Study on modified J-integral range and its correlation with fatigue crack growth

Abstract

The research work reported in this paper is concerned with the studies on J-integral range ( ΔJ)/modified value of J-integral range ( ΔJ ∗ ) and to relate them to crack tip opening displacement (CTOD)/crack opening displacement (COD) and fatigue crack growth rate ( da dN ). Experiments were performed on 6063-T6 Al-alloy in both as-received and annealed states for different stress ratios ( R). CTOD and COD were measured using surface measurement technique by a small COD gauge, by mounting it near a fatigue crack tip at different positions behind the crack tip and at the crack tip respectively. It was found that ΔJ is a function of CTOD/COD. When conventional concept of J-integral is modified by introducing notional crack concept for considering the plasticity at the crack tip, the experimental results are found to come closer to the ΔJ ∗ values (found theoretically by notional crack concept). The fatigue crack propagation law though conventionally expressed as a function of stress intensity range ( ΔK) can also be expressed as a function of ΔJ ∗ . The experimental results for both states of the material (as-received and annealed) show that for large range of da/ dN, ΔJ ∗ is able to provide a satisfactory correlation in the form of power law and the crack growth rate equation is found to be independent of R.