S Bueno, C. Baudin/Composites: Part A 40(2009)137-143 ondary cracks perpendicular to the layers in the second A30 layer a (Fig. 7b). From such load value(55 N, Fig. 5), the failure stress for A10题 the discerned formation of secondary cracks was calculated from the stress distribution under 3-points bending on a prismatic bar formed by layers with different elastic properties 40 by assuming a propagating crack with a length equal to the location of the cen- tre of the second A30 layer(a/w=0.76: Fig. 7b). From the failure stress, and taking into account a general stress intensity formula- This value is slightly lower than that reported for the most per forming crack-deflecting laminates(15-18 MPa m"/[9, 28, 35-39) constituted by Sic and graphite. Nevertheless, it is comparable to values reported for the stationary state of transformation-tough- ened ceramics(=6 MPa m"/[42]. 9-12 MPa m/2[43)), which are A10 the toughest oxides. 4100um The work of fracture value calculated from the area under the semistable fracture curves(Fig. 5. 62 +3 Jm higher(=26%) than that obtained by calculation, taking into ac count the additive character of the work of fracture from the work aob of fracture values of monolithic materials of composition Al0 same way, and the surface fraction of the crack corresponding to each layer(S10≈0.83ands3o≈0.17,ands10≈0.6,530≈0.4for specimens with notch lengths of 0. 4 and 0.8, respectively). This fact A30 reveals a synergic effect of the laminated structure on the mechan ical behaviour of the material The most important difference of the laminate posed here and that of other laminates with high capability for crack deflection is that the crack deflection and branching pro- cesses occur at local level, as demonstrated by the micrograph of fractured samples in Figs. 5 and 6. As a consequence, the new de- sign for ceramic laminates proposed allows reaching high apparent A10 toughness and work of fracture while maintaining the structu integrity of the piece after the initiation of crack propagation under 100 shear stresses as those that develop in wear applications. Further improvements of the proposed structure will be reached by lami- nated designs with larger numbers of thinner layers to originate linate. Polished later. of the samples. The sense of crack propagation was from the bottom to graceful fracture he micrographs (a) Crack branching and deflection of the main crack due to its interaction with the pre-existing microcracks ar Acknowledgments rrow shows the point at which the new crack is started. (b)Crack branching and multiple cracking are shown. The authors acknowledge the support of the Project MEC MAT2006-13480C02-01, Spain. A30 layer(Fig. 7b)will allow the specimens to deform without References increasing the load, as observed in the load-displacement curves before the complete failure. [1 Clegg W] Design of ceramics laminates for structural applications. Mater Sci Strength values are summarised in Table 1. The strength values for the layered material were lower than those of the A10 monolith [21 Gunnison KE, Sarikaya M, Liu J, Aksay IA Structure-mechanical property which can not be explained at this point, as the expected residual Baer e Sariaya M, editors. Hie ally structured materials materials stresses were very low and, moreover, they should have been par research society. Pittsburgh, Penn. USA: DA Tirrel Editors: 1992. tially released as discussed above. Special care should be taken in [3 Aksay IA, Sarikaya M. Bioinspired proces composite materials. In: Soga order to improve the quality of the microstructure of the external amics toward the 21st century. Tokyo: The Ceramic layers in this kind of structures. Nevertheless, strength values of [4] Hiltner A, Sung K, Shin E, Bazhenov S, Im J. Baer E Polymer microlayer the laminate were higher than those of the monolith of the same composites. In: Aksay IA, Baer E, Sarikaya M, editors. Hierarchically structured composition as that of the internal A30 layers while presenting a laterals. Materials research society. Pittsburgh, Penn. USA: DA Tirrel rong interfaces. J Am Ceram Soc 1995: 78(4): 1125-7. 4. Concluding remarks [61 Sakai M, Bradt RC Graphical methods for determining the nonlinear fracture From the load-displacement curves obtained and the observed fracture paths it is possible to calculate the apparent [7 Lutz EH, Brunings St E, Steinbrech RW. Steel-re ma ceramics. A the point of failure of the studied laminate [9, 28 was arrested at about 55 N, and then, the specimens 如~邮 [81 Rao MP. Sanchez-Herencia [91 Clegg W]. Kendall K, Alford NMcN, Birchall JD, Button Tw. A simple way to ther deformation due to multiple deflection and forn make tough ceramics. Nature 1990: 347: 45-57A30 layer (Fig. 7b) will allow the specimens to deform without increasing the load, as observed in the load–displacement curves before the complete failure. Strength values are summarised in Table 1. The strength values for the layered material were lower than those of the A10 monolith which can not be explained at this point, as the expected residual stresses were very low and, moreover, they should have been par￾tially released as discussed above. Special care should be taken in order to improve the quality of the microstructure of the external layers in this kind of structures. Nevertheless, strength values of the laminate were higher than those of the monolith of the same composition as that of the internal A30 layers while presenting a significantly higher toughness. 