Continuum principles

It has long been established that the type of failure exhibited in materials is a function, not only of the magnitude of the applied stress or the basic material characteristics, but also of the way in which they are applied. Thus for annealed EC grade aluminium we might expect an elongation of ~30% if tested in tension, of ~400% if cold rolled and several 1000% if extruded. It was Von Karman [1] who first demonstrated the effect of hydrostatic stresses on the mode of yielding when he demonstrated the ductility of marble under the application of high compressive hydrostatic stresses and this has since been confirmed by observing other more typical cases such as notched bars where the ductility appears to be reduced in the presence of high tensile hydrostatic stresses. In both rolling and extrusion the stresses are in each case predominantly compressive and in extrusion are compressive in all three directions.