Laminate Behavior for SiC Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

Abstract

The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained ∼30 vol% fraction of aligned 142-μm diameter SiC fiber in a porous RBSN matrix. Three types of laminates studied were unidirectional laminates ([0]8, [10]8, [45]8, and [90]8), cross piled laminates [02/902]s, and angle plied laminates [+452/−452]s. Each laminate contained eight fiber plies. Results for the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane elastic moduli and strength. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Matrix crack deflection along the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. Inplane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for [02/902]s and [+452/−452]s laminates showed that their matrix failure strains were similar to that for [0]s laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.