Fig. 19. Composite fastening process development summary. effort on fastening braces to beam caps began. Many approaches were considered; those listed in Figure 19 were subjected to limited development testing and evaluation. Induction Weld Using a weld head developed by NASA-LaRC, Grumman under contract to evaluate the device found that it produced excellent joints. Induction currents heat the part at the joint interface until the resin melts and fuses together. Packaging presents no problem and power consumption is extremely low. RFI may be a problem, although this is still to be investigated. Material Evaluation As discussed earlier, our first composite processing development efforts met with somewhat disastrous results. After reviewing our goals, we decided to try some alternate approaches. The material to be investigated had to satisfy the following simple requirements: • Structurally sound in a space environment, including vacuum, thermal and radiation exposure; • No out-gassing during forming in space or during its operational lifetime; • Simple to preprocess into the required strip stock laminate; • Long ground storage life; • Easy to handle. Thermoplastics seemed to satisfy these general requirements (3). (Thermosets present strip laminate processing, storage and handling problems since they have to remain in their uncured state until formed.) Figure 20 shows the thermoplastic composite materials which were evaluated. Acrylic was selected because it not only met our structural baseline (strength and modulus of elasticity to be as close to or better than that of aluminum) but also because it is a resin system which lends itself to continuous prepregging of graphite and strip laminate production, including: • Monomer/polymer blend liquid at room temperature; • Excellent fiber wetting characteristics; • Monomer and polymer are readily available — in tank car quantities, if required; • Monomer and polymer are relatively low in cost — a factor which makes them attractive for both research and development, and production.
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