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Mode II fatigue crack growth tests as well as tests in sequential Mode Ithen Mode II were performed on ferritic-pearlitic steel. For ΔKII ranging from 7 to43MPa√m, bifurcation occurs after 12 to 450 μm coplanar growth, at a decreasingspeed. By contrast, hundreds of microns of constant speed coplanar growth wereobtained under sequential Mode I then Mode II loading, for ΔKII = 20MPa√m and ΔKIranging from 0.25 to 1.0 ΔKII. The crack growth rate is a simple sum of thecontributions of each mode for ΔKI equal to 0.25ΔKII but above this value, a synergeticeffect is found. The mechanism of this fast propagation mode is discussed in the light ofstrain range maps ahead of the crack tip obtained by digital SEM images correlationand elastic-plastic finite elements calculations. The stability of the crack path accordingto the maximum growth rate criterion is demonstrated.