Date of Award
Master of Science in Mechanical Engineering (MSME)
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To find material systems that offer low density and high strength, stiffness or toughness, hierarchically designed material systems have provided a promising research area. This thesis lays the groundwork for designing efficient micro-architectured material systems by characterizing size effects for 3d printed polymer parts. Two methods were used to analyze data from 3-point bend tests for specimens of varying size: the load-separation method was used for finding the point of crack growth initiation and Bazant’s method was used to find shape independent strength at failure. The strength values were used as inputs for finding size independent material constants within a thermodynamic scaling law model. Such material constants were found to not be universally applicable across the size scales being considered because the scaling law displayed non-monotonic behavior. The strength values showed a local maximum before decreasing at the smallest length scales. These results are compared to similar analytical and experimental results for both quasi-brittle materials and metals. Effects due to fractal crack propagation and specimen homogeneity are ruled out in consideration of the evidence for multiple size effects. Support is provided to show that the PolyJet process produces layers of varying elastic modulus, the thickness of which act as a characteristic length scale and reverse the energetic scaling law. The study concludes with recommendations for further work to confirm this hypothesis.
Bell, Darren Thomas, "Size scaling of strength and toughness for 3D printed polymer specimens" (2016). Open Access Theses. 832.