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Results of an experimental and analytical investigation of the compressive strength of unidirectional boron-epoxy composite material are presented. Observation of fiber coordinates in a boron-epoxy composite indicates that the fibers contain initial curvature. Combined axial compression and torsion tests were conducted on boron-epoxy tubes and it was shown that the shear modulus is a function of axial compressive stress. An analytical model which includes initial curvature in the fibers and permits an estimate of the effect of curvature on compressive strength is proposed. Two modes of failure which may result from the application of axial compressive stress are analyzed—delamination and shear instability. Based on tests and analysis, failure of boron-epoxy under axial compressive load is due to shear instability.
composite materials, boron, epoxy resins, compressive strength, shear strength, shear modulus
Structural materials engineer, NASA-Langley Research Center, Hampton, Va.