Development of translaminar fracture toughness testing methods for composite materials

Abstract

The work presented within this thesis concerns measurement of the fracture toughness associated with the translaminar, fibre-breaking, failure modes of composites. Loading cases of mode I tension, mode I compression and mixed mode I/II tension and shear are considered. Fracture toughness measurement for translaminar tensile failure is investigated using the compact tension specimen. A detailed analysis of data reduction schemes concludes that a modified compliance calibration technique is the most appropriate, in terms of reproducibility of results and simplicity. Investigation of specimen in-plane size, thickness and lay-up effects indicates a thickness dependence. Specimen fracture surfaces reveal that the increase in measured toughness for specimens with thicker 0° plies is due to an increased amount of fibre pull-out. Furthermore, it is found that true measurements of fracture toughness are obtained from specimens with initial notch radii less than 250 μm, for the IM7/8552 material system. Fractography reveals that the critical notch root radius is dictated by the 0° plies alone. A four-point bend specimen is used to measure the fracture toughness associated with translaminar compressive failure. This method enables the failure mode to be triggered in isolation, making it superior to other methods in the literature. Microscopy of failed specimens reveals a shear driven fibre failure at 45° to the initial notch; the cause of this failure mode is investigated using a micromechanical finite element model. Mixed-mode translaminar tensile/shear fracture is investigated using a modified compact tension configuration and a specially developed fixture. SEM of the fracture surfaces of failed specimens reveals damage mechanisms specifically caused by the introduction of a mode II component of loading which accompany a significant increase in the damage zone size. R-curves are generated which suggest that the specimen configuration used here is appropriate for characterising fracture toughness at low proportions of mode II.