If γ is the surface energy density of a cylindrical rod of radius formula_1 and length formula_2 at high temperature and a constant uniaxial tension formula_3, then at equilibrium, the variation of the total Gibbs free energy vanishes and we have:formula_4where formula_5 is the Gibbs free energy and formula_6 is the surface area of the rod::formula_7Also, since the volume (formula_8) of the rod remains constant, the variation (formula_9) of the volume is zero, i.e.,:formula_10Therefore, the surface energy density can be expressed as:formula_11The surface energy density of the solid can be computed by measuring formula_3, formula_1, and formula_2 at equilibrium.