Yield Strength  
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When a material yields without an increase in stress, it is often referred to as being perfectly plastic.

The graph plots stress in p s i versus strain in inch per inch. The vertical axis ranges from 0 to 80,000 and the horizontal axis ranges from 0 to 0.25. The graph starts at the origin, shoots vertically up to the point (0, 46,000) on the vertical axis which is labeled yield point, goes horizontally to the right till (0.05, 46,000), goes up and to the right with approximately constant steepness till (0.2, 67,000), goes up and to the right with decreasing steepness till (0.225, 69,000), goes down and to the right till (0.25, 67,000) and ends. The portion of the graph between the vertical axis and the point (0.05, 46,000) is labeled yielding and the region on the top right near the peak of the curve is labeled perfectly plastic.

A material that behaves in the manner depicted here is said to have a yield point. The yield point is the stress at which there is an appreciable increase in strain with no increase in stress.

Not every material has a yield point.