Multiaxial fatigue behaviour of laserbeam-welded thin steel and aluminium sheets under proportional and non-proportional combined loading

Laserbeam-welded overlapped tubular joints made from structural steel St35 (S235 G2T), artificially hardened aluminium alloy AlSi1MgMn T6 (EN AW 6082 T6) and self-hardening aluminium alloy AlMg3.5Mn (EN AW 5042) were investigated under combined proportional and non-proportional axial and torsional loading with constant amplitudes in the range of 2 × 104–1 × 107 cycles. The multiaxial fatigue behaviour of these joints was assessed by the notch stress concept with the reference radius rref = 0.05 mm. The equivalent stresses, especially considering the fatigue life reducing influence of non-proportional loading in comparison to proportional loading, were calculated by a recently developed hypothesis, which is called the Stress Space Curve Hypothesis (SSCH). This hypothesis is based on the time evolution of the stress state during one load cycle.