Some stress-strain curves obtained by deforming the samples in tension at a constant strain-rate are shown in the diagram below. These curves show both the elastic and plastic range for the materials.

The elastic deformation for all the samples except the rubber correspond to small strain values, so that the initial slopes of these curves give the Young's modulus of the material. The point at which the material changes from elastic to plastic behavior is known as the Yield Point, and the yield stress, s_y , is a characteristic quantity for the material. Engineering design normally tries to keep applied stresses during use below about half of the yield stress. If the material is to be bent or shaped during manufacture, stresses larger than the yield stress will be necessary.

From: Guy, "Introduction to Materials Science," McGraw Hill (1972)