Dissertation

Micromechanical Behaviour of Alumina Fibre/Epoxy Model Composites

Sinclair, Rebecca

2001

Overview

The micromechanical behaviour of model composites containing alpha-alumina fibres in epoxy has been studied using fluorescence spectroscopy. Using calibration constants determined for fibre strain versus alumina fluorescence peak position, strain and stress distributions in the embedded fibres were measured in-situ as the composites were loaded.Three types of alumina fibre, Nextel 610, PRD-166 and Saphikon, were characterised using scanning electron microscopy, x-ray diffraction and fluorescence spectroscopy. The fibre diameters range from 12 to 130 μm. PRD-166 has a significantly rougher surface than the others. This fibre has a randomly oriented polycrystalline structure, while Nextel 610 has a degree of texture. Saphikon is a single crystal with the long axis of the unit cell oriented parallel to the fibre axis. The fluorescence spectrum of all three fibres is composed of two intense peaks at about 14400 and 14431 cm-1, which shift to higher wavenumbers in approximately linear relationships with axial strain and stress.The fragmentation test, in which dumbbells containing single alumina fibres, fully embedded in epoxy resin were subjected to tensile stress, was performed whilst simultaneously measuring fluorescence spectra in a distribution along the embedded fibre length. This enabled the evaluation of the alumina epoxy interface. The interfacial shear strengths of the fibre-epoxy interfaces were found to be 40 MPa for Nextel 610, 38 MPa for PRD-166 and 18 MPa for Saphikon. The predictions of shear lag models were found to fit the data well. It was necessary to deduce certain parameters of the models from the measured data. Therefore the models are of limited use as predictive tools.In addition the axial and radial stress distributions were derived for Saphikon using stress- bandshift rates for the different crystallographic directions of ruby crystals. The axial stress scaled approximately linearly with strain, while the radial stress distributions provided a new insight into interfacial behaviour. The radial stress increased with applied load until interfacial debonding occurred, and then become zero for debonded parts of the interface.The push-in test, in which compressive stress is applied to the end of single, partially embedded fibres, was also performed using fluorescence spectra to measure strain and stress distributions in the embedded fibres. A protruding section of the fibre was used as a ‘push rod’ for the application of stress to the embedded fibre. A shear lag model was used to fit the data. The strain in Saphikon was found to be affected by a compressive radial component. When the axial strain was calculated this was found to fit the model better.The Broutman test, in which modified dumbbells containing single, fully embedded fibres are subjected to compressive load, was performed using fluorescence spectroscopy to evaluate the fibre strain distributions for the first time. The nucleation of debonding due to radial tension across the interface was detected using the radial stress distribution before it became visible using optical methods.The multiple fibre fragmentation test, in which a dumbbell containing several fully embedded parallel fibres is subjected to tensile stress, was performed for Saphikon in epoxy resin. The large diameter of the Saphikon fibre enabled fluorescence spectra to be obtained as a transverse distribution, and therefore strain, axial stress and radial stress variations were obtained across the width of the embedded fibre. The radial stress was found to be relieved on the side nearer to a broken fibre due to the stress field surrounding a damage site.

• Publisher: ProQuest Dissertations & Theses
• Subject: Materials science
• ISBN: 9798371993762