$-6\mathrm{.}16$ Repeat Problem $6\mathrm{.}13$ using a material of D$6$AC steel at $-40F.$

$6\mathrm{.}13$ A thick $4340$ steel plate at room temperature is loaded as in Figure P$6\mathrm{.}13$ to a grossarea stress ${}_g$ of $0\mathrm{.}73S_y.$ For dimensions $t=1in.,2w=6in.,$ and $\frac{a}{2}c=0\mathrm{.}25,$ estimate the critical crack depth, $a_{cr},$ at which rapid fracture will occur.

$6\mathrm{.}16$ ONLY