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This paper presents an assessment of various tests conducted to characterize individual fibrous composite laminae in such a way that the strengths and stiffnesses of cross plied laminates made from those laminae can be predicted analytically, without the need to confirm the predictions by testing each and every cross-plied laminate. In the paper, tests for in-plane shear, longitudinal tension and compression, and transverse tension and compression are treated. Both strengths and elastic moduli are covered. Only in the ±45° laminate test are there orthogonal fibers to prevent premature failure from widespread cracking of the resin matrix. All the other tests have fibers in only one direction. The transverse tension tests in particular, but even the fiber-dominated longitudinal tests, are shown to be currently unsatisfactory but could be improved by including appropriate orthogonal fibers. Doing so would prevent any structurally insignificant microcracks in the resin from joining up and causing the specimens to fail catastrophically before completing their task of fully characterizing the laminae. With such revised test data, it is possible to predict the strength of cross-plied structural laminates far more accurately than by tests on unidirectional laminae, as has been done in the past.
composite materials, lamina properties, laminate properties, testing, coupons, strength-prediction, laminate theory
Douglas Aircraft Co., McDonnell Douglas Corp., Long Beach, CA