In the paper, the evolution of mechanical characteristics of structural materials in the process of uniaxial loadings are experimentally investigated. The energy balance was studied in a series of tensile and creep experiments of caprolon specimens, taking into account the division of the work of external forces into the energy of volume change and the energy of shear deformations. Simultaneous registration of heat release and changes in the coefficient of transverse deformations by the optic-acoustic method allowed us to estimate the contribution of volume changes and shear deformations to the energy balance during quasi-static loading of caprolon specimens. The obtained results demonstrated the contribution of shear deformations to the change of latent energy at the stages of linear and nonlinear behavior of caprolon during deformation.