Analysis of Crack Growth under Cyclic Loading with Mean Stress by Means of Elastic-Plastic Finite-Element Method.

Abstract

The stress-strain field was assessed by means of the elastic-plastic finite-element method in the vicinity of the crack tip of the through-thickness-center cracked specimen subjected to cyclic loading under test conditions where ratchet deformation occurs. Then the cyclic J-integral range was calculated and its validity was examined. Both the maximum and minimum axial total strains at the crack tip during one stress cycle were greatly increased when ratchet deformation was generated. The path independence of ΔJop failed with the beginning of ratchet deformation, where ΔJop is cyclic J-integral range corresponding to load ranges above a crack opening point. Cyclic J-integral range ΔJ calculated through Dowling's conventional method, which was plotted against the crack tip opening displacement range ΔΦt calculated at 200μm behind crack tip, was situated on the smaller ΔJ side under test condition accompanied with ratchet deformation than under that without ratchet deformation. Meanwhile, the modified parameterΔJ/(1-Φminc/Φmaxc)0.2 showed a good linear correlation with ΔΦt under all the test conditions with and without ratchet deformation. The earlier work in which one of the authors showed that the elastic-plastic fatigue crack growth rates were controlled quite well by means of the parameter ΔJ/(1-Φminc/Φmaxc)0.2 might be realized in this way.