  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.
