D. Koch et al. Composites Science and Technology 68(2008)1165-1172 1167 sic/sic,cⅥ +45°-45° 3 2 0 [12]风闾8]10冈冈图3图3[2 Porous matrix Dense matrix Tensile Test WMC WIC C/C. LPI 0°90° Fig. 4. WMC-WIC classification of different composites according to their stiffness ratio Eoo/E4so. Results from literature [2, 3, 5-8, 10, 12]and own measurements(marked with [x] Size effects are also not really significant and low scattering of strength is observed +45°-45° 3. Weak matrix composites, WMC Infiltration processes as LPl, Lsl, or CSI to provide the matrix in CMCs lead to microstructures which are charac- Strain[°%] terized by a fine porosity and therefore a low stifness. The significantly reduced matrix stifness and strength, com- Fig. 3. Representative stress versus strain curves of (a)WIC(CVI SiC/ pared to WIC, enables debonding processes and thus dam Si[2]and(b)WMC(LPIC/C) from axial(090° and diagonal(±45°) loaded tensile tests age tolerance even in the case of a strong fiber-matrix interface as the cracks which propagate through the matrix easily deviate close to the fiber surface through the matrix. layers occurs, the mechanical properties may change and This concept again corresponds to the theoretical analysis lead to an increase of the relative fracture energy of fiber of He and Hutchinson(Fig. 1)where the large difference and interface. Acceptable changes of the interfacial proper- between the Youngs moduli of the stiff fiber and the weak ties without provoking brittle behavior can be discussed matrix allows a much stronger bonding and a higher ratio using the borderline in Fig. 1. Hence, the interphase has between the fracture energies of interface and fiber. while to accomplish not only mechanical functions in order to the matrix fails at low stresses the composite can still be provide debonding: it also has to fulfill thermal and envi- loaded well above the matrix cracking strength as long as ronmental boundary conditions as e.g. sufficient oxidation the overall load can be transferred to the fibers. However resistance. Fig. 2 shows the effect of oxidation of the fiber as the redistribution of stresses from the fiber to the matrix matrix interface in a SiC/DiMOx composite as manifested does not take place in a significant manner final failure of by single fiber push-in tests. It shows that due to the forma- the composite occurs when the fibers do not fail locally tion of silica at the interface after oxidation at 900C initial restricted but in a large volume of the component. Thus, debonding is no longer possible at sufficiently low stresses. the mechanical behavior can no longer be described by a This leads to brittle failure of the composite as debonding micromechanical approach. Furthermore the mechanical is prevented behavior of WMC is dominated strongly by the properties It can be concluded that WIC with relatively strong and of the fibers and therefore the mechanical performanc stiff matrices and obligatory fiber coating provide high depends on their orientation and the loading direction fracture toughness values. Following the micromechanical As the matrix is not able to carry significant load, low mechanisms of debonding, these CMCs are relatively notch strength will be obtained under compression or loading insensitive with the highest achievable stress being fairly mode with an angle between fiber orientation and loading independent of the fiber and applied stress orientations. direction.layers occurs, the mechanical properties may change and lead to an increase of the relative fracture energy of fiber and interface. Acceptable changes of the interfacial proper￾ties without provoking brittle behavior can be discussed using the borderline in Fig. 1. Hence, the interphase has to accomplish not only mechanical functions in order to provide debonding; it also has to fulfill thermal and envi￾ronmental boundary conditions as e.g. sufficient oxidation resistance. Fig. 2 shows the effect of oxidation of the fiber matrix interface in a SiC/DiMOx composite as manifested by single fiber push-in tests. It shows that due to the forma￾tion of silica at the interface after oxidation at 900 C initial debonding is no longer possible at sufficiently low stresses. This leads to brittle failure of the composite as debonding is prevented. It can be concluded that WIC with relatively strong and stiff matrices and obligatory fiber coating provide high fracture toughness values. Following the micromechanical mechanisms of debonding, these CMCs are relatively notch insensitive with the highest achievable stress being fairly independent of the fiber and applied stress orientations. Size effects are also not really significant and low scattering of strength is observed. 3. Weak matrix composites, WMC Infiltration processes as LPI, LSI, or CSI to provide the matrix in CMCs lead to microstructures which are charac￾terized by a fine porosity and therefore a low stiffness. The significantly reduced matrix stiffness and strength, com￾pared to WIC, enables debonding processes and thus dam￾age tolerance even in the case of a strong fiber–matrix interface as the cracks which propagate through the matrix easily deviate close to the fiber surface through the matrix. This concept again corresponds to the theoretical analysis of He and Hutchinson (Fig. 1) where the large difference between the Young’s moduli of the stiff fiber and the weak matrix allows a much stronger bonding and a higher ratio between the fracture energies of interface and fiber. While the matrix fails at low stresses the composite can still be loaded well above the matrix cracking strength as long as the overall load can be transferred to the fibers. However, as the redistribution of stresses from the fiber to the matrix does not take place in a significant manner final failure of the composite occurs when the fibers do not fail locally restricted but in a large volume of the component. Thus, the mechanical behavior can no longer be described by a micromechanical approach. Furthermore the mechanical behavior of WMC is dominated strongly by the properties of the fibers and therefore the mechanical performance depends on their orientation and the loading direction. As the matrix is not able to carry significant load, low strength will be obtained under compression or loading mode with an angle between fiber orientation and loading direction. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 50 100 150 200 250 300 350 400 Tensile Test SiC/SiC, CVI +45°/ -45° 0°/ 90° Stress [MPa] Strain [%] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 50 100 150 200 250 300 350 400 Tensile Test C/C, LPI +45°/ -45° 0°/ 90° Stress [MPa] Strain [%] Fig. 3. Representative stress versus strain curves of (a) WIC (CVI SiC/ SiC) [2] and (b) WMC (LPI C/C) from axial (0/90) and diagonal (±45) loaded tensile tests. 0 1 2 3 4 5 6 E0 / E45 WHIPOX Nicalon/SiC Nicalon/SiC Nicalon/MAS C/SiC Nextel 610 /Mullite + Alumina Nextel 720 /Mullite + Alumina (Al C/C C/C, SIGRABOND C/C O2 3-ZrO2 )mc/Al O3 2 C/C [6] [5] [12] [x] [8] [7] [10] [x] [x] [3] [3] [2] WMC WIC Porous matrix Dense matrix Fig. 4. WMC–WIC classification of different composites according to their stiffness ratio E0/E45. Results from literature [2,3,5–8,10,12] and own measurements (marked with [x]). D. Koch et al. / Composites Science and Technology 68 (2008) 1165–1172 1167