Supervisorer: Bent F. Sørensen, DTU Vindenergi - Casper Kildegaard, LM Wind Power - Christian Berggren, DTU Mekanik - Rasmus C. Østergaard, LM Wind Power
Eksaminatorer: Lars Pilgaard Mikkelsen, DTU Vindenergi - Henrik Myhre Jensen, Aarhus Universitet, Alberto Barroso Caro, University of Seville
Titel: Adhesive Joints in Wind Turbine Blades
The industrial goal of this PhD project is to enable manufacturing of larger wind turbine blades by improving the existing design methods for adhesive bonded joints. This should lead to improvement of the current joint design such that more efficient wind turbine blades can be produced. The main scientific goal for the project is to develop new- and to improve the existing design rules for adhesive joints in wind turbine blades. The first scientific studies of adhesive joints were based on stress analysis, which requires that the bond-line is free of defects, but this is rarely the case for a wind turbine blade. Instead a linear-elastic fracture mechanics based approach is applied since it is appropriate to assume that a crack can initiate and propagate from a pre-existing defect.
The project was divided into three sub projects. In the first sub project an experimental approach was applied to test transverse cracks in the adhesive that evolve due to a combination of mechanical- and residual stresses. A new experimental approach was developed to measure residual stresses in the adhesive in several different ways. In the second sub project, tunneling cracks in adhesive joints were analyzed numerically and experimentally. Simulations with a new tri-layer finite element model showed that the energy release rate of the tunneling crack could be reduced by embedding a socalled ‘’buffer-layer’’ with a well-chosen stiffness and thickness. A new approach was in combination with a generic tunneling crack tool used to predict the cyclic crack growth rate for a tunneling crack in the adhesive joint of a wind turbine blade. The model predictions were in agreement with measurements from a full scale blade fatigue test. In the third sub project, crack deflection at interfaces was investigated experimentally. In order to design the experimental test, new models of the four-point single-edge-notch-beam specimen were developed to ensure stable crack growth and thus enable that crack deflection could be observed during loading. Based on the experimental tests, a novel approach was developed to determine the mode-I cohesive strength of a bi-material interface.
The results and new approaches developed through this PhD project can be used to improve the current design methods and design rules for adhesive joints in wind turbine blades.