Lifecycle Reusable Functional Digital Materials

The Digital Material Skins framework considers various manufacturing methods to apply towards the lifecycle of pressure vessel applications, from initial design to final production. Properties including recyclability, reparability and reconfigurability were considered from inception of the design process, to better incorporate these requirements and prove that a structure could be manufactured out of discrete parts for later reuse and repairability in the mission architecture of spacecraft hardware. Analysis and study of pressure vessel skin and applications, and additional consultation were conducted with professors at MIT including Paul Lagace on composites, Jeffrey Hoffman on Spacecraft Systems, and Neil Gershenfeld on manufacturing methods of Digital Material Skins, and MSFC staff including John Vickers as lead mentor for advanced manufacturing methods, Alan Nettles on composite fabrication, Neil Meyers and Robert Wingate on pressure vessel joining methods, and Kenneth Cooper for additive manufacturing methods. Successful use of facilities and resources throughout the yearly activity allowed for the development of a reusable, recyclable strategy for a pressure vessel skin, and many ideas were synthesized and adumbrated into a lifecycle strategy for future designer’s and engineer’s to consider for space applications. Composite Manufacturing of Digital Material Skins The Digital Material Skin (DMS) design would dictate everything about the process of molding and curing the parts themselves. If the DMS was designed as a 2D DMS unit, then the part could conceivably be fabricated by cutting out the shape of the design in each ply, and then laying up all the plies for the final thickened piece, or larger plies could have been stacked and then trimmed according to the 2D DMS unit. The latter method was used to initially stack all composite plies as larger thickened pieces, and then an exacto knife and also a multi-ply cutting system was used to trim the flat unit. This flat unit would then be applied to a mold and cured in a small oven. The guidelines considered for the laminates include the ply angles which would dictate strength based on anisotropy or isotropy, and the number of plies would dictate thickness. Homogeneous laminates were used with a single orientation of ply that were stacked in cross section by rotating the ply for greatest strength in the following directions: +90°, +45°, 0°, -45°, +90°. This unit would then be placed around a mold, which in this case was an aluminum 1/8” ruler. In another example, each ply was added in one orientation one at a time wrapped around the mold. The entire unit including the DMS unit and mold were inserted each time into a vacuum bagging system and compression applied at 1atm. This proved slightly more intensive but produced a cleaner part with edges filleted to allow another unit to fit in perfectly once cured. Some of the pieces were kept in the vacuum bagging system for 24 hours for better compaction and curing. The oven used was a small conventional oven with the ability to set rate of heating, along with max. and min. temperatures to start with. The units were 2”x 2” for x and y, and were heated up to 250 over a 4 hour period. The manufacturing method for the cross section designed would best fit an extrusion process whereby a long stable mold would accommodate a continuous laid out composite material which would cure in place, then be removed and due to the same cross-section would alternately be cut in the z-axis depending on the size required for the DMS unit. Another issue encountered included maintaining the same part requirements for many of the units created, since uneven distribution of heat and compaction tended to warp or create higher or lower compacted areas in the units, which proved to fit too loose, too tight, or to not fit at all. Future questions similar to manufacturing methods for larger composite structures include: How do we control all of these parameters for a system that has hundreds of pieces to fabricate, and how do you do that in such a way, as to make sure that every DMS unit is exactly the same and what process is most conducive to that? This is highly dependent on the DMS unit geometry, which in all cases will be a standard repetitive unit. Additive Manufacturing of Digital Material Skins An SLA process was initially used to print the DMS units. An SLA process uses a UV curable resin and a UV laser that prints and cures entire cross-sections one layer at a time. The advantage of this process is speed of model turn around time, while a major disadvantage is cost and post processing of the parts. An EBM printer was used to print the same parts out of titanium alloys. The EBM printer requires a lot of maintenance and the machine was not usable the majority of the time, due to cleaning and servicing of the machine. The FDM machine proved to be the most stable of all the printer’s, with a turnaround time of half a day depending on part complexity and size. The FDM prints thermoplastics and eutectic metals, and deposits materials one drop at a time, increasing time to print and lowering resolution of the parts. The FDM used in the Additive Manufacturing Lab was a 3D systems Dimension printer, which produced the parts that were eventually used to conduct tensile tests on an older model Instron machine. The tests were conducted with the aid of Alan Nettles, with duration of up to 30min per coupon. The videos of the test results illustrate that the units failed at the joints. The joints which were the overlapped areas of each DMS unit never broke in any of the experiments. As the coupon was pulled from both ends, the filleted cornered flaps simultaneously opened up little by little until they reached a full 90°. At this stage, the units sheared and joints slid off from one another one by one until both sides were completely unengaged. The joint needed to incorporate a snap fit connection in the plane of the folded flap, thereby incorporating a tertiary joining system, separate from the primary flaps, and secondary tongue and groove joints for the unit to unit connections. Additive manufacturing has recently been used for custom and complex geometries that otherwise require more intensive traditional manufacturing methods and tools. It is also a great way to test design concepts rapidly earlier in the design process. The questions incurred include: how do you scale up and use 3D printers to print hundreds of parts, instead of a couple custom units, and also make sure that these parts are all produced the same, while reducing costs of time, materials used, and servicing of these machines?