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MECHANICAL FASTENING AND ADHESIVE 28 BONDING D. W. Oplinger 28.1 INTRODUCTION It would be difficult to conceive of a structure that did not involve some type of joint. Joints often occur at a transition between a major composite part, where most of the structural performance is generated, and a metal feature, which is introduced to allow for very high localized bearing contact for which the com- posite has inadequate strength or durability. In aircraft such a situation is represented by artic- ulated fittings on control surfaces as well as on wing and tail components which require the ability to pivot the element during various stages of operation. Tubular elements such as power shafting often use metal end fittings for connections to power sources or for articulation at points where changes in direction are needed. In addition, assembly of the structure from its constituent parts will involve either bonded or mechanically fastened joints or both. Joints represent one of the greatest chal- lenges in the design of structures in general and in composite structures in particular. The reason for this is that joints entail interruptions of the geometry of the structure and often material discontinuities, which almost always produce local highly stressed areas, except for certain idealized types of adhesive joint such as scarf joints between similar materials. Stress concentrations in mechanically fastened joints Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 are particularly severe because the load trans- fer between elements of the joint have to take place over a fraction of the available area. For mechanically fastened joints in metal struc- tures, local yielding, which has the effect of eliminating stress peaks as the load increases, can usually be depended on; such joints can be designed to some extent by the 'P over A' approach, i.e. by assuming that the load is evenly distributed over load bearing sections so that the total load (the 'I") divided by the available area (the 'A') represents the stress that controls the strength of the joint. In organic matrix composites, such a stress reduction effect is realized only to a minor extent, and stress peaks predicted to occur by elastic stress analysis have to be accounted for, especially for one-time monotonic loading. 28.2 MECHANICALLY FASTENED JOINTS COMPARED WITH ADHESIVE JOINTS In principle, adhesive joints are structurally more efficient than mechanically fastened joints because they provide better opportuni- ties for eliminating stress concentrations; for example, advantage can be taken of ductile response of the adhesive to reduce stress peaks. Mechanically fastened joints tend to use the available material inefficiently and are charac- terized by sizeable regions where the material near the fastener is nearly unloaded, which must be compensated for by regions where
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