Multiaxial Fatigue Behaviour of A356-T6
  Aluminum alloy A356-T6 was subjected to fully reversed cyclic loading under tension, torsion and combined loading. Results indicate that endurance limits are governed by maximum principal stress. Fractography demonstrates long shear mode III propagation with multiple initiation sites under torsion. Under other loadings, fracture surfaces show unique initiation sites coincidental to defects and mode I crack propagation. Using the replica technique, it has been shown that the initiation life is negligible for fatigue lives close to 1E6 cycles for combined loading. The natural crack growth rate has also been shown to be comparable to long cracks in similar materials. 
M. J. Roy, Y. Nadot, D. M. Maijer and G. Benoit
cond-mat.mtrl-sci