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Laminate Strain Allowables as Design Parameters in Teaching an Introductory Course on Composite Materials



A common practice in the aerospace industry is to use a laminated composite failure criterion based only upon laminate strain allowables, for which failure in any lamina denotes first-ply failure. Therefore, first-ply failure is a single event that can be used in composite design. The main assumptions of a balanced and symmetric laminate subjected to membrane forces constant along the edges (i.e., no external bending or twisting), are utilized. In order to prepare our students for future work and generate interest in the field of composites, we have introduced an active component into our introductory composites course in which student teams fabricate, test and analyze laminates to determine laminate strain allowables. Four student teams comprised of 3-4 members were given different laminate configurations with varying percentages of 0°, ±45°, and 90° plies. Students were required to fabricate their laminates using unidirectional carbon fiber/epoxy prepreg. The primary outcomes of this activity were to (a) enable students to use their own design laminate code to determine the critical stress that can be applied so that the given stress resultants can be supported without exceeding the “allowable” strains, (b) compare the results to the measured data and (c) introduce students to a common and important industry practice in composite analysis and strength prediction. The exercise demonstrates that the critical laminate strain is not a function of the laminate configuration, but of the percentage of plies in the four orthogonal directions, thereby validating the use of laminate strain allowables as design parameters.

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