Abstract
Interest in tidal power is continuously increasing due to its huge potential for energy generation. This has led to the emergence of tidal turbine designs often inspired from earlier developments in the wind turbine industry. In comparison with land-based structures such as wind energy devices, development of marine energy conversion devices places more emphasis on material properties. Maintenance cost at sea is high and for marine energy supply to become economically viable, long term durability and reliability of these materials must be considered. Fluctuating loads due to the tidal flow are a common characteristic of marine energy conversion devices and they must withstand both extreme one-off and fatigue loads during their lifetimes in an aggressive environment, therefore it is expected that long term environmental fatigue performance will be a major consideration in their design. Fibre reinforced composites can provide the required hydrodynamic performance of tidal turbine blades and they are relatively low-cost and low-weight candidate materials for blade construction, however limited information is available to predict material properties and failure modes under coupled environmental conditions and oscillating loading. This study fills this gap by introducing a methodology for prediction of the fatigue behaviour of composite tidal turbine blades. The methodology combines: (a) a hydrodynamic model to calculate the distributions of tangential and axial forces on the blade; (b) a Finite Element structural model to predict the mechanical and fatigue behaviour of blade and (c) cyclic and quasi static tests at laboratory scale in sea water to generate realistic mechanical data for modelling. All tests have been performed in conjunction with the Digital Image Correlation technique in order to evaluate strain distribution and strain localization. Damage characterization and failure analysis based on this method was performed to identify damaged zones on the test coupon surface and to follow failure mode development during the fatigue life. For simplicity, and regarding the typical loading on a tidal turbine blade, the three point bending fatigue test was selected from different types of standard tests to determine fatigue life and failure mechanisms of a FRP composite. A submerged fatigue test rig, capable of exerting cyclic loads of up to 15kN was designed and manufactured to enable fatigue tests to be carried out on multiple GFRP composite test coupons in sea water. The influence of the sea water environment on fatigue behaviour and failure mechanisms was characterized using failure analysis techniques such as Scanning Electron Microscopy and Energy-dispersive X-ray VII spectroscopy. Microscopic techniques were used to reveal information about the multiple failure mechanisms on the specimen fracture surface. The stiffness parameter was considered as an important index to monitor damage evolution during each fatigue test. Also for further verification of microscopic and DIC results, the Xray Micro-computed tomography technique was used to characterize the internal damage and the geometry of flaws including delamination. The Finite Element method was used to conduct stress analysis on composite test coupons and composite tidal turbine blades. This was done in order to interpret the experimental results and to examine the failure modes of the test specimens under static loads and also to predict failure of composite blades under both static and cyclic loads. Ply-by-ply stress analysis and the TsaiHill failure criterion were employed for the strength and failure prediction. Fatigue properties of a GFRP composite in a form of strain-life diagrams showed that sea water can considerably reduce the fatigue life of GFRP composite and the main failure mechanisms associated with environmentally affected fatigue of GFRP composite are delamination, degraded fibre/matrix interface and fibre breakage. Furthermore, it was found that delamination and de-bonded fibre/matrix play an important part during the fatigue test, therefore using a composite resistant to delamination and de-bonding can significantly increase the life of composite components. Finally, on the basis of experimentally generated fatigue data (Strain-Life diagram) and Finite Element structural analysis of blade, the fatigue life of the blade was estimated to be about between 4-5 years. This means that if replacing tidal turbine blades every 5 years is economically viable, this low cost composite (G10) can be a good candidate for tidal turbine systems.