What does a stress-strain curve for a metal look like and what does each part mean?

Stress is force divided by area, this is plotted on the y axis. Strain is extension divided by original length, this is plotted on the x axis. In the process of recording data to plot a stress-strain curve, the metal is stretched until it breaks.The first part of the graph is linear, this represents the elastic region. In the elastic region, stress and strain are proportional, Hooke's Law will apply and Young's Modulus of the metal can be calculated. At the end of the linear region, yield stress is reached and the material starts to act plastically. The graph curves because stress and strain are no longer proportional. Ultimate tensile strength is reached at the maximum of the graph and then the graph ends when the material breaks.Extension for students who understood adequately: Explanation of the unloading graph and hysteresis loops.