Stage I crack directions under in-phase axial–torsion fatigue loading for AISI 304L stainless steel

• AISI 304L stainless steel was subjected to in-phase biaxial fatigue.
• The S–N curves are remarkably horizontal.
• The biaxial fatigue curve is better fitted with an elliptical quadrant.
• The crack direction during Stage I crack growth is close to the Mode I direction.

Smooth AISI 304L stainless steel specimens were tested in fatigue. The tests were axial, torsional and in-phase axial–torsional, all of them under load control with R = −1. S–N   curves were constructed following the ASTM E739 standard and the fatigue limits were obtained following the method of maximum likelihood proposed by Betinelli. The ratio between the pure torsion and tension fatigue limits, τFL/σFLτFL/σFL, was 0.91, close to 1, which is a typical value for fragile materials. The fatigue limits determined by the biaxial tests were represented on σ-τσ-τ axes. These experimental values were well fitted with an elliptical quadrant, although this fitting is considered appropriate for ductile materials. The crack direction along the surface was analysed using a microscope, with special attention dedicated to the crack initiation zones. The crack direction along the first grains, or Stage I, was much closer to the Mode I direction than to the Mode II direction.