Failure Problems In Composites
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FAILURE PROBLEMS IN COMPOSITES
FREDERICK J. MCGARRY Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139
ABSTRACT Fiber reinforced polymer matrix composites have become very useful in chemical processing systems, in transportation applications such as automobiles, aircraft and boats, in electrical hardware and in sports equipment. Failures in these rarely involve gross cohesive fracture; usually leaks develop, the stiffness decreases, local delamination occurs, the dielectric properties degrade or the strength declines. Almost all of these failures can be traced to local cracking, either at the fiber matrix interface or within the matrix itself. The cracks are generated by mechanical loads, by thermal excursions or by fluid absorption, because the relevant properties of the fiber and the matrix often differ by orders of magnitude. Current technology attempts to avoid the cracks by maximizing the fiber-matrix adhesion and great progress has been made in this area. An alternative interposes a thin compliant layer, rubber, between the two constituents thereby reducing the stress concentrations which exist because of their greatly different properties. Cracking is inhibited, composite strength is increased and its energy absorption also rises; if the rubber layer is thin (a few thousand Angstroms) no loss of stiffness or heat resistance is evident.
Introduction As discussed here, composites will refer to fiber reinforced plastic matrices, the latter usually crosslinked thermnosetting polymers. The fiber volume fraction, either continuous or discontinuous, will be high enough to render the composite structural - load carrying - rather than simply decorative or cosmetic. Emphasis will be given to manufactured rather than natural fibers: glass, carbon and synthetic high polymers of various types. (This will ignore vastly larger numbers and types of naturally occurring composites, though some important teachings from them will be discussed.) Composites have become widely used, valuable engineering materials. Both the chemical processing industry and the oil Mat. Res. Soc. Symp. Proc. Vol. 255. (01992 Materials Research Society
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industry are large consumers of them, principally because of their light weight and superior corrosion resistance. Aircraft, high specific missiles and space hardware also exploit their properties: strength-to-weight and stiffness-to-weight ratios Sandwich or monocoque which are among the best available. constructions provide boats with minimum joints and seams, low capital construction methods and good resistance to deterioration Composites offer land transport in the marine environment. vehicles and systems the economy of parts consolidation, low and relative chemical tooling costs, styling flexibility In all of these application areas the materials, inertness. procedures, equipment and methods have evolved to a tolerable of and, sometimes, a high level level of efficiency basis, composites have effectiveness; on a
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