Assessment of wrinkles impact on the failure of laminates for wind turbine blades

Wind turbine (WT) blades are susceptible to various defects that may occur during manufacturing or later during operation. Among the manufacturing defects, wrinkles are detrimental to the blade structural performance, particularly under fatigue loading. Wrinkle defects can result from different stages of the manufacturing process, assuming various shapes, arising at different locations of a WT blade. These defects may be confined to thick sections, posing challenges in detection and resulting in unfeasible repair. Detecting and repairing all wrinkle defects within a blade can be a difficult and lengthy process on an industrial production scale. This means that WT manufacturers currently have to adopt very conservative designs. Therefore, evaluating the severity of different types of wrinkles in a feasibility analysis is necessary to make wind energy even more cost competitive. One potential solution is to apply verified numerical tools able to predict the behavior of various configurations of wrinkle defects under fatigue loading to assess whether the defect can remain within the blade structure during operation or whether the repair is necessary. In the present thesis, a methodology is developed for selecting and representing a physical wrinkle defect and embedding it into a numerical modeling framework along with experimental testing protocols to investigate the impact of wrinkles on the failure of a WT blade under fatigue loading conditions.
The methodology elaboration was developed around evaluating potential wrinkle defects characteristic of a wind turbine blade at different severity types. The artificial defects are achieved through a method that emulates one of the steps in the manufacturing process of a real blade which is identified as a potential source of wrinkles. Following this, a modeling framework has been designed to translate the true defect into a finite element model (FEM), encompassing its real geometrical features. Wrinkle defects can present different failure mechanisms under fatigue loading, i.e., delamination, fiber kinking, and matrix cracking. Post-mortem analysis of WT blades with wrinkle defects often displays delamination as a recurring failure mechanism and is therefore considered the primary failure mechanism in this thesis. A Paris-type fatigue cohesive formulation is applied within the modeling framework represented as a material interface in the FEM. A two-step model links the coupon specimen scale to support the modeling at the blade scale. Experimental fatigue testing protocols are elaborated to verify the implemented numerical framework and assess the validity of the underlying assumptions. The investigation indicates that wrinkles with the same configuration but different severity types exhibit significant differences in fatigue life when subjected to equivalent strain amplitude. The methodology and modeling framework has demonstrated their validity in predicting the fatigue fracture pattern for the defect under investigation.