Surface flaw behavior in a round bar under combined tension and torsion loading

The subject for studies is a steel bar of circular cross-section with straight-fronted edge transversal notch undergoing fatigue loads. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth behaviour is studied under axial and combined axial+torsional fatigue loading. Results show that the cyclic Mode III loading superimposed on the cyclic Mode I leads to a fatigue life reduction. In parallel to the experimental activity, numerical calculations are performed based on three-dimensional DBEM-analysis to determine the crack path (based on three distinct criteria from literature) and fatigue life. The experimental fatigue crack growth curve obtained from round bar specimens have been compared with the numerical predictions. The computational DBEM results are found to be in good agreement with the experimental findings.