Mechanically-Fastened Joints for Advanced Composites — Phenomenological Considerations and Simple Analyses

Abstract

The various factors affecting the strength of bolted or riveted joints in advanced composites are discussed qualitatively. The mechanisms associated with each failure mode-tension. bearing. shearout. cleavage. and pull·through-are explained. The work is based mainly on experimental observations. from which conclusions have been drawn to integrate the study. A simple theory to account for the elastic stress concentrations at loaded bolt holes. in terms of the various geometric parameters. is deduced from experimental and analytical evidence for isotropic materials. Further tests on composite materials are used to deduce correlation factors which relate this theory to the limited. but significant. stress concentration relief observed prior to failure. The combination of these theories permits a considerable generalization from limited test data to geometries for which no test data are available. but does need some testing for that specific composite material and fiber pat· tern. The interaction of stress concentration factors caused by bearing and bypass loads is explained. The theories above are shown to cover such combined load cases. as at multi-row bolted joints. and along spar caps and consistency with test data is shown. Not all load cases are usually covered by test programs. so the use of this theory to extend results to such cases as bearing loads applied orthogonally to the bypass load is explained. along with cautions about known limits on such techniques. No attempt is made to compile a comprehensive data bank of test data. but the examples presented do cover a sufficiently diverse range of fiber patterns to elucidate the known failure phenomena. The need to consider advanced composites not as homogeneous ortho-tropic materials but as two-phase mixtp.res of fibers and resin is highlighted by the observations on the benefits and problems associated with interference-fit fasteners in composites. The sequence of first failure of the resin (by delamination or dis bonding of the fibers) and second of the fibers (by tension) is shown to explain the strength increases resulting from nonlinearity. 543 E. M. Lenoe et al. (eds.), Fibrous Composites in Structural Design