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Micromechanical Prediction of Tensile Damage for Ceramic Matrix Composites Under High Temperature.
Final rept. 15 Aug 90-15 Feb 94
CITY COLL NEW YORK DEPT OF MECHANICAL ENGINEERING
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This report documents the experimental and theoretical results obtained in studying the tensile damage behavior of ceramic matrix composites namely NicalonCAS II composites at room and elevated temperature. First the composite Specimens were machined into dog-bone shape and polished to increase efficacy of observation in the SEM. Then the specimens were placed inside the chamber of a scanning electron microscope SEM and subject to tensile loading at room and higher temperatures up to 700 C. At each temperature, the load was increased until failure of the specimen and the progression of damage was observed and recorded from first crack to total failure. Damage usually started with matrix cracking, followed by debonding along fiber-matrix interface, fiber pullout and finally fiber breakage. At each load level, the displacement was measured, resulting in a stress-strain curve with linear and non-linear portions.Similar behavior was observed at higher temperatures. Theoretical models to predict and simulate the observed behavior were also developed. A singular integral formulation with periodic matrix cracking and interface debonding explains well the fact that embedded matrix cracks almost always propagate to the interface and are arrested by the fibers . The results obtained from finite element model with multiple rows of matrix cracks and debonding at the fibermatrix interface, compare extremely well with the experimental data.
APPROVED FOR PUBLIC RELEASE