Improved design basis for large wind turbine blades, phase 3

The major results concerning compressive strength of composite panels are: - The establishment of buckling maps that show the border between local and global buckling modes. A good agreement was obtained for the numerical predictions and the observed behaviour of the panel tests are generally found; - From the experiments it is found that large and deep delaminations caused local buckling and instant failure. The major findings concerning materials joints between dissimilar materials are: - an equation was derived for the selection of the optimal layer thickness ratio of bimaterial specimens, so that the curvature of bi material specimens can be maximised and the misfit stress can be determined with the greatest accuracy; - fracture mechanics characterisation of bimaterial interfaces (by DCB-UBM) bimaterial specimens was successfully achieved. Both the J value at the initiation of cracking and the J value during steady-state cracking were found to be almost constant for mode mixities in the range of 0 to 50 and increased with higher values of the mode mixity angle reaching a value about three times higher; - symmetric bimaterial medium size specimens were manufactured and tested in uniaxial tension. The propagation of cracks along the material interface (mixed mode interface cracking) was monitored, included the applied stress at which interface cracking initiated; - an analytical model was developed for the prediction of the stress level for the initiation of interface cracking in the bimaterial medium size specimens subjected to uniaxial tension. The predicted stress level, based on fracture data from the DCB-UBM tests at an assumed mode mixity of 52, was compared with the stress values found experimentally. The stress level found experimentally was found to be higher than the predicted. The major results concerning cohesive laws are: - the automatic creation of cohesive elements between solid shelle element layers for the modelling of delamination of laminates and debonding of structural components. This implementation makes it possible to effectively generate models of large layered composite structures in which imperfections due to debonded regions are present. The major result concerning hierarchical modelling is: - A framework for the damage and failure analysis including numerical preprocessing, solution and post-processing of large three-dimensional laminated composite structures with geometric and material nonlinearities under mixed-mode loading has been established. The project was focused on the development of new design methods for wind turbine blades, so that uncertainties associated with damage and defects can be reduced. The following topics with respect to failure modes were covered: Buckling-driven delamination of load-carrying laminates, cracking along interfaces in material joints, implementation of cohesive laws in finite element programmes and hierarchical finite element models. Methods and major research results of the project are summarised. Some future goals for future research activities are briefly discussed