Computional Programing Method For Rotor Blade Buckling Strength Determination

The dimensions of large wind turbines under development still increase, which is partly driven by the offshore- requirements of a ’high energy capture per turbine unit’. The relatively large amount of structural mass, and the dominating weight loads associated with increasing dimensions result in a trend towards material-efficient blade design. The consequence of this trend is that the blade becomes a thin-walled structure that is sensitive to ’buckling’; the geometric instability of the blade cross-section. For the design of large size rotor blades it becomes necessary to verify the resistance against buckling, also called ’buckling strength’. The verification of the buckling strength can be done with non-linear finite element packages, FEM. These FEM packages however, require a lot of structural detail as input which is not always available. For this reason and also because ’design towards buckling’ requires many (fast) analyses for structural variations, one may use simpler more dedicated tools, in which the blade is represented with sectional models. For each of the sectional models it is assumed that it is part of a long prismatic structure, which is reasonable for the part of the blade outside of the largest chord. In the past ECN has been involved in European and Dutch research projects on the development and verification of buckling-load prediction tools. A description is given of the so-called ’Design rules’ that require little computational effort. These ’Design rules’ are addressed to buckling of curved composite panels (such as in a rotor blade structure) and can therefore be applied in the pre- design stage of rotor blades and of other slender composite structures. Various codes are used as buckling tools for improved conditions and implemented in the rotor-blade design. The improvements are mainly addressed to the modelling of buckling of sandwich layup, and the inclusion of shear loading such that the buckling of shear webs it predicted more realistic. The final aim of these developments is to bring simple and fast buckling load prediction tools in the design process. This must allow optimisation of the number of shear webs, and the location of each shear web. These tools also help the designer to find the layer-stacking sequence with the strongest buckling strength, and the (minimum) required stiffness and thickness of the core of sandwich panels.