O.N. Grigoriev et al. / Composites: Part B 37(2006)530-54 解新A Fig. 6. Structure of strong(400-600 MPa)composites with porous SiC layers. 400-600 MPa), and the composites strength exceeds the further t that u in response to mechanical Taking into trength of monolithic ceramics. There is an unusual inverse account the numerical values of al-mechanical relationship between strength of a composite and hardness of characteristics for each layer of the Ite were not iC layers(Fig. 7). The decrease in hardness is determined by determined experimentally, the following estimates of the the relevant growth of porosity in silicon carbide. Hence, the elastic moduli (E)of the layers were assumed as a first increase of composite strength with the growth of porosity of approximation for further calculations: EA=50 GPa(material one of its component is stipulated by a high relaxation ability A) and EB=50 GPa(material B), which is justified by the and, apparently, high magnitude of critical fracture strains. object of this study The composition and strength of composites prepared using SiCUFos powders with small amount of additives and perspective for the high-temperature applications are given 3.3.1. Linear approach Table 4 Test results for the laminated specimen were used to obtain The same as in case of SiCMs powders, materials with large the experimental load F--deflection w curve for its middle mismatch of thermal expansion between layers(the composites cross-section (Fig. 8). This nonlinear curve exhibits the and 2)have low strength--60-65 MPa Decrease in thermal portions of drastic relaxation of applied loads xpansion mismatch between layers(composites 3 and 4) The analysis of the diagram demonstrated that elastic eliminates microcracking and allows for increasing strength of moduli EA and Eb of the layers were initial ones and they composites up to 300-400 MPa. It should be noted that for all cannot be used for getting analytical results fitted to layered composites, a standard deviation and fluctuation factor experimental data. At the same time, the comprehensive 2-4). It clearly demonstrates that the laminated structure makes based on g-e diagrams for each of those materials. This would geneities)which should result in the increase of Weibull procedure for calculating composite beams [21, 221 parameters and reliability Let us first consider the calculations for the composite specimen based on initially preset elastic moduli, i.e. let us 3.3. Experimental and theoretical studies on the nonlinear make calculations by linear approximation. Their results are stress-strain state of a laminated ceramic composite Composites designed as a package of twelve alternating layers of more rigid(material A) and less rigid (material B) HV SiC layers materials were used in investigations. Material A is a dense TiB2-B4C layer, and material B is a porous SiC layer(relative 5 HHV TiB porosity up to 40%o) Previous experimental investigations of the above laminated composite demonstrated the inelastic pattern of its defor- mation, which is probably caused by microcracking in the structure as a result of internal stresses in composite layers [6] At the present stage of investigation, the mechanical behavior of the composite was analyzed regardless of the initial stress- strain state determined by the above residual stresses and its Fig. 7. Relations between strength of composites and hardness of their layers.(400–600 MPa), and the composites strength exceeds the strength of monolithic ceramics. There is an unusual inverse relationship between strength of a composite and hardness of SiC layers (Fig. 7). The decrease in hardness is determined by the relevant growth of porosity in silicon carbide. Hence, the increase of composite strength with the growth of porosity of one of its component is stipulated by a high relaxation ability and, apparently, high magnitude of critical fracture strains. The composition and strength of composites prepared using SiCUF05 powders with small amount of additives and perspective for the high-temperature applications are given in Table 4. The same as in case of SiCM5 powders, materials with large mismatch of thermal expansion between layers (the composites 1 and 2) have low strength—60–65 MPa. Decrease in thermal expansion mismatch between layers (composites 3 and 4) eliminates microcracking and allows for increasing strength of composites up to 300–400 MPa. It should be noted that for all layered composites, a standard deviation and fluctuation factor of strength is much lower, than for monolithic ceramics (Tables 2–4). It clearly demonstrates that the laminated structure makes it possible to control effectively the size of defects (inhomo￾geneities) which should result in the increase of Weibull parameters and reliability. 3.3. Experimental and theoretical studies on the nonlinear stress–strain state of a laminated ceramic composite Composites designed as a package of twelve alternating layers of more rigid (material A) and less rigid (material B) materials were used in investigations. Material A is a dense TiB2–B4C layer, and material B is a porous SiC layer (relative porosity up to 40%). Previous experimental investigations of the above laminated composite demonstrated the inelastic pattern of its defor￾mation, which is probably caused by microcracking in the structure as a result of internal stresses in composite layers [6]. At the present stage of investigation, the mechanical behavior of the composite was analyzed regardless of the initial stress– strain state determined by the above residual stresses and its further change in response to mechanical loading. Taking into account that the numerical values of physical–mechanical characteristics for each layer of the composite were not determined experimentally, the following estimates of the elastic moduli (E) of the layers were assumed as a first approximation for further calculations: EAZ50 GPa (material A) and EBZ50 GPa (material B), which is justified by the object of this study. 3.3.1. Linear approach Test results for the laminated specimen were used to obtain the experimental load F—deflection w curve for its middle cross-section (Fig. 8). This nonlinear curve exhibits the portions of drastic relaxation of applied loads. The analysis of the diagram demonstrated that elastic moduli EA and EB of the layers were initial ones and they cannot be used for getting analytical results fitted to experimental data. At the same time, the comprehensive theoretical investigation of composite deformation should be based on s–3 diagrams for each of those materials. This would allow for the evaluation of the stress–strain state by a known procedure for calculating composite beams [21,22]. Let us first consider the calculations for the composite specimen based on initially preset elastic moduli, i.e. let us make calculations by linear approximation. Their results are Fig. 7. Relations between strength of composites and hardness of their layers. Fig. 6. Structure of strong (400–600 MPa) composites with porous SiC layers. 536 O.N. Grigoriev et al. / Composites: Part B 37 (2006) 530–541