M B. Ruggles-Wrenn et al/ Composites Science and Technology 66(2006)2089-2099 Fig. 13. Fracture surfaces of a Nextel610/Monazite/Alumina specimen tested in creep at 900C(creep stress= 120 MPa, time to rupture=432, 175 s) showing:(a) Fibers debonding from the matrix and a matrix crack propagating around the fibers and (b) residue of the monazite coating and small pieces of matrix attached to the exposed fibers surfaces also exhibit matrix sockets left by pullout of single brittle-type fracture dominated the fracture surfaces. while filaments as well as of small bundles. Along with the some pullout of fiber bundles was present, the extensive regions of extensive pullout, fracture surfaces contain small pullout of individual fibers was not observed regions of flatter, more coordinated fracture topography with little or no fiber pullout, as shown in Fig. 12(d). Close 5. Concluding remarks examination reveals that most of the fibers fracture on dif- ferent planes, suggesting that a single crack front did not 5.1. Monotonic tension cause this fracture topography. The nearly planar fracture surface topography of the 90 plies is also seen in The tensile stress-strain behavior of N610/A and N610/ ig. 12(d). This micrograph also reveals a higher matrix M/A composites was investigated and the tensile properties volume and sparse fiber distribution at the edge of the measured at room and elevated temperatures. At tempera ply. Fig 13 demonstrates the efficacy of monazite coating tures <1100C, the stress-strain behavior of the uncoated in providing crack deflection. The matrix crack seen in fiber composite is nearly linear elastic until failure, while Fig 13(a)is effectively deflected and the crack front does the behavior of the N610/M/A composite becomes nonlin not cut across. but rather meanders around the fibers. a ear as the strain exceeds 0%. The addition of monazite higher magnification view in Fig. 13(b) shows residue of coating results in near 50% improvement in strength; how- the monazite coating and small pieces of matrix attached ever, it also causes a 34% decrease in modulus and a three- to the exposed fibers fold increase in failure strain. At 1200C, both composites In contrast to the " brushy" fracture surfaces of the exhibit highly nonlinear stress-strain behavior and a signif- N610/M/A, the fracture surfaces of the N610/A composite icant decrease in strength and modulus. The use of mona were considerably more planar(see Fig 14). Flat regions of zite coating results in 37% improvement in UTS Fig. 14. Fracture surface of a Nextel 610/alumina specimen tested in creep at 900C(creep stress=80 MPa, time to rupture= 19,995 s).(a) Regions of planar fracture with no individual fiber pullout dominate the fracture surface and (b) fiber bundle pullout and planar fracture of the 90 plies are visiblesurfaces also exhibit matrix sockets left by pullout of single filaments as well as of small bundles. Along with the regions of extensive pullout, fracture surfaces contain small regions of flatter, more coordinated fracture topography with little or no fiber pullout, as shown in Fig. 12(d). Close examination reveals that most of the fibers fracture on dif￾ferent planes, suggesting that a single crack front did not cause this fracture topography. The nearly planar fracture surface topography of the 90 plies is also seen in Fig. 12(d). This micrograph also reveals a higher matrix volume and sparse fiber distribution at the edge of the ply. Fig. 13 demonstrates the efficacy of monazite coating in providing crack deflection. The matrix crack seen in Fig. 13(a) is effectively deflected and the crack front does not cut across, but rather meanders around the fibers. A higher magnification view in Fig. 13(b) shows residue of the monazite coating and small pieces of matrix attached to the exposed fibers. In contrast to the ‘‘brushy’’ fracture surfaces of the N610/M/A, the fracture surfaces of the N610/A composite were considerably more planar (see Fig. 14). Flat regions of brittle-type fracture dominated the fracture surfaces. While some pullout of fiber bundles was present, the extensive pullout of individual fibers was not observed. 5. Concluding remarks 5.1. Monotonic tension The tensile stress–strain behavior of N610/A and N610/ M/A composites was investigated and the tensile properties measured at room and elevated temperatures. At tempera￾tures 61100 C, the stress–strain behavior of the uncoated fiber composite is nearly linear elastic until failure, while the behavior of the N610/M/A composite becomes nonlin￾ear as the strain exceeds 0.1%. The addition of monazite coating results in near 50% improvement in strength; how￾ever, it also causes a 34% decrease in modulus and a three￾fold increase in failure strain. At 1200 C, both composites exhibit highly nonlinear stress–strain behavior and a signif￾icant decrease in strength and modulus. The use of mona￾zite coating results in 37% improvement in UTS. Fig. 13. Fracture surfaces of a Nextel610/Monazite/Alumina specimen tested in creep at 900 C (creep stress = 120 MPa, time to rupture = 432,175 s) showing: (a) Fibers debonding from the matrix and a matrix crack propagating around the fibers and (b) residue of the monazite coating and small pieces of matrix attached to the exposed fibers. Fig. 14. Fracture surface of a Nextel 610/alumina specimen tested in creep at 900 C (creep stress = 80 MPa, time to rupture = 19,995 s). (a) Regions of planar fracture with no individual fiber pullout dominate the fracture surface and (b) fiber bundle pullout and planar fracture of the 90 plies are visible. M.B. Ruggles-Wrenn et al. / Composites Science and Technology 66 (2006) 2089–2099 2097