ovember 2006 Oxide Fiber Composites Unnotched E苏 entration C Displacement(mm) Fig 23. Notched and unnotched flexural response of a porous matrix. 510 hree-dimensional orthogonal weave composite. The fiber volume frac- tions are 21%. 15%. and 3% in the transverse. longitudinal. an Notch Length, a, (mm) through-thickness directions, respectively( Courtesy J. H. Weaver) where oo is the unnotched tensile strength and ach is a charac teristic length scale, dictated by the work of fracture, the elastic concentration moduli, and the test geometry ( details presented in Sidebar C) Upon evaluating the pertinent values of ach for each of the three 4% omposites, the results are re-plotted using the non-dimensional 画o6 parameters oN/oo and ao/ gch(Fig. 21(b). In this form, the test results collapse onto essentially a single band, broadly consistent with Eq(19) VIIL. Delamination ON/oo =(1+tao/ach)12 Whether damage tolerance is achieved by porous matrices weakly bonded coatings, or fugitive coatings, fracture resistar under loads normal to the fiber directions remains low b lam inates of unidirectional tapes or 2D fabrics are particularly vul- 0.0 0.5 nerable to delamination. Under mode I and mixed mode I/ll Normalized notch length, a/ach loadings(the latter obtained in edge-notched flexure), tape lam- Fig.21.Notched strength of porous matrix continuous-fiber ceramic inates of dense matrix CFCCs(e.g, SiC-fiber glass-ceramics) composites(Adapted from Mattoni and Zok) exhibit an initiation toughness 20 J/m+, increasing gradually with increasing crack length and ultimately reaching a steady state value a40-200Jm4 dependent on the extent of bridging mina within a particulate mullite-alumina matrix), the length of shear bands emanating from the notch tip and the extent of fiber Dy in-plane fibers. 6263 Similar results have been obtained pullout decrease dramatically. Concurrently, the degree of notch porous-matrix CFCCs: the mixed mode toughness being 45-65 ensitivity increases. For instance, for a notch length of 12 m J/m at steady state(Fig. 22) The obvious way to improve delamination resistance is to by only 25% from the unnotched value, in contrast, the one with he strongest matrix exhibits a reduction of 60% cedents exist in polymer-matrix composites, oxide CFCCs with The notch sensitivity can be rationalized on the basis of es- example of a porous matrix reinforced by a 3D orthogonal tad is predicted te fonlewsss s In general, the notched strength interlock fiber weave is presented in Fig. 23. With a mere 3% largely suppressed and the notched strength is essentially iden- tical to the unnotched value. Delamination becomes evident only at large displacements, well beyond that at the peak load. In the latter domain, multiple cracks form between the layers of plane fibers and propagate stably from the notch tip B IX. Concluding Remarks From their infancy a decade ago, oxide composites have emerged as viable candidates for high-temperature thermostruc- tural applications. Their rapid evolution is attributable to recent 6 C developments in materials and microstructural concepts, both phological stability at the targeted service temperatures. Amor these the discoveries of monazite as a coating material for oxide fibers and the porous-matrix concept as an alternative to coat- OcA ings have taken center stage. Alternate strategies for producing eak interfaces as well as cost-effective methods of coating Displacement (mm) complex fiber architectures are emerging. These are expecte Fig 22. D Ccoauie i. Hi weaver us matis, two-dimensonal f to play a significant role in future material designs and their laminate mplementation in engineering systemsmina within a particulate mullite–alumina matrix), the length of shear bands emanating from the notch tip and the extent of fiber pullout decrease dramatically. Concurrently, the degree of notch sensitivity increases. For instance, for a notch length of 12 mm, the strength of the composite with the weakest matrix is reduced by only 25% from the unnotched value; in contrast, the one with the strongest matrix exhibits a reduction of 60%. The notch sensitivity can be rationalized on the basis of es￾tablished cohesive zone models. In general, the notched strength sN is predicted to follow57,58: sN sO ¼ 1 þ pao ach
1=2 (19) where so is the unnotched tensile strength and ach is a charac￾teristic length scale, dictated by the work of fracture, the elastic moduli, and the test geometry (details presented in Sidebar C). Upon evaluating the pertinent values of ach for each of the three composites, the results are re-plotted using the non-dimensional parameters sN/so and ao/qch (Fig. 21(b)).56 In this form, the test results collapse onto essentially a single band, broadly consistent with Eq. (19). VIII. Delamination Whether damage tolerance is achieved by porous matrices, weakly bonded coatings, or fugitive coatings, fracture resistance under loads normal to the fiber directions remains low.61 Lam￾inates of unidirectional tapes or 2D fabrics are particularly vul￾nerable to delamination. Under mode I and mixed mode I/II loadings (the latter obtained in edge-notched flexure), tape lam￾inates of dense matrix CFCCs (e.g., SiC–fiber glass–ceramics) exhibit an initiation toughness 20 J/m2 , increasing gradually with increasing crack length and ultimately reaching a steady￾state value 40–200 J/m2 , dependent on the extent of bridging by in-plane fibers.62,63 Similar results have been obtained in porous-matrix CFCCs: the mixed mode toughness being 45–65 J/m2 at steady state (Fig. 22). The obvious way to improve delamination resistance is to incorporate through-thickness fibers. Although impressive pre￾cedents exist in polymer-matrix composites, oxide CFCCs with 3D fiber architectures remain in their infancy. An illustrative example of a porous matrix reinforced by a 3D orthogonal interlock fiber weave is presented in Fig. 23. With a mere 3% volume fraction of through-thickness fibers, delamination is largely suppressed and the notched strength is essentially iden￾tical to the unnotched value. Delamination becomes evident only at large displacements, well beyond that at the peak load. In the latter domain, multiple cracks form between the layers of in-plane fibers and propagate stably from the notch tip. IX. Concluding Remarks From their infancy a decade ago, oxide composites have emerged as viable candidates for high-temperature thermostruc￾tural applications. Their rapid evolution is attributable to recent developments in materials and microstructural concepts, both for enabling damage tolerance and for ensuring long-term mor￾phological stability at the targeted service temperatures. Among these, the discoveries of monazite as a coating material for oxide fibers and the porous-matrix concept as an alternative to coat￾ings have taken center stage. Alternate strategies for producing weak interfaces as well as cost-effective methods of coating complex fiber architectures are emerging. These are expected to play a significant role in future material designs and their implementation in engineering systems. Fig. 21. Notched strength of porous matrix continuous-fiber ceramic composites. (Adapted from Mattoni and Zok56). Fig. 22. Delamination in a porous matrix, two-dimensional fabric laminate. (Courtesy J. H. Weaver). Fig. 23. Notched and unnotched flexural response of a porous matrix, three-dimensional orthogonal weave composite. The fiber volume frac￾tions are 21%, 15%, and 3% in the transverse, longitudinal, and through-thickness directions, respectively (Courtesy J. H. Weaver). November 2006 Oxide Fiber Composites 3321