Welded joints made of ductile materials show a significant reduction in fatigue life when subjected to non-proportional loading compared to proportional loading. While existing research on non-proportional loading strongly focuses on a 90° phase shift between normal and shear stress, which is widely considered to be most damaging, complex loading other than out-of-phase is rarely and insufficiently studied. This paper presents fatigue tests under various forms of non-proportional loading to distinguish the effects on fatigue life. These forms include different phase shifts, different frequencies, different R-ratios of normal and shear stress as well as an alternating application of both stress components. Finally, hypothesis testing is applied to differentiate whether different forms of non-proportional loading have different effects on fatigue life.