Abstract
Composites processing to repeatably high levels of dimensional accuracy and mechanical performance remains a key challenge due to process-induced stresses that arise during the curing process. Primary sources of residual stresses during composite processing are attributed to the anisotropy of the materials, tool-part interactions and chemical cure shrinkage (CCS) of the polymer matrix.The aim of this research project was to study the dimensional change of epoxy resins during the curing process. A non-contact laboratory-based Digital Image Correlation (DIC) technique was developed to measure displacements of a resin surface as the cross-linking reaction proceeds. In parallel, thermo-chemical characterisation was undertaken on the resin system using differential scanning calorimetry to describe the degree of cure state variable and estimate the gel point of the polymer. The major research challenge overcome was the application of a speckle pattern on the surface of the initially liquid resin that remained intact as the polymer crosslinked into a glassy solid. A patterning methodology was developed where coloured high-density polyethylene was ground into a sieve and applied by tapping once the sample reached the required test temperature, which was up to 90°C in this study.
Results show a linear dependence of CCS with degree of cure, where values of 4 to 6% linear shrinkage were measured. The DIC results were compared to PVT-alpha measurements made at the University of Nantes, and good agreement was found. Using DIC for CCS measurements of epoxy resins was limited to isothermal conditions from the early stages of curing, otherwise speckle pattern disintegration occurred.
The influence of mould edge effects and pre-gelation shrinkage behaviour on data was evaluated. Surface tension at the edge of the mould required sufficient sample diameter to create a flat surface for data collection free from edge effects. Despite these limitations, DIC was shown to be a viable technique to measure the CCS of epoxy resins prior to gelation, which was only previously possible by bespoke PVT-α testing.
| Date of Award | 27 Sept 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | James Kratz (Supervisor) & Janice M. Dulieu-Barton (Supervisor) |