Fatigue Crack Growth Behavior on Power Shafts with Semi-Elliptical Surface Cracks

摘要

An analysis of the influence of steady torsion loading on fatigue crack growth rates on shafts is presented. Tests were carried out on cylindrical specimens in DIN Ck45k steel, using a testing machine especially designed and constructed simulating the real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from a chordal notch on the cylindrical specimen surface, was measured for several Mode Ⅲ/ Mode Ⅰ ratios. Results have shown a significant reduction on the crack growth rates when a steady torsion is superimposed to cyclic Mode Ⅰ. This fact seems to be related with a decreasing of △K effective at crack tip due to crack closure effect and the zig-zag path of crack growth as a consequence of torsion. For this type of cracks, under mixed-mode loading, the shaft lifetime increases with torsion loading. A Finite Elements 3D analysis have also shown that the Stress Intensity Factors of semi-elliptical surface crack values at free surface crack tip depends on the direction of applied steady torsion but it is independent at the maximum crack depth. This study provides a contribution for a better understanding of crack growth rates under mixed-mode load conditions on pre-cracked rotor shafts in order to predict remaining lifetimes and to estimate the risks in operation conditions.