The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiberdominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porousmatrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.

The present study explores the effects of exposure in a laboratory combustor on microstructural stability and property retention of an all‐oxide fiber‐reinforced ceramic composite. The material consists of a porous mullite–alumina matrix and Nextel 720 fibers in an eight‐harness satin weave. To assess the effects of matrix strength, two matrix conditions are used, distinguished from one another by the amount of alumina added through precursor impregnation and pyrolysis (1.8% and 4.8%). In both cases, the dominant damage mode upon exposure involves interply delamination along the panel midplane. However, significant reductions in the rate and extent of cracking are obtained in the material with higher alumina content: a result of the higher delamination resistance. Mechanical tests performed on exposed specimens reveal a slight (10–20%) reduction in tensile strength along the fiber direction and a comparable increase in shear strength. These trends suggest some sintering of the matrix upon exposure. Examinations of fracture surfaces provide additional supporting evidence. Implications for long‐term performance and strategies for imparting improvement in microstructural stability and delamination resistance are discussed.

The effects of matrix strength on the notched and unnotched tensile properties of a family of porous-matrix oxide composites are examined both experimentally and theoretically. Experiments are performed on three composites, distinguished from one another by the amount of binding alumina within the matrix. Increases in alumina concentration produce elevations in unnotched tensile and shear strengths, but the benefits are offset by an increase in notch sensitivity. The degree of notch sensitivity is rationalized on the basis of a model that accounts for interactions between notch tip tensile and shear bands. The model predictions are cast in terms of the ratio of the notch length to a characteristic bridging length scale. These results, in turn, form the basis for a simple analytical formula for notched strength, accounting for effects of elastic anisotropy and finite sample size. The utility of this formula in predicting notched strength is assessed. Issues associated with bridging law shapes and bridging length scales are addressed. The effect of alumina concentration on notch sensitivity is discussed in terms of its influence on the bridging length scale, dictated by the interplay between the unnotched tensile strength, the longitudinal Young's modulus, the degree of in-plane elastic anisotropy, and the fracture energy. The net result is a decreasing bridging length scale and hence increasing notch sensitivity as the matrix is strengthened with alumina.

This study focuses on the interfacial properties of a family of porous matrix oxide composites with uncoated fibers. Measurements of debond energy and sliding stress are made using a modified version of the established fiber push-in test. Modifications include the following: (i) use of a sphero-conical indenter (not a sharp-tipped one) to produce only elastic deformation of the fibers, and (ii) analysis of the loop width (instead of absolute displacements) to ascertain interface properties. The method obviates the need for indentation tests on reference (non-sliding) fibers. It also mitigates the problems associated with the elastic deformation of the surrounding matrix. The measured debond toughnesses (about 0.05 J/m 2 ) are about two orders of magnitude lower than the fiber toughness. This ensures that debonding will occur when a matrix crack impinges on a fiber. Additionally, the sliding stresses are in the same range as those reported for C-coated Nicalon fibers in glass-ceramic matrices (about 5 MPa). The latter results are qualitatively consistent with the observed damage tolerance in these two seemingly disparate systems, as manifested in the degree of fiber pullout as well as the notch sensitivity of tensile strength.

A theoretical framework is developed for the notched strength of ceramic composites that exhibit rising fracture resistance. It is based on established concepts of crack stability under stress-controlled loadings. On using a linear representation of the resistance curve (expressed in terms of an energy release rate), straightforward analytical solutions are obtained for the strength as well the amount of stable crack growth preceding fracture and the associated fracture resistance. Calculations are performed for several test configurations commonly used for material characterization, including single-and doubleedge-notched tension, center-notched tension, and single-edgenotched bending. The results reveal salient trends in strength with notch length and specimen geometry. An assessment of the theory is made through comparison with experimental measurements on an all-oxide fiber composite. Transitions in the degree of notch sensitivity with notch length are identified and explored. The utility of the theoretical results both for rationalizing the trends in measured notched strength and for guiding experimental studies of notch sensitivity is demonstrated.