LXXXVIII. On the state of stress in a plastic-rigid body at the yield point

Summary The yield point of a plastic-rigid body is defined as the moment when deformation first becomes possible as the load is increased. The practical significance of the yield-point load for an actual plastic-elastic body is discussed. It is then shown that part of the plastic zone at the yield point, namely the part where deformation occurs, depends only on the current surface tractions and not on the previous loading programme. By means of the principle of maximum plastic work and its complimentary minimum principle (Hill 1950) approximations from above and below to the yield-point load can be obtained; some examples are given. Recent American work on the plastic limit design of structures is critically reviewed and shown, when properly regarded, to be a particular application of the principle of maximum plastic work.