Journal of the American Ceramic Society-Kovar et al. Vol. 80. No. 10 (a) Crosshead DIsplacement, d(mm) nu between the lavers 30°off-axis 90°off-uxis shead Displacenent, d (n) 2 mm mn Fig. 10.(a) Stress-deflection response is shown for uniaxially aligned specimens tested at 30 and 90 orientations. Side surface of the specimen determined by the strength of the BN-containing cell boundary hat for on-axis orientations. Also, because very little crack rather than the Si3N cell. Failure occurred at a nominal stress deflection is required to propagate a crack completely through of 70 MPa for the specimen tested at 90 and at 145 MPa for the specimen when cells are oriented perpendicular to the axis the specimen tested at 30. Because the BN cell boundary is of the applied load, little energy is absorbed during the fracture much weaker than the Si3N4 cell, the strength of uniaxially process fibrous monoliths tested off-axis are much lower than 3) Tensile Failure by Combination of Cell and Cell-Boundary fracture The side surface of a specimen with a [0/45/90 architec- the load-deflection behavio ture is shown in Fig. 11(a)after testing. The specimen shows stress. which the original specimen would hay extensive delamination cracking between the 0 plies. Because of the orientation of weak bn cell boundaries the delaminationdetermined by the strength of the BN-containing cell boundary, rather than the Si3N4 cell. Failure occurred at a nominal§ stress of 70 MPa for the specimen tested at 90° and at 145 MPa for the specimen tested at 30°. Because the BN cell boundary is much weaker than the Si3N4 cell, the strength of uniaxially aligned fibrous monoliths tested off-axis are much lower than that for on-axis orientations. Also, because very little crack deflection is required to propagate a crack completely through the specimen when cells are oriented perpendicular to the axis of the applied load, little energy is absorbed during the fracture process. (3) Tensile Failure by Combination of Cell and Cell-Boundary Fracture The side surface of a specimen with a [0/±45/90] architec￾ture is shown in Fig. 11(a) after testing. The specimen shows extensive delamination cracking between the 0° plies. Because of the orientation of weak BN cell boundaries, the delamination § Once fracture begins, signaled by nonlinearity in the load–deflection behavior, beam theory cannot be used to relate load to stress. The apparent stress levels are reported as the ‘‘nominal stress,’’ which the original intact specimen would have experienced at that load. Fig. 10. (a) Stress–deflection response is shown for uniaxially aligned specimens tested at 30° and 90° orientations. Side surface of the specimen tested at (b) a 90° orientation and (c) a 30° orientation. Fig. 9. (a) Flexural response for a uniaxially aligned specimen tested in the 0° orientation. Apparent flexural stress is defined as the load sustained by the specimen divided by the original cross-sectional area. (b) Side surface of this specimen after testing, showing extensive delamination cracking between the layers of cells. 2478 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10