Fatigue Crack Growth and Closure of Stainless Casting SCS6 and Carbon Steel Casting SC49 for Hydraulic Turbine Runner

Abstract

In hydraulic turbine runner, unexpected accidents due to fatigue often occur. In this study, in order to clarify fatigue crack growth and closure properties on SCS6 and SC49 used as hydraulic turbine runner over 27 years, fatigue crack growth test under the stress ratio R-constant were carried out using compact tension specimens of both materials. Fatigue crack growth rate and crack closure were measured by means of an unloading elastic compliance method. The results are summarized as follows:(1) Although the stress ratio dependency on the fatigue crack growth rate is found in both materials, the influence is recognized remarkably in SCS6 except for threshold value. (2) Although the threshold stress intensity factor ΔKth of SCS6 is almost same as that of SC49 when R<0.5, the value of stress ratio of SCS6, when the R-dependency of growth rate disappears, was found to be higher than that of SC49. (3) Fatigue crack growth rates of SCS6 and SC49 could be correlated with the effective stress intensity factor range ΔKeff, and da/dN-ΔKeff relationships of both materials agree well in the low and mid growth rate region except for threshold value. In high ΔKeff, however, the rapid acceleration of crack growth rate due to the change of growth mechanism to cleavage fracture was found only in SC49.