Micro-mechanistic analysis of failure processes in carbon fibre-reinforced polymer composites using synchrotron imaging and digital volume correlation

Abstract

Carbon fibre-reinforced polymer (CFRP) composites have gained significant attention for many lightweight structural applications due to their combination of high stiffness, high strength, and low density. However, predicting failure in CFRPs remains a significant engineering challenge, complicated by their heterogeneous, multiscale, and intrinsically brittle nature. This challenge becomes even more pronounced in multidirectional composites, where microscale damage initiating at the fibre, matrix, and interface evolve through complex, multi-stage process, ultimately making the material susceptible to catastrophic failure. It is well recognised that the final tensile failure of multidirectional composite coincides with the failure of fibres oriented in the loading direction. This underscores the importance of understanding longitudinal tensile failure in 0-degree fibre plies of unidirectional (UD) composites.

Accordingly, it is essential to understand how fibre breaks initiate and evolve into clusters in the 0-degree plies, ultimately leading to final failure. A key mechanism determining this fibre break development is the load redistribution around fibre breaks in 0-degree plies, a process that has historically been understood primarily through model predictions. Advances in X-ray computed tomography, particularly synchrotron radiation computed tomography, have substantially expanded the ability to characterise load redistribution and to investigate fibre break development by enabling in situ 3D imaging of bulk composites at a resolution sufficient to distinguish individual fibre breaks.

Building on these advances, this thesis leveraged high-resolution in situ synchrotron imaging to advance the understanding of load redistribution around fibre breaks and the associated fibre break development process in UD CFRPs under tensile loading. Two state-of-the-art synchrotron imaging methods with different voxel sizes, 650 nm propagation-based tomography and 150 nm holotomography, were utilised. Each imaging methods, in combination with Digital Volume Correlation (DVC), played a distinct and complementary role in achieving the three main research objectives of this thesis: (i) Understanding how local microstructural variations interact with fibre breaks and drive their development into clusters, together with the accompanying microscale damage, (ii) Characterising load redistribution around single fibre breaks under different interfacial shear strengths using DVC, and (iii) Assessing the reliability of 650 nm-based DVC measurements via DVC benchmarking with 150 nm holotomography.

The key contributions of this thesis emerged through the achievement of these objectives. First, this study represents the first application of 150 nm holotomography for in situ monitoring of fibre break initiation and their progressive development into clusters driven by fibre misalignment and resin-rich pockets, while also enabling the clear visualisation of interfacial debonds and matrix microcracks that were not typically resolved using synchrotron imaging techniques commonly used in composite research. Second, the 650-nm DVC measurements provided a reliable experimental basis of strain recovery length (i.e. the characteristic length over which broken fibre fully recovers stress), which can help advance longitudinal tensile strength models. Their physical plausibility was further corroborated by observations from 150 nm holotomography, as well as predictions from analytical and numerical models. Third, the cross-resolution benchmarking strengthened the validity of the 650-nm DVC measurements by leveraging the higher spatial resolution available in the 150 nm scans. More importantly, it provided on the key areas in which DVC approaches for mapping strains around fine-scale damage, such as fibre breaks, require improvement to obtain strain fields that more closely reflect the underlying physical behaviour. Taken together, these contributions are closely interconnected within the overarching research objective of this thesis, deepening the mechanistic understanding of load redistribution and the fibre break development process.

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Identifiers

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

ORCID for Meisam Jalalvand: orcid.org/0000-0003-4691-6252

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