J.M. Ehrman et aL/Ce tes Science and Technology 67(2007)1425-1438 1433 fracture planes of both specimens are not well defined. The which shows matrix material still bonded to the pulled 0o fiber tows break over a wide range of axial locations in out fiber general spanning the entire width of the specimen. The While regions of both brushy and nearly planar failure fibers in the 0 tows in each cloth layer exhibit random fail- are present in all fracture surfaces, the balance of these ure producing brushy fracture surfaces. Note that the spec- two fracture topographies within a given fracture surface imen tested in air, which achieved a fatigue run-out and is influenced by the test type(see Fig 12). The fracture sur failed in the subsequent tensile test, has a considerably face produced in fatigue(Fig. 12a)is dominated by areas of longer damage zone than the specimen tested in steam, uncorrelated fiber fracture where individual fibers are produced damage zones ranging from I to 13 mm in in fatigue with 10 s hold(Fig. 12b) still exhibits multiple length. It is noteworthy that specimens which exhibited areas of uncoordinated brushy failure, areas of planar frac- longer lifetimes invariably produced longer damage zones. ture are also visible. Coordinated fracture becomes more All fracture surfaces obtained in this effort contain bru- prevalent as the hold time increases to 100s(Fig. 12c) shy regions of fibrous fracture as well as regions of flatter, Finally, the fracture surface produced in creep(Fig. 12d) more coordinated fracture topography. Typical features of is dominated by areas of nearly planar failure. It is seen the composite microstructure are shown in Fig. ll. An that extensive fiber pullout is produced in cyclic loading overall view of a fracture surface is presented in Fig. lla. and predominantly planar fracture, in creep. Once a hold Fig. Ilb shows pullout of individual fibers. Note that the time is introduced into a fatigue cycle, regions of coordi- locations of the fiber failure within an individual tow, nated failure are seen. The planar fracture topography and consequently the lengths of fiber pullout exhibit a becomes more prevalent with increasing hold time broad distribution Region of coordinated fracture of the Fracture surfaces obtained in the 125 MPa creep-fatigue 0o tows is seen in Fig. llc. Coordinated fiber fracture typ- interaction tests in steam are shown in Fig. 13. The fatigue ally is indicative of a single crack front passing through fracture surface exhibits mostly uncorrelated fiber failure. the tow. Evidence of the strong fiber-matrix bond, charac- while the creep fracture surface is almost entirely planar teristic of the present composite can be seen in Fig. Ild, However, the fracture surfaces produced in fatigue with 如mM 1.0mm Fig. 12. Fracture surfaces of the N720/A CMC tested at 1200C in air with omax= 125 MPa: (a)fatigue, (b )fatigue with 10-s hold, (c) fatigue with 100-s hold, and (d) creepfracture planes of both specimens are not well defined. The 0 fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The fibers in the 0 tows in each cloth layer exhibit random fail￾ure producing brushy fracture surfaces. Note that the spec￾imen tested in air, which achieved a fatigue run-out and failed in the subsequent tensile test, has a considerably longer damage zone than the specimen tested in steam, which failed after 0.23 h. Specimens tested in this effort produced damage zones ranging from 1 to 13 mm in length. It is noteworthy that specimens which exhibited longer lifetimes invariably produced longer damage zones. All fracture surfaces obtained in this effort contain bru￾shy regions of fibrous fracture as well as regions of flatter, more coordinated fracture topography. Typical features of the composite microstructure are shown in Fig. 11. An overall view of a fracture surface is presented in Fig. 11a. Fig. 11b shows pullout of individual fibers. Note that the locations of the fiber failure within an individual tow, and consequently the lengths of fiber pullout exhibit a broad distribution. Region of coordinated fracture of the 0 tows is seen in Fig. 11c. Coordinated fiber fracture typ￾ically is indicative of a single crack front passing through the tow. Evidence of the strong fiber–matrix bond, charac￾teristic of the present composite can be seen in Fig. 11d, which shows matrix material still bonded to the pulled out fiber. While regions of both brushy and nearly planar failure are present in all fracture surfaces, the balance of these two fracture topographies within a given fracture surface is influenced by the test type (see Fig. 12). The fracture sur￾face produced in fatigue (Fig. 12a) is dominated by areas of uncorrelated fiber fracture where individual fibers are clearly discernable. While the fracture surface produced in fatigue with 10 s hold (Fig. 12b) still exhibits multiple areas of uncoordinated brushy failure, areas of planar frac￾ture are also visible. Coordinated fracture becomes more prevalent as the hold time increases to 100 s (Fig. 12c). Finally, the fracture surface produced in creep (Fig. 12d) is dominated by areas of nearly planar failure. It is seen that extensive fiber pullout is produced in cyclic loading and predominantly planar fracture, in creep. Once a hold time is introduced into a fatigue cycle, regions of coordi￾nated failure are seen. The planar fracture topography becomes more prevalent with increasing hold time. Fracture surfaces obtained in the 125 MPa creep–fatigue interaction tests in steam are shown in Fig. 13. The fatigue fracture surface exhibits mostly uncorrelated fiber failure, while the creep fracture surface is almost entirely planar. However, the fracture surfaces produced in fatigue with Fig. 12. Fracture surfaces of the N720/A CMC tested at 1200 C in air with rmax = 125 MPa: (a) fatigue, (b) fatigue with 10-s hold, (c) fatigue with 100-s hold, and (d) creep. J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438 1433