The yield point of crystalline aggregate, like mild steel, is comparatively high, although a single crystil is not so strong. It is because, in crystalline aggregate, the orientations are varied and plastic deformations are restrained by the surrounding neighbours. As the restraint in the surface layer is weaker, the strees-strain relation at that part will be different from that at the inner part ; thus a sort of boundary layer must be taken into account when the stress is sharply concentrated. Experiments on the yield points of mild steel strips with holes under tension were carried out. The stress in the surface layer must be lower than that calculated by elasticity. Assuming that the maximum true stress in the material is equal to the elastically calculated stress at a certain depth from the surface of the hole, and applying the criteria of yielding of mild steel hitherto advanced, the theory and experiments conform very closely. The depth taken in this case was 0.30mm.
