Molecular Dynamics Simulations of Tensile, Fatigue and Creep Loading on Polymers
MetadataShow full item record
This PhD study is part of a larger research program within the composite and polymers group to investigate long term properties of polymers. The research on polymers is growing as a continuing effort to explore their advantage as an alternative to more traditional materials. Investigating the mechanical behaviour of polymers at the atomistic/molecular scale will provide a better understanding of the failure characteristics under various temperatures and loading conditions, such as static, creep and fatigue loads. Molecular dynamics (MD) simulations can predict material properties based on classical equations of motion and the atomistic force field between atoms/molecules. MD simulations can show important molecular mechanisms during deformation, such as potential energy and structural geometry development. Therefore, MD simulations are a potential tool to simulate new polymeric materials before they are made in a laboratory and to predict the polymer´s mechanical properties. The effects of temperature and strain rate on the deformation behaviour of Polycarbonate (PC) and amorphous Polyethylene (PE) have been reproduced by MD simulations. An all-atom model of Bisphenol A Polycarbonate (BPA-PC) and a united atom model of amorphous polyethylene (PE) were used. Tensile tests at various temperatures and strain rates were simulated. Calculated mechanical properties from the simulation results, such as Young’s modulus, Poisson’s ratio and yield stress, are similar to experimental measurements performed by other researchers using much lower strain rates. The time– temperature superposition principle was applied to produce master curves for the amorphous PE model. These simulated master curves compare fairly well to master curves produced from laboratory experiments. Therefore, developing a master curve can address the discrepancies in strain rates between the simulations and the experiments. Differences in the numbers of monomers and chains, the degree of crystallinity and molecular orientation lead to discrepancies in the mechanical properties between MD simulations and experiments. Simulations resembling strain controlled loading fatigue tests on amorphous PE model in tension-tension mode were performed to study the effect of the R-ratio and mean strain on the mechanical responses. The simulations were able to produce qualitatively similar behaviour to the experimental results, for instances 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. Cyclic loading changes the total potential energies of the system especially the van der Waals energy. These changes contribute significantly to the increase of the stiffness of the system with increasing number of cycles. 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 along the loading axis as the loading progresses. The relationship between creep and tensile fatigue was investigated for amorphous PE. Stress controlled tensile cyclic loading under various R-ratios and constant stress (equal to creep) was simulated. The simulations were able to produce qualitatively similar behaviour as observed in experiments, for instances strain-softening behaviour and hysteresis loops in the stress-strain curves. Increasing R-ratio loading reduces mean strain while constant stress loading produces the lowest mean strain. Creep gets enhanced when combined with cyclic fatigue. These trends were properly reproduced even though simulations were done at much higher stress (or strain) rates than laboratory experiments and an amorphous PE model was used while the real material is typically a semi crystalline polymer. The simulations predict that the molecular mechanisms of creep and fatigue are the same. Fatigue and creep loading significantly change the van der Waals and dihedral potential energies. These changes are caused by movements of the polymer chains, creating more un-twisted dihedral angles and the unfolding of polymer chains along the loading direction.