Official Answer
Answer
'Cupping' or 'dishing' is a form of elastic/plastic deformation where the ring's cross-section twists under an axial load. This happens when the thrust load is applied at a point far from the groove support (e.g., against a large radius on the retained part). The ring acts like a Belleville washer. While the ring might not exit the groove, the 'cupping' leads to axial 'play' in the assembly. This play causes 'pounding' or impact loading during operation, which eventually leads to fatigue of the groove or the ring itself. Failure analysis involves measuring the 'dish' angle; a permanent dish indicates that the bending stress exceeded the material's yield point: $\sigma_b = _x000c_rac{M imes c}{I} > \sigma_y$.
