Abstract
Understanding the fatigue damage mechanisms in composite materials is of great importance in the wind turbine industry because of the very large number of loading cycles rotor blades undergo during their service life. In this paper, the fatigue damage mechanisms of a non-crimp unidirectional (UD) glass fibre reinforced polymer (GFRP) used in wind turbine blades are characterised by time-lapse ex-situ helical X-ray computed tomography (CT) at different stages through its fatigue life. Our observations validate the hypothesis that off-axis cracking in secondary oriented fibre bundles, the so-called backing bundles, are directly related to fibre fractures in the UD bundles. Using helical X-ray CT we are able to follow the fatigue damage evolution in the composite over a length of 20 mm in the UD fibre direction using a voxel size of (2.75 µm)3. A staining approach was used to enhance the detectability of the narrow off-axis matrix and interface cracks, partly closed fibre fractures and thin longitudinal splits. Instead of being evenly distributed, fibre fractures in the UD bundles nucleate and propagate locally where backing bundles cross-over, or where stitching threads cross-over. In addition, UD fibre fractures can also be initiated by the presence of extensive debonding and longitudinal splitting, which were found to develop from debonding of the stitching threads near surface. The splits lower the lateral constraint of the originally closely packed UD fibres, which could potentially make the composite susceptible to compressive loads as well as the environment in service. The results here indicate that further research into the better design of the positioning of stitching threads, and backing fibre cross-over regions is required, as well as new approaches to control the positions of UD fibres.
Highlights
Composite wind turbine rotor blades undergo a very large number of fatigue loading cycles during service [1]
X-ray computed tomography (CT), the overall damage distribution along the composite specimen can be monitored in relation to the composite microstructure at different stages of its material by Jespersen et al [8,14,15], it is expected that fatigue damage is most likely to initiate from regions where the backing bundles cross-over; it is important to confirm whether this is generally true for this material system or whether there are other factors that might localise fatigue damage
Owing to the extended length that can be viewed at high resolution by helical X-ray CT, the overall damage distribution along the composite specimen can be monitored in relation to the composite microstructure at different stages of its fatigue life
Summary
Composite wind turbine rotor blades undergo a very large number of fatigue loading cycles during service [1]. As a result, their fatigue performance is a major design factor as high cycle fatigue (HCF) damage can result in unexpected catastrophic failure. It is important to understand damage evolution during fatigue, the key damage mechanisms and the interaction between them for the unidirectional (UD) composites employed in wind turbines. Unlike fatigue crack initiation and propagation in homogeneous materials, various damage mechanisms occur cooperatively in composites under cyclic loading, including fibre fracture, matrix cracking, debonding. Establishing a time evolving 3D map of the complex fatigue damage modes in relation to local microstructure will contribute to the establishment and validation of models of fatigue failure able to better predict the safe life of such composites
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