" Mechanical internal friction dissipation" is of most importance in introducing viscoelasticity of polymer. But some details of its derivation process were eliminated from the textbook

which result in a difficult understanding of this part for the students. In order to achieve better teaching results

the authors divided the strain under cyclical stress into two parts

namely ideal elastic deformation and ideal viscous flow respectively

then derived the energy consumption in a deformation cycle and the energy storage at any given time; and cleared "why the system has a maximum energy storage in 1/4 cycle" and the mechanical internal friction dissipation was obtained. Practices show that the vast majority of students can be have an easy acceptance of the derivation in this way

and have a good understanding of the concept of mechanical internal friction dissipation.