The copper bi$-$crystal shown below has a grain boundary of $(110)$ orientation and an interfacial grain boundary energy of $0\mathrm{.}8\frac{J}{m^2}.$ The surface energy of $Cu(110)$ is $2\mathrm{.}4\frac{J}{m^2}.$ The Young's modulus of copper is $130$ GPa $.$

Calculate the critical Griffith value of the stress intensity, $K_G,$ in plane stress $($in units of MPa$\sqrt[\phantom{2}]{m})$ for grain boundary fracture.

$0\mathrm{.}64$

$0\mathrm{.}85$

$0\mathrm{.}79$

$0\mathrm{.}72$