The retardation effect of static torsion on fatigue crack growth in strip steel

Abstract

AbstractThis study is aimed at analysing the effect of static torsion on Mode I fatigue crack growth behaviour. Experiments have been conducted under constant torque or constant twist angle with different load ratios on SAPH440 and DP780 strip steels. The experimental results showed that the application of the static torsion or twist angle extended the fatigue life of the specimens in all cases. The fatigue life showed a positive relationship with the applied torque or twist angle until some critical value. No additional improvement in fatigue life was found after the critical value. The mechanism of the retardation due to static torsion was discussed and supported by the microscope fractography analysis. An effective stress intensity factor expression was proposed based on the fractography analysis to estimate the fatigue crack growth under constant torque in the low torque condition. The parameters introduced in the expression were found to have relationships with the dimensionless quantity related to torque, tensile load, and specimen geometry. The proposed expression achieved reliable estimates of the SAPH440 and DP780 constant torque experimental results.