Influence of Mean Stress and Stress Variation on Fatigue Strength of Stainless Cast Steel SCS5 for Hydraulic Turbine Runner

Abstract

In hydraulic turbine runners, accidents due to cyclic loading often occur. In this study, in order to clarify the fatigue properties and fracture mechanism of stainless cast steel SCS5 cut out from the hydraulic turbine runner vane for about 22 years, fatigue tests under constant amplitude with positive mean stress and variable amplitude loading were carried out by using tension-compression specimens. The results are summarized as follows : (1)The S-N curves of aged SCS5 used in this study have wide scatter. (2) From SEM observation of the fracture surface, fatigue crack initiation site were found to be the casting defect which consists of dendritic phase and shrinkage cavity. (3) It is possible to evaluate the fatigue life of aged SCS5 by using the defect size √area and the initial stress intensity factor range calculated by casting defect size. The value of the threshold stress intensity factor range obtained in this study were 3.8 MPa·m1/2. (4) Under repeated two-step loading pattern, a fatigue crack was found to grow when the stress intensity factor range was lower than the threshold stress intensity factor range of under constant amplitude loading.