0 200 400 600 800 0 10 20 30 40 50 0 50 100 150 200 Tensile Strength (MPa) % Elongation at Break UV Exposure Hours 3.3. Tensile Testing Results 3.4. Thermal Degradation 3.4.1 Nanoindentation and Tensile Testing Results Figure 7: Variation of tensile strength and percentage elongation at break of nylon fibers with number of UV exposure hours. Figure 6: WAXS Images of UV exposed nylon fibers, (a) unexposed (b) 48 hours (c) 96 hours (d) 144 hours. (a) (b) (c) (d) Tensile testing was carried out on UV exposed nylon fibers to determine the tensile strength and elongation at break. Nanoindentation is not capable of measuring those two property values. Using the stress-strain diagrams, the percentage elongation at break and tensile strength of UV exposed fibers as a function of exposure hours is obtained and shown in Figure 7. The unexposed nylon fibers had around 40% percentage elongation at break. A significant reduction could be observed in both tensile strength and percentage elongation at break with increase of number of UV exposure hours. This loss of toughness on photo-oxidation would be crucial for many applications of nylon fibers in industrial textiles. Variation of Young’s modulus of thermally exposed fibers from the center to surface is shown in Figure 8. A trend of decreasing Young’s modulus can be noticed on the surface at the beginning and later on reduction tends to spread to the interior after three and four weeks of exposure,. Especially at three weeks of exposure, Young’s moduli were noticed to be prominently lower in the first three indentation points closer to surface. A significant decrease of Young’s modulus was observed throughout the fiber except the center at four weeks of exposure. Engineering stress strain graphs for the thermally exposed nylon is shown in Figure 9. The slope of the curve at 1% strain was observed to be approximately same for all the fibers. This suggests that the deterioration of mechanical properties measured by elastic modulus cannot be determined using tensile testing. Exposed fibers were tested until they failed and percentage elongation at break and tensile strength was measured. Both tensile strength and percentage elongation at break tend to decrease with increase of thermal exposure time. 233
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