Analysis of a hybrid composite pressure vessel using multi-scale computed tomography techniques

Abstract

In this work multi-scale CT techniques have been developed to characterise the material structure of PMCs, from the whole engineering structure geometry down to individual fibre level. The techniques have first been applied to a 'model' aerospace grade carbon/epoxy notched laminate loaded in-situ in tension to failure. The material structure and damage mechanisms of internally pressurised experimental cylinders have then been investigated and compared to the notched laminate. It was found the damage accumulation of both samples to be comparable, where fibre breaks were the dominant strength controlling mechanism. The data provides, to the authors knowledge, the first direct internal 3D measurement of the accumulation of fibre damage for commercial CFRP materials under structurally relevant load conditions. A high level of confidence is placed in the measurements, as the failure processes are viewed internally at the relevant micromechanical length-scales, as opposed to previous indirect and/or surface-based methods. Whilst fibre breaks are the dominant composite damage mechanism considered in the work, matrix damage was also seen to occur in advance of extensive fibre breaks. The formation of clusters of broken fibres were observed at high loads in both sample types. The largest clusters were observed in the notched laminate sample, consisting of a group of eleven breaks and a group of fourteen breaks. In comparison, clusters of only four neighbouring breaks were observed in the pressure vessel samples. A correlation between fibre volume fraction and fibre breaks was found, in which higher fibre volume fractions result in higher fibre break densities. No strong correlation was found between the location of matrix damage and fibre breaks in both sample types. Initial analysis showed some correlation between fibre breaks and voids, however further work has been recommended to confirm this. A simple 3D Finite Element Analysis was carried out for the pressure vessel to give an understanding of the stress partitioning through the composite layers and confirm the damage found experimentally. The data sets of the accumulation of fibre breaks with load provide evidence to validate or inform existing micromechanical models, for two different carbon fibre systems, where previous experimental findings are limited. A detailed comparison of the results of this work and the multi-scale micromechanical model of Blassiau and co-workers has been made, in which the underlying assumptions have been discussed

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Identifiers

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

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