Micro-mechanical contributions to interlaminar toughness in particle-toughened CFRPs

Abstract

The main objective of this thesis was to increase the understanding of interlaminar toughening mechanisms in particle-toughened interlayers within carbon/epoxy laminates. High-resolution Synchrotron Radiation Computed Tomography (SRCT)and Synchrotron Radiation Computed Laminography (SRCL) allowed the crack tip micro-mechanisms to be observed in situ under Mode I and Mode II quasi-static loading conditions. Fracture toughness tests were undertaken to establish the ranking of the ten different material systems, which were compared in terms of the micro-mechanisms observed. Two different intermediate modulus fibres were investigated, with combinations of three different particle types dispersed within the interlayers.

The work showed that interlaminar failure in the materials involves a complex process zone, rather than a singular crack tip. Three distinct crack wake bridging mechanisms were identified, namely; fibre-bridging, epoxy-bridging, and particle-bridging ligaments. It was determined that an interlaminar crack path provided a high Mode I and Mode II fracture toughness. The ligament-rich and tortuous crack path appeared to provide higher energy dissipation than the comparably smooth intralaminar failure at the ply interface and associated
fibre-bridging mechanisms. Quantitatively, the work showed that a larger number of bridging ligaments in particle containing interlayers correlated to a higher Mode I fracture toughness. Provided that the particles in question could maintain an interlaminar crack path, the particle size and type had a less significant effect on the Mode II toughness.

Digital Volume Correlation (DVC) was employed to quantify interlayer strains head of a Mode I crack, showing that the toughening particles can be used as effective markers to enable displacement tracking. A finite element (FE) model was used to explore key variables that were identified experimentally to have an effect
on the crack path. The fibre interface strength, particle cohesive strength, density and distribution were shown to affect crack paths. The results implied that the role of the particles is to alleviate the stresses at the ply interface by de-bonding or fracturing internally, following which additional toughness may be generated via the formation of bridging ligaments as failure occurs within the interlayer. Overall, the work is intended to support material development and lead to better predictive capabilities for these materials that are increasingly used in primary aerospace structures.

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Identifiers

ORCID for Mark Spearing: orcid.org/0000-0002-3059-2014

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