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An investigation into the mixed mode delamination behaviour of brittle composite laminates Kanji, Karim Mohamed Jamal

Abstract

Delamination is a prevalent composite laminate failure mode. It is of particular concern to the aerospace industry where laminated composites have found widespread usage in critical applications. Delamination growth has been widely studied, with Linear Elastic Fracture Mechanics (LEFM) being the most common approach taken to predict delamination behaviour, typically through global parameters such as specimen geometry and applied load measured away from the actual crack tip. However, internal mechanisms, such as fibre bridging, can occur within the structure thus affecting the transfer of the globally applied conditions to the actual crack tip. Thus, measurements made at the actual crack tip, referred to as local measurements, can be compared to the delamination behaviour predicted from the global analysis. The objective of this thesis is to better understand the relationship between the globally predicted and the actual local crack tip behaviour under mixed mode loading. Of particular interest is the mixed mode region at low shear (mode, II) loads where the global tensile opening (mode I) loads exceed the pure mode I fracture toughness of the material. An experimental loading j ig developed by Paris et al. (2001) is used in this work. The j ig can be used inside a scanning electron microscope (SEM) to allow for simultaneous local and global analysis of a specimen. Previous pure mode experiments (Paris, 1998) showed that in the absence of fibre bridging mode I global predictions match the actual local crack tip conditions. Global mode II loads, however, induce a local mixed mode condition where the mode I component is created due to the surface roughness as the crack surfaces slide over each other. In this work, mixed mode experiments on unidirectional AS4/3501-6 carbon fibre/epoxy laminates show that global applied shear loads are transferred directly to the crack tip. The crack surface roughness effects seen initially under pure mode II loading also manifest themselves under mixed mode loading resulting in a significantly higher local mode I component than predicted globally. As such, current test practices and global data reduction schemes are inadequate and do not provide a complete picture of delamination behaviour. The total local critical strain energy release rate is also significantly higher than that determined globally and observed in the literature. The higher local failure loads observed are consistent with fractographic evidence in the literature that indicates failure at the mixed mode conditions examined here is similar to much tougher mode II dominated failure.

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