The present thesis is devoted to numerical modelling of thermomechanical phenomena occurring during curing in the manufacture of large fibre reinforced polymer matrix composites with thick laminate sections using vacuum assisted resin transfer moulding (VARTM). The main application of interest in this work is modelling manufacturing induced shape distortions and residual stresses in commercial wind turbine composite blades. Key mechanisms known to contribute to shape distortions and residual stress build-up are reviewed and the underlying theories used to model these mechanisms are presented. The main mechanisms of thermal-, chemical- and mechanical origin are; (i) the thermal expansion mismatch of the constitutive composite materials, layer and tooling, (ii) chemical cure shrinkage of the composite matrix material and (iii) the tooling (i.e. the mould, inserts etc.) influence on the composite part. In the modelling approach taken in the current study, 1D and 3D thermomechanical models are utilized. A 1D thermomechanical model in a finite difference (FD) framework, capable of predicting heat transfer, internal heat generation, cure degree development, as well as process induced in-plane strains and residual stresses is initially presented. This 1D model is the framework for the first attempt at a void growth model, capable of predicting the laminate through-thickness discretized void size distribution, as a function of processing parameters. Using a 3D thermomechanical finite element (FE) model in ABAQUS, different constitutive modelling approaches are investigated, including a cure hardening instantaneous linear elastic (CHILE) approach, a viscoelastic approach and a path-dependent approach. The latter is a limiting case of viscoelasticity. These approaches are investigated with regards to their accuracy in predicting process induced strain and stress development in thick section laminates during curing, and more precisely regarding the evolution of the composite thermoset polymer matrix mechanical behaviour during the phase transitions experienced during curing. The different constitutive approaches are utilized in various case studies and compared, where possible, to experimental results from measured in situ internal total strains in laminates using embedded fibre Bragg grating (FBG) sensors. Due to reasonable model accuracy, ease of implementation and use of relatively simply obtained material characterization data, the CHILE and path-dependent approaches are found to be most favorable. It is shown that use of the viscoelastic approach to accurately predict process induced strains and stresses in modelling manufacturing cases where mild tooling constraints on the composite part exist, is not viable. In a final case study, process induced shape distortions in a commercial wind turbine blade root subsection, courtesy of LM Wind Power A/S, are analyzed using the CHILE constitutive approach. It is shown how large non-uniform through-thickness part thermal- and corresponding cure gradients are the main driving factors for process induced shape distortions.
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Nielsen, Per Hørlyk, Hattel, Jesper Henri, Løgstrup Andersen, Tom, Branner, Kim