Creep lifetime under constant load and constant stress: Theory and experiment

Creep tests are usually performed under constant load. The remarkable reduction of time to rupture due to the increase of true stress with the elongation of the specimen and with necking is estimated on the basis of the theories by Hoff, Brooklyn, and Kachanov. The influence of necking in tests under constant load and constant stress is investigated using the finite element method (FEM) with specimens containing an initial geometric imperfection. Results of the two types of tests show specimens have different sensitivities to these imperfections. To verify the theoretical statements, two types of creep tests under constant stress were performed in which the behavior of the AISI 316 L(N) austenitic steel at 700 degrees C was investigated. In the first type, tests under constant homogenized stress, necking of the specimen was ignored. In the second type, tests under constant true stress, the mean axial stress in the minimal cross-section was held constant. To perform this type of test, an optical measuring technique was employed to determine the minimal diameter of the specimen during the test. The observed data, in particular the time to rupture, are compared with those from creep tests performed under constant fond. Remarkable differences are observed. Theory and experiments confirm that problems arise if creep tests under constant load on ductile materials yielding too conservative data are interpreted with lifetime rules or continuum damage models.