Determination and modeling of viscoelastic-viscoplastic behavior of polymeric materials

Due to the increased use of polymeric materials, much research has focused on understanding their viscoelastic and viscoplastic behaviors and modeling their mechanical behavior under different loading conditions. High density polyethylene (HDPE) has been widely used in the piping industry as a raw material. Even though there are large numbers of experimental and analytical investigations on HDPE, a few of them has examined the effects of manufacturing techniques on the small and finite deformations. This study is constituted over two main parts: expertimental and modeling in order to understand the deformation mechanisms and mechanical behavior of HDPE. In the part of experimental study, the mechanical behaviors of HDPE associated with manufacture methods are investigated. The nonlinear mechanical behavior of HDPE is investigated using samples obtained from extruded HDPE pipe and compression molded sheet. Extruded and compression molded specimens are constituted the same raw materials. Extensive experimental work has been performed to characterize the nonlinear time dependent response of the material. The tensile loading-unloading behaviors with three different strain rates are investigated. In addition, creep and relaxation behavior of HDPE at the beginning of elastic, viscoelastic and viscoplastic regions are investigated. In the part of modeling, the viscoelastic and viscoplastic behaviors of HDPE under uniaxial monotonic, cyclic loading, multiple creep and relaxation are modeled using the modified viscoplasticity theory based on overstress (VBO). The visco-elastic modeling capabilities of the modified VBO are investigated by simulating the behavior of semicrystalline HDPE under uniaxial compression tests at different strain rates. Afterwards, the experimental results of extruded HDPE have been simulated using the finite viscoplasticity theory based on overstress (FVBO) model. In addition, the macro-mechanical model containing microstructural variables which is introduced by Boyce et al. (2000) is used to simulate the aforementioned behavior of HDPE. The simulation results of these two models are compared to the experimental data. Comparing overstress model and Boyce model in levels of finite and small deformation, reveals that FVBO is capable of modeling all investigated viscous behaviors: rate dependency, creep, relaxation. However, the model introduced by Boyce can model only loading behavior, but not unloading and creep and relaxation behavior. Boyce et al. (2000) model is capable of modeling large deformation, however, the response of the model at small deformation is not good enough. In the second part of modeling study, VBO model is extended to account for crystallinity rate ( ? ) on mechanical behavior of semicrystalline polymers. The modifications on VBO are done considering the semicrystalline polymeric material as a composite material since it consists of amorphous and crystalline phases. Amorphous and crystalline phase resistances are arranged in two different analog models: amorphous stiffness and flow are in parallel and series with crystalline phase. Apart from many existing work in the literature, not only uniaxial loading are modeled but also creep and relaxation behaviors are simulated. It is shown that when amorphous and crystalline phase resistances acting in parallel are considered in the model, creep, relaxation and uniaxial loading and unloading behaviors can be simulated well using the modified VBO. In the final part of the investigation, the material parameters of VBO model are optimized optimization method based on genetic algorithms. With this optimisation method, parameters are obtained more accurately and a relatively fast way. Keywords: Polimeric materials, viscoelasticity, viscoplasticity, modelling, axial tension, creep, relaxation experiments.