Answer
Shear failure occurs when the axial load $P$ exceeds the shear strength of the ring's cross-section. Forensic evidence of shear failure includes a clean, polished appearance on the sheared surface of the ring, often with a distinct 'step' between the part of the ring that remained in the groove and the part that was forced out. This differs from 'dishing' failure, where the ring would show signs of bending and twisting. Measurement of the groove depth $d$ and the ring thickness $T$ after the event is crucial. If the ring sheared, it often indicates an unexpected impact load or a significant underestimate of the static thrust. One must also inspect for 'galling' on the ring faces, which suggests high-pressure sliding occurred before the final shear event.
