Material characterization of long-term stress relaxation in a semi-crystalline polymer material : An experimental and numerical study

Abstract: As the plastic and packaging industry is looking to increase the longevity of plastic products as well as recycling used material, there is a need to understand how material properties respond and change during long periods of mechanical loading. Physical tensile experiments on thin plant-based High-Density Polyethylene (HDPE) are conducted with the intent of capturing relaxation behavior from a short-term (1-3 hours) and long-term (29-56 days) perspective. Experimental tests aiming to capture short-term relaxation behavior prior to necking at various loads are made on a MTS Qtest100 tensile-machine in the laboratory at BTH. Long-term experiments are conducted on a custom-built tensile machine stationed in the author’s apartment.   Data gathered from the experiments are swiftly converted into true stress and strain based on the derived mathematical expressions in preparation for computer simulations, i.e. modeling the behavior using two expressions and the Finite Element Method (FEM) in the general purpose FE-software AbaqusTM R2020. The loading curve, i.e. uniform deformation, prior to geometrical necking, was modeled using the Ramberg-Osgood expression and captured the mechanical non-linear behavior accurately. Two expressions are initially used to capture the stress decay, referred to as relaxation behavior: the first one is Guiu and Pratt and the second one is a data-generated Four Parameter Logistics (4PL) expression. A comparison between the two expressions, show that the 4PL expression captures the entire short-term behavior of the experiments. The 4PL expression could also predict the long-term behavior without further calibration. The Guiu and Pratt expression could not predict the behavior as accurately as the 4PL expression.   Using the converted physical data to calibrate a Parallel Rheological Framework (PRF) model in the MCalibration software proved to be time consuming. A combination of the Ramberg Osgood and 4PL expression is used to re-create the converted physical experiment data which reduces both noise and size of the datasets dramatically. The calibration time was significantly reduced because the datasets were much smaller. With a material model calibrated using the re-created data, simulations could be conducted in Abaqus, creating a virtual twin of the physical experiments. Results from the physical experiments are compared to the results of the virtual simulations proving that the PRF model can capture the relaxation behavior shown in the short-term experiments. The model also works for long-term relaxation behavior and only a slight increase in stress relaxation compared to the physical experiments was observed.