UC Davis

This dissertation documents the research work accomplished to experimentally determine the surface displacements resultant from the manifestation of the internal stresses caused by the thermal flows within the body of a structure created during the act of Fused Filament Fabrication (FFF) manufacturing processes. One of the most critical technical issues plaguing additive manufacturing (AM) technologies are the effects of warping and dimensional variations found print-to-print in manufactured structures. These issues arise as a result of the creation of manufacturing-induced residual stresses within the body of the structure being created. To overcome these issues, it is necessary to first understand how these stresses manifest and effect the structure of interest during the build process. To establish this level of understanding one needs to utilize a method to locate, quantify, and measure surface displacements over the entire build process in order to develop a means to mitigate the residual stresses causing these problems. This dissertation documents the experimental and analytical processes created to perform these essential measurements. Application of this novel experimental and analysis framework allows for the understanding to be generated as to the behavior of the residual stress induced displacements as well as determining the most significant manufacturing parameters leading to the creation of these displacement events. Through the execution of the experimental and analysis work presented within the following dissertation, an in situ displacement measurement capability was developed for small scale AM FFF manufacturing systems. As part of the design of this overall measurement capability, it was first required to determine the best modalities available to accurately sense and record the surface displacements developed within the AM structure being assembled during manufacturing. Through a thorough literature review and targeted preliminary experimentation, it was determined that a “no touch” and “no interference” displacement measurement technique was the only viable method to capture full field warping and deformation events created due to thermal discontinuities occurring during the AM process, because it does not interfere with the process. These measurements were accomplished through the use of digital image correlation (DIC) systems which, within the methodology employed, are capable of quantifying the full field surface displacement of the test specimen. This data-capture capability provided a means to determine the magnitude and location of displacement events as they occurred and changed during the act of part manufacture. This novel application of in situ DIC monitoring in the 3D printing of small-scale structures was adapted from published works of researchers from the University of Tennessee, Oak Ridge National Laboratory, Vanderbilt University, and The Institute for Advanced Composites Manufacturing Innovation (IACMI). This DIC approach allows for the measurement of surface displacements without the application of an external speckle pattern to the surface of the test specimen. Instead, this application of the DIC technology requires the natural surface roughness of the test specimen itself to act as a replacement for an externally applied speckle pattern. This methodology was found to produce displacement measurements with high correlation to those produced by DIC analysis using an externally applied speckle patterns. Pairing this sensing capability with innovative analysis provided the insight needed to analyze the deformation behavior of the constructed test structure and to work through the determination of the manufacturing parameters that provided the greatest statistical contribution to the generation of those deformation/warping events. Overall, the experimental and analysis process created through the course of this research effort and detailed within this dissertation provide for the generation of a foundational understanding of manufacturing-induced residual stresses within the body of FFF polymer structures. The highly robust process designed within this research effort is capable of being adjusted and customized to allow for the displacement assessment of structures composed of a wide variety of filament types across a broad selection of manufacturing systems. This will thereby provide the research and manufacturing communities with an enduring and, I believe, crucial technique.