2424 B. Davies et al load (a) Fig. 6.(a) Bridging fiber tows in test loaded as in on.(b)In-situ optical micrograph from test as in Fig. 4 at net section stress f MPa 50m ApOA Fig 8.(a) Fractured test piece of LaPO4 matrix composite. (b) Higher magnification of region of fracture surface from same specimen as(a). Dark circular area is fracture surface of sap- phire fiber. Smooth light region is LaPO4 coating in cylindrical hole (going into page from right to left)remaining after pull out of the other half of the fractured fiber sapphire weave direction. However, the large nonlinear response has not been observed in tensile loading parallel to the 0 or 90 weave directions. In this case, the composites may show a very small non Fig.7. Cross-section of hot-pressed LaPOa-matrix composite linearity before the peak load(nonlinear compo- ( secondary electron image). nent of strain less than 10% of the total strain) and a jagged fracture surface associated with some pullout of fiber tows. However, failure is cata or other porous matrix composites in systems strophic and the extensive pullout of individual where strong local bonding between matrix and fibers, such as in Fig. 5, is not observed. Therefore, fibers would be expected(SiO2/Al2O3,6 Al2O3/ it is evident that, while the porous nature of the mullite, 8 .9AlPO4 ) The strengths of such compo- matrix in the Al2O3/ LaPO4 composite most likely sites under loading parallel to the 0 or 90 direc- contributes to the nonlinear response, the presence of tions have been reported in the same range, while the weakly bonded LaPO phase at the fiber-matrix notch-insensitivity along with nonlinear stress- interface greatly enhances the damage-tolerant strain response and noncatastrophic failure have behavior of the composite been reported under flexural loading and under Although porous-matrix composites may be tensile loading in a direction at 45 to the 0/90 adequate for many applications, certain uses benefor other porous matrix composites in systems where strong local bonding between matrix and ®bers would be expected (SiO2/Al2O3, 6 Al2O3/ mullite,8,9 AlPO4 10). The strengths of such compo￾sites under loading parallel to the 0 or 90 direc￾tions have been reported in the same range, while notch-insensitivity along with nonlinear stress￾strain response and noncatastrophic failure have been reported under ¯exural loading and under tensile loading in a direction at 45 to the 0/90 weave direction.9 However, the large nonlinear response has not been observed in tensile loading parallel to the 0 or 90 weave directions. In this case, the composites may show a very small non￾linearity before the peak load (nonlinear compo￾nent of strain less than 10% of the total strain) and a jagged fracture surface associated with some pullout of ®ber tows. However, failure is cata￾strophic and the extensive pullout of individual ®bers, such as in Fig. 5, is not observed. Therefore, it is evident that, while the porous nature of the matrix in the Al2O3/LaPO4 composite most likely contributes to the nonlinear response, the presence of the weakly bonded LaPO4 phase at the ®ber±matrix interface greatly enhances the damage-tolerant behavior of the composite. Although porous-matrix composites may be adequate for many applications, certain uses bene￾Fig. 7. Cross-section of hot-pressed LaPO4-matrix composite (secondary electron image). Fig. 6. (a) Bridging ®ber tows in test specimen loaded as in Fig. 4, interrupted before complete separation. (b) In-situ optical micrograph from test as in Fig. 4 at net section stress of 8 MPa. Fig. 8. (a) Fractured test piece of LaPO4 matrix composite. (b) Higher magni®cation of region of fracture surface from same specimen as (a). Dark circular area is fracture surface of sap￾phire ®ber. Smooth light region is LaPO4 coating in cylindrical hole (going into page from right to left) remaining after pull￾out of the other half of the fractured ®ber. 2424 J. B. Davies et al