Assuming the formation of plastic hinges at the quarter points of a ring subject to a diametral compressive force, a comparison is made between experimental collapse loads and those predicted by use of a simple expression based on the use of plastic rectangular stress blocks, for some common materials; good correlation is obtained with aluminium.

A member may be said to fail when it yields completely at several positions and so allows collapse as a mechanism to occur. The smallest load at which this happens marks the end of useful life of the member. In the case of very thin rings loaded at the ends of a diameter in compression, as shown in Fig. 1, it is obvious that the sections of greatest bending moment are at A, B, C and D. These moments will continue to increase with increasing diametral load.