A three$-$point bend beam shown in Figure $1$ was tested to measure the value of $G_F.$ The beam was loaded downward as shown in the figure. The dimensions of the beam are $S=800mm,b=100mm,$ and $t=100mm.$ The initial notch length $a_0$ was equal to $50mm.$ The values of $W_0=620N.mm$ and ${}_0=1\mathrm{.}6mm$ were measured. The density of concrete is $2400k\frac{g}{m^3}.$ Calculate the value of $G_F.$