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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Structure and properties of glass fibre reinforced cements.

Author: BUSHBY, Andrew

Year: 1991

Supervisor(s): Craig Davies

The properties of composites consisting of a matrix of ordinary Portland cement, fine sand and powdered mica, reinforced with a random two-dimensional array of alkali resistant glass fibre were investigated. Materials were manufactured in the form of flat sheets using a pre-mixing techniques and with fibre volume fractions in the range 0-4% . The materials were characterised in terms of structure, using optical and electron microscopy (SEM) techniques, and some simple physical properties. Mechanical tests were performed on materials in two conditions, water saturated and oven dried at 100oC.
Three point bend tests were performed on the materials, generating load-deflection curves. Both the maximum force and the area under the curve were seen to increase with increasing fibre content, particularly for volume fractions greater than 2% . Fracture mechanics parameters were determined for the materials using notched bending and compact tension specimens conforming with ASTM E399. Values of apparent critical stress intensity factor (Ka) were determined using a number of existing anlayses which demonstrated the difficulties in determining geometry and crack length independent toughness parameters. The Ka values obtained were found to be critically dependent on the interpretation of specimen compliance data.

The cracking behaviour of the composites was studied extensively using polished sections of the notched three point bend specimens, optical examination of the surface cracks and SEM examination of the fracture surfaces. A change in behaviour from single cracks to branched and multiple cracks was noted with increasing fibre content and crack length. The complexity of the cracking process on a microscopic level was observed, together with the effect of interface behaviour on the interaction of cracks and fibre bundles.

The implications of the results were discussed in terms of the role of fribres, the assessment of material properties and fracture mechanics type descriptions of the fracture process. A new method of interpreting compliance data the `equivalent crack model' was proposed giving further information on the structure of cracks and allowing a more specimen independent evaluation of Ka values.