The material stress - strain curve is the graphic representation of the equivalent stress and strain states (engineering or real) obtained in a mechanical characterisation tensile test. As a consequence, the stress - strain curve is the simplified description of the material constitutive equation.
The stress - strain curve presented is typical of a metallic material. Several important points can be observed:
The elastic limit (yield point) is the point of transition between elastic and plastic deformation in the material, so from this point on the curve, permanent deformations are present. This point often coincides with the proportional limit (up to this point on the curve, linearity can be assumed between stress and strain). In this case linear elastic behaviour is considered, whereas when elastic limit is significantly above the proportional limit, then the behaviour is non-linear elastic.
If the material is loaded above the elastic limit, then plastic deformation increases when load is also increased (strain hardening), up to a point known as ultimate stress. When this point is reached, the force decreases and the material deformation leads to rupture. In this last part of the curve, the deformation is often quite located at one section of the sample, consequently a phenomenon known as "necking" takes place.