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Micromechanics Failure Criteria for Composites

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[Technical Report, Final Report]

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Theoretical and experimental studies are presented of the micromechanics failure criteria for the transverse tensile and shear strengths of composites. Parameters accounted for in the failure theories include fiber properties, matrix properties, fiber content, void content, volume fraction of ineffective fibers, internal triaxial stresses due to fibers and voids, and the interaction of stresses from fibers and voids. Equations are developed for predicting the influence of voids on elastic properties and internal stresses in solids containing voids. The voids are assumed to be cylindrical and arranged in a square array. Pertinent expressions are given for the Youngs moduli, shear moduli, and Poissons ratios of solids with voids. Approximate expressions are also presented for the internal stresses in solids containing voids. The theoretical results on the influence of voids on internal stresses and mechanical properties are verified experimentally using photoelastic models. Existing theory is used to predict the internal triaxial stresses in fiber-reinforced composites. Moreover, using superposition, the ineraction of internal stress concentrations due to fibers and voids is established theoretically and verified experimentally using photoelastic models. Test models are also employed to establish the strength of the material as influenced by porosity and matrix ductility. The results on fiber-void stress interaction and on the strength of brittleductile solids in combination with modified Hencky Von Mises Distortion Energy Criteria are used in formulating failure theories for composites subjected to transverse and shear loading. Test data from actual composites are used for final verification of the failure theories Types of composites fabricated and tested in transverse tension and interlaminar shear include glass epoxy, glass phenolic, graphite epoxy, graphite phenolic, and boron epoxy.


Subject Categories:

  • Laminates and Composite Materials

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[A, Approved For Public Release]