Load path effect on fatigue crack propagation in I+II+III mixed mode conditions – Part 1: Experimental investigations

Abstract This paper is devoted to the analysis of the load path effect on mixed mode I + II + III fatigue crack growth in a 316L stainless steel. Experiments were conducted in mode I + II and in mode I + II + III. The loads were applied using the six actuators servo-hydraulic testing machine available at LMT-Cachan and the crack growth rate was measured using digital image correlation. The topographies of the crack paths were determined post-mortem using a numerical optical microscope. The fracture surfaces were also examined at high magnification using a scanning electron microscope (SEM). The load paths used in the experiments were chosen so as to be equivalent with respect to most of the fatigue crack growth criteria, in particular with those based on Δ K eq = α Δ K I n + β Δ K II n + γ n Δ K III n since the same maximum, minimum and mean values of the stress intensity factors were used for each loading path. In addition, load paths were constructed by pairs, either so that the extreme values of the stress intensity factors are attained simultaneously, or so as to display the same cumulative “length”. The main result of this set of experiments is that very different crack growth rates and crack paths are observed for load paths that are however considered as equivalent in most fatigue criteria. In addition, it was shown that the load path can have a very significant effect on the crack growth rate even if the crack path is not significantly different. The comparison of the results of the experiments conducted in mode I + II and in mode I + II + III, also allowed to show that the addition of mode III loading steps to a mode I + II loading sequence is increasing the fatigue crack growth rate, even when the crack path is not significantly modified. And finally, the SEM observations of the fracture surface showed that in non-proportional mixed mode conditions, a complex system of slip bands is formed at crack tip and is used to promote crack growth.