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Title: Radio frequency excited carbon dioxide laser processing of carbon fibre reinforced composites : experimental and theoretical analyses of the fume content and expansion dynamics
Author: Leach, Matthew B.
ISNI:       0000 0004 5367 1393
Awarding Body: University of Hull
Current Institution: University of Hull
Date of Award: 2014
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In this thesis a detailed analysis of long pulse CO₂ laser interactions with carbon fibre reinforced polymer composites (CFRPs) is presented. In particular, the fume dynamics and the fume contents are determined. Knowledge of both of these aspects of laser processing of CFRPs is important for the development of specialist fume extraction and treatment systems. The study has been conducted in a progressive manner; firstly the individual components of the composite have been characterised in respect of their thermal and optical properties, then their individual interaction with laser light quantified, and finally the whole composite has been subject to the same laser processing tests. The materials are PAN-based carbon fibres (T300, Toray Carbon Fibers America Inc.), an epoxy resin (RS-M135, PRF Composite Materials) and a 50 : 50 combination by volume as a complete CFRP from Goodfellow Cambridge Ltd. The laser processing of CFRP is known to be a challenge due to the vastly differing thermal and optical properties between the components of the composite. Despite this, it has been studied widely due to its highly desirable properties in high-technology industries and drilling with an assortment of laser sources has proved it to be a viable option. The contribution of this work is to put aspects of the laser interaction on a firmer experimental and theoretical basis in the medium irradiance regime. In particular, there is little published work on the origin and composition of fume from laser processed CFRPs and this thesis documents studies that address this knowledge gap. The fume from the carbon dioxide laser ablation of CFRPs has been found to be primarily phenol, CO₂, CO, water, methane and an aldehyde which is most likely formaldehyde. Specific regimes under which carbon fibres can be released from the surface of both the bare carbon fibre weave and the CFRP have been identified. Predominantly, this is when the fibre is cut in two places using the laser beam in a scanning mode of operation as would be used for trepanning holes, for example. A method of tracking the motion of the ejected fibres in a time resolved way using fast imaging has been shown, but also in a non-time resolved fashion by observing the incandescent streaks from them. The evidence suggests that fibres cannot be released from a single ablation site unless they are near the edge of the material, and also that any fibres released when using PAN fibres and a Gaussian beam are unlikely be thinned to the point where they are hazardous to human health. This is due to a concomitant swelling of the fibre at moderate fluences which increases its size significantly. They could however be a source of debris on the surface of the sample. By using time-resolved interferometry, the sensitivity of the plume image capture system was improved over the typical shadowgraphic method to the point that gas phase decomposition can be observed very early on in the laser interaction. This technique can be applied to finding the moment during the laser interaction at which carbon fibre or epoxy resin starts to decompose. The two materials produce very different fume compositions and the results vary significantly over a piece of CFRP as the thickness of the epoxy over-layer changes due to the woven carbon fibre structure beneath. A theoretical model has been used to analyse the data and is shown to give good agreement with the time-to-onset of thermal decomposition as a function of laser irradiance as well as closely predicting the threshold laser fluence. The time-resolved images of the plume have also been used to plot the progression of the ejecta with time so that their velocity can be measured thus informing the conditions required for efficient extraction. The fluence threshold for laser-induced damage to carbon fibre weave was measured and was found to have a threshold 1.5x higher when the fibre orientation was perpendicular to the polarization of the laser as opposed to parallel. This result was supported by calculations of the reflectivity of the fibre at the laser wavelength, which found that 1.6x more energy was absorbed by fibres that are perpendicular to the polarization of the laser. As there is a large array of fibre types, matrix compositions and laser sources, the work presented here is detailed in such a way as to allow these techniques to be applied to other systems in the future.
Supervisor: Snelling, Howard Victor Sponsor: Purex International Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physics