Fig. 20. Thermoplastic materials requirements evaluation. The other materials tested did not, in general, meet the performance requirements. For example: • Structural — much lower strength and modulus of elasticity than desired; • Preprocessing — poor fiber wetting. Press forming of strip stock required, thus limiting length available. Woven graphite was chosen as the fiber medium because it is readily available and easy to handle. When processed as a graphite/acrylic composite, it gives good strength and stiffness properties and also forms easily. The thermal performance of this particular composite is also quite good for passive structure in low earth orbit (Figure 21). Through testing, we have demonstrated that though the strength of the material begins to fall off somewhat at the elevated temperature, compressive load testing at room temperature indicated a load carrying capability 180% greater than aluminum at room temperature and 120% at the elevated temperature. The coefficient of thermal expansion of this woven graphite/acrylic is 10% of that of aluminum. Recent electron bombardment testing in Grumman's Van de Graff facility has indicated that the material would have about a 40-year life in low earth orbit (LEO). Ultraviolet exposure testing is still to be conducted. While the graphite/acrylic satisfies the structural requirements for a passive structure (one which carries non-heat generating or radiating components or experiments), there still exists a need for a composite which could operate in a higher temperature regime. Work has therefore been continued with determining the forming process parameters associated with graphite/polyethersulfone laminates. The preliminary results have been encouraging (Figure 18), although further study is required.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==