresis Behavior of Type 304L Stainless Steel in Air and PWR Water, at 150°C and 300°C

Abstract

This is a study of the 10⁷ cycle fatigue limit of Type 304L Stainless Steel, as measured in fully reversed (R=-1) load-controlled tests, at 150°C and 300°C, in air and PWR water. The staircase method was used to determine the fatigue limit. The tests run here utilized a cycle frequency of 1.818Hz and are compared to other tests from the literature that were run at 30Hz. The fatigue limit measured in the tests run at the high frequency was higher than that measured here. This is explained by measurements of the strain developed during cycling, using the different cycle frequencies. The tests run at the higher frequencies yielded lower strains for a given stress and, as expected, this resulted in higher fatigue limits. Using 10⁷ cycles to define a run-out also led to a lower fatigue limit. These results are important as most previous fatigue limit measurements utilized 10⁶ cycles or less to define a run-out, and when lives as long as 10⁷ cycles are used the tests are generally run at high cycle frequencies, thus leading to higher fatigue limits than those measured here.

This study utilized hysteresis loops measured during load controlled cycling, which illustrate the complex nature of the cyclic deformation that can develop. At 300°C, and at high stresses, ratcheting (an increase in the average strain) was observed, but at low stresses retrograde motion (a decrease in the average strain) was observed. The development of these strains should be an important consideration in any calculation of the behavior of actual components. These strains were also used to develop cyclic stress-strain curves, which can be used for design calculations and to correlate strain and load-control tests.