| Fibrous-monolithic ceramics are a class of material with many similarities to layered ceramic composites. Like layered composites, fibrous monoliths depend on a weak interphase to promote crack deflection and energy absorption, avoiding catastrophic failure. However, in a fibrous monolith, the interphase surrounds fiber-like "cells" of the strong phase, forming a continuous, 2-dimensional honeycomb network. In the most simple architecture, all cells are aligned unidirectionally. More complex architectures are easily produced by varying the orientation of successive layers relative to each other. The Young's modulus of the unidirectional architecture is predicted accurately along principal axes using a "brick" model, while the modulus at angles between 0{dollar}spcirc{dollar} and 90{dollar}spcirc{dollar} is predicted using laminate theory. Laminate theory may also be used to accurately predict the Young's modulus of multidirectional architectures such as a cross-ply 0{dollar}spcirc{dollar}/90{dollar}spcirc{dollar} and a quasi-isotropic {dollar}0spcirc/{lcub}pm{rcub}45spcirc/90spcirc.{dollar} Unidirectional fibrous monolithic ceramics are linear elastic in flexure until the first major failure event. The flexural strength of the unidirectional architecture tested at orientations between 0{dollar}spcirc{dollar} and 90{dollar}spcirc{dollar} is observed to fall into three distinct regions. Between 0{dollar}spcirc{dollar} and 10{dollar}spcirc{dollar} the strength is a constant 450 MPa, but between 10{dollar}spcirc{dollar} and 45{dollar}spcirc,{dollar} it gradually drops to 80 MPa. Above 45{dollar}spcirc{dollar} the strength remains essentially constant. Between 0{dollar}spcirc{dollar} and 30{dollar}spcirc{dollar}, the strength is accurately predicted using the Maximum Stress theory. Above 30{dollar}spcirc,{dollar} the strength is predicted using the Tsai-Hill model. The multidirectional architectures exhibit nonlinearity in flexural loading prior to the peak stress. Cyclic loading experiments indicate that this nonlinearity is a result-of microcracking in the boron nitride cell boundaries of the off-axis layers. The cross-ply architecture exhibits a strength of 334 {dollar}pm{dollar} 35 MPa, while the quasi-isotropic has a strength of 255 {dollar}pm{dollar} 22 MPa. The models developed to describe the unidirectional architecture may be extended to predict upper and lower bounds on the strength of multidirectional architectures. |
