Browsing by Author "Bucknall, Clive"

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    Effects of phase separation in crosslinked resins containing polymeric modifiers
    (Cranfield University, 1984-03) Partridge , I. K.; Bucknall, Clive
    The study reported in this thesis concerns the relationship between morphology and the properties of multicomponent epoxy and methacrylate resins. The thermodynamic and kinetic aspects of phase separation in reactive, thermosetting, systems are explored and applied to the investigation of blends of 'high performance' epoxy resins with polyethersulphone. The mechanical properties of the same cured blends are shown to be insensitive to considerable variations in composition, cure and the resulting morphology, both in the bulk-resin and in carbon fibre-epoxy laminate forms. Room temperature fracture and yield behaviour of highly cross linked and rubber modified methacrylates, of rubber modified dysfunctional epoxies, as well as of the above mentioned epoxy/polyether sulphone blends, is investigated. Special emphasis is placed on a comparison of the kinetics of tensile creep between the different systems. Loss of ductility in the highly cross linked resins is shown to result in a very limited scope for toughness enhancement by the usual rubber toughening methods.
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    Effects of rubber particle cavitation on the yielding of high impact polystyrene
    (Cranfield University, 1997-09) Yang, Hsiao-Hsi; Bucknall, Clive
    A pre-damage method was developed to determine whether rubber particle cavitation is responsible for craze initiation, or vice versa. Tensile tests were carried out on pre-strained high impact polystyrene (HIPS) specimens which had been annealed above 100 °C to heal any crazes formed in the PS matrix during pre-straining. Moderate prestraining followed by annealing was found to reduce the yield stress of the HIPS, but not the post-yield flow stress. These observations are related to cavitation for the rubber particles, which results in a fibrillar structure within the rubber membranes of a typical "Salami' particle. The reduction in yield stress provides evidence for primary chain scission in the rubber phase during yielding: on subsequent loading of the annealed tensile bar, the rubber particle exhibits a reduced resistance to cavitation because less energy is required to form a void. It was found in the follow-up pre-straining tests on HIPS blends and at different testing temperatures that the critical pre-strain is a function of rubber content and temperature. Comparable effects are seen in creep tests. Small levels of pre-straining have little effect if the specimen is not annealed before reloading. The TEM work also confirms rubber particle cavitation as a rate-determining step in the deformation of HIPS and supports the view that cavitation precedes crazing. A modified cavitation model is proposed to account for the deformation of a cavitated particle under tension. The predictions of this model agree with the experiments that the resistance of the particle is weakened by cavitation and crazes could initiate at the lower tension. The technique of pre-straining and annealing specimens, before submitting them to conventional tensile tests, provides valuable insight into the mechanisms and kinetics of toughening.
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    Fracture of modified urethane - methacrylate resins
    (1993-03) Zhang, Xiangcheng; Bucknall, Clive
    Fracture and toughening mechanisms in rubber modified and hybridized urethane-methacrylate resins have been investigated. Fracture mechanisms are defect-dominated in the unmodified resin. The relationships between defect size and fracture strength are characterized through the critical stress intensity factor KIC. Low fracture toughness and high crack sensitivity of the unmodified resin is due to lack of plastic deformation at the crack tip. A 10-fold increase in fracture resistance in the resin has been achieved through rubber modification. The main reason for the improvement is due to occurring of intensive plastic deformation in the presence of rubber, which effectively eases stress concentrations and spreads them away from the crack tip. Deformation mechanisms in rubber-modified resins are shear-dominated. Cavitation of rubber plays a key role in inducing shear deformation in the matrix. Fracture processes in rubber-modified resins start from coalescence and linkage of voids initiated inside rubber particles within rubber domains, which leads to final fracture in the resin matrix. Further increase in KIC was also obtained by incorporation of filler in a matrix toughened with rubber. This increase is not due to the effect of crack front pinning but due to increase in Young’s modulus in the presence of rigid filler. The same deformation and fracture mechanisms operate in the hybrid resins as in the rubber-modified ones.
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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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    Prediction of explosive decompression damage in elastomer seals
    (Cranfield University, 1999-07) Routh, James Mathew; Bucknall, Clive; Ho, Emily; Groves, Steve
    Elastomer seals are widely used in industry for containing gases and liquids. Seal failure can have significant environmental and financial implications far beyond the cost of the seal itself. Explosive decompression failure can cause catastrophic cracking of an elastomer seal, causing leakage. The objective of this research was to develop a modelling methodology to predict the onset of crack damage in elastomer seals under various operational conditions. The modelling methodology uses the finite element method to determine the performance of various elastomers under decompression conditions. The model takes into account seal and groove geometry, the non-linear behaviour of the material and the operational conditions seen by the seal. The model predicts crack initiations, locations and the orientation of the propagation. The model can also calculate the safe decompression time - required for no damage in the elastomer seal by using the methodology in reverse. To carry out the modelling methodology, certain input data are required. The data was determined by designing and constructing specialist test rigs. A permeation testing facility was developed to determine the diffusion, solubility and permeation characteristics of elastomers subject to gas pressure. The physical behaviour of the elastomers was determined through extensive uniaxial and equibiaxial tensile testing. The nature of the failure initiation points is determined by microscopic analysis of seal sections. Decompression tests were performed to validate the output of the model. Comparison of the model outputs with the decompression tests show a good correlation between the prediction and the occurrences, orientations and positions of cracks. The ability to predict damage in a quantitative manner was previously not available. The methodology will be developed into a knowledge-based software tool for use in industry to predict damage and develop new materials to resist explosive decompression.
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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.
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    Toughening tetrafunctional epoxy resins with thermoplastics
    (Cranfield University, 1988-12) Gilbert, A. H.; Bucknall, Clive
    The study described in this thesis examines how modification with different thermoplastics affects the structure and properties of a tetrafunctional epoxy re5ín_ Polyetherimide (PEI) is found to give the best improvement in fracture properties without loss in Youngs Modulus and the PEI/epoxy system is used as the basis for further study. The influences of PEI concentration, initial cure temperature, test temperature and the presence of a second thermoplastic additive, are investigated. The information provided gives insight into the likely mechanisms of toughening in tetrafunctional epoxy/thermoplastic blends. Flory-Huggins Lattice Theory is used to describe miscibility behaviour for a number of curing thermoplastic/epoxy blends and the predictions compared with the actual morphologies observed. Further, the sensitivity of the expected miscibility behaviour to fluctuations in Flory Huggins interaction parameter X12 and number-average molecular weight Mn of the thermoplastic, is considered. Dynamic mechanical analysis is used to monitor the changing viscoelastic properties of curing thermoplastic/epoxy blends, allowing investigation of the way different thermoplastics influence the state transformation profile of a curing epoxy resin.