3320 Journal of the American Ceramic Society--Zok No. I 90M10A E8巴o 70M/30A 70M/0A ◆60M40A Class Il Penetration Class I ClassⅢl Matrix Cracking Multipe matriⅸx Shear Banding Aging time, t(h Fiber Pullout g. 19. Classification of damage mechanisms in continuous (b fiber ceramic composites. Nevertheless. even afte e re tains about 80% of its init of this comt stability of the all-mullite neasurements vIl. Notch Sensitivity Notch insensitivity is a key attribute of high-performan CFCCs. It allows design of components with complex geomet- ric features and with attendant stress concentrations using prin- 10 ciples developed for metallic materials. As with metals, stress concentrations in CFCC structures are mitigated by inelastic deformation, albeit through markedly different mechanisms Fig. 17 mong the dominant modes of deformation experienced by 260 GPa. The solid lines represen CFCCs(Fig. 19).Classes I and Ill are prevalent in the oxides dictions. E on of the predictions to∑=o≈1 That is, notched fracture proceeds either by the propagation of a cal time, Ic, at which crack penetration is predicted to occur. (b) dominant mode I crack, accompanied by fiber bridging, or by Corresponding effects of concentration of precursor-derived alumina the development of shear bands parallel to the direction of load- particulate mullite ing, followed by fiber rupture. Broadly, the former is character istic of materials with dense matrices and relatively strong interfaces. It provides the least plastic dissipation and the 250 strongest notch sensitivity. The latter prevails in porous-matrix Mullite matrix composites and can mitigate notch sensitivity when the extent of (Simon, 2005) shear banding is large in relation to the notch length The state of the matrix plays a dominant role in notch sen- sitivity, especially in porous-matrix materials. Experimental measurements on three such materials, with varying matrix strength levels, and accompanying photographs of the teste 100 Mullite-alumina matrⅸx (Carelli et al., 2002) specimens are shown in Figs. 20 and 21(a). As the strength of the matrix is increased(by addition of a precursor-derived alu- Alumine VA=0% VA=4% VA=8% 8HSW Nextel tu llite matrix (Simon, 2005) 01009001000110012001300 Aging temperature(°C tched tensile specimens, show Fig 18. Effects of sition on the streng of delamination and fiber pullout. Materials consist of data from Nextel 720 fibers in a particulate mullite-alumina matrix, and strength- ferences in the fibe fractions(ranging from 38% to 45%) ened by precursor-derived alumina(concentration, VA)(Adapted from rengths have been to a volume fraction of 40% Mattoni and zokNevertheless, even after exposure at 13001C, the composite re￾tains about 80% of its initial strength. The superior performance of this composite is attributable to the enhanced morphological stability of the all-mullite matrix. VII. Notch Sensitivity Notch insensitivity is a key attribute of high-performance CFCCs. It allows design of components with complex geomet￾ric features and with attendant stress concentrations using prin￾ciples developed for metallic materials. As with metals, stress concentrations in CFCC structures are mitigated by inelastic deformation, albeit through markedly different mechanisms. Among the dominant modes of deformation experienced by CFCCs (Fig. 19),55 Classes I and III are prevalent in the oxides. That is, notched fracture proceeds either by the propagation of a dominant mode I crack, accompanied by fiber bridging, or by the development of shear bands parallel to the direction of load￾ing, followed by fiber rupture. Broadly, the former is character￾istic of materials with dense matrices and relatively strong interfaces. It provides the least plastic dissipation and the strongest notch sensitivity. The latter prevails in porous-matrix composites and can mitigate notch sensitivity when the extent of shear banding is large in relation to the notch length. The state of the matrix plays a dominant role in notch sen￾sitivity, especially in porous-matrix materials. Experimental measurements on three such materials, with varying matrix strength levels, and accompanying photographs of the tested specimens are shown in Figs. 20 and 21(a).56 As the strength of the matrix is increased (by addition of a precursor-derived alu￾Fig. 19. Classification of damage mechanisms in notched continuous- fiber ceramic composites. Fig. 20. Macrophotographs of edge-notched tensile specimens, show￾ing the extent of delamination and fiber pullout. Materials consist of Nextelt 720 fibers in a particulate mullite–alumina matrix, and strength￾ened by precursor-derived alumina (concentration, VA). (Adapted from Mattoni and Zok56). Fig. 17. (a) Effects of aging time and composition in particulate mul￾lite–alumina on the crack deflection parameter P, using fiber properties Gf 5 15 J/m2 and Ef 5 260 GPa. The solid lines represents model pre￾dictions. Extrapolation of the predictions to P5 o  1 yields the crit￾ical time, tc, at which crack penetration is predicted to occur. (b) Corresponding effects of concentration of precursor-derived alumina in particulate mullite. Fig. 18. Effects of matrix composition on the strength retention of Nextelt 720 fiber composites. (data from12,52,43). Because of slight dif￾ferences in the fiber volume fractions (ranging from 38% to 45%), strengths have been normalized to a volume fraction of 40%. 3320 Journal of the American Ceramic Society—Zok Vol. 89, No. 11