Hierarchical FEM of Wind Turbine Blades

 

The subject of the Ph.D. is analysis of large composite structures with respect to prediction of failure by use of the finite element method. The overall aim of the project is to develop a general computational design tool for estimation of failure in composite structures. Such a design tool can be used in the industry to design optimized and more reliable composite structures than possible today.

 

Failure in composite structures is often caused by local damage effects, e.g. delamination. A hierarchical FEM approach is used to model both the global structural response and the local response near load critical regions. The global response is modeled by solid shell elements with in-plane and transverse stress components. Based on stress/strain based initiation criteria, load critical regions are automatically refined through the thickness, and cohesive elements are embedded between the solid-shell element layers in order to simulate delamination.

 

The hierarchical finite element design tool will be implemented in an in-house developed FE software, named MUST. At present, a 3D solid-shell element has been developed together with an adaptive mesh generator by use of the Lagrange multipliers method. Currently, the solid-shell element formulation is expanded to include geometrical non-linearity. Cohesive elements and different cohesive zone models describing initiation and propagation of mixed mode delamination have been implemented in MUST. FE models of crack test specimens will be used to investigate the precision of the cohesive zone method by comparison with experimental data. This work will be done in collaboration with Bent F. Sørensen, Materials Research Department, Risø.

 

A FE model of a wind turbine blade will be used to investigate the influence of imperfections on the strength of the structure. Likewise, the interaction between buckling and material damage will be investigated on this model. Finally, the developed design tool will be evaluated by comparison with experimental results, e.g. from a collapse test of a section of a wind turbine blade.