Over the past decade, "additive manufacturing" (AM) has seen an exponential increase in investment, infrastructure, services and materials available. The advantages of AM parts are reduced weight, reduced number of components and reduced manufacturing time. Conversely, in the application of AM to produce electrical components, the level of technical readiness remained relatively low. Magnetic and conductive elements have been successfully prototyped, but to obtain a machine or assembly "entirely" manufactured by FA, the printing of robust electrical insulation is essential. The need to use higher voltages on aircraft has two main applications: either to improve the fuel efficiency of conventional propulsion, or to enable the complete electrification of the propulsion system. Propulsion system electrification can be a combination of all-electric systems or high-power density gas turbine systems to power an electric propulsion system. This thesis focuses on two aspects of electrical insulation produced by AM in the context of airborne applications: (1) the evaluation methodology, and (2) the usefulness of AM materials in aerospace electrical insulation applications. Three AM processes were examined: stereolithography (SLA), selective laser sintering (SLS) and fusion deposition modelling (FDM). Tests for conductivity, complex permittivity, partial discharge in volume and dielectric rupture were carried out on samples printed in different orientations. The dielectric characterization was carried out over a wide range of frequencies (DC to 1 MHz) and temperatures (-65o C to +200o C; depending on the material). A dielectric fracture resistance model was developed and applied to the test results. The method was an extension of Schneider (2013) and introduced the concept of "critical energy fluence" for the threshold characteristic of a material to facilitate the propagation of filament fracture. This method provided a means of extrapolating test data to arbitrary thicknesses as well as permittivity disturbances. In general, the materials studied exhibited acceptable dielectric behavior, with the exception of FDM which produced structures with large empty spaces in the structure. A "composite" dielectric structure was inadvertently examined to the extent that the materials printed by FDM were a matrix of polyetherimide and air. Photopolymer (SLA) and thermoplastic polymer (SLS) offered poor dielectric performance above 80°C corresponding to the softening temperatures of the materials. No clear advantages over existing insulation systems were demonstrated, other than that they allow complex multifunctional designs (dielectric, structural and thermal). Dielectric holdings exceeding 30 kV/mm for a thickness of 1.0 mm were observed for SLA e samplest SLS, with dielectric stiffness scale laws explored over a thickness range of up to 5.0 mm. For orthogonal construction orientations (construction XY and YZ), no discernible anisotropic dielectric behavior was observed. In the structures printed by FDM, the extended internal voids exhibited anisotropic behavior due to the nature of the shape and orientation of the voids. As built, the FDM method is unusable in applications whose voltages approach the Paschen minimum. |
In the application of AM to aerospace vehicles, AM materials and processes have reached full technical maturity. Currently, AM produced items are utilized on commercial jet aircraft. The uses range from complex engine fuel nozzles to cabin interior structural provisions. The benefits of AM parts are weight reduction, reduction of the number of components, and reduced manufacturing time. The necessity for higher voltages has two different motivations; either to enable improvements in conventional propulsion fuel efficiency, or to full "electrification" of the propulsion system. Propulsion system electrification can be a combination of all-electric or using high power density gas turbine systems to power an electric propulsion system. This paper focuses on two aspects of AM produced electrical insulation within the context of airborne applications: (1) methodology for assessment, and (2) utility of AM materials in aerospace electrical insulation applications. Three AM processes were examined; stereolithographic (SLA) selective laser sintering (SLS) and Fused Deposition Modeling (FDM). Conductivity, complex permittivity, volume partial discharge, and dielectric breakdown tests were performed on samples printed in different orientations. Dielectric characterization was performed over wide a range of frequencies (dc to 1 MHz) and temperatures (-65oC to +200oC; material permitting). Models were developed both for the partial discharge of a given void size within a dielectric medium, and for dielectric breakdown strength. Regarding dielectric strength a extended method from Schneider was used to develop and expression for the critical energy fluence for the propagation of filament breakdown. These models were applied to test data for validation. The critical energy fluence (filamentary electromechanical breakdown) offered a useful means of extrapolating test data to a given thickness of material. The partial discharge methods were not as successful, and the inherent limitations of such models are discussed. Generally, the materials studied exhibited acceptable dielectric behavior apart from FDM which produced structures with extensive voids. The photopolymer (SLA) and thermoplastic polymer (SLS) offered poor dielectric performance above 80oC. No clear advantages over existing insulation systems were evident, other than enabling complex multifunction designs (dielectric, structural and thermal). Dielectric strengths exceeding 30 kV/mm for a 1.0 mm thickness were exhibited in both SLA and SLS samples, with strength scaling laws explored over a range of thicknesses up to 5.0 mm. For orthogonal build orientations (XY- and YZ-build) no discernible anisotropic dielectric behavior was observed. In the FDM structures printed, the extensive internal voids did exhibit anisotropic behavior by the nature of void shape and orientation. As printed, the FDM method is unusable in applications that approach the Paschen minimum. |