The mechanical behavior of a woven fiber-reinforced ceramic matrix composite: Relationships between constituent and macroscopic properties

Improvements in jet engine efficiency and aircraft performance require the development of new lightweight high temperature materials. Fiber reinforced composites manufactured with ceramic constituents, typically referred to as ceramic matrix composites, are promising candidates. Ceramics can withstand the high temperatures and the fiber reinforcements can prevent brittle failures. Research is needed to identify inexpensive processing techniques, optimal constituents, and effective design methods.; McDonnell Douglas Technologies Inc. (MDTI) has developed a low cost ceramic matrix composite--one possible solution. They pass an eight-harness-satin weave, SiC cloth through an aluminum-phosphate slurry to produce lamina which can be laid-up in complex geometries. To assess the potential of this composite, research was conducted to evaluate its mechanical behavior. Specifically, the relationships between the constituent and macroscopic properties were investigated. The research included extensive mechanical testing on the constituent, 1D, and 2D materials and the characterization of the damage which developed. Analysis and modeling were performed to interpret the results.; Bundle testing of the Nicalon fibers demonstrated that the fiber strength was degraded by the heat cleaning used to remove the fiber sizing. Weibull parameters were established for the as-received and heat treated fibers and indicated that the heat cleaning reduced the potential strength of the composite by 61%.; The stress-strain behavior of the biaxially reinforced CMC was non-linear and marked by stiffness reductions, hysteresis, and permanent strains. These characteristics were directly correlated to damage in the longitudinal plies as quantified by a single damage parameter--the product of the matrix crack spacing and the interfacial sliding stress.; The fiber-matrix interface had contrary effects on the tensile and shear strength of the composite. In tension, a weak interface promoted extensive debonding which prevented catastrophic crack propagation and yielded higher strengths. In material with a weak interface, the average tensile strength was 195 MPa, and the toughness exceeded 10.9 MPa{dollar}surd{dollar}m. Whereas, these values were 95 MPa and 5.4 MPa{dollar}surd{dollar}m in material with a strong interface. Shear loads promoted large inelastic deformations which were attributed to rigid body sliding at intralaminar crack and delamination surfaces. The shear strength was directly correlated to the interface strength as measured with push-out tests. Stronger interfaces resulted in higher shear strengths.