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ENVIRONMENTAL EFFECTS ON COMPOSITES 36 Ann F. Whitaker, Miria M. Finckenor, Hary W. Dursch, R.C. Tennyson and Philip R. Young 36.1 INTRODUCTION Composite usage has increased dramatically over the last three decades due to the advan- tages of light weight, specific strength and stiffness, dimensional stability, tailorability of properties such as coefficient of thermal expansion and high thermal conductivity. Environmental effects on these properties may compromise a structure and must be consid- ered during the design process. Because of the variety of uses, the compos- ite environment cannot be exactly defined. This chapter details the major environmental concerns for the composite designer, problems encountered with these environments in the past, and some materials or protective systems effectively used. The use of trade names, how- ever, does not constitute endorsement, either expressed or implied, by the authors. 36.2 HISTORICAL PERSPECTIVES Use of composites in commercial aircraft increased under two NASA programs, the Flight Service Evaluation Program and the Aircraft Energy Efficiency Program, begun in the 1970s. These programs included evalua- tion of environmental effects on the composite parts of the Boeing 727, McDonnell Douglas DC-10 and Lockheed L-1011 commercial air- craft. Elevators, rudders, ailerons, horizontal Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 stabilizers, vertical fins and fairings were flight-tested with annual inspections. Satisfactory performance of the composite materials was noted over fourteen years, with some parts experiencing more than 39 000 hours of flight loads. Also evaluated were composite parts from military aircraft such as the C-130 center wing box, S-76 tail rotors and horizontal stabilizer, 206L fairing, doors, and vertical fin and the CH-53 cargo ramp skin. Boron/epoxy, graphite/epoxy, Kevlar/epoxy and Nomex honeycomb were used in these aircraft and helicopter components. 36.3 ENVIRONMENTS AND EFFECTS 36.3.1 BIOLOGICAL ATTACK Biological attack on composites may consist of fungal growth or marine fouling. As reported in the literature, fungal growth does not appear to be as damaging as the wet condi- tions that promote growth. Fungicide has been mixed in with resins to retard this growth. Marine boring organisms do not appear to attack glass-reinforced composites. Even though marine organisms will grow on com- posite surfaces, mechanical properties do not appear to be affected, and the fouling can be removed by scraping (Fried, 1969). Composites with graphite fibers have been used in medical applications for both internal and external purposes. Internal composite structures, such as artificial joints or plates for bone fracture support, must be biocompatible
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Environments and effects 811 or the material may degrade over time.
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