Christian Berggreen: Compression Loaded Composite Elements in Large Wind Turbine Blades
The presentation will be divided into two parts dealing with:
- Design related issues of compression loaded members in future large wind turbine blades.
- Prediction of defect tolerance and imperfection sensitivity of compression loaded members in turbine blades.
Part 1 will address the advantages of applying sandwich construction as opposed to traditional single skin composites in the flanges of the load-carrying spar in a future 180 m wind turbine rotor. A parametric finite element model is used to analyze two basic designs with single skin and sandwich flanges, respectively. Buckling is by far the governing criterion for the single skin design. Introducing sandwich construction results in a globally more flexible structure making tower clearance the critical criterion. Significant weight reduction (up to 22.3%) and increased buckling capacity is obtained.
In Part 2, tests will be presented which have been performed on square composite plates under in-plane compression. The plates, which are similar to those in the load carrying spar of a turbine blade, had a width-to-thickness ratio close to the value for which the elastic critical load and the load for compressive fibre failure over a complete section would be equal, giving the maximum sensitivity to initial geometric imperfections. Some of the plates were manufactured with no intentional imperfections or defects, others with an intentional initial out-of-plane geometric imperfection. An advanced digital photogrammetry measurement system was used to monitor deformations of the tested plates. The responses were also calculated by means of geometrically non-linear finite element analysis. With the assumption of rotationally fixed edges, the calculated elastic critical loads were significantly higher than those deduced from the measurements. Closer examination revealed that the loaded edges of the plates rotated significantly during the tests. It was found necessary to include in the analysis the observed variation of edge rotation with applied in-plane displacement. Although material non-linearity was not modelled, some conclusions concerning the failure sequence were drawn from the analyses.
Christian Berggreen (MSc., PhD.Eng) is Assistant Professor, Composite Lightweight Structures Group, Department of Mechanical Engineering, Technical University of Denmark