TABLE OF CONTENTS 
Elastic design, avoiding plasticity, ensures that the cabin of the car does not deform in a crash. Plasticity absorbs the energy of impact, and allows metals to be shaped and polymers to be molded. (Image of crash testing courtesy AutoNews; image of hot rolling courtesy of Tanis Inc., Delafield, WI.)
Stiffness-limited design, described in Chapter 5, is design to avoid excessive elastic deflection. Strength-limited design, our concern here, is design to avoid plastic collapse. That generally means design to avoid yield, arranging that the component remains elastic throughout, when it is called elastic design. Elastic design is not always possible or necessary: local yielding may be permissible provided general yield is avoided.
That is half the picture. The other half is design to permit controlled plastic collapse. The safety of modern cars relies on the front of the car absorbing the kinetic energy in a collision by plastic deformation. And the manufacturing processes of metal rolling, forging, extrusion and pressing use plastic flow. Here, strains are large, elastic deformation irrelevant, and the focus is on the forces and work necessary to achieve a prescribed change of shape (see cover picture).
Plasticity problems are solved in more than one way. When yield is to be avoided, we analyze the elastic state of stress in a component and make sure that this nowhere exceeds the yield strength. Full plasticity, by contrast, requires general yield. Then the mechanism of plastic collapse must be identified and the...
