Modelling the Pultrusion Process of Off Shore Wind Turbine Blades

This thesis is devoted to the numerical modelling of the pultrusion process for industrial products such as wind turbine blades and structural profiles. The main focus is on the thermo-chemical and mechanical analyses of the process in which the process induced stresses and shape distortions together with the thermal and cure developments are addressed.

A detailed survey on pultrusion is presented including numerical and experimental studies available in the literature since the 1980s. Keeping the multi-physics and large amount of variables involved in the pultrusion process in mind, a satisfactory experimental analysis for the production requires considerable time which is obviously not a cost-efficient approach.

Therefore, the development of suitable computational models is highly desired in order to analyse the process for different composite manufacturing aspects such as heat transfer, curing and solid mechanics. In order to have a better understanding of the processing polymer behaviour in pultrusion, the chemo-rheology of an industrial “orthophthalic” polyester resin system specifically prepared for a pultrusion process has been characterized.

The curing behaviour is first characterized using differential scanning calorimetry (DSC). Isothermal and dynamic scans are performed to develop a cure kinetics model which accurately predicts the cure rate evolutions. The resin viscosity and the gelation point are subsequently obtained from rheological experiments using a rheometer.

Based on this, a resin viscosity model as a function of temperature and degree of cure is developed and it is found to predict the measured viscosity correctly. The temperature- and cure-dependent elastic modulus of the resin is determined using a dynamic mechanical analyzer (DMA) in tension mode. A cure hardening and thermal softening model is developed and a least squares non-linear regression analysis is performed.

The predicted best fit results are found to agree quite well with the measured data. The temperature and degree of cure distributions inside the processing material have been calculated using the developed thermo-chemical numerical process models and subsequently used in the mechanical analysis of the pultrusion.

The effects of the thermal contact resistance (TCR) at the die-part interface of a pultruded part are investigated using a two dimensional (2D) thermo-chemical model. It is found that the use of a variable TCR is more convenient than the use of a constant TCR for the simulation of the process. The 3D thermo-chemical modelling strategies of a thermosetting pultrusion process are investigated considering both transient and steady state approaches.

So far in the literature, the pultrusion process of a relatively thick composite having a curved cross sectional geometry such as the NACA0018 blade profile has not been modelled numerically. Hence, a numerical simulation tool embracing the blade manufacturing process has been developed in this thesis.

The effects of the heater configuration and pulling speed on the pultruded blade profile have been addressed by means of the devised numerical simulation tool. In addition to the efficient thermo-chemical models developed in this thesis, stateof-the-art mechanical models have also been developed by the author to predict the process induced stresses and shape distortions in the pultrusion process.

Together these models present a thermo-chemical-mechanical model framework for the process which is unprecedented in literature. In this framework, the temperature and degree of cure fields already calculated in the thermo-chemical model are mapped to the quasi-static mechanical model in which the finite element method is employed.

In the mechanical model, the composite part is assumed to advance along the pulling direction meanwhile tracking the corresponding temperature and degree of cure profiles. Modelling the pultrusion process containing both uni-directional (UD) roving and continuous filament mat (CFM) layers has not been considered in the literature up to now.

A numerical simulation tool embracing the thermo-chemical and mechanical aspects of the pultrusion for industrial, pultruded products is hence developed in the present work. Various case studies have been carried out using the devised numerical simulation tool. The residual stresses and shape distortions in pultrusion of an industrial rectangular hollow profile and L-shaped product are predicted.

The deformation pattern as well as the corresponding magnitudes are found to agree with the real pultruded profiles. In addition, the internal stresses at the web flange junction of a pultruded I-beam are addressed which includes a more complex layer orientation. The manufacturing aspects of the pultrusion process such as the residual stresses and distortions are combined with the subsequent service loading scenario for a pultruded wind turbine blade profile (NACA0018).

The effects of the residual stresses on the internal stress level after the loading analysis are investigated. A pulling force model has specifically been analysed including gelation effects and the shrinkage induced effects. The compaction, viscous and frictional forces have been predicted for a pultruded composite rod.

The viscous drag is found to be the main contribution in terms of the frictional force to the overall pulling force, while the contribution due to material compaction at the inlet is found to be negligible. Process optimization studies have been carried out in order to improve the production rate and the quality.

For this purpose, the mixed integer genetic algorithm (MIGA) is developed to optimize the process by finding the optimum heater configuration. Moreover, a multi-objective optimization problem (MOP) is implemented to the thermo-chemical analysis to minimize the energy consumption and maximize the productivity of the process simultaneously.

Probabilistic analyses are also performed to investigate the effect of uncertainties in the process parameters on the product quality by using Monte Carlo simulations, response surface method and first order reliability method.