Fatigue of Glass Fibre Composites in Marine Renewable Energy

Abstract

Glass fibre reinforced polymers (GFRP) are obvious candidate materials for use in the structures of ocean energy devices due to their corrosion resistance, high strength and low cost. The polymers normally used in GFRP can absorb up to 5% water by weight when immersed for long periods and this can reduce the tensile strength of the material by 25% or more. The thesis describes a combination of experiments and modelling studies undertaken to (i) quantify the degradation in fatigue strength due to moisture saturation in a number of candidate materials, (ii) seek to understand the damage mechanisms which are important in the degradation of the material strength by moisture saturation and (iii) predict the degradation in tidal turbine blade life due to water saturation. The experimental work involved the fabrication of quasi-isotropic (QI) coupons of vinyl-ester or epoxy / E-glass and vinyl-ester / Advantex-glass using the vacuum assisted resin transfer moulding process followed by post curing at elevated temperature for 4 hours to ensure full cure of the laminate. Approximately one half of those coupons were acceleration aged in warm water for up to 2 years to simulate immersion in 12° C seawater for 15 to 20 years. The rest were stored at normal room temperature and humidity for a similar length of time. Constant amplitude fatigue testing of both dry and wet coupons established the stress-life curves for the materials and thereby quantified the degradation in the fatigue strength due to water saturation of the materials. The modulus of the coupons was also monitored during the fatigue testing to establish the damage levels due to fatigue cycling. A number of other experimental studies were also performed to investigate the effects of applied stress during ageing and different glass fibre material on the fatigue strength of the material. A preliminary fatigue design methodology for tidal turbine blades was developed using a tidal velocity model, a hydrodynamic forces model, a structural finite element model and a strain-life fatigue model. The methodology is applied here for the preliminary design of a three-bladed tidal turbine concept, including tower shadow effects, and comparative assessment of pitch- and stall-regulated control with respect to fatigue performance. This methodology was also used to predict the effect of moisture saturation on blade life for both pitch- and stall-regulated turbines. A multiaxial fatigue damage model for fibre reinforced polymer composite materials has also been developed. The model combines (i) fatigue-induced fibre strength and modulus degradation, (ii) irrecoverable cyclic strain effects and (iii) inter-fibre fatigue. The inter-fibre fatigue aspect is based on a fatigue-modified version of the Puck multiaxial failure criterion for static failure. The model is implemented in a user material finite element subroutine and calibrated against fatigue test data for unidirectional glass fibre epoxy. Validation is performed against the fatigue tests on epoxy / E-glass coupons in the experimental programme. The model is successfully validated across a range of stress levels. The model predicts both modulus degradation and fatigue life of GFRP laminates during constant amplitude fatigue cycling. It also predicts the fatigue strength knockdown factor due to moisture saturation of the materials.