Tidal turbine blades are subjected to significant thrust and torsional loadings due to the high density of the seawater in which they operate. These thrust loadings lead to high bending moments at the blade root, which can prove to be a serious design constraint for these devices and can have implications with respect to cost-effectiveness and scalability. This work presents a combined hydrodynamic-structural design methodology for a commercial scale (1.5 MW) tidal turbine. A hydrodynamic analysis of the blade is carried out to determine force distributions along the blade span under normal and extreme operating conditions. Using output from the hydrodynamic model, a pre-processor for computing blade structural properties is used to determine the strain distribution along the blade spar caps. The strain distributions from this analysis are then compared with a finite element model of the blade which is then used to compare the structural performance of glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) as spar cap materials.
Tidal turbine blades experience significant fatigue cycles during operation and it is expected that fatigue strength will be a major consideration in their design. Glass fibre reinforced polymers (GFRP) are a candidate low-cost material for this application. This paper presents a methodology for preliminary fatigue design of GFRP tidal turbine blades. The methodology combines (i) a hydrodynamic model for calculation of local distributions of fluid-blade forces, (ii) a finite element structural model for prediction of blade strain distributions, (iii) a fatigue damage accumulation model, which incorporates mean stress effects, and (iv) uniaxial fatigue testing of two candidate GFRP materials (for illustrative purposes). The methodology is applied here to the preliminary design of a three-bladed tidal turbine concept, including tower shadow effects, and comparative assessment of pitchregulated and stall-regulated control with respect to fatigue performance. INTRODUCTIONThe emerging field of ocean energy is naturally turning to composite materials because of their perceived non-corrosive properties in the harsh saltwater environment as well as their high specific strength and stiffness. Tidal turbines are to the forefront due to their reliable and predictable power delivery to the grid and the absence of overload conditions. A number of different designs of tidal turbines are already at utility scale trials. A low-solidity, two-bladed turbine, similar to wind turbines, and a high solidity, multi-blade, ducted rotor are presently undergoing customer testing [1]. Wave energy converters are also under consideration with a number of devices at full size prototype stage [2]. Glass-fibre reinforced polymers (GFRP) are candidate low-cost materials for the blades of tidal turbines and the energy collection surfaces of wave devices. It is therefore important to understand the durability and performance of such materials for these applications. A 10 rpm tidal turbine would see approximately 4 million revolutions per year and wave devices will encounter approximately the same number of waves. Therefore fatigue failure will need to be considered in the design of these devices.The structural properties of GFRP materials depend on orientation of the fibres, polymer type and fibre/polymer volume fraction. One of the main advantages of fibre reinforced materials is the ability to align the strong, stiff fibres with the main loads and thereby use the material to its maximum advantage. There are situations, however, particularly in relation to emerging technologies e.g. ocean energy, where (i) the loads are not very well understood, or (ii) the loads are complex and multi-directional in nature, for which a class of fibre-reinforced laminates called quasi-isotropic (QI) can be used. The typical QI layup has equal numbers of fibres at 0, 45, 90, and 135 degrees although many other configurations are possible. Often QI laminates are used for an entire structure or they can be implemented locally on other laminate types by addin...
Abstract:A multiaxial fatigue damage model for fibre reinforced polymer composite materials is presented. 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. A programme of uniaxial fatigue tests on quasi-isotropic glass fibre epoxy laminates is presented for validation of the novel fatigue damage methodology. The latter is successfully validated across a range of stress levels. IntroductionGlass-fibre reinforced polymers (GFRP) are candidate low cost materials for use in ocean energy structures. Quasi-isotropic (QI) laminates are useful where (i) the loads are not very well understood, or (ii) the loads are complex and multi-directional in nature, both of which are relevant to the ocean energy (e.g. tidal turbine) application, which is a novel application. It is anticipated that long-term durability of materials will be a key factor in the success of candidate ocean energy devices. Hence, the fatigue of QI laminates is investigated as here part of a larger research programme investigating the fatigue behaviour of GFRP laminates while immersed in seawater. Micromechanical approaches to fatigue modelling are preferred because they offer the possibility of being able to model any laminate configuration with any combination of applied loads. A convenient level for modelling is the single unidirectional (UD) ply level as these models can then be combined in a computer simulation of any laminate configuration. Talreja described the fatigue damage mechanisms in a UD ply based on strain levels and subsequently when it is embedded in a laminate [1]. It was found that matrix and interfacial cracking occurred first parallel to the fibres, in the most off-axis ply, caused by a combination of transverse normal and shear loading on the ply. In epoxy/E-glass cracking started with localised transverse fibre debonding at approximately 0.12% strain in 90° plies and became full width (of test coupon) and of ply thickness by 0.42% strain [2] (termed inter fibre fracture (IFF) [3] ). Fatigue cycling of an epoxy/E-glass laminate between 0 and 0.33% strain was found to initiate IFF in the 90° ply at the coupon edges around 3,500 cycles and these fractures propagated in both length and density at a steady rate [4]. Higher peak (cyclic) strains were found to increase the rate and maximum density of IFF in 90° plies. Analysis of IFF initiation strain in plies at angles other than 90° is facilitated using composite laminate theory (CLT) to calculate the strains in the ply orientation. For example, Talreja
Tidal turbine blades are subject to harsh loading and environmental conditions, including large thrust and torsional loadings, relative to wind turbine blades, due to the high density of seawater, among other factors. The complex combination of these loadings, as well as water ingress and associated composite laminate saturation, have significant implications for blade design, affecting overall device design, stability, scalability, energy production and cost-effectiveness. This study investigates the effect of seawater ingress on composite material properties, and the associated design and life expectancy of tidal turbine blades in operating conditions. The fatigue properties of dry and water-saturated glass fibre reinforced laminates are experimentally evaluated and incorporated into tidal blade design. The fatigue lives of pitch-and stall-regulated tidal turbine blades are found to be altered by seawater immersion. Water-saturation is shown to reduce blade life about 3 years for stall-regulated blades and by about 1 to 2 years for pitch-regulated blades. The effect of water ingress can be compensated by increased laminate thickness. The tidal turbine blade design methodology presented here can be used for evaluation of blade life expectancy and tidal device energy production.
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