Modelling of the thermal chemical damage caused to carbon fibre composites

Abstract

Previous investigations relating to lightning strike damage of Carbon Fibre Composites (CFC), have assumed that the energy input from a lightning strike is caused by the resistive (Joule) heating due to the current injection and the thermal heat ux from the plasma channel. Inherent within this statement, is the assumption that CFCs can be regarded as a perfect resistor. The validity of such an assumption has been experimentally investigated within this thesis. This experimental study has concluded that a typical quasi-isotropic CFC panel can be treated as a perfect resistor up to a frequency of at least 10kHz. By considering the frequency components within a lightning strike current impulse, it is evident that the current impulse leads predominately to Joule heating. This thesis has experimentally investigated the damage caused to samples of CFC, due to the different current impulse components, which make up a lightning strike. The results from this experiment have shown that the observed damage on the surface is different for each of the different types of current impulse. Furthermore, the damage caused to each sample indicates that, despite masking only the area of interest, the wandering arc on the surface stills plays an important role in distributing the energy input into the CFC and hence the observed damage. Regardless of the different surface damage caused by the different current impulses, the resultant damage from each component current impulse shows polymer degradation with fracturing and lifting up of the carbon fibres.

This thesis has then attempted to numerically investigate the physical processes which lead to this lightning strike damage. Within the current state of the art knowledge there is no proposed method to numerically represent the lightning strike arc attachment and the subsequent arc wandering. Therefore, as arc wandering plays an important role in causing the observed damage, it is not possible to numerically model the lightning strike damage. An analogous damage mechanism is therefore needed so the lighting strike damage processes can be numerically investigated. This thesis has demonstrated that damage caused by laser ablation, represents a similar set of physical processes, to those which cause the lightning strike current impulse damage, albeit without any additional electrical processes.

Within the numerical model, the CFC is numerically represented through a homogenisation approach and so the relevance and accuracy of a series of analytical methods for predicting the bulk thermal and electrical conductivity for use with CFCs have been investigated. This study has shown that the electrical conductivity is dominated by the percolation effects due to the fibre to fibre contacts. Due to the more comparable thermal conductivity between the polymer and the fibres, the bulk thermal conductivity is accurately predicted by an extension of the Eshelby Method. This extension allows the bulk conductivity of a composite system with more than two composite components to be calculated. Having developed a bespoke thermo-chemical degradation model, a series of validation studies have been conducted. First, the homogenisation approach is validated by numerically investigating the electrical conduction through a two layer panel of CFC. These numerical predictions showed initially unexpected current flow patterns. These predictions have been validated through an experimental study, which in turn validates the application of the homogenisation approach.

The novelty within the proposed model is the inclusion of the transport of produced gasses through the decomposing material. The thermo-chemical degradation model predicts that the internal gas pressure inside the decomposing material can reach 3 orders of magnitude greater than that of atmospheric pressure. This explains the de-laminations and fibre cracking observed within the laser ablated damage samples. The numerical predictions show that the inclusion of thermal gas transport has minimal impact on the predicted thermal chemical damage. The numerical predictions have further been validated against the previously obtained laser ablation results. The predicted polymer degradation shows reasonable agreement with the experimentally observed ablation damage. This along with the previous discussions has validated the physical processes implemented within the thermo-chemical degradation model to investigate the thermal chemical lightning strike damage.

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