Oxide composites of Al203 and LaPO4 2423 extension measurements; however, the strain is very ing occurred in the LapOa matrix, and pieces of the nonuniform within the gauge area. The stress-strain matrix pulled out with the fibers as in Fig 8(a) curve shows substantial nonlinearity in the loading higher magnification micrographs such as Fig 8(b) portion and continuously decreasing load beyond show that debonding and sliding occurred at the the peak. Test specimens without notches showed fiber-matrix interface [scratch marks due to fiber similar stress-strain curves, with strengths, in the sliding can be seen in the LaPOa matrix in Fig range 200 to 220 MPa 8(b), in the hole left behind after pulling out the The nonlinear response is associated with exten- fiber ive pullout of fibers, as seen from the separated ends of the test specimen in Fig. 5. Failure of the specimen occurred by separation of the matrix 4 Discussion along an approximately planar section between the ends of the two notches and pullout of individual Under tensile loading parallel to the 0 or 90 weave fibers over lengths up to ml cm from both sides of direction, the AlO3/LaPO4 matrix composite the fracture plane. Small but finite loads were sup- shows nonlinear stress-strain response, with the ported by these bridging fibers at crack opening nonlinear component of strain at the peak load displacements of several mm. Examples of intact being several times the linear elastic strain extra- pecimens loaded beyond the peak in the stress- polated from low loads. Failure is non-cata strain curve are shown in Fig. 6: at a crack opening strophic, with fiber pullout over distances of l of Nl mm in Fig. 6(a),(showing bridging fibers) cm. Strengths, in the range 200-250 MPa, are and at a crack opening of 3 mm in Fig. 6(b)(in- unaffected by the presence of severe notches. These situ micrograph with net section stress 8 MPa) characteristics differ from the responses reported 3.2 Fully dense LaPO4-matrix composite The microstructure of the hot pressed LaPO4-matrix composite is shown in the polished cross-section of 0.01 0.02 Fig. 7. The matrix contains a few isolated pores but is mostly fully dense LaPO4. There is no apparent degradation of the sapphire fibers or reaction with the matrix Micrographs of the specimen depicted in Fig. 2 after loading to failure are shown in Fig. 8. Pullout of the sapphire fibers occurred over lengths of sev eral mm, with most fibers pulling out from one side of the fracture surface. Although extensive crack. Fig 4. Tensile test results from co 1g. Fig. 3. Microstructure of composite from Fig. 1(secondary Fig. 5. Separated test specimen from test in Fig. 4extension measurements; however, the strain is very nonuniform within the gauge area. The stress±strain curve shows substantial nonlinearity in the loading portion and continuously decreasing load beyond the peak. Test specimens without notches showed similar stress±strain curves, with strengths, in the range 200 to 220 MPa. The nonlinear response is associated with exten￾sive pullout of ®bers, as seen from the separated ends of the test specimen in Fig. 5. Failure of the specimen occurred by separation of the matrix along an approximately planar section between the ends of the two notches and pullout of individual ®bers over lengths up to 1 cm from both sides of the fracture plane. Small but ®nite loads were sup￾ported by these bridging ®bers at crack opening displacements of several mm. Examples of intact specimens loaded beyond the peak in the stress± strain curve are shown in Fig. 6: at a crack opening of 1 mm in Fig. 6(a), (showing bridging ®bers) and at a crack opening of 3 mm in Fig. 6(b) (in￾situ micrograph with net section stress 8 MPa). 3.2 Fully dense LaPO4-matrix composite The microstructure of the hot pressed LaPO4-matrix composite is shown in the polished cross-section of Fig. 7. The matrix contains a few isolated pores, but is mostly fully dense LaPO4. There is no apparent degradation of the sapphire ®bers or reaction with the matrix. Micrographs of the specimen depicted in Fig. 2 after loading to failure are shown in Fig. 8. Pullout of the sapphire ®bers occurred over lengths of sev￾eral mm, with most ®bers pulling out from one side of the fracture surface. Although extensive crack￾ing occurred in the LaPO4 matrix, and pieces of the matrix pulled out with the ®bers as in Fig. 8(a), higher magni®cation micrographs such as Fig. 8(b) show that debonding and sliding occurred at the ®ber±matrix interface [scratch marks due to ®ber sliding can be seen in the LaPO4 matrix in Fig. 8(b), in the hole left behind after pulling out the ®ber]. 4 Discussion Under tensile loading parallel to the 0 or 90 weave direction, the Al2O3/LaPO4 matrix composite shows nonlinear stress±strain response, with the nonlinear component of strain at the peak load being several times the linear elastic strain extra￾polated from low loads. Failure is non-cata￾strophic, with ®ber pullout over distances of 1 cm. Strengths, in the range 200±250 MPa, are una€ected by the presence of severe notches. These characteristics di€er from the responses reported Fig. 4. Tensile test results from composite of Fig. 1. Fig. 5. Separated test specimen from test in Fig. 4. Fig. 3. Microstructure of composite from Fig. 1 (secondary electron image). Oxide composites of Al2O3 and LaPO4 2423