4. Concluding remarks From the load–displacement curves obtained and the observed fracture paths it is possible to calculate the apparent toughness at the point of failure of the studied laminate [9,28]. As previously discussed for the load–displacement curve in Fig. 5, the load drop was arrested at about 55 N, and then, the specimens admitted fur￾ther deformation due to multiple deflection and formation of sec￾ondary cracks perpendicular to the layers in the second A30 layer (Fig. 7b). From such load value (55 N, Fig. 5), the failure stress for the discerned formation of secondary cracks was calculated from the stress distribution under 3-points bending on a prismatic bar formed by layers with different elastic properties [40] by assuming a propagating crack with a length equal to the location of the cen￾tre of the second A30 layer (a/W = 0.76; Fig. 7b). From the failure stress, and taking into account a general stress intensity formula￾tion [41], an apparent toughness of ffi12 MPa m1/2 was calculated. This value is slightly lower than that reported for the most per￾forming crack-deflecting laminates (15–18 MPa m1/2 [9,28,35–39]) constituted by SiC and graphite. Nevertheless, it is comparable to values reported for the stationary state of transformation-tough￾ened ceramics (6 MPa m1/2 [42], 9–12 MPa m1/2 [43]), which are the toughest oxides. The work of fracture value calculated from the area under the semistable fracture curves (Fig. 5, 62 ± 3 Jm2 ) was significantly higher (26%) than that obtained by calculation, taking into ac￾count the additive character of the work of fracture, from the work of fracture values of monolithic materials of composition A10 (35 ± 3 Jm2 [16]) and A30 (53 ± 4 Jm2 [15]) processed in the same way, and the surface fraction of the crack corresponding to each layer (s10 0.83 and s30 0.17, and s10 0.86, s30 0.14 for specimens with notch lengths of 0.4 and 0.8, respectively). This fact reveals a synergic effect of the laminated structure on the mechan￾ical behaviour of the material. The most important difference of the laminated structure pro￾posed here and that of other laminates with high capability for crack deflection is that the crack deflection and branching pro￾cesses occur at local level, as demonstrated by the micrographs of fractured samples in Figs. 5 and 6. As a consequence, the new de￾sign for ceramic laminates proposed allows reaching high apparent toughness and work of fracture while maintaining the structural integrity of the piece after the initiation of crack propagation under shear stresses as those that develop in wear applications. Further improvements of the proposed structure will be reached by lami￾nated designs with larger numbers of thinner layers to originate graceful fracture. Acknowledgments The authors acknowledge the support of the Project MEC MAT2006-13480 C02-01, Spain. References [1] Clegg WJ. Design of ceramics laminates for structural applications. Mater Sci Technol 1998;14(6):483–95. [2] Gunnison KE, Sarikaya M, Liu J, Aksay IA. Structure-mechanical property relationships in a biological ceramic-polymer composite: nacre. In: Aksay IA, Baer E, Sarikaya M, editors. Hierarchically structured materials materials research society. Pittsburgh, Penn. USA: DA Tirrel Editors; 1992. [3] Aksay IA, Sarikaya M. Bioinspired processing of composite materials. In: Soga N, Kato A, editors. Ceramics toward the 21st century. Tokyo: The Ceramic Society of Japan; 1991. p. 136–49. [4] Hiltner A, Sung K, Shin E, Bazhenov S, Im J, Baer E. Polymer microlayer composites. In: Aksay IA, Baer E, Sarikaya M, editors. Hierarchically structured materials. Materials research society. Pittsburgh, Penn. USA: DA Tirrel Editores; 1992. p. 141–50. [5] Prakash O, Sarkar P, Nicholson P. Crack deflection in ceramic/ceramic laminates with strong interfaces. J Am Ceram Soc 1995;78(4):1125–7. [6] Sakai M, Bradt RC. Graphical methods for determining the nonlinear fracture parameters of silica and graphite refractory composites. In: Bradt RC, Evans AG, Hasselman DPH, Lange FF, editors. Fracture mechanics of ceramics, vol. 7. New York: Plenum; 1986. p. 127–42. [7] Lutz EH, Brunings St E, Steinbrech RW. Steel-reinforced plasma ceramics. A new multilayer design. Ceram Eng Sci Proc 1998;19(3):457–65. [8] Rao MP, Sánchez-Herencia AJ, Beltz GE, McMeeking RM, Lange FF. Laminar ceramics that exhibit a threshold strength. Science 1999;286(5437):102–5. [9] Clegg WJ, Kendall K, Alford NMcN, Birchall JD, Button TW. A simple way to make tough ceramics. Nature 1990;347:45–57. Fig. 7. Characteristic crack paths in the studied laminate. Polished lateral surfaces of the samples. The sense of crack propagation was from the bottom to the top of the micrographs. (a) Crack branching and deflection of the main crack in the first A30 layer due to its interaction with the pre-existing microcracks are shown. The arrow shows the point at which the new crack is started. (b) Crack branching and multiple cracking are shown. 142 S. Bueno, C. Baudín / Composites: Part A 40 (2009) 137–143