Experimental analysis of stresses in curved sandwich structures

Some studies indicate that the cross-section of large wind turbine blades subjected to wind and gravity loads will ovalise due to the Brazier effects [1,2]. This feature could however not be verified by numerical experiments of a FE model of a fictive 61.5 m blade, based on the NREL 5MW reference turbine [3], subjected to simplified load cases. Accurate prediction of failure modes of sandwich structures based on finite element calculations are highly important if accurate predictions of such features in large wind turbine blades are to be investigated. In this study, an experimental setup designed to cause deflection and stress patterns in the core similar to what might be achieved by the Brazier effect in a wind turbine blade subjected to severe wind loads has been carried out. The test set up will highlight stress in the through-thickness direction, causing classical shell theory to be circumspect as a modelling tool. By designing specimens with different foam core material properties, stiffness and strength, different failure modes were observed in the test. Through the use of combinations of solid and shell elements and geometrically nonlinear analyses, the experimental effects were shown to be predictable by an otherwise linear FE model.