Santhan et al. /Composites: Part A 32(2001)1095-110 1101 200 Stress c as hot-pressed, (75 GPa) D1200°c,2h(74.5GPa) 100 1300°c,1000h,cyc.(92GPa) 0 H 1300.C, 1000h, const. (91 GPa) -0.20.00 040.6 1,012141,6 hot-pressing as well as after heat-treatments was determined composite. The Youngs modulus was reduced from 106 by three-point-bending tests of the composites at room to 87 GPa(after 1200C/2 h) and strength from 230 to temperature. For the calculation of the Youngs modulus for 200 MPa. Whereas cyclic prolonged heat-treatment at each composite, the linear elastic part of the stress/strain 1300C resulted in slight increase in the Youngs modulus curves was taken. The stress/strain curves and calculated (117 GPa) but decrease in the strength (170 MPa) Youngs modulus are shown in Fig. 6(a)-(c) Continuous heat-treatment at 1300.C resulted in a severe For the reference composite with initial carbon mono layer decrease of fracture strength (140 MPa). Some load and the carbon/Al -double layer composite, the 1200'C capability after the load maximum was maintained in all heat-treatment resulted in slight changes in ultimate fracture carbon(fugitive)/ZrO2-double layer composites except for strength and Youngs modulus compared with their the prolonged continuously heat-treated composite counterparts as-hot-pressed composite(Fig. 6(a) and(c)) Both hot-pressed and 1200C/2 h heat-treated reference opposites exhibited some load capability during further 4. Discussion straining after reaching the load maximum. In contrast, the prolonged (1000 h) and continuous heat-exposure at For the interpretation of the fibre/matrix-interaction in a 1300.C resulted in the reference composite an increased composite, the Youngs modulus of the composite may be Youngs modulus(84 GPa)and strength(120 MPa)but considered. Assuming that the interphase fulfils the require fully brittle fracture. Cyclic heat-treatment of the composite ment for a good load transfer between the fibre and the at 1300.C for 1000 x I h gave the same elastic modulus and matrix, the Youngs modulus of the composite can be calcu- almost the same fracture strength (160 MPa)as con- lated using the rule of mixtures For our composites, having inuously heat-treated composite, however, it should be 35 vol. %6 Nextel 720-fibers with a mo 260 GPa and 65 vol %o porous matrix with a modulus, Em, apability remains during further straining of the cyclic tested of 46 GPa, Youngs modulus of the composite, EcMc, is sample. Both composites with double layer interphase gave remarkably higher ultimate fracture strength and Youngs modulus, compared to the reference composite(only carbon ECMC=EV+ EmV (1) mono layer)before and after prolonged heat-treatment. ECMc=260GPa×0.35+46GPa×065=12lGPa Somewhat different stress/strain behaviour was observed for the carbon(fugitive )/ZrOr-c omposite Youngs modulus, Er, for the Nextel 720-fibers were (Fig. 6b), compared with the 1200C/2 h heat-treated determined experimentally in a study after heat-treating   ,     ,                   J     KL                3    & ,  -  3    &   KL   ,  J (12F12 J      ,         36'5+      8'))
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