“…The strain energy per unit volume, E, stored in a material is equal to the area under the stress-strain curve from the zero strain to the elastic strain, E, Equation (1) applies to both linear and non-linear cases. For linear case Q = LE, Equation (1) can be rewritten as or, alternatively So, for n arbitrary structures with respective stresses a, (i = 1,2,...,n), the corresponding strain energies E, (i = 1,2,...,n) are is the total energy of the system before composition.…”