The New Jersey Institute of Technology's
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Title:	Nondestructive evaluation of 3d printed, extruded, and natural polymer structures using terahertz spectroscopy and imaging
Author:	Clark, Alexander T.
View Online:	njit-etd2022-016 (xix, 118 pages ~ 4.4 MB pdf)
Department:	Committee for the Interdisciplinary Program in Materials Science and Engineering
Degree:	Doctor of Philosophy
Program:	Materials Science and Engineering
Document Type:	Dissertation
Advisory Committee:	Federici, John Francis (Committee chair) Gatley, Ian (Committee member) Ahn, Ken Keunhyuk (Committee member) Chester, Shawn Alexander (Committee member) Barat, Robert Benedict (Committee member) Barden, Phillip (Committee member)
Date:	2022-05
Keywords:	3D printing Amber Polymers Spectroscopy Terahertz Tomography
Availability:	Unrestricted
Abstract:	Terahertz (THz) spectroscopy and imaging are considered for the nondestructive evaluation (NDE) of various three-dimensional (3D) printed, extruded, and natural polymer structures. THz radiation is the prime candidate for many NDE challenges due to the added benefits of safety, increased contrast and depth resolution, and optical characteristic visualization when compared to other techniques. THz imaging, using a wide bandwidth pulse-based system, can evaluate the external and internal structure of most nonconductive and nonpolar materials without any permanent effects. NDE images can be created based on THz pulse attributes or a material’s spectroscopic characteristics such as refractive index, attenuation coefficient, or the level birefringence present within. The evaluation processes for polyethylene gas pipes and amber specimens lack efficient and accurate NDE techniques while 3D printed polymer structures currently have no standardized NDE methods. The primary focus of this research is to determine and evaluate the use of THz spectroscopy and imaging as a NDE technique for a variety of polymers extruded mechanically and naturally. Results indicate the refractive indices, attenuation coefficients, and level of birefringence of several 3D printing filaments including copolyester (CPE), nylon, polycarbonate (PC), polylactic acid (PLA), and polypropylene (PP) may change depending on the printing parameters. THz spectroscopy is used to measure relative permittivity of printed ceramic samples with various sintering temperatures. THz imaging proves to be a successful method to diagnose print head misalignment in ceramic nanoparticle jetting printing processes. Proper diagnosis of surface level defects on polyethylene (PE) gas pipelines is achieved along with preliminary joint fault imaging and 3D visualization by creating an interactive detailed map of surface level defects. THz NDE imaging, combined with tailored refractive index matching materials, can construct tomographic images and 3D reconstructions of multi-million-year-old amber. Visual and THz birefringence images are created to determine stress direction within extruded PE and amber. These results suggest that THz spectroscopy and imaging have multiple confirmed uses in the NDE of polymer structures, both mechanically and naturally fabricated.