Thesis-2013-Forrester-Chen. (8.89 MB)
High strain rate compression testing of polymers: PTFE, PCTFE, PVC and PMMA
thesisposted on 21.11.2013, 11:53 authored by Hsuan-Hsiou (Chen) Forrester
The mechanically compressive flow stress sensitivities of various polymers are investigated at high strain rates above 103 s-1. Temperatures near the glass transition temperature are investigated and the polymer stress-strain responses have been studied from ambient temperature to 100ºC. Previous work has reported peaks in flow stress as a function of strain rate [Al-Maliky/Parry 1994, Al-Maliky 1997]. The analyses showed rapid increases of flow stress followed by a sudden drop at elevated strain rates, which is unlike the well known linear relationship documented at the low strain rates. The mechanics and stipulation of what bring about this phenomenon, or the types of polymers influenced are still unclear. Two fluoropolymers, polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE), and two vinyl polymers, polyvinylchloride (PVC) and polymethylmethacrylate (PMMA), are chosen for this study. PTFE, PCTFE and PVC are semi-crystalline polymers with different percentage of crystallinity contents, whereas PMMA is an amorphous polymer. The glass transition temperature, Tg, is the characteristic of the amorphous content in polymers, which has been suggested to influence the flow stress peaks [Swallowe/Lee 2003]. Tg of the semi-crystalline polymers are within the test temperature range. High strain rate compression tests have been carried out using the split Hopkinson pressure bar (SHPB). This is a well-established method for determining the stress, strain, and strain rate of materials. The strain rate range of interest is 103 s-1 to 105 s-1 where the strain rate sensitivity has previously been identified [Al-Maliky/Parry 1994, Al-Maliky 1997, Walley/Field 1994]. Two thermal analyses techniques are used to quantify the dependency of the viscoelastic behaviour in relation to time and temperature. Differential scanning calorimetry (DSC) measures the enthalpy of the polymers to show how the materials are affected by heat, and Dynamic mechanical analysis (DMA) is used to characterise the time-temperature dependence of the elastic storage and loss moduli of the polymers A total of 42 PCTFE, 44 PTFE, 45 PVC and 55 PMMA specimens were tested using the SHPB system, with the strain rate varying between 1600 s-1 and 6100 s-1. Initial results for PMMA have been reported [Forrester/Swallowe 2009]. The rate of strain where specimens begin to show crazing is identified. The value of yield stress increases with the increase of strain rate and the decrease in temperature. Large strain hardening can be seen in all three semi-crystalline polymers at higher strain rates. The temperature rise during plastic flow of compression is calculated by the stress-strain rate curves. In this thesis, the emphasis is on the relation of yield/flow stress to strain rate as the polymers deform under high strain compression. The mechanism behind the cause of high strain rate deformation responses for amorphous to semi-crystalline polymers in ductile state is discussed, with a view to understanding the sensitivity of yield/flow stresses as a function of strain rate. Also, the modelling of the polymers has been carried in order to alleviate doubts about the validity of the real experimental results that may arise due to the nature of the decomposition of the polymers. It has been shown that the strain energy density pulses through the sample in response to the compression wave in various circular intensities.