Effects of water absorption on the strain rate sensitive properties of glass fibre reinforced polymers

Thesis

The present thesis introduces, for the first time, a comprehensive numerical and experimental investigation of the combined effects of strain rate and water exposure on composite laminates and their constituents epoxy resin matrix and E-glass fibres. Long-term exposure to wet environments and the application of loads at various speeds are critical challenges for marine composites, as both strain rate and water absorption affect their mechanical behaviour. This thesis addresses the problem by investigating the influence of water absorption on the strain rate dependent behaviour of glass fibre reinforced polymers. Unlike previous studies focusing on the consequences of water on the properties of the composite or the resin matrix only, the strategy behind this investigation considers the experimental and numerical study of the effects of water on every constituent of the composite.

Specimens were immersed in water and salt-water baths at 50° C to accelerate water absorption, and subsequently tested at different strain rates. Experiments showed that all constituents and the composite are strain rate dependent regardless the water pre-conditioning. In addition, all tests suggested that wet environments have detrimental effects on the resin matrix, fibre bundles, and composite laminates. Water absorption affected their elastic modulus, tensile strength, short beam strength, and caused swelling of the matrix and damage to the matrix-fibre interface. Only E-glass single fibres were found to be indifferent to water pre-conditioning. For fibres, a novel experimental technique named Sound Measurements, based on sound acquisition, proved its applicability to gather information about the distribution of fibres strengths within a bundle.

Simulations of the constituents showed how water affects the parameters of the Arruda-Boyce model for polymers and the Weibull model for fibre bundles. The comparison of the experimentally measured properties of the composite with those obtained by homogenisation suggested that matrix-fibre interface must have suffered damage from water absorption. Finally, a numerical study of water-induced stresses due to matrix swelling and viscoelastic relaxation was conducted and determined the history of stresses arising during the water absorption process due to the expansion of the matrix.

Authors: Quispe, G

• DOI: 10.5287/ora-qx1dk7vmr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford