M.B. Ruggles-Wrenn, P D. Laffey / Composites Science and Technology 68(2008)2260-2266 a and clean, indicating that only a single fiber layer is associated with delamination, some rough areas exposing debris and fiber break age are also visible In the course of delamination the departing fi- a bers leave distinct troughs in the remaining matrix (see higher magnification views in Fig. 9b and c). Small amounts of the matrix material remain bonded to the fibers exposed during delamination y contrast, the fracture surface of the dNS specimen tested in compression following 100 h of prior creep at 6.5 MPa at 1200C case includes considerable fiber fracture. The rougher fracture su face shows increased damage in fiber tows, frequently exposing T=1200°c,Air multiple 0 /90 fiber layers. Only some limited areas show clean delamination of a single fiber layer from the matrix-rich regions 0.00 0.25 (Fig. 10a). n in Fig. 10c, the failure also involves extensive Compressive Strain (%) damage to the matrix. Recent studies [33, 34 demonstrated that for a composite con- b sisting of Nextel720 fibers in a porous alumina matrix, a porosity T=1200°c, Stean reduction of -6% was observed after a 10-min exposure at 1200C, which was caused by additional sintering of the matrix. It is likely 24 h at o MPa in steam that additional sintering of the matrix occurred during the 100 h creep test at 6.5 MPa. The resultant strengthening of the matrix is manifested in the retained ILSS of the composite Results in Table 2 show that the ILSS increased after prior creep at 6.5 MPa. The strengthening is also manifested in the change of the failure mech- anism. The failure of the pre-crept composite involves extensive fi ber fracture, while the as-processed material fails predominantly through matrix damage and interply delamination. 100 h at 4 MPa in steam The fracture surfaces obtained in steam(Figs. 11 and 12)are considerably more violent and rough than those obtained in air. The fracture surface produced in creep at 6.5 MPa(Fig. 11a and Compressive Strain(%) b)as well as the fracture surface obtained in compression test on a specimen subjected to 100 h of prior creep at 4 MPa(Fig. 12a ig. 8. Effects of prior creep in interlaminar shear on interlaminar shear stress- and b)reveal extensive fracture of fiber tows. As seen in Figs. 11c compressive strain behavior of N720/A ceramic composite at 1200C: (a)in air and and 12c, the amount of matrix material remaining bonded to the b)in steam. exposed fibers is greater than that in the specimens tested in air. In fact, it appears that during tests of over 100 h duration con- of the woven 0 /90 fiber layers from the matrix-rich regions a ducted in steam, the fiber tows become bonded together by the pears to be the primary mechanism of interlaminar shear failure. matrix material and then fail in coordinated fashion. It is possible While most of the fracture surface shown in Fig. 9a is fairly smooth that the sintering of the matrix is accelerated in the presence of 1. 0 mm Fig 9. Fracture surface of the DNS specimen tested in compression to failure at 1200C in air. Fig. 10. Fracture surface of the dNS specimen tested in compression to failure following 100 h at 6.5 MPa at 1200C in air.of the woven 0/90 fiber layers from the matrix-rich regions ap￾pears to be the primary mechanism of interlaminar shear failure. While most of the fracture surface shown in Fig. 9a is fairly smooth and clean, indicating that only a single fiber layer is associated with delamination, some rough areas exposing debris and fiber break￾age are also visible. In the course of delamination the departing fi- bers leave distinct troughs in the remaining matrix (see higher magnification views in Fig. 9b and c). Small amounts of the matrix material remain bonded to the fibers exposed during delamination. By contrast, the fracture surface of the DNS specimen tested in compression following 100 h of prior creep at 6.5 MPa at 1200 C in air (see Fig. 10a and b) reveals that the failure mechanism in this case includes considerable fiber fracture. The rougher fracture sur￾face shows increased damage in fiber tows, frequently exposing multiple 0/90 fiber layers. Only some limited areas show clean delamination of a single fiber layer from the matrix-rich regions (Fig. 10a). As seen in Fig. 10c, the failure also involves extensive damage to the matrix. Recent studies [33,34] demonstrated that for a composite con￾sisting of NextelTM720 fibers in a porous alumina matrix, a porosity reduction of 6% was observed after a 10-min exposure at 1200 C, which was caused by additional sintering of the matrix. It is likely that additional sintering of the matrix occurred during the 100 h creep test at 6.5 MPa. The resultant strengthening of the matrix is manifested in the retained ILSS of the composite. Results in Table 2 show that the ILSS increased after prior creep at 6.5 MPa. The strengthening is also manifested in the change of the failure mech￾anism. The failure of the pre-crept composite involves extensive fi- ber fracture, while the as-processed material fails predominantly through matrix damage and interply delamination. The fracture surfaces obtained in steam (Figs. 11 and 12) are considerably more violent and rough than those obtained in air. The fracture surface produced in creep at 6.5 MPa (Fig. 11a and b) as well as the fracture surface obtained in compression test on a specimen subjected to 100 h of prior creep at 4 MPa (Fig. 12a and b) reveal extensive fracture of fiber tows. As seen in Figs. 11c and 12c, the amount of matrix material remaining bonded to the exposed fibers is greater than that in the specimens tested in air. In fact, it appears that during tests of over 100 h duration con￾ducted in steam, the fiber tows become bonded together by the matrix material and then fail in coordinated fashion. It is possible that the sintering of the matrix is accelerated in the presence of 0 2 4 6 8 10 12 0.00 0.05 0.10 0.15 0.20 0.25 Shear Stress (MPa) Compressive Strain (%) T = 1200 ºC, Air As-processed 100 h at 6.5 MPa in air 0 2 4 6 8 10 12 0.00 0.05 0.10 0.15 0.20 0.25 Shear Stress (MPa) Compressive Strain (%) T = 1200 ºC, Steam 24 h at 0 MPa in steam 100 h at 4 MPa in steam Fig. 8. Effects of prior creep in interlaminar shear on interlaminar shear stress– compressive strain behavior of N720/A ceramic composite at 1200 C: (a) in air and (b) in steam. Fig. 9. Fracture surface of the DNS specimen tested in compression to failure at 1200 C in air. Fig. 10. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 6.5 MPa at 1200 C in air. 2264 M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266