Application of large amplitude oscillatory shear for the analysis of polymer material properties in the nonlinear mechanical behavior

The results of studying the nonlinear viscoelastic properties obtained through the generation of large strain amplitudes are interpreted via plotting of Lissajous-Bowditch figures in two different systems of coordinates: (i) stress versus strain and (ii) the derivative of stress with respect to the phase angle versus strain. The former system yields the integral characteristic of dissipative loss in the deformation cycle, and the latter yields a measure of elasticity of a material. The generality of such approach to analyze large deformations, even in the case of an extremely complex shape of the nonlinear response, is due to the fact that it is not related to the a priori choice of any rheological constitutive equation. The developed method was tested on supramolecular structures and polymer solutions and melts. Novel results allow estimation of the character of nonlinearity development, i.e., the dependences of pseudoplasticity, dilatancy, and stiffening or softening of the medium on the shear. The comparison between the proposed measures of nonlinearity and large strain nonlinearity characteristics described in the literature shows that the integral characteristics are in good qualitative agreement with other measures of nonlinearity. However, in some cases, the proposed approach gives a more objective and consistent estimation than other measures of nonlinearity give.