J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 requires long beams and assumes pure bending. The general conclusion is that the short beam bend test may be used for comparison of similar materials for inter laminar shear strength The results for interlaminar shear strength showed apparent shear strength dependence of the span to thickness ratio, Work of others has also shown this and concluded that the short beam bend test does not char- acterize a quantitative interlaminar shear strength. 25.28 This is due to the localized loading conditions at the loading anvils and due to the short beam length. It could be pointed out that the short beam prevents 100pm application of St. Venant's principle, which states that stresses become uniformly distributed in a body at a (a sufficient distance from the loading points. A short beam does not provide sufficient distance to allow the stress distribution to be uniform in the body and so the measured shear strength changes with the span length for a given thickness. Also, as discussed above, the absolute magnitude of the shear strength is uncertain due to the nature of the localized loading The 'nominal interlaminar shear strength for the materials tested varied from 8 to 11 MPa. This is com- parable to the alumina and mullite matrix composites made by others. One observation was that lower strengths were measured in specimens that appeared to have more flaws on side edges of the beam. This sug- gests the possibility that flaws in the matrix micro- structure(voids or cracks) may reduce the interlaminar shear strength, but this trend is not clear nor readily quantifiable. In work by others, lower porosity (or Fig. 10. Fracture surface near the tensile surface of a +/-450 fiber higher densities in the matrix)caused increased inter orientation composite containing 70v% cubic ZrO2(8m%Y203)/ 30v% mullite sintered at 1250C for 5 h in HCL. The composite was laminar shear strength at the cost of notch insensitivity. 2 tested in the in-plane 3-point bending configuration. Very little fiber The effort to characterize and improve the inter- pullout is evident for in-plane testing in this fiber orientation. (a) laminar shear strength of all-oxide composites was due Arrows show where the crack propagated along fibers before cutting to initial measurements of the properties of the compo- across part of the tow of fibers. (b)Arrows show that fracture of the site in bending to measure a tensile strength. In early fibers was not on a common plane but still independently failed fibers with less disintegration of surrounding matrix tests, not reported here, the composite failed in an interlaminar shear manner. By reducing the thickness of the powder layer between the fiber cloth layers, which is loading can be reduced by placing compliant materials not characterized here, the interlaminar shear strength between the beam and the loading anvil, as attempted was improved to that reported in Fig 3. It appears that here, so that stresses will be distributed more uniformly with improved processing tensile failure may now limit over a larger area at the loading point 26 This can be the performance of the composite for potential applica tolerated in this test because it is the shear stress that is tions. Very short spans(or intense localized bending) of interest rather than the tensile stress on the lower are needed to produce delamination failure. Further work may determine that 3-D architectures and weaving The short beam bending test that has been used to of layers of the fibers could be effective ways of characterize the interlaminar shear strength of fiber improving this property of the composite if it is necessary composites is ASTM Standard D2344. However, based on finite element modeling experiments and analytical 4.2. Notch sensitivity and in-plane bend testing analysis it is suggested that this type of testing is not accurate for comparing substantially different materials The flexure test provides a measure of the tensile or for design purposes. 25.27.28 This is due to the localized strength of the material. The 0/90 orientation had and complicated loading at the loading anvils and due flexure strength of about 165 MPa which is comparable to to differences from Euler-Bernouilli beam theory, which the strengths obtained previously with a porous aluminaloading can be reduced by placing compliant materials between the beam and the loading anvil, as attempted here, so that stresses will be distributed more uniformly over a larger area at the loading point.26 This can be tolerated in this test because it is the shear stress that is of interest rather than the tensile stress on the lower beam. The short beam bending test that has been used to characterize the interlaminar shear strength of ®ber composites is ASTM Standard D2344. However, based on ®nite element modeling experiments and analytical analysis it is suggested that this type of testing is not accurate for comparing substantially di€erent materials or for design purposes.25,27,28 This is due to the localized and complicated loading at the loading anvils and due to di€erences from Euler±Bernouilli beam theory, which requires long beams and assumes pure bending. The general conclusion is that the short beam bend test may be used for comparison of similar materials for inter￾laminar shear strength. The results for interlaminar shear strength showed an apparent shear strength dependence of the span to thickness ratio. Work of others has also shown this and concluded that the short beam bend test does not char￾acterize a quantitative interlaminar shear strength.25,28 This is due to the localized loading conditions at the loading anvils and due to the short beam length. It could be pointed out that the short beam prevents application of St. Venant's principle, which states that stresses become uniformly distributed in a body at a sucient distance from the loading points. A short beam does not provide sucient distance to allow the stress distribution to be uniform in the body and so the measured shear strength changes with the span length for a given thickness. Also, as discussed above, the absolute magnitude of the shear strength is uncertain due to the nature of the localized loading. The `nominal' interlaminar shear strength for the materials tested varied from 8 to 11 MPa. This is com￾parable to the alumina and mullite matrix composites made by others.26 One observation was that lower strengths were measured in specimens that appeared to have more ¯aws on side edges of the beam. This sug￾gests the possibility that ¯aws in the matrix micro￾structure (voids or cracks) may reduce the interlaminar shear strength, but this trend is not clear nor readily quanti®able. In work by others, lower porosity (or higher densities in the matrix) caused increased inter￾laminar shear strength at the cost of notch insensitivity.26 The e€ort to characterize and improve the inter￾laminar shear strength of all-oxide composites was due to initial measurements of the properties of the compo￾site in bending to measure a tensile strength. In early tests, not reported here, the composite failed in an interlaminar shear manner. By reducing the thickness of the powder layer between the ®ber cloth layers, which is not characterized here, the interlaminar shear strength was improved to that reported in Fig. 3. It appears that with improved processing tensile failure may now limit the performance of the composite for potential applica￾tions. Very short spans (or intense localized bending) are needed to produce delamination failure. Further work may determine that 3-D architectures and weaving of layers of the ®bers could be e€ective ways of improving this property of the composite if it is necessary. 4.2. Notch sensitivity and in-plane bend testing The ¯exure test provides a measure of the tensile strength of the material. The 0/90 orientation had a ¯exure strength of about 165 MPa which is comparable to the strengths obtained previously with a porous alumina/ Fig. 10. Fracture surface near the tensile surface of a +/ÿ45 ®ber orientation composite containing 70v% cubic ZrO2 (8m% Y2O3)/ 30v% mullite sintered at 1250C for 5 h in HCl. The composite was tested in the in-plane 3-point bending con®guration. Very little ®ber pullout is evident for in-plane testing in this ®ber orientation. (a) Arrows show where the crack propagated along ®bers before cutting across part of the tow of ®bers. (b) Arrows show that fracture of the ®bers was not on a common plane but still independently failed ®bers with less disintegration of surrounding matrix. J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618 615