QUESTION $2$

A randomly oriented, shorter$-$fibre$-$reinforced composite plate having a central throughthickness crack of length $2$ a is subjected to a uniaxial stress $,$ as shown in figure $1.$ The composite has a fracture toughness $K_{\frac{?}{C}}=45$MPa$-m^{\frac{1}{2}}$ and a critical crack size $a_c=15mm.$

$2\mathrm{.}1$ Determine the critical stress that will cause crack growth.

$2\mathrm{.}2$ Determine the strain energy released by the crack, if the composite plate material has an elastic modulus of $30$ GPa and a plate thickness $t=2cm.$