64 M. Schaefer MPa). Elongation at break was not as high as reported in manufacturer’s data (8.2 % in x-y direction, not reported for zdirection). As expected, lengthwise printing gave lower strength and ductility. Mixed results make is difficult to say which is “better” between flat-printed and edgewise. Flat printed had higher elongation but edgewise had higher tensile strength. Experiment 2 – Strain Rate Trends shown in the tests on strain rate demonstrate all three expected behaviors. As strain rate increases the ultimate tensile strength increases. As strain rate increases, the elongation to rupture decreases. As strain rate increases, there is a slight increase in the elastic tensile modulus. Test results are shown in Figure 5 and summarized in Table 3. Fig. 5 Stress-strain results comparing strain rates. Flat-printed samples, tested in as-printed condition. Table 3 Summary of mechanical properties, comparing 3 different strain rates tested. Crosshead (strain rate) Modulus, E [MPa] Tensile Strength [MPa] Elongation at Break [%] 0.5 mm (1%) /min. 1,750 42.9 5.1 15 mm (30%) /min. 1,870 51.1 4.9 500 mm (1000%) /min. 1,990 56.5 4.8 Experiment 3 – Annealing of FDM Samples, post-printing Test results for this part of the experiment were counter to what was expected. Most significant was an approximately 30% reduction in elongation at break. It was expected that annealing after printing might improve elongation. Both strength and elastic modulus were improved, but only slightly (2 to 4% increase in strength). Figure 6 shows comparison for two of the annealed samples versus earlier data from the print orientation study. This part of the experiment was most problematic, both in terms of devising a method for annealing and with generating clear trend in the results. A first attempt at annealing was done without the holding fixture and resulted in severe dimensional changes in the specimens. Using a holding fixture to restrain samples during annealing and cooling helped solve that issue. However, the entire process was quite cumbersome and time consuming. Annealing response study needs further development to: 1) improve the ease-of-use of the holding fixture and 2) revise the annealing steps to come up with a more favorable response. Ideally, there would be increase in both strength and elongation. Variations in annealing temperature and cooling rate are the key parameters. Both these factors require further study and experimentation.
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