\(Ar=b\cdot lt\ \left(1\right);\ \ \ \ \ \ \ \ lt=l_0-\left(u\cdot ml\right)\ \ \ \left(2\right);\ \ \ \ \ \ k_H=G.Ar\cdot\frac{\left(1-\frac{Pl}{Pcr}\right)^2}{h_{rubber}}\ \ \left(3\right)\)