Figure. 4 shows a stress-strain curve for a metal under tension. Initially the strain produced rises proportionally to the applied stress (ie. Hooke’s law is obeyed). Then after a given stress the stretched intermolecular bonds begin to break and reform, leading to plastic deformation, with loss of proportionality. As stress continues to increase the material undergoes strain hardening, necking and eventually failure.
Figure 4. Stress-strain curve demonstrating the principles of Hooke's law, the points of the proportional limit, elastic limit, yield point, ultimate tensile strength and failure.
The area of yielding can be subdivided into three definite points:
- Proportional limit = point at which Hooke’s law is no longer obeyed.
- Elastic limit = point at which the plastic deformation starts to occur.
- Yield point = point at which 0.2% plastic strain has occurred.
Identifying the exact points at which proportionality is lost or plastic deformation occurs is difficult. Calculating when 0.2% residual strain is present after the load has been released is relatively simple. For this reason the yield point is most commonly used in everyday practice.