Investigation of damage in laminated carbon fibre composites using high resolution computed tomography

Abstract

Laminated fibre reinforced polymer matrix composites have been used in design and manufacture for more than 50 years, exploiting desirable material properties such as high specific strength and stiffness, enabling large weight savings to be made on structural components. To take full advantage of this class of materials a comprehensive knowledge of behaviour under different service conditions is required. This thesis illustrates the degree to which this is currently achieved, and describes the motivation and progression of an experimental and theoretical analysis of the static damage growth in carbon fibre reinforced polymers.

Notched carbon fibre-epoxy cross-ply composite samples have been manufactured and loaded in uni-axial tension. Synchrotron radiation computed tomography (SRCT) has been used to characterise in 3-D the initiation and evolution of damage during in situ loading. Characteristic splitting, off-axis matrix cracking, interlaminar cracking and fibre failure within the samples were identified and the interaction of the damage mechanisms during crack growth has been evaluated. Splitting in the plies aligned with the loading direction was studied in greater detail, including measurements of crack opening displacement and shear deformation at crack flanks.

3-D finite element models of splitting have been developed based on the observed damage and specimen microstructure from the SRCT results. Thermal residual stress and mechanical loading conditions were simulated for comparison with the experimental findings. Effects of local microstructural inhomogeneities were also embedded in models of varying complexity to assess the degradation of the results or model predictions due to simplifications or homogenisation. Significant discrepancy was found between the measured experimental data and finite element predictions due to simplifications in the model. Likely candidates for the over-prediction of crack growth include the effects of transverse ply cracks, delaminations and the lack of symmetrical damage formulation. Of particular significance is the confirmation that, via qualitative observations and quantitative data extraction, SRCT has facilitated the first known instance of direct full field comparison of model predictions for composite damage for a practical engineering layup.

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Identifiers

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

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