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Fracture toughness measures resistance to crack advance under specified loading and environmental conditions. In linear elasticity, $K_c$ is the critical stress intensity factor. For opening-mode loading, a valid plane strain measurement is denoted $K_{Ic}$, with units $\mathrm{Pa}\sqrt{\mathrm{m}}$, commonly $\mathrm{MPa}\sqrt{\mathrm{m}}$. It differs from a tensile yield strength: a crack changes the near-tip stress concentration.

For a fixed geometry with $K_I=Y\sigma \sqrt{\pi a}$, the threshold $K_I=K_{Ic}$ givesFor example, $K_{Ic}=10\mathrm{MPa}\sqrt{\mathrm{m}}$, $\sigma =100\mathrm{MPa}$ and $Y=1$ give $a_c\simeq 3.18\mathrm{mm}$ in this idealized geometry. The crack-tip plastic zone must remain small for the elastic intensity description to apply; extensive plasticity in continuum mechanics requires a different characterization.^[ref]

A thin elastic plate model may instead prescribe a critical front curvature ${\kappa }_f$. Such a model parameter cannot be substituted numerically for $K_{Ic}$ without a relation specific to that fracture model.