Proportional Limit  
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The graph plots stress in p s i versus strain in inch per inch. The horizontal axis ranges from 0 to 0.1 and the vertical axis ranges from 0 to 80,000. The graph starts at the bottom left of the viewing window at the origin, goes up and to the right with constant steepness till the point (0.050, 50,000). The graph then goes up and to the right with a reduced positive slope till (0.1, 60,000). A straight line is drawn passing through (0, 0) and (0.0040, 43,700) and it overlaps the linear portion of the curve.

Recall the unload/reload behavior shown earlier. As long as the stress in the material remains below the proportional limit, no permanent damage will be caused during loading and unloading.

In an engineering context, this means that a component can be loaded and unloaded many, many times and it will still behave just like new.