Fatigue Crack Propagation in Mode II and Sequential

ABSTRACT. Mode II fatigue crack growth tests as well as tests in sequential Mode I then Mode II were performed on ferritic-pearlitic steel. For ∆KII ranging from 7 to 43MPa√m, bifurcation occurs after 12 to 450 µm coplanar growth, at a decreasing speed. By contrast, hundreds of microns of constant speed coplanar growth were obtained under sequential Mode I then Mode II loading, for ∆KII = 20MPa√m and ∆KI ranging from 0.25 to 1.0 ∆KII. The crack growth rate is a simple sum of the contributions of each mode for ∆KI equal to 0.25∆KII but above this value, a synergetic effect is found. The mechanism of this fast propagation mode is discussed in the light of strain range maps ahead of the crack tip obtained by digital SEM images correlation and elastic-plastic finite elements calculations. The stability of the crack path according to the maximum growth rate criterion is demonstrated