Fatigue lifetime assessment of glass fibrereinforced composite wind and tidal turbine blades
Kazemi Vanhari, Afrooz
Kazemi Vanhari, Afrooz
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Publication Date
2023-11-24
Type
Thesis
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Abstract
Wind and tidal turbines have garnered significant interest due to their capability to exploit wind and water currents as sources of renewable energy. These turbines are constituted of various components, amongst which the blades are a crucial element. Blades are instrumental in capturing energy from wind or tidal currents and transforming it into rotational motion for electricity generation. Nonetheless, the constant exposure of turbine blades to harsh environmental conditions presents a substantial challenge regarding durability and performance. Fatigue, resulting from cyclic loading and fluctuations in stress, is a frequent occurrence that compromises the structural integrity of turbine blades over time. Failures due to fatigue can result in operational interruptions, costly repairs, and diminished efficiency in energy production. The overarching objective of this thesis is to formulate enhanced methodologies for estimating the fatigue life of composite structures, paying special attention to wind and tidal turbine blades, whilst minimising dependence on exhaustive experimental testing. Initially, a novel estimation method is presented for fitting fatigue data using the Sendeckyj model, thereby contributing to a more robust S-N curve for all fatigue regions in composites. This new method employs regression techniques; specifically, it utilises an exponential model for the low-cycle fatigue region and a power-law model for the high-cycle fatigue region for parameter estimation. Comparisons between the output parameters of the original Sendeckyj model and the novel estimation method that shapes the S-N curve, utilising various glass fibre laminates and polymer resins, indicate that the novel method furnishes more accurate predictions of composite fatigue behaviour. Subsequently, the refined Sendeckyj model is employed as a foundation for the development of a new strategy, which estimates the residual strength parameter in the Schaff and Davidson model through a mathematical algorithm. This obviates the need for residual strength tests and facilitates the direct computation of composite residual strength using the S-N curve. Validation with multiple datasets confirms the effectiveness of this strategy. Notably, for well-scattered fatigue life data, the estimation aligns almost within one standard deviation from the mean of the measured data. The new strategy is incorporated into an algorithm representing a new strength-based method for estimating the fatigue life of a 13-metre wind turbine blade constructed of glass fibre-reinforced powder epoxy. This algorithm accommodates load sequences, strength conversion (tensioncompression interplay) effects, and employs Rainflow counting, the improved Sendeckyj model, and a piecewise constant life diagram. Upon validation of the finite element (FE) representation of the blade, a comparison with Miner’s rule illustrates that this new method predicts fatigue life earlier in the life of the blade by incorporating material degradation behaviour and other factors, while requiring computational and experimental resources comparable to Miner’s rule. However, the strength-based method exhibits limitations in accounting for interactions among different stress factors. To rectify this, it is amalgamated with a multi-axial stress-based approach, known as the FTPF (Failure Tensor Polynomial in Fatigue) method. This novel approach is applied to an 8-metre tidal turbine blade constructed of similar material and demonstrates the potential of reliably assessing fatigue life under multiple loading phases. Lastly, in tandem with the fatigue analysis, a cavitation analysis is undertaken to evaluate the impact of DNVGL-ST-0164 design load cases on both fatigue and cavitation of the aforementioned tidal turbine blade, without directly coupling their effects. The fatigue analysis concentrates on the unidirectional composite layers located 3 meters from the root, which are identified as an area of considerable fatigue risk through the novel approach. The results suggest that transient startup and shut down processes at cut-out tidal speeds significantly contribute to fatigue damage. However, an increase in the rotational speed of the actual pitch mechanism seems to mitigate this damage. In contrast, the cavitation analysis, executed using AeroDyn software, encompasses the entirety of the blade. This analysis identifies tip cavitation occurrences for operating conditions where tidal speeds exceed the rated speed and in transient conditions at the cut-out tidal speed. Therefore, transient startup and shutdown procedures at cutout tidal speeds are the primary contributors to fatigue and cavitation damage. Consequently, novel design recommendations are suggested to alleviate these damages, inclusive of the elimination of the cut-out tidal speed. In summary, this thesis developed improved methods for estimating the fatigue life of composite structures, specifically focusing on wind and tidal turbine blades, with the goal of reducing experimental test data while reliably predicting the fatigue state of materials. The improved methods proposed in this study are validated using multiple fatigue databases and subsequently applied to real-world examples, namely a 13-meter wind turbine blade and an 8-meter tidal turbine blade.
Publisher
NUI Galway