Toughness (energy needed for rupture per volume) K<sub>C</sub> determined from the tension test curve, for a ductile material (top) and a fragile material (bottom).
Note that we should use the true stress and strain, and thus <i>not</i> the conventional curve; however, qualitatively, the use of the conventional curve can explain the difference between fragile and ductile.
<a href="//commons.wikimedia.org/wiki/User:Cdang" title="User:Cdang">Cdang</a>
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