The two-dimensional stress distributions in orthotopic composite beam under 4-point flexure are analyzed by replacing the concentrated load by distributing pressure in the vicinity of loading nose and by expanding the stress function for compatability equation in terms of Fourier series. The validity of analytical results is verified experimentally by using the photoelastic film-coating technique on unidirectional reinforced plastics. It is shown that the local stress distributions around the loading points are complicated and are very different from those predicted by elementary beam theory; and hence, the reliability of flexural strength and interlaminar shear strength defined by elementary beam theory are in question. It is suggested that the 4-point flexure is reasonable and recommendable as a standard testing method compared with the 3-point flexural one.

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