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Performance of multi-component polymers at high strain rates

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posted on 22.11.2012, 10:12 by Andrew Prudom
More and more, advanced polymer and composite materials are being applied in engineering situations where a high resistance to loading at high rates of strain, such as by impact or blast deformation, are a vital requirement. Specific examples exist in the fields of defence and sport research and development for personal, and in the case of the former, vehicular, protection. There are obvious advantages to the use of polymer materials for these applications in augmenting the more widely used metals and ceramics, most notably the evident reduction in weight, and it is believed that with suitable nano-reinforcement these materials may exhibit improved combat survivability. The current study concerns the effect that nano-reinforcements in the form of Carbon Black, Titanium Dioxide, Exfoliated Hectorite Nanoclay and Carbon Nanotubes; have upon the high strain rate mechanical properties of structural variants of Polyethylene (Linear Low Density Polyethylene, LLDPE; High Density Polyethylene, HDPE; Ultra-High Molecular Weight Polyethylene, UHMWPE) and blends of UHMWPE and HDPE. The testing samples were manufactured using a novel process developed in the Loughborough University Materials Department, which has produced well-dispersed specimens. The formed nanocomposite samples were studied using an in-house four-bar Split Hopkinson Pressure Bar (SHPB) system for high strain rate performance, instrumented dropweight for intermediate strain rates and a conventional commercial Hounsfield H50KM universal testing machine for quasi-static strain rate compressive tests. The experimental results recorded for un-reinforced materials are used as a reference to allow comparative analysis of any effect the nano-reinforcements or the blending process have upon the structure, performance and properties of the composite material. From the mechanical testing, it was seen that the stress-strain behaviour of Polyethylene is highly strain-rate-dependent, as plots of the average representative yield stress as a function of strain rate show a bilinear relationship when plotted on a logarithmic strain rate scale, with the gradient of the curve rising sharply at around 103s-1. Concerning the addition of the nanofiller materials, it was seen that there was an increase in the flow and yield stresses and the energy absorption characteristics of the resulting composite with the magnitude dependent upon whether it was a pure or blended polymer that was reinforced. Of the aforementioned fillers it was seen that the addition of Carbon Nanotubes in the small concentrations studied resulted in the greatest increase in properties compared to the pure polymers, closely followed by the Carbon Black fillers. Also of note, the un-reinforced blended samples showed significant increases in flow stress, yield stress and energy absorption when compared to the constituent UHMWPE and HDPE polymers. Additionally, a complete set of Differential Scanning Calorimetry and density measurements were made before testing to assess any changes in the properties after reinforcement or blending, and to help in the interpretation of the results from the different mechanical tests.



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© Andrew John Prudom

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A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.

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