March 2002 Efects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite 000hat1200c 1000hat1100°c 1000hat1000°c 000 0003 0.004 Tensile Strain 10 u0z0 (b) Aging Temperature(C) (c) As-Processed 48% Nextel 720 fiber (Jurf and Butner, 2000 3 mm 0°/90 Fig 3. Macrophotographs of the 0/90 tensile specimens in two orthog- 20040060080010001200 onal views: (a) as-processed, (b)after aging for 1000 h at 1100.C, and(c Aging Temperature(C) after aging for 1000 h at 1200C Fig. 2. Effects of thermal aging on the tensile properties of the 0/90 composite. The modulus was calculated from the slope of the initial linear strain is accommodated by additional opening displacement of the rtion of the stress-strain curve over a stress range of =50 MPa. racks such that the net average strain is approximately zero. This hypothesis is supported by the measurements of the composite porosity, which indicated no significant change after any of the between fiber tows. They arrest at the interface with the longitu- aging treatments. Higher-magnification SEM examinations of the dinal fibers (oriented perpendicular to the crack plane) and matrix microstructure did not reveal any other obvious changes penetrate only slightly into the transverse tows. Following aging, due to aging the pattern of matrix cracks remains essentially the same, with the Representative stress-strain curves for the as-processed and the exception that the cracks tend to grow into the transverse tows and aged specimens in the 0 /90 orientation are plotted in Fig. 2(a) their opening displacement increases somewhat(see, for example, The variations in the Youngs modulus, E, and the ultimate tensile the cracks on the right side of Fig. 1(b)). These features are strength, u, with aging temperature are summarized in Figs. 2(b) believed to be due to some matrix densification in the matrix and (c). The only significant change is the slight increase in th segments contained between the cracks along the direction per- modulus, from 60 GPa in the as-processed condition to 70 GPa pendicular to the cracks. Since this shrinkage is constrained by the after the 1200C aging treatment. The tensile strength and the (dense) fibers in the adjacent longitudinal tows, the shrinkage failure strain, Es, remained unchanged; the averages and standardbetween fiber tows. They arrest at the interface with the longitu￾dinal fibers (oriented perpendicular to the crack plane) and penetrate only slightly into the transverse tows. Following aging, the pattern of matrix cracks remains essentially the same, with the exception that the cracks tend to grow into the transverse tows and their opening displacement increases somewhat (see, for example, the cracks on the right side of Fig. 1(b)). These features are believed to be due to some matrix densification in the matrix segments contained between the cracks along the direction per￾pendicular to the cracks. Since this shrinkage is constrained by the (dense) fibers in the adjacent longitudinal tows, the shrinkage strain is accommodated by additional opening displacement of the cracks such that the net average strain is approximately zero. This hypothesis is supported by the measurements of the composite porosity, which indicated no significant change after any of the aging treatments. Higher-magnification SEM examinations of the matrix microstructure did not reveal any other obvious changes due to aging. Representative stress–strain curves for the as-processed and the aged specimens in the 0°/90° orientation are plotted in Fig. 2(a). The variations in the Young’s modulus, E, and the ultimate tensile strength, u, with aging temperature are summarized in Figs. 2(b) and (c). The only significant change is the slight increase in the modulus, from 60 GPa in the as-processed condition to 70 GPa after the 1200°C aging treatment. The tensile strength and the failure strain, εf , remained unchanged; the averages and standard Fig. 2. Effects of thermal aging on the tensile properties of the 0°/90° composite. The modulus was calculated from the slope of the initial linear portion of the stress–strain curve, over a stress range of 50 MPa. Fig. 3. Macrophotographs of the 0°/90° tensile specimens in two orthog￾onal views: (a) as-processed, (b) after aging for 1000 h at 1100°C, and (c) after aging for 1000 h at 1200°C. March 2002 Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite 597