July 1998 nterface Properties in High-Strength Nicalon/C/SiC Composites 40001 4000 3500 3500 3000 3000 32500 2500 2000 2000 1500 500 P:90/2.2/28/Pull 1000 P:50/50/28/Pu00 500 500 0,5 Inelastic Displacement(micrometers) dicted curved of bridging fiber stress versus characteristics as the push-out curves of Fig. 3 ession as in pushout: slightly after the region Ivill trans ment for composite tension tests(Type Il boundary conditions2), assuming the n to the secondary crack is assumed (somewhat arbitrarily) to occur at the same which is -2.2 GPa in tension. Fiber strengths in composite tension tests ly 1.8-2 GPa, hence, composite failure can be to occur before transition to the secondary crack flection is usually assumed to be literally that: the matrix crack near the coating/fiber interface, this observation seems to pro- p propagates into the interfacial region, then turns parallel to the fiber surface. However, it has been suggested that an in- in weak-interface equence of events for the deflection process opposites terfacial crack that develops in a composite under tension may dditional consideration of the failure process tempers th initiate as a mode I crack in the tensile(normal to the fiber conclusion somewhat and provides interesting speculation on ace)stress field ahead of the crack tip, 8,26 and observations he degree to which oxide coatings can be expected to provide on model laminate materials have confirmed the existence of protection against fiber oxidation. Consider the tensile failure such a failure sequence. 7 Because the microscopic details of process that is illustrated in Fig. 5, which shows a matrix crack fracture are difficult to probe experimentally, the exact se- quence of events in real fibrous composites has remained a The consequent debonding crack propagates some distance in matter of speculation. Nevertheless, it is a matter of some im- the coating away from the matrix crack plane. With subsequent portance in understanding the design and analysis of coating loading, the matrix crack by passes the fiber and the debonding systems. In one case, the interfacial fracture will be determined crack propagates somewhat farther within the coating. At that by the radial tensile strength, whereas in the other case, it will point, the crack is bridged by a fiber that is still coated by the be determined by the interfacial shear strengths. Understanding remaining intact portion of the coating. This remaining coating the process is also important to interpret test data. For example, could provide some protection; however, unless the coating is fiber push-in/pull-out tests may measure a somewhat different extremely strain tolerant, it must fail in tension with subsequent property than that which determines debonding during com- loading, which introduces another mode I crack. That crack posite failure may deflect into another mode Il crack that has traversed par- The carbon coatings in both composites are considered to be allel to the fiber axis; however, the process must repeat until the same, except perhaps very near the coating/fiber interface the coating is completely cracked and there is a debonding In the case of an untreated-fiber composite failing in tension, crack in the fiber/coating interface. The only possibility for the weak coating/fiber interface region fails, whereas the coat- ing itself does not, which implies that the interface must fail strains)as the fiber(1%)1.e, much more strain tolerant than before the crack enters the coating, because of the stress field would be expected from equivalent bulk materials. Although in front of the crack. If the crack had traversed through the thin coatings can be expected to exhibit increasing strain-to- coating, it would have been deflected in the coating. The proof failure with decreasing thickness(for example, see Hu et a1. 27) of this statement is provided by the treated-fiber materials, in such high values may be problematic for approaches such which the cracks deflect in the coating before reaching the those which use porous oxides. This scenario implies two con- fiber/coating interface. Provided that the debonding in the sequences: (i) even though crack deflection occurred in the weak-interface(untreated-fiber)composites is truly at or very coating, post-failure analysis may show cracks in the coating/flection is usually assumed to be literally that: the matrix crack tip propagates into the interfacial region, then turns parallel to the fiber surface. However, it has been suggested that an in￾terfacial crack that develops in a composite under tension may initiate as a mode I crack in the tensile (normal to the fiber surface) stress field ahead of the crack tip,8,26 and observations on model laminate materials have confirmed the existence of such a failure sequence.7 Because the microscopic details of fracture are difficult to probe experimentally, the exact se￾quence of events in real fibrous composites has remained a matter of speculation. Nevertheless, it is a matter of some im￾portance in understanding the design and analysis of coating systems. In one case, the interfacial fracture will be determined by the radial tensile strength, whereas in the other case, it will be determined by the interfacial shear strengths. Understanding the process is also important to interpret test data. For example, fiber push-in/pull-out tests may measure a somewhat different property than that which determines debonding during com￾posite failure. The carbon coatings in both composites are considered to be the same, except perhaps very near the coating/fiber interface. In the case of an untreated-fiber composite failing in tension, the weak coating/fiber interface region fails, whereas the coat￾ing itself does not, which implies that the interface must fail before the crack enters the coating, because of the stress field in front of the crack. If the crack had traversed through the coating