什么是 Extension, stretch, shear strain？

A line of material of length $\ell_0$ deformed to the length $\ell$ has the extension $e = (\ell - \ell_0)/\ell_0$ and the stretch $S = \ell/\ell_0 = 1 + e$; a shortening is a negative extension, a stretch below 1. Two lines of material that were perpendicular and now make the angle $90{}^{\circ} - \psi$ have been sheared by the angle $\psi$, and the shear strain is $\gamma = \tan\psi$.