This thesis presents the results of an investigation into the effects of hygrothermal conditioning on the mechanical properties, and fatigue properties of epoxy based composites reinforced with carbon, glass and Kevlar 49 fibres. Cross-plied laminates (0/90) of these materials of nominal volume o fraction 60% were conditioned by drying at 60°C, by exposure to a 65% RH atmosphere at room temperature and by boiling in water. The effects of conditioning on the tensile and shear strengths and on the tensile fatigue response are discussed. The effects of exposure to an extreme diurnal cycle and to the ultra violet in isolation on the tensile properties are also discussed. The 0/90 tensile properties of the three laminates are relatively little affected by the environmental conditioning except for the case of GRP exposed to boiling water, when corrosion damage to the glass fibres significantly reduces the composite strength, and in the KFRP in which the strength is reduced by complete drying. The +/-45 strengths are more sensitive to the effects of moisture, however, it appears that the optimum strength is obtained after conditioning in the 65% RH environment. Acoustic emission monitoring of the tensile tests shows distinctive differences between KFRP and the two other types of composite and permits the identification of characteristic effects of moisture on the tensile failure mechanisms of all three materials. Tensile fatigue tests have been carried out on the laminates in the 0/90 orientation. The CFRP shows no effect of conditioning on the fatigue behaviour, and in the GRP only the boiling water conditioning affects the results. The tensile fatigue of the KFRP is affected both by boiling and by drying, the latter being the most severe. The fatigue response of the KFRP shows a dramatic down turn at lives in excess of 105 cycles. This effect appears to reflect the ease with which mechanical damage is sustained by the aromatic polyamide fibres. The residual strengths of the laminates after fatiguing is discussed and possible mechanisms for the damage accumulation in the materials during fatiguing are given.
|Date of Award||1984|