In-situ measurements of delamination crack tip behaviour in composite laminates inside a scanning electron microscope
Creators: name: Paris, Isabelle
Publisher: name: University of British Columbia
Year: 2009

description: Delamination is an important failure mode for composite laminates. As it affects the mechanical response of the structure and is difficult to detect from the surface, this type of damage is of great concern, particularly in the aerospace industry. The topic of delamination growth has received much attention, with Linear Elastic Fracture Mechanics (LEFM) being the most common approach to predict the behaviour of a crack from the global applied conditions. However, local perturbations such as resin rich regions, fibre bridging and friction have been noticed by many investigators. Thus global applied loads are often not transposed directly into equivalent local crack tip conditions. Moreover, there is currently considerable controversy about the exact nature of mixed-mode fracture behaviour. Therefore, the objective of this thesis is to measure the load and displacement applied to a specimen and, at the same time, the crack tip behaviour, in order to establish a quantitative relation between them. An experimental loading jig designed to fit inside a scanning electron microscope (SEM) has been developed. Mode I, mode TJ and mixed-mode loadings can be applied. The applied loads and displacements are measured and the images obtained from the SEM are stored. After the test, the crack opening and shear displacements are calculated from the applied loads and displacements using LEFM, and are compared with those measured from the images. Mode I and mode II tests have been conducted that show good agreement between LEFM predictions and measurements. For a brittle material, the behaviour remains linear elastic up to failure. The comparison of the crack faces displacements with the ones obtained from a finite element analysis are also excellent. The effect of the increase in fracture toughness with mode I crack growth on the local crack tip behaviour has been studied. As the crack grows, the magnitude of the measured crack opening displacement profiles is reduced. The assumption that fibre bridging keeps the crack closed is thus confirmed experimentally and quantitatively. As reported by other investigators, 45° microcracks are created ahead of the crack tip under mode II loading. When the load is increased, the ligaments created by the microcracks bend and finally the microcracks coalesce, while more microcracks are created ahead. The growth of this damage zone has been measured and modeled using an analogy with the plastic zone in metals. The stress-displacement curve of the damaged material has also been deduced from the experimental results using a Dugdale approach. One of the most interesting findings of the mode TJ tests is the presence of significant crack opening displacements even though the loading is supposed to induce pure shear. The amount of opening varies with the surface roughness of the crack. This can explain the large scatter observed by many investigators in GIIc data, as the tests are not really pure mode U tests, but in fact mixed-mode tests with various proportions of mode I. The determination of the widely used mode II material toughness is therefore questioned.