PhD, EngD, MPhil and MSc by research theses (SIMS)

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Browsing PhD, EngD, MPhil and MSc by research theses (SIMS) by Author "Bucknall, Clive"

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    High strain deformation and ultimate failure of HIPS and ABS polymers
    (1997) O'Connor, Bernard; Bucknall, Clive
    The role of the rubber particle in the ultimate failure of High Impact Polystyrene and Acrylonitrile Butadiene Styrene was investigated by modifying the rubber content and the shear modulus of the rubber phase in the materials. The rubber content in a commercial grade HIPS and a commercial grade ABS, both with 8 wt. % rubber, was varied by blending with general purpose polystyrene and general purpose poly(styrene-aciylonitrile) respectively. The shear modulus of the rubber phase was varied through blending the materials with sulfur or irradiating the materials with gamma irradiation. Dynamical mechanical thermal analysis confirmed that the Tg of the rubber phase increased with increasing sulfur content. It was found that with decreasing rubber content or increasing rubber shear modulus, the yield, flow and breaking stresses and the elastic modulus of the composite increased, while the failure strain decreased. In a similar experiment to Sjoerdsma and Boyens (1994), the statistics of failure of the materials were investigated with respect to rubber content and rubber shear modulus. Batches of specimens numbering not less than 20 were extended under a constant applied stress until failure occurred. A custom designed creep rig was built to carry out several long term creep tests simultaneously. From these tests it was concluded that the probability of failure increased as the stress on the rubber increased and underpinning this, is a novel discussion of the high strain deformation and the mechanism controlling failure in HIPS and ABS. This conclusion was discussed in terms of rubber content and rubber shear modulus and a model was developed which describes the maximum failure strain in terms of these variables. The level of applied stress was also found to have an effect on the probability of failure. It was found that the success achieved by Sjoerdsma and Boyens (1994), in correlating failure strain data for a single grade of HIPS, could not be repeated when their model was applied to another grade of HIPS. The tw oparameter Weibull equation gave an improved correlation between the failure of HIPS and the strain on the material. Analysis of the relationship between the experimental failure strain distribution and the Weibull distribution revealed that the mean stress on the rubber phase at failure may be a better basis for achieving a Weibull distribution.
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    Rubber particle cavitation in toughened Poly(methyl methacrylate)
    (2001-01) Rizzieri, Rosalba; Bucknall, Clive
    Theoretical and experimental investigations were performed on multiphase polymers; especially rubber toughened (RT) poly(methyl methacrylate) (PMMA) to explore cavitation in the rubbery phase. The main objectives of this project: i. To identify experimental methods to effectively detect rubber particle cavitation. ii. To relate intrinsic toughness with rubber properties (e.g. rubber type, particle morphology, rubber content, particle size). iii. To study the relationships between different pre-treatments, and control the onset of cavitation. Thermal contraction measurements, dynamic mechanical analysis, creep and fracture tests were the techniques adopted. Results from those different methods were examined, compared, and related to a specifically devised mathematical model. They were found consistent. Thermal contraction measurement presents valuable information about the progress of cavitation after pre-strain. It shows extensive rubber cavitation at low longitudinal strain (about 2 - 3%), which is sufficient to produce permanent damage, not recoverable by annealing. Dynamic mechanical procedure estimates the resistance of the soft phase to cavitation in response to mechanically and thermally generated stresses. It can be used to detect distributions of stress and strain within the soft phase after cavitation. The dynamic mechanical tests, supported by electron microscopy, provide further insight into the cavitation mechanism. It is suggested that a complete failure of the rubber will allow any internal stresses to relax, and the rubber glass transition temperature (Tg) to become independent of the tensile stress on the specimen. If the particles remain intact, the loss peak will shift to lower temperature with increasing triaxial tension as the rubber free volume increases in response to a growing dilatational volume strain. To any inbetween state, regarding rubber phase partial failure, will correspond a loss peak in the temperature range defined by Tg of the stretched rubber and the one of the relaxed rubber (upper limit). A major advantage is that thermal contraction measurements and dynamic mechanical tests provide an observation method for the onset of cavitation as a separate process, without the complications that arise when shear yielding or multiple crazing occur at the same time. Analysis based on the energy-balance model suggested multiple cavitation as a possible mechanism for complex particle morphology (e.g. salami or hard-soft-hard core-shell). These results are consistent with experimental data.