$2-4$Given the strain tensor,

${}_{ij}=\left[\begin{array}{ccc}-0\mathrm{.}006& -0\mathrm{.}002& 0\\ -0\mathrm{.}002& -0\mathrm{.}004& 0\\ 0& 0& 0\end{array}\right]$

Find:

$(1)$ principal strain;

$(2)$ principal strain direction;

$(3)$ octahedral shear strain;

$(4)$ Strain invariant.