The combination and transition of a shear to opening type of failure mechanism has been reported by for dominant mode II loading of notched or cracked plates (metallic alloys and polycarbonate). The present paper addresses additional aspects of the phenomenon in relation to mode-mixity for actual fatigue cracks in polycarbonate specimens. Two distinct experimental setups are used and systematically compared throughout the work, by means of numerical simulations and scanning electron fractographic analysis. The first setup allows for dominant mode II loading with a minor mode I component (side impact of cracked plates). A preliminary numerical study accounting for contact and friction between the fatigue crack flanks is carried out. This study shows that, due to the fatigue crack, the initial loading mode is close to “pure mode II” followed at a later stage by mixed-mode. The experiments show that by varying the impact velocity, adiabatic shear band formation and fracture can be provoked and controlled, as observed in previous work. For these experiments, a threshold value of KII (4 MPa m1/2) is proposed to trigger shear band formation in the adiabatically heated crack-tip material. The second setup allows for dominant mode I loading with a minor mode II component, using compact compression specimens (CCS). Here, mixed-mode is experienced from the onset of the crack-tip loading throughout the experiment. The comparative fractographic analysis shows that identical characteristic failure mechanisms operate irrespective of the specimen geometry (i.e. mode-mix) and crack-tip nature (notch or crack) for a similar range of impact velocities. Specifically, a shear failure mechanism is also observed in those “mode I” (CCS) specimens which were impacted at a higher velocity. This comparative study therefore extends the phenomenon of failure mode transition to general mixed-mode loading.