Molecular dynamics simulations of strain-controlled fatigue behaviour of amorphous polyethylene

Fatigue of amorphous polyethylene under low strain was simulated using molecular dynamics. The united atom approach and the Dreiding force field were chosen to describe the interaction between monomers. Molecular dynamics simulations resembling strain-controlled loading fatigue tests in tension-tension mode were performed to study the effect of the R-ratio and mean strain on the mechanical responses. Laboratory fatigue experiments in strain/displacement control were performed at room temperature, and the results were compared to the simulation results. The simulations are able to produce qualitatively similar behaviour to the experimental results, for instance, mean stress relaxation, hysteresis loops in the stress�strain curve, and change in the cyclic modulus. They also show that stress relaxation is enhanced by cyclic loading. The simulations show that cyclic loading changes the total potential energies of the system, especially the van der Waals potential. The changes in the van der Waals potential energy contribute significantly to the increasing of the stiffness of the system. Some changes in dihedral angles with lower energy configurations are observed; however, bond distances and angles do not change significantly. The chains tend to unfold slightly along the loading axis as the fatigue loading progresses.