Explorative Investigation into the Thermal and Mechanical Properties of 2D Tubular Braided Composites with an outlook on the future of Green Composites
Creators: Ead, Ahmed Samir
Publisher: name: University of Alberta Library
Year: 2024

<p>Due to their tailorable mechanical and physical properties, 2D tubular braided composites (TBCs) have many potential applications in fields including aerospace, construction and medicine. Over the past 70 years, many studies have thoroughly investigated several aspects of braided composites including their manufacturing processes, the geometrical features and patterns of braided composites and the resulting mechanical properties. Although extensive literature exists on these materials, some gaps remain that present an obstacle to introducing these materials in their applications. The thermal expansion behaviour of TBCs has been sparsely experimentally and analytically investigated. Further, although mechanical properties are well documented, large variability is often seen between different studies for identical samples. Lastly, green composites have been proposed as more environmentally friendly alternatives to traditional synethic composites, however, remain largely unexplored. These specific gaps were addressed in this thesis.<br>
In the first part of the thesis, the lack of research on the coefficient of thermal expansion is addressed. A literature review is presented, summarizing the experimental, analytical and numerical studies looking into thermal expansion behaviour of textile composites. The gaps from this literature review highlight the need to investigate the thermal expansion behaviour of 2D tubular braided composites. To do this, a novel non-contact experimental method is proposed and a classical laminate plate theory (CLPT) based analytical model is developed. Kevlar®/epoxy TBC samples were manufactured at three braid angles (35, 45 and 55 degrees) and heated to 150 °C. To measure the deformation of the samples, two-dimensional digital image correlation (DIC) was used. A CLPT-based model was then developed to calculate the longitudinal CTE and compare it to the existing available data on Kevlar®/epoxy composites and the collected experimental data. Results showed the ability of DIC to measure the coefficient of thermal expansion of 2D TBCs and the ability of the model to predict the thermal expansion behaviour of these materials.<br>
In the second part of the thesis, the variability in documented tensile properties of TBCs is addressed. Two hypotheses are proposed as potential explanations to this variation. The first is the displacement rate defined as the rate at which the sample is loaded during tensile tests. ASTM standards do not specify this rate, but rather specify a timespan under which the sample should fail. To test this hypothesis, an experimental study was conducted. Kevlar®/epoxy TBC samples were manufactured at three braid angles (35, 45 and 55 degrees) and tested at three displacement rates (1, 2 and 6 mm/min). DIC was used to measure the strain of the samples. The collected data was used to calculate the elastic moduli of the tested samples. Results showed that the higher strain rate (6 mm/min) resulted in larger deviations in the elastic moduli, particularly for the larger braid angles tested (45 and 55 degrees). These results suggest that to reduce variability in recorded mechanical properties, lower displacement rates in the range of 1 mm/min to 2 mm/min should be used. The second proposed hypothesis is stress-free aging time defined as the time between sample manufacture and testing. Literature has shown that the epoxy can fully set after the curing process is complete, altering the mechanical properties. To test this hypothesis, an experimental study was conducted. Kevlar®/epoxy TBC samples were manufactured at three braid angles (35, 45 and 55 degrees) and tested at three stress-free aging times (0, 2 and 6 weeks). DIC was used to measure the strain of the samples. The collected data was used to calculate the elastic moduli and the strength of the tested samples. Results of the work showed that longer stress-free aging times resulted in increased modulus and strength of the samples, providing a potential explanation to the variation documented in the mechanical properties of TBCs between different studies.<br>
In the third and final part of the thesis, the potential of green composites as viable alternatives to synthetic composites is addressed. A literature review is conducted to assess the environmental impact of green composites using a life cycle analysis approach. The literature review discusses the relative differences between green composites and synthetic composites from procurement to use to end-of-life. Results of the literature review qualitatively strongly suggest that green composites are superior in terms of their environmental impact. Further studies should characterize the properties of green braided composites.</p>