Applications of advanced imaging and simulation methods in analysing delamination fracture in fibre reinforced composite materials.

Abstract

The Mode I interlaminar fracture toughness plays a key role in improving the impact resistance of carbon fibre reinforced polymer composites due to the contribution of damage response mechanisms determined in part by the relative competition between interlaminar and intralaminar crack paths. In turn, the resultant damage tolerance and resistance after impact can be attributed to the ability of the composite to suppress the initiation and propagation of delamination damage under the impact site. Keeping a crack in the toughened interlayer of a composite can reduce the extent off impact damage and improve the residual strength properties. However, the mechanisms that determine whether a crack stays in the toughened interlayer or transitions to weaker interface regions are uncertain. This study focuses on the damage response mechanisms that contribute to the interlaminar fracture toughness of particle-toughened carbon fibre reinforced polymer composites at the meso to macroscale. The double cantilever beam experimental test and complementary imaging methods are used to quantify the toughness response of different material systems and map the underlying mechanisms which may be contributing to the change in toughness response. Computational modelling is used to investigate the relationship between mesoscale interlayer geometry variation, relative strength values, critical strain energy release rate, and stochastic cohesive damage parameter variability. The results demonstrate that interlayer geometry is a factor in determining the crack path and the toughness response during Mode I loading. The relative competition between crack initiation mechanisms in potential crack paths is another key factor that determines interlaminar to intralaminar crack path transition, where a comparatively small difference in adhesive or cohesive interface strength i.e. less than ten percent is enough to facilitate the transition. Additionally, this work shows the three-dimensional characteristic behaviour of crack transition, where the presence of tow gaps; agglomerations of toughening particles or conversely the sparsity of them can lead to local thickening and thinning of the interlayer aligned with the direction of the fibres respectively, which can increase the likelihood of crack path transitions in a plane orthogonal to the dominant crack growth direction. Overall, the work presented in this thesis provides mechanistic insight into how previously identified microscale variations in particle-toughened composites lead to mesoscale architectural variations in interlayer geometry which in turn contribute to the interlaminar to intralaminar crack path transitions. Furthermore, through the work presented in this thesis and previous research, a link between these identified crack path transitions and the resulting toughness response mechanisms has been postulated and used to provide mechanistic theories as to the knock-on to the overall R-curve response under carefully defined conditions. This work suggests that by increasing the uniformity and thickness of the interlayer, the crack transition can be mitigated, improving the predictability of the crack path, and the system's toughness response.

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Identifiers

ORCID for Keiran Ball: orcid.org/0000-0003-2628-0891

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

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