2482 Journal of the American Ceramic Society-Kovar et al. Vol. 80. No. 10 VI. Energy Absorption energy dissipation in fibrous m hs are considered: creation of crack area and frictional sliding of cracked cells The work-of-fracture that is determined during a flexural test is a measure of the energy dissipation capacity of a material. (1) Energy Absorption Due to Cracking There are two potential sources of energy dissi composites suggests that the primary mechanism for diss pating during the growth of delamination cracks, However a e cracking: cracking of cells and cracking of cell boundaries. The contribution to the energy absorption due to cracking, We,is given by careful accounting of the total energy absorbed during a flex ural test indicates that there is substantially more energy ab. W=w, +w sorbed during a flexural test than can be attributed to the cre- ation of crack area alone, leading to the suggestion that another dissipative process also must be active. 4 Two mechanisms of where WL is the contribution from cracking of cells, w, the With G A. Hilmas, Advanced Ceramics Research, Tucson, Arzon y Panel C. A Versatile Tool for Obtaining Submillimeter Structures e The processing methods used to manufacture fibrous nd cell boundaries. MFCX systems are characterized by unique, submillimeter architectures to be o ny number of the size of the first and second filament. A 2 mm/2 mm are very versatile, alloy system has -2400 cells/cm, each-1 15 um in size, whereas ricated. For example, sheets of filament can be rotated with a I mm/l mm system has 9500 cells/cm2. Figure C2 illus- respect to one another to form a multiaxial architecture, as trates an example from a 0.85 mm/0.85 mm MFCX system, two extrusion steps that can be used to create much fine The structure of the cell or the cell boundaries also can be cells. The first extrusion step produces spaghetti-sized modified to introduce features. For example, in Fig. C3 nary filaments that are 0. 85-2 mm in diameter. These fila layer fibrous monolith is shown that consists of Si,N ments are cut and rebundled to form a second feedrod. For cells with bn cell boundaries that contain a thin web of example, 380 primary filaments of I mm diameter are used Si3 N4 reinforcement. Fibrous monoliths also can be com- to create the second feedrod. The second extrusion step bined with conventional laminates to form unique structures produces a filament with a much finer substructure of cells that contain elements of each, as shown in Fig. C4 30u 250um Fig. C2. Cross-section view of a fibrous monolith fabricated vi a multiple coextrusion process showing that cell sizes as fine as 40 um can be achieved. Fig. Cl. Low-magnification SEM composite showing three tions of a fibrous monolith with a [0/90] architecture. SiaNa Cell 50 um m monolithVI. Energy Absorption The work-of-fracture that is determined during a flexural test is a measure of the energy dissipation capacity of a material. Previous theoretical work on layered ceramics38,43 and layered composites39 suggests that the primary mechanism for dissi￾pating energy in these materials occurs by the creation of crack area during the growth of delamination cracks. However, a careful accounting of the total energy absorbed during a flex￾ural test indicates that there is substantially more energy ab￾sorbed during a flexural test than can be attributed to the cre￾ation of crack area alone, leading to the suggestion that another dissipative process also must be active.44 Two mechanisms of energy dissipation in fibrous monoliths are considered: creation of crack area and frictional sliding of cracked cells. (1) Energy Absorption Due to Cracking There are two potential sources of energy dissipation due to cracking: cracking of cells and cracking of cell boundaries. The contribution to the energy absorption due to cracking, Wc, is given by Wc = WL + Wi = GLAL + GiAi (6) where WL is the contribution from cracking of cells, Wi the Panel C. A Versatile Tool for Obtaining Submillimeter Structures With G. A. Hilmas, Advanced Ceramics Research, Tucson, Arizona The processing methods used to manufacture fibrous monoliths are very versatile, allowing any number of unique, submillimeter architectures to be designed and fab￾ricated. For example, sheets of filament can be rotated with respect to one another to form a multiaxial architecture, as shown in Fig. C1. Multifilament coextrusion (MFCX)5 has two extrusion steps that can be used to create much finer cells. The first extrusion step produces spaghetti-sized pri￾mary filaments that are 0.85–2 mm in diameter. These fila￾ments are cut and rebundled to form a second feedrod. For example, 380 primary filaments of 1 mm diameter are used to create the second feedrod. The second extrusion step produces a filament with a much finer substructure of cells and cell boundaries. MFCX systems are characterized by the size of the first and second filament. A 2 mm/2 mm system has ∼2400 cells/cm2 , each ∼115 mm in size, whereas a 1 mm/1 mm system has 9500 cells/cm2 . Figure C2 illus￾trates an example from a 0.85 mm/0.85 mm MFCX system, with 40 mm cells at a number density of 71,000 cells/cm2 . The structure of the cell or the cell boundaries also can be modified to introduce features. For example, in Fig. C3, a trilayer fibrous monolith is shown that consists of Si3N4 cells with BN cell boundaries that contain a thin web of Si3N4 reinforcement. Fibrous monoliths also can be com￾bined with conventional laminates to form unique structures that contain elements of each, as shown in Fig. C4. Fig. C1. Low-magnification SEM composite showing three sec￾tions of a fibrous monolith with a [0/90] architecture. Fig. C2. Cross-section view of a fibrous monolith fabricated via a multiple coextrusion process showing that cell sizes as fine as 40 mm can be achieved. Fig. C3. Cross-section view of fibrous monolith fabricated with a thin web of Si3N4 reinforcing the BN cell boundary. Fig. C4. Low-magnification optical micrograph showing hybrid structures containing layers of Si3N4 separated by layers of fibrous monolith. 2482 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10