Effect of mean stress on closure and growth of short cracks in low cycle fatigue.

Abstract

Low cycle fatigue tests were performed under load controlled conditions with tensile mean stress, and short surface crack growth was investigated in association with crack closure. The short crack growth rate was successfully analyzed in terms of the cyclic J integral range ΔJ and the strain intensity factor range ΔKε, which were estimated by disregarding the crack closure, in a practical sense. The detailed discussion, however, showed that crack closure should be taken into consideration in the estimation of ΔJ even for short cracks since an effective load ratio U is about 0.85 especially in the load controlled test with a load ratio of R=-1.0. The crack tip opening displacement range ΔΦt is a more important parameter for controlling short crack growth than ΔJ and ΔKε. It was noticed that the da/dN vs. ΔΦt plot of a short crack was situated on the higher side of da/dN, compared with that of a through-thickness center crack.