Influence of Curing Cycle on the Build-up of Residual Stresses and the Effect on the Mechanical Performance of Fibre Composites

Due to environmental and economic factors, over the last decades there has been a progressive shift from fossil fuels to renewable energy sources such as wind and solar. Even though these energy sources are already very competitive, the increasing demand for energy requires a continuous effort for further reduce the energy cost and make them even more competitive. In wind energy, this can be attained by increasing the output power (larger wind rotor blades) and extend the structure lifetime service while reducing the need for maintenance work. The wind rotor blades are one of the most important structural components in wind turbines and are made essentially of fibre reinforced polymer composite materials. Wind blades are constantly subjected to loads which vary in intensity due to wind fluctuations. This eventually leads to development of fatigue damage on the material decreasing its strength and ultimately resulting in failure. The root of fatigue damage can be related with the growth of small cracks within the composite material which can initiate in the matrix and eventually propagate into the fibre reinforcement thus affecting the component´s strength and structural integrity. One of the main causes for crack development is related to the formation of internal stresses that develop during the curing stage. Thus the curing process is a critical step which has ramifications on the fatigue life of the composite material in the wind turbine blade. This work essentially explores the influence of curing cycles on the development of residual stresses and their effect on the mechanical performance of the composite material. The curing reaction of an epoxy resin was characterized and modelled in order to design different curing cycles yielding different magnitudes of residual stresses, either by varying the temperature at which matrix and fibres bond (here denoted as gelation temperature) or by varying the final cure state. The influence of the curing cycles on the residual stresses was experimentally investigated using a customized experimental setup of an unconstrained neat resin where higher curing induced strains measured can be associated with higher residual stresses in composites. The results showed that higher gelation temperatures result in higher residual stresses and that these can also be reduced by not fully cure the material. The same curing cycles were applied in the manufacturing of composites test plates composed of the same epoxy and unidirectional non-crimp glass fibre fabric with the respective residual stresses being estimated from micromechanical models. Mechanical testing showed that the residual stresses have no effect on the static tensile and shear properties of the composite material. On the other hand, fatigue is strongly affected by the magnitude of residual stresses. Tensile fatigue and four point bending fatigue test results showed that lower residual stresses resulting from lower gelation temperatures improves the fatigue life of the composites. However, it was not possible to conclude if reducing the residual stresses by partially curing the material, a better fatigue performance could be obtained. Depending on the fatigue load applied, reducing the residual stresses by 50% increases the number of cycles to failure by a factor of 3 while a reduction by 75% in residual stresses increases the number of cycles to failure by a factor of 10. From the fatigue results a relation established between the number of cycles to failure and residual stresses in the matrix showed that the fatigue load becomes less relevant with increasing residual stresses. In addition, the fatigue life of the material for different loads can be predicted based on a linear dependency found between the S-N curves parameters and the matrix residual stresses. The findings from this research showed that controlling the material temperature during processing is critical to reduce the magnitude of residual stresses. The gelation temperature and the composite service temperature were found to be important key parameters to consider when designing optimal curing cycles in terms of reduced processing times and enhanced mechanical performance, particularly for improvement in fatigue properties. This would allow extending the service life of wind blades and at the same time reducing the need for maintenance work and associated costs.