P.W.M. Peters et al. /Journal of the European Ceramic Society 20(2000)531-535 Table 4 Elastic constants and intralaminar shear strengths of different CMCs with a porosity content of 10-15% Process Material Er GPa Em GPa Ey GPa Matrix contr. to En% E, GPa Gn GPa S MPa 21.8 CVI SiC/SIC E∥=ErVr+EmV linearly to 145.2 MPa at 800C. Above 800C a stronger in the absence of porosity. Taking Er(Nextel 610)=331 drop occurs. The modulus is maintained around 100 GPa? and Em=220 GPao for mullite the resulting GPa up to a maximum temperature of 1000C.At modulus of E/= 271 GPa is substantially higher than 1200 C a strong drop in modulus takes place. The the value given in Table 4 stress-strain curves at the different temperatures are Due to porosity and the presence of the probably not practically linear up to failure with the tendency for continuous Si-O-C network in which the mullite pa larger non-linearity at larger temperatures. Non-linear cles are embedded, the matrix contributes only 6% of type of failure giving rise to the before-mentioned quasi the theoretical modulus of mullite ductility can be a result of A comparison made in Table 4 with other CMCs(C/c and Sic/SiC) shows that this behaviour is characteristic (a) matrix cracking for CMCs produced by the pyrolysis of a polymer matrix b) fibre failure resulting in loose bundle type of failure (PP). If the matrix is produced by chemical vapour It has been discussed that the matrix contributes little impregnation(CVI) a continuous stiff matrix with a max- to the modulus of the composite Matrix cracking, thus, imum contribution of the matrix [ Em(1-Vp-Vr)] can hardly give rise to a quasi-ductile behaviour. The can be possible. I only phenomenon which can produce a substantial The shear modulus Gl, can be determined with the amount of non-linearity is fibre failure due to a loose aid of a tensile test on a +45-specimen, if the strain in bundle type of failure. Condition for this type of failure and transverse to the loading direction, &n and et is a low capability of transferring stresses between fibre respectively is measured. Fig. 5 shows the result of such and the matrix an experiment. The shear modulus follows from This low stress transfer capability usually leads to a considerable pull-out. Microscopical observations of G=a/(E1-E1) (2) fracture surfaces, given in Fig. 7, indicate in agreement with the occurrence of little non-linearity only limited whereas the intra-laminar shear strength is given by pull-out of broken fibres, mainly near porous areas. No substantial difference between the fracture surfaces pro- S=Omax/2 ( duced at room temperature and e.g. at 800C test tem- perature occurred. Fracture surfaces illustrate that a The results of the tensile tests at temperatures ranging homogeneous fugitive interface with a theoretical thick from room temperature to 1200oC are indicated in Fig. ness of 0. l um is not present around all fibres but local 6. The strength of the(0/90/0/90/0/90)s specimens at room temperature measures 177. 4 MPa and decreases -+-modulus 目 「 transverse40 axial 80 40140050 01015 200400600 010001200 strain Fig. 6. Tenile strength and modulus of Nextel 610/Mox crossply Fig. 5. Stress-strain curve of +45 specimen at room temperature. laminate as a function of the test temperature.E== ˆ EfVf ‡ EmVm …1† in the absence of porosity. Taking Ef (Nextel 610)=331 GPa9 and Em ˆ 220 GPa10 for mullite the resulting modulus of E== ˆ 271 GPa is substantially higher than the value given in Table 4. Due to porosity and the presence of the probably not continuous Si±O±C network in which the mullite parti￾cles are embedded, the matrix contributes only 6% of the theoretical modulus of mullite. A comparison made in Table 4 with other CMCs (C/C and SiC/SiC) shows that this behaviour is characteristic for CMCs produced by the pyrolysis of a polymer matrix (PP). If the matrix is produced by chemical vapour impregnation (CVI) a continuous sti€ matrix with a max￾imum contribution of the matrix [ ˆ Em…1 ÿ Vp ÿ Vf†] can be possible.11 The shear modulus G==;? can be determined with the aid of a tensile test on a ‹45-specimen, if the strain in and transverse to the loading direction, "l and "t respectively, is measured. Fig. 5 shows the result of such an experiment. The shear modulus follows from: G ˆ = "…† … l ÿ "t 2† whereas the intra-laminar shear strength is given by S ˆ max=2 …3† The results of the tensile tests at temperatures ranging from room temperature to 1200C are indicated in Fig. 6. The strength of the (0/90/0/90/0/90)s specimens at room temperature measures 177.4 MPa and decreases linearly to 145.2 MPa at 800C. Above 800C a stronger drop occurs. The modulus is maintained around 100 GPa up to a maximum temperature of 1000C. At 1200C a strong drop in modulus takes place. The stress-strain curves at the di€erent temperatures are practically linear up to failure with the tendency for larger non-linearity at larger temperatures. Non-linear type of failure giving rise to the before-mentioned quasi￾ductility can be a result of (a) matrix cracking (b) ®bre failure resulting in loose bundle type of failure. It has been discussed, that the matrix contributes little to the modulus of the composite. Matrix cracking, thus, can hardly give rise to a quasi-ductile behaviour. The only phenomenon which can produce a substantial amount of non-linearity is ®bre failure due to a loose bundle type of failure. Condition for this type of failure is a low capability of transferring stresses between ®bre and the matrix. This low stress transfer capability usually leads to a considerable pull-out. Microscopical observations of fracture surfaces, given in Fig. 7, indicate in agreement with the occurrence of little non-linearity only limited pull-out of broken ®bres, mainly near porous areas. No substantial di€erence between the fracture surfaces pro￾duced at room temperature and e.g. at 800C test tem￾perature occurred. Fracture surfaces illustrate that a homogeneous fugitive interface with a theoretical thick￾ness of 0.1 mm is not present around all ®bres but local Table 4 Elastic constants and intralaminar shear strengths of di€erent CMCs with a porosity content of 10±15% Process Material Ef GPa Em GPa E== GPa Matrix contr. to E==% E? GPa G//? GPa S MPa PP C/C 331 35 206 0 0 6 21.8 PP N610/Umox 325 220 163 6 54.4 23.0 36 CVI SiC/SiC 200 469 265 100 130 65 ± Fig. 5. Stress-strain curve of ‹45 specimen at room temperature. Fig. 6. Tenile strength and modulus of Nextel 610/Umox crossply laminate as a function of the test temperature. 534 P.W.M. Peters et al. / Journal of the European Ceramic Society 20 (2000) 531±535