The present paper introduces the typical results from a series of fatigue tests under non-proportional mixed mode loads. Three groups of load conditions are covered, namely cyclic mode I plus steady or quasi steady mode II, cyclic mode II plus steady mode I and, cyclic mode I plus cyclic mode II with varied loading path. The tests were performed with notched thin-walled tubes in mild steel. Two types of crack growth mechanism are recognized. The mode I type propagation occurs to the tests where the cyclic portions of the fatigue loads are proportional. The non-mode I type propagation occures to the tests where the cyclic portions of the fatigue loads are non-proportional. Crack surface fractograph, crack growth direction and crack growth rate are presented, for which significant differences are noticed over the two growth mechanisms.
Structural integrity against fatigue damage is a big concern in offshore engineering. Because of the geometrical complexity of the structures and the random combinations of loads from different sources, a fatige crack in an offshore structure is inavoidably exposed to mixed mode load conditions. Furthermore with the random load combinations, the components of the mixed mode loads, such as the mode I and mode II components, do not vary in phase therefore the non-proportional mixed mode condition is envisaged. The investigations on mixed mode fatigue crack growth can be traced back to 1969 (Iida and Kobayashi, 1969). While the majority of the works thereafter were carried out under proportional mixed mode I and mode II conditions, investigations associated with non-proportional loads were also performed, such as Hourlier and Pineau (1981), Stanzi et al(1989), Bold et al(1991), Otsuka and Aoyama (1993), Wong et al (1996) and, Otsuka et al(1996). It was evidenced that the knowledge accumulated with proportional mixed mode fatigue