Impact of Compact Tension Specimen Size on Fracture Toughness of FFF-Processed Thermoplastics J. B. Mira1, V. Restrepo2, B. Vajipeyajula1, A. E. Patterson1,2 1 Manufacturing and Mechanical Engineering Technology, Department of Engineering Technology and Industrial Engineering, Texas A&M University, College Station, TX, USA 2 Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA ∗ Joseb00@tamu.edu Keywords: Additive manufacturing; experimental mechanics; testing methods and standards The ASTM D5045 standard is one of the most widely-used and cited methods for estimating the fracture toughness of thermoplastic materials. This standard is simple to implement and does not depend on J-integrals and similar tools to estimate fracture toughness. It is well-established in the literature that additively-manufactured (AM) thermoplastic materials fail to satisfy the criterion in this standard for plane strain condition, but there is also a common observation that there is no a strong dependence of the sample size on the fracture toughness values; this suggesting that these materials reach plane strain at a much smaller geometry than the ASTM D5045 criterion specifies. Given how important this consideration is for design, the present study explored this question by completing a series of experiments on three common thermoplastic materials (acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and polycarbonate (PC)) manufactured using the fused filament fabrication (FFF) process. The CT specimen was used with three different sample sizes (W = 30, 40, and 50 mm, B = 6, 8, and 10 mm - none of which met the plane strain criterion specified in the standards) with the test run 3 times for a total of 27 tests. Using ANOVA, it was found that there was no statistically significant effect based on sample size. One of the sample runs (W = 30 mm, B = 6 mm) was then repeated with B = 3 mm (3 more tests per material, n = 9) to further test the effect of thickness. As with the first set of experiments, no statistical difference was found in the fracture toughness values. It was concluded that, at least within the size range studied, there is no significant difference in output values based on sample size and that the tests were repeatable across different sample sizes. This suggests that either the samples reached plane strain condition at a smaller size than expected by the standard or that there is not a large difference between plane strain and plane stress conditions for FFF-processed CT samples. Future work in this area will focus on more extreme size differences and other materials. 64
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