Ultimate Strength of Wind Turbine Blades under Multiaxial Loading

Modern wind turbine rotor blades are sophisticated lightweight structures, optimised towards achieving the best compromise between aerodynamic and structural design as well as a cost efficient manufacturing processes. They are usually designed for a lifetime of minimum 20 years, where they must endure a variety of weather conditions including uncontrollable, extreme winds without developing damage and fracture.
The trend in the development of wind turbines is towards larger, more efficient wind turbines, placed offshore, where access is difficult and repairs costly. In consequence, failures in the rotor blade usually lead to long downtimes. Therefore, it is of great importance that the turbines operate reliably and that robust methods are available to predict damage initiation and growth under multiaxial loading conditions. The purpose of this PhD project is the investigation of multiaxial loading effects and its influence on the ultimate strength of typical wind turbine rotor blade structures and to develop methods to perform reliable prediction of failure. For this purpose, origin and consequence of some of the typically occurring failure types in wind turbine rotor blades are investigated. The research aims on predicting more accurately when and how blades fail under complex loading. The main contribution from this PhD study towards more reliable and robust operating wind turbine systems can be divided into two fields. One part covers numerical modelling approaches and the other part deals with failure origin and effects. The research, covering the numerical part, is done with the purpose to investigate the limitation of state-of-the-art numerical prediction methods and to improve existing simulation methods by combining different existing techniques, capable to predict the ultimate strength of wind turbine rotor blades under multiaxial loadings. Failure origin and effects are studied numerically and experimentally with the purpose to investigate root causes of blade failure and to find generalities for their origin. The main contributions from this PhD study covering the numerical part are the demonstration of a subset simulation approach for large scale delamination in the cap of a wind turbine rotor blade, making it possible to determine more precisely critical delamination sizes and load levels for delamination growth onset and propagation in dependency of the through the thickness location. Another modelling approach shows a modelling strategy, where shell and solid elements where combined with the purpose to estimate the strain energy release rate of transversely orientated crack in the trailing edge for different loading conditions. Furthermore, state-of-the-art failure criteria are studied and their limitations demonstrated by comparing numerical and experimental results of a full scale blade loaded to ultimate failure. The main contributions from this PhD thesis dealing with failure origin and effects are the determination of generalities of failure. For buckling driven delaminations, delamination onset and propagation could be determined. For trailing-edge failure, a characterisation of effects of geometrical non-linear cross section deformation and trailing-edge wave formation on the energy release rate was shown. Furthermore, a sequence of trailing edge buckling leading to sandwich failure and finally causing ultimate blade failure were demonstrated.