Date of Award

8-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Dr. Garrett J. Pataky, Committee Chair

Committee Member

Dr. Joshua D. Summers

Committee Member

Dr. Huijuan Zhao

Abstract

Cellular materials are known for being lightweight as well as deforming in unique ways. Cellular materials have become more viable due to additive manufacturing (AM). AM cellular materials are easier to fabricate compared to traditional cellular materials and AM cellular materials are not as limited in geometry as traditional fabrication methods were. AM materials were studied in this paper in a two-phase approach. Phase 1 focused on the global mechanical properties of AM cellular materials. Phase 2 focused on the crazing of AM thermoplastic glassy polymers and how additive manufacturing affects the behavior or cellular materials. Because cellular materials do not have a consistent cross sectional area throughout the material, there is not a standard cross sectional area to use for property calculations. The author introduced an effective area for in-plane loading that normalized cellular materials by the amount of area present to allow accurate, direct comparisons between cellular materials of different unit cell geometries, unit cell dimensions, cellular materials of different stock material and comparisons between cellular material and solid materials. Strains calculated from DIC displacement measurements were used to validate the behavior observed using the effective area compared to how the cellular material was actually deforming. It was observed that the AM honeycomb material crazed at the plastic hinges that formed. Crazing was studied in AM acrylonitrile butadiene styrene (ABS) and extruded ABS to compare how crazing behavior differed in AM materials versus extruded materials. Extruded ABS crazes were thin with an average width of 10 μm and appeared simultaneously throughout the cross section of a dog bone specimen when the macro crazing threshold stress was reached. AM ABS crazes were an order of magnitude wider with an average width of 100 μm and appeared at one or two locations when the macro crazing threshold stress was reached. Further crazing spread from the original craze locations as the material was further strained. Using DIC to detect macro crazing in AM ABS dog bone specimens and MicroCT scans to locate voids in the specimens, crazing was discovered to initiate in the large voids inherent in the AM process. Understanding how AM thermoplastics deform is critical for the development of using AM thermoplastic cellular materials.

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