Experimental and numerical investigations of the cure shrinkage for predicting cure-induced wrinkles aimed for wind turbine blade manufacturing

The increase in the size of wind turbines means that wind turbine blades exceed well over a hundred meters today. This increase in size puts a lot of pressure on the blade manufacturing process. Mitigating flaws in the blade produced is challenging as the blades become larger. Flaws that are not easily attributed to a phenomenon challenge the repeatability of the manufacturing process. Therefore, the mitigation of defects in the manufacturing process is a field of research of high importance as it will be increasingly relevant to ensure uniform and repeatable wind turbine blade production without defects. Wrinkle defects are common in wind turbine blade production and appear during many process stages. Wrinkles of even a couple of centimetres in a blade of more than a hundred meters have a high impact on the structural performance of the blade. The stiffness and fatigue properties of the blade are known to suffer significantly and challenge the turbine blades, which can suffer failure prematurely relative to their planned design life.
This PhD project aims to predict the occurrence of wrinkles specific to the curing process. The process of deformations appears due to variations in expansion caused by thermal gradients, resulting in the undulation of fibres and the creation of a wrinkle, i.e., a cure-induced wrinkle. Investigations into wrinkles are a subject where the cause is not obvious. The research into the different disciplines of the manufacturing process and their attributes is relevant to mapping the causes of wrinkles. The novelty of wrinkles induced in the curing process as a possible cause is highly relevant to map this field of defects. The project demonstrates the concept of a cure-induced wrinkle through a simulation framework. This framework was developed through extensive experimental investigations into the cure behaviour of epoxy resin for wind turbine blade application. This is followed by experimental investigations into the cure shrinkage by neat experimental procedures. The measured cure shrinkage is then used to validate and map the shortcomings of the modelling. The model’s ability to capture the exothermal heat release from the resin ensures a good baseline for predicting shrinkage. The curing shrinkage was then captured precisely by mapping the complex nature of the resin thermal expansion and the chemical shrinkage. The well-validated model is then applied to the study of cure-induced wrinkles. The occurrence of wrinkles was mapped using several parameters: length of the composite laminate, constraining effects, and, most importantly, the influence of pre-cure temperature. The latter is this thesis’s key result, as accurately controlling the temperature during the pre-cure is crucial to limiting wrinkles as a mitigation strategy.
The results of the wrinkle predictions can be used by engineers and researchers across different industries working with composite manufacturing. This project worked within the specific scope of wind turbine blade manufacturing. However, the influence of curing on defects such as wrinkles is not limited to wind turbine blades; experimental procedures and modelling can be applied in other industries.