Microscale Fracture of Composite Materials for Wind Turbine Blades

Karolina Martyniuk

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Due to the increase in wind turbines size it is essential that weight savings due to design changes do not compromise the reliability of the rotor blades. The reliability can be increased by improving design rules and the material models that describe the materials properties. More reliable materials models can be developed if the understanding of the microscale damage- the first stage of material failure- is increased. Therefore it is important to characterize materials’ microstructures and micro-cracks initiation and propagation.The microstructure of fibre reinforced composite materials which are the most extensively used in the rotor blades, has been shown to play an important role on the overall response of the material. The properties of a fibre/matrix interface have been found to have a significant influence on the macroscopic behavior of composites. Therefore, the characterization of the fibre/matrix interface has received a considerable attention in the research. So far, however, most of studies related to the fibre/matrix interface focus on the interface subjected to shear stresses acting parallel to the fibre direction. Thereby, the fibre/matrix interface has been characterized in terms of interfacial shear strength or Mode II interface fracture toughness. However, for the fibres oriented off the principle stress direction, fibre/matrix debonding occurs under mixed mode conditions. The mode mixity of an interfacial crack tip is not considered in many studies. It has been shown, that the fracture parameters of an interface between dissimilar materials depends on mode mixity of the crack tip. Therefore, the fracture parameters of the fibre/matrix interface must be determined in terms of mode mixity. Experimental investigation must be conducted in order to provide reliable parameters for micromechanical models.The present PhD study is concerned with the experimental investigations of the microcracks initiation and propagation in the glass fibre composites with the aim of measuring the input parameters required for micromechanical modelling. A special attention is given to the determination of fracture parameters for the glass fibre/matrix interfacial debonding and the interfacial crack kinking into the matrix by two different approaches, linear elastic fracture mechanics (LEFM) and cohesive zone approach.The fibre/matrix debonding was investigated experimentally in 2D and in 3D in specimens containing a single fibre embedded in matrix subjected to a transverse stress. 2D in situ observations in the scanning electron microscope (SEM) allowed for an early detection of the debonding initiation (at the free surface of the sample), and measurements of the debond angles and normal opening displacements as functions of applied stress. The same fracture tests were carried out at the synchrotron facility where the debonding initiation and propagation were observed in 3D using X-ray tomography. The debonding was found to initiate at the free surface of the sample and subsequently it propagated into the sample along the fibre. Once the debond depth exceeded the length of two fibre diameters, the unstable crack growth (tunnelling) occurred.The measurements obtained from testing conducted in the SEM linked with the numerical results available in the literature led to the determination of the mixed mode fracture energy of the glass fibre/matrix interface by LEFM analysis. The fracture energy for nominal Mode I was found to be ~0.2 J/m2 and ~0.4 J/m2 for the interfacial crack arrest and crack propagation respectively; for nominal Mode II they were found to be ~2 J/m2 and ~3 J/m2 respectively.Numerical modelling of the fibre/matrix debonding was conducted by the cohesive zone approach using augmented finite element method (A-FEM). Model predictions coupled with the experimental measurements from tests carried out in the SEM allowed for the mixed mode cohesive law parameters identification and mixed mode fracture energy determination for the glass fibre/matrix interface. The Mode I cohesive peak stress and critical opening displacement have been found to be in the range of 0.75-5 MPa and <0.2 µm respectively. The Mode I fracture energy of the glass fibre/matrix interface was estimated to be ~0.1 J/m2. The interface Mode II fracture energy is found to be ~4 times of the fracture energy for Mode I. From the model results it was found, that the interfacial debonding initiation and propagation does not depend only on the fracture energy of the interface but is also sensitive to the cohesive law parameters within the range explored in this study.The fibre/matrix interfacial crack kinking was analysed by coupled experimental (in 2D and in 3D) and numerical approach (in 2D). It was shown by the SEM observations of the free-edge of the sample that the kinking occurs when the debond reaches characteristic angle ~60° with respect to the applied stress direction. 3D observations by X-ray tomography supported that hypothesis showing kinking occurrence only close to the sample free-edge, where the debond angle characteristic for kinking has been reached. 2D numerical simulations of the interfacial crack kinking into the matrix carried out by cohesive zone approach showed that the strength of the matrix affected the position of the crack kinking.
Original languageEnglish
PublisherDepartment of Energy Conversion and Storage, Technical University of Denmark
Number of pages174
Publication statusPublished - 2014

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