什么是 Pure shear, simple shear？

A pure shear is a homogeneous strain that keeps the area and keeps its principal directions fixed in the rock: $F = \begin{pmatrix} k & 0 \\ 0 & 1/k\end{pmatrix}$ in those axes, with $k > 1$. A simple shear moves every point parallel to one direction, the shear direction, by $\gamma$ times its distance from a fixed plane parallel to it: $F = \begin{pmatrix} 1 & \gamma \\ 0 & 1\end{pmatrix}$.