A schematic diagram for the stress-strain curve of low carbon steel at room temperature is shown in the figure. Several stages show different behaviors, which suggests different mechanical properties. Materials can miss one or more stages shown in the figure or have different stages to clarify. In this case, we have to distinguish between stress-strain characteristics of ductile and brittle materials. The following points describe the different regions of the stress-strain curve and the importance of several specific locations.
Yield Strength – Yield Point
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is where nonlinear (elastic + plastic) deformation begins. Before the yield point, the material will deform elastically and return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behavior termed a yield point phenomenon. Yield strengths vary from 35 MPa for low-strength aluminum to greater than 1400 MPa for high-strength steel.
In many situations, the yield strength is used to identify the allowable stress to which a material can be subjected. This criterion is not adequate for components that have to withstand high pressures, such as those used in pressurized water reactors (PWRs). The maximum shear stress theory of failure has been incorporated into the ASME (The American Society of Mechanical Engineers) Boiler and Pressure Vessel Code, Section III, Rules for Construction of Nuclear Pressure Vessels to cover these situations. This theory states that failure of a piping component occurs when the maximum shear stress exceeds the shear stress at the yield point in a tensile test.
