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Three-point and four-point flexure tests are the commonly used strength and fracture toughness tests for brittle materials because of the simplicity of the technique and the ease with which the specimens can be fabricated. However, the formation of matrix cracks, single or multiple, on the tensile side of a ceramic matrix composite beam specimen destroys the macroscopic uniformity of the beam, and the use of the flexure formula for bending stress calculations results in large errors. In this study, an analytical model is developed to give a more accurate prediction of the behavior of a unidirectional ceramic matrix composite beam exhibiting multiple matrix cracking. A combination of stress intensity factor and micromechanics approaches is employed in determining the failure modes, the true compressive and tensile stresses in the composite, and the shear stresses at the crack tip. The functional parameters are the applied load, the beam geometry, and the microstructural properties of the onstituents The model predicts significantly different stresses than those obtained using the flexure formula.
composite materials, thermal properties, mechanical properties, ceramic matrix, unidirectional composites, failure modes, flexure specimens, true maximum stresses
Associate professor, University of Missouri Rolla, Rolla, MO