Fatigue crack initiation and propagation behavior in Al-7075 alloy under in-phase bending-torsion loading

Fatigue crack initiation and propagation behavior were investigated on a tubular specimen of Al 7075 alloy subjected to multiaxial mixed mode bending-torsional loadings. Tests under pure bending and pure torsional loadings were also conducted to segregate the effect of multiaxiality. A single crack nucleated on the plane of maximum shear stress in all the tests, except for pure torsion. For pure bending, crack propagated on the plane of maximum shear stress in a mixed mode condition tracing an inverted S-shaped trajectory; whereas, crack trajectory under combined bending-torsional load consisted of mode I dominant region, transition region, and pure mode II region. Furthermore, analyses of fracture surfaces were conducted to determine the micro-mechanisms governing crack initiation and propagation behavior.