Stress intensity factors KI, KII, KIII, Keq, induced at the crack tip of CT specimens subjected to torsional loading

Abstract

The paper herein presented shows the Stress Intensity Factors (SIF), KI, KII, KIII, and Keq, calculated for Compact Tension (CT) specimens with different thicknesses (B), namely 2.5 mm; 3 mm; 5 mm, 7.5 mm, and 10 mm, that were subjected to three torsional loads (T): 6 N.m; 7.5 N.m; 9 N.m. Fatigue pre-crack subjected to torsional loading was found to be predominantly under Mode-II loading, and a crack branch was observed.The CT specimens, which were analysed using the Finite Element Method (FEM), were then modelled with two cracks that had grown experimentally from a fatigue pre-crack along two directions (+70º and -70º), and for several crack lengths (a/L=0; a/L=0.25; a/L=0.50; a/L=0.75; a/L=1.0). Therefore, equivalent SIF, Keq, were calculated from the numerical results of KI, KII, ans KIII, and a polynomial regression function was determined in function of the thickness of the specimen (B), the crack length (a/L) and the applied torque (T). It was observed that even in the case of a Mode-III loading is applied, Mode I was locally dominant at the branched crack tip, followed by Mode III and Mode II, in that order of relevance. It was also observed that the maximum equivalent SIF occurred at the two outer lateral surfaces of the CT specimens under test, resulting in high crack growth rates in those locations, and minimum at the midplane of the specimen.