Methods were developed for dividing the total mechanical work of deformation and the deformation corresponding to it into elastic-reversible and viscoelastic-plastic components which suggests use in both calculation of the resistance of chemical fibres in the dynamic deformation regime and for assessing the ability of materials to resist a mechanical effect and recover the initial shape during use. Incorporation of a coefficient for the irreversibility of deformation allows separating the viscoelastic and plastic components of deformation after the total deformation of chemical fibres is broken down into the constituent components.