Application of SBF Test to Fatigue Damage Assessment of Type 316 Steel

Abstract

Abstract A new fatigue testing technique, the small bulge fatigue (SBF) test, was recently developed by some of the present authors. In this technique, a cyclic oil pressure is alternatively applied to both surfaces of a small disk-type specimen (8 mm in diameter) with flat and concave surfaces. This SBF test is a kind of biaxial fatigue test as well as the disk bending fatigue (DBF) test. In this study, damaged specimens were subjected to the SBF test to examine the adaptability of this test to the fatigue damage assessment of actual components. To prepare these damaged specimens, Type 316 austenitic stainless steel was subjected to the DBF test using large disk specimens (87 mm in diameter) by considering that most of components were generally operating under multiaxial loading condition. The test was stopped at several stages of the fatigue life. Then, the SBF test specimens were taken from the surfaces of the interrupted DBF test specimens. It was expected that fatigue life of the SBF tests corresponded to the degrees of fatigue damage induced by the DBF test. The SBF test results showed that the fatigue life of the damaged SBF specimen, which contained microcracks formed by the DBF test, was only one tenth as long as that of the undamaged specimen. It was noteworthy that some microcracks introduced by the DBF test were opened and became clearly visible after the SBF test, although they were closed and not visible just after the DBF tests.