Mechanical Behavior of Hierarchical Synthetic Composites
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MECHANICAL BEHAVIOR OF HIERARCHICAL SYNTHETIC COMPOSITES
GEORGE MAYER Institute for Defense Analyses, 1801 N. Beauregard Street, Alexandria, VA 22311
ABSTRACT Diverse microstructures observed in rigid natural composites have been linked to mechanical properties that are equal to and, at times, superior to those which have been designed into synthetic composite materials comprised of organic, metallic, and ceramic constituents. The constitution, properties, and microstructures of the natural and synthetic materials are described, along with what is known and what is unknown about the constituent components, and interfaces. Hierarchical approaches to designing with and the mechanical analyses of both natural and synthetic composites will be described. INTRODUCTION Activities in the area of hierarchical structures have been catalyzed by the work of Baer and his colleagues [1-3]. The examples of tendon hierarchy (Figure 1) and the properties of other hierarchical materials at various levels of structure have been established. TENDON HERARCHY ,-fty EM
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Figure 1. Tendon Hierarchyll] Many other examples have been cited in recent years of the structural hierarchies of natural or biological materials and their complex relationships with properties of materials [4]. A broader interpretation of structural hierarchy in science, art and history has been provided by C.S. Smith [5] in which the pervasiveness of the topic is noted. For this paper, the segment of hierarchical structures that will be considered relates and contrasts the mechanical behavior of rigid natural composites to synthetic composites. Examples will be given of similarities and differences between these families of composites, and problems which remain to be addressed will be noted. RETROSPECTIVE The main purposes of stiff, structural, natural composites are to provide protection, shape, and support, and to serve as jointed limbs and weapons. Mat. Res. Soc. Symp. Proc. Vol. 255. 01992 Materials Research Society
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Composite materials were first constructed of two or more materials in combination in order to provide higher levels of performance than a monolithic material alone was able to supply. Early civilizations made use of rigid composite materials as laminated bows employed for strength, and straw and mud mixtures for building blocks. They recognized simplistically that certain combinations of materials were useful for strength and toughness. The Composite Bow Examples of the exploitation of combinations of useful properties of natural materials can be found in the composite bow [6]. Here, tensile strength, compressive strength, energy storage, and resiliency are required for proper function. Use of natural composite materials was made, probably by trail-and-error in Europe as early as 2700 B.C. Yew wood was, and
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