Notch Sensitivity of Ceramic Composites with Rising Fracture Resistance

Abstract

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.

Strength and Notch Sensitivity of Porous‐Matrix Oxide Composites

“…The composites were comprised of seven layers of Nextel 720 fiber cloth in an 8‐harness satin weave and a porous matrix of mullite and alumina. The processing route was essentially identical to that described in earlier articles 17,18 . Briefly, the matrix was introduced in two steps: (i) vacuum infiltration of a slurry containing 80% mullite and 20% alumina particles into the fiber preform and (ii) impregnation and pyrolysis of an alumina precursor solution (Al 2 Cl(OH) 5 ).…”

“…The processing route was essentially identical to that described in earlier articles. 17,18 Briefly, the matrix was introduced in two steps: (i) vacuum infiltration of a slurry containing 80% mullite and 20% alumina particles into the fiber preform and (ii) impregnation and pyrolysis of an alumina precursor solution (Al 2 Cl(OH) 5 ). The effective volumetric yield of the precursor solution upon conversion to g-alumina, determined in the manner described below, was 6.4%.…”

“…Notwithstanding progress in the development of fiber coatings and environmental barrier coatings for use with SiC/SiC composites, oxide systems remain top contenders for applications requiring long service lives (>10 4 h) in oxidizing environments, albeit at operating temperatures (about 1200°C) that are some 100–200 K below the targeted levels for SiC/SiC. This has motivated numerous activities in material development and assessment in industrial 1–5 and government laboratories, 6–12 as well as academia 13–19 …”

“…This has motivated numerous activities in material development and assessment in industrial [1][2][3][4][5] and government laboratories, [6][7][8][9][10][11][12] as well as academia. [13][14][15][16][17][18][19] The present article focuses on tensile strength and notch sensitivity of one particular class of oxide composite: that in which damage tolerance is enabled by a porous matrix, rather than through the conventional route involving fiber coatings. It builds on recent experience in composite fabrication and microstructural control, through appropriate selection of matrix constituents and their topologies.…”

Carbon–Carbon Composites

“…The notch effects, such as stress concentration, local damages, stiffness, and strength loss, are critical mechanical properties for structure designation. The notch effects of advanced materials are initially investigated in the ceramic matrix composites (CMCs) [1][2][3][4][5]. Three classes of stress redistribution mechanisms can lead to the excellent notched strength performance of CMCs: class I, matrix cracking and fiber failure; class II, matrix cracking; class III, shear damage by matrix cracking [2].…”