Fatigue damage initiation and propagation in al – 7075 under combined bending and torsion loading

Fatigue crack initiation and propagation of Al – 7075 was investigated under multiaxial in-phase bending and torsional loading. A thin-walled tubular specimen was designed to achieve a combination of mode I, and mode II loadings for studying the mixed mode bendingtorsion fatigue behavior. Analysis of crack trajectory and crack growth modes was conducted for various amplitudes of bending and shear stresses at a constant value of stress amplitude ratio (λ). Tests under pure bending and pure torsional loading were also conducted to gain a better insight into the mechanisms governing the combined bending-torsion load and to segregate the effect of multiaxiality from uniaxial bending or torsional loading. Irrespective of the loading conditions, a single crack nucleated on the plane of maximum range of shear stress in both pure bending and combined bending-torsion loading. For the pure bending case, crack propagated on the plane of maximum shear stress and an inverted S-shaped antisymmetric crack trajectory was observed. Crack propagation behavior in the combined loading case can be characterized by dividing crack trajectory into three regions: mode-I dominant mixed mode region, crack transition region, and mode II region. Using SEM, analysis of fractured surfaces for in phase bending-torsional loading was also conducted at different stress levels to compare and corelate macroscale aspect of fatigue fracture with the microscale fracture mechanisms.