Engineers have long used stressstrain curves to uncover a host of material properties. The curves are created by plotting the results of tensile strength tests of material samples, putting stress (force divided by area) on the yaxis and strain (stretch divided by gage or original length) on the xaxis. Some of the key material properties the curve can reveal include the material’s elastic limit, along with the elastic and plastic ranges, the yield point, ultimate and rupture strengths, and the moduli of resilience and toughness.
Sometimes, however, engineers must interpolate between data points to get those performance figures. Often a stiffnessstrain curve tensile test result defines some material properties more precisely. It puts stiffness (change in stress divided by change in strain) on the yaxis and strain on the xaxis. In effect, it graphs the slope of the stressstrain curve as a function of strain. The two graphs show the stressstrain curve (left) and the stiffnessstrain curve (right) of a lowcarbon steel. On the traditional stressstrain curve, engineers must measure the slope of the initial portion of the curve to determine the stiffness. On the stiffnessstrain curve, engineers directly take the yintercept as the stiffness value. The stiffnessstrain plot used the same data as the stressstrain curve, but for the clearest representation, engineers might need to use and expanded scale. They can do this by using more data points from the test results on curved portions and parts of the graph with the features of interest. Or they can increase the number of data points by using a program that generates intermediate point as using a suitable algorithm. 
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