The mixed mode fatigue testing was carried out by applying the same phase and arbitrary ratio of torsional loading to tensile loading.
The threshold stress intensity factor and fatigue fracture toughness decreased with an increase of ΔKII0 for initial crack.
On the other hand, the stable fatigue crack growth region was found to be devided into IIa region of low growth rate and IIb region of accerated fatigue crack growth rate. Both regions were influenced by mixed mode loading.
The crack initiation life Ni and propagation life Nf had similar characteristic in terms of (1+Δτ/Δσ)/√ΔKI02+3ΔKII02=(1+ΔKII0/ΔKI0)/√ΔKI02+3ΔKII02, where ΔKI0 and ΔKII0 are the stress intensity factor amplitude of initial crack. The value of Ni/Nf was found to be about 0.12.