## 2 Materials and Methods
### Adaptable Fan Casing Design  
The new fan casing is referred to as the adaptable fan casing due to its design intent of incorporating swappable inserts as the flowpath surrounding the rotor. The overall architecture of the assembly is made up of three permanent metal components constructed of 6061 aluminum, and six interchangeable 3D printed inserts made from a high-temperature ceramic-like thermoset plastic called PerFORM. The major components are shown installed around the fan in the render in Figure 2. The metal components required precision machining to ensure the concentricity of the final build and to aid in assembly. Additionally, alignment pins were incorporated on both sides to align the AM inserts. The inserts have notches on both the forward and aft sides to accommodate the pins. Aluminum was chosen as the metal build material due to its corrosion resistance, machinability, mechanical properties, and low density compared to stainless steel. The lower density minimized the overhung mass over the rotor and made installation smoother. The AM inserts are built through a manufacturing process called SLA. SLA is a vat polymerization process during which a liquid resin is cured in lay  
Figure 1: Fan rig render with rotor cutaway. Figure 2: AM fan casing installed over the rotor (**a**) along with an exploded view (**b**) for reference. light [9]. As the layers cure, complex components can be constructed. The layer thickness produced by the light governs the geometric capabilities such as feature size and surface finish. It also dictates component size, printing time, and mechanical strength. This plastic presented significant risk and design challenges in its incorporation over the rotor. The ultimate tensile strength of the PerFORM is significantly lower compared to other aerospace metals. Furthermore, the tolerances on the prints are slightly higher. A typical fan casing may have a profile tolerance of around 0.010\({}^{\prime\prime}\) (0.25 mm), while the tolerance on the inner radius of the AM inserts is roughly 0.020\({}^{\prime\prime}\) (0.5 mm). However, the expected payoff due to associated cost, lead time, and available component complexity was worth the endeavor. Table 2 shows representative values of lead time, cost, and feasible part complexity for AM plastic, AM metal, and traditionally machined components. Both AM plastic and AM metal present significant cost and lead time reductions compared to machining. Furthermore, 3D printed parts do not require drawings with individual features called out, which further reduces the development time. Printing in plastic offers the most outstanding advantage as it has a lead time on the order of weeks and undercuts the cost of AM metal components by a factor of two. In the span of one month, it is conceivable to print, implement, and test a new flowpath design if AM plastic inserts are used. The monetary values listed in Table 2 take into account the associated material cost. Further considerations in the cost-benefit analysis included long-term maintenance costs and replacement cycles. The fatigue life of the PerFORM was evaluated experimentally and found to be adequate [10]. The number of blade pass events that occur during data collection for a fan map is on the order of 100 million. Fatigue life assessments for thermoset plastics like the PerFORM are difficult because they are typically based on linear elastic materials. To effectively collect aerodynamic data, inserts should be expected to last through roughly 60 h worth of data collection and numerous stall cycles. The fan was stalled multiple times at different corrected speeds and approximately 45 h were accumulated on the inserts used for this project. There were no signs of fracture in the inserts upon inspection and they can likely endure further testing. Any advantage of an aluminum fan casing in terms of fatigue life would be irrelevant in this case because the required hours before failure of the AM inserts would likely be higher than the required testing hours.