Investigation Of Failure Mechanisms In A Wind Turbine Blade Root Sub-Structure

Typically, high safety factors and large quantities of material are applied in the root of wind turbine blades because of the high loads and the complex interaction between steel bushings, pre-cured elements, and large changes in thickness and stiffness. The amount of material in the root can be reduced if more knowledge is obtained regarding the strength and failure mechanisms in the root due to these interactions. This cannot be tested through coupon testing, and it is too expensive to perform full scale tests. However, sub-structure tests if performed correctly, can provide valuable and realistic results at the fraction of the cost of a full-scale test. Therefore, this work focuses on testing of sub-structures from the root end of wind turbine blades at the transition from the thick root laminate to the thinner main laminate.
Some wind turbine blade manufacturers include pre-cured tapered beams in the root to reduce the time required to place the large quantity of material in the mould and to decrease manufacturing defects in these elements. However, this entails the risk of introducing other manufacturing defects during the Vacuum Assisted Resin Transfer Moulding process such as resin pockets and fibre wrinkles. Through this work it is sought to determine the effect that these manufacturing defects can have on the strength properties of the sub-structure. The sub-structures used in this work are cut out from actual wind turbine blades, meaning that the manufacturing defects in this work are not artificially made, but a consequence of the actual manufacturing processes.
A special purpose loading fixture and grips have been developed to apply realistic loads, comprised of a normal and transverse load and a moment. Furthermore, the loading fixture ensures that the sub-structure does not fail in the tab area before the desired failure mode occurs. The failure of interest is delamination at the transition area from the pre-cured element to the main laminate. Therefore, the strength of the sub-structure is quantified based on the load at delamination onset between the pre-cured element and the rest of the sub-structure. The point in time and space of the delamination onset is determined based on data from DIC measurements and high speed camera images.
The experimental tests are compared to numerical simulations where parametric studies are performed to create a framework in which the strength of the bonding between the pre-cured beam and the rest of the blade can be determined based on the severity of different manufacturing defects such as resin pockets and wrinkles.
This work will provide better understanding of the failure mechanisms in the root of wind turbine blades, and hereby enable better predictions of the strength of the root. This can in turn decrease the required amount of material in the root and reduce the manufacturing cost of future wind turbine blades.