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The University of Southampton

Research project: 3D Imaging Of The Tensile Failure Mechanisms Of Carbon Fibre Composites

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Synchrotron radiation computed tomography (SRCT) has been used to analyse and quantify the tensile failure mechanisms in carbon fibre/epoxy composites. Two specimen types are analysed – small, aerospace grade coupons, loaded in situ, and filament wound matchstick samples taken from incrementally loaded hoop-wound and scanned “post mortem”. The effects of fibre, matrix and interfacial properties on the initiation and accumulation of fibre breaks are analysed. Attention is focused on the formation of clusters of broken fibres, as the development of a critical size of interacting fibre breaks is believed to be the strength-defining failure event.

In situ tensile loading rig at the Swiss Light Source TOMCAT beamline.
Tensile loading rig

Carbon fibre/epoxy composites offer superior properties for structural applications, including high specific stiffness and strength, good fatigue behaviour, and excellent corrosion resistance. The use of these materials as replacements for their traditional metal counterparts has increased considerably over the last couple of decades. However, the optimal design of a composite structure is often not achieved, due to uncertainties surrounding the prediction of failure.

Resultant SRCT scan of a cross-ply carbon fibre/epoxy composite showing damage accumulation
SRCT scan showing damage

The tensile strength of continuous fibre FRPS is controlled by the fibre properties, and more specifically the stress transfer mechanisms surrounding broken fibres. The failure process of a composite undergoing tensile failure has two distinguishing features (1) multiple failure sites, distributed through the structure and (2) a sequential progression of failure as a function of increasing load or time duration under applied load. Failure occurs due to the accumulation and interaction of individual fibre breaks, which develop as the material is loaded until a group of interacting breaks reaches a critical size and the specimen fails catastrophically. The strength of a specific structure ultimately depends on the chance dispersion or clustering of failure sites. The understanding of this damage initiation and propagation in composites, and the role of the material parameters in the transition from subcritical to critical damage is vital in material development and design applications.

Associated research themes

Composite materials – materials and surface engineering

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