The stress-strain curve contains no higher stress than the ultimate strength. When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals).
Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.” If this stress is applied and maintained, fracture will result.
This corresponds to the maximum stress that can be sustained by a structure in tension. The ultimate tensile strength is the maximum on the engineering stress-strain curve. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Between the proportional limit and the yield point the Hooke’s Law becomes questionable between and strain increases more rapidly. For tensile and compressive stress, the slope of the portion of the curve where stress is proportional to strain is referred to as Young’s modulus and Hooke’s Law applies. The proportional limit corresponds to the location of stress at the end of the linear region, so the stress-strain graph is a straight line, and the gradient will be equal to the elastic modulus of the material. The following points describe the different regions of the stress-strain curve and the importance of several specific locations. In this case we have to distinguish between stress-strain characteristics of ductile and brittle materials. To clarify, materials can miss one or more stages shown in the figure, or have totally different stages. There are several stages showing different behaviors, which suggests different mechanical properties. A schematic diagram for the stress-strain curve of low carbon steel at room temperature is shown in the figure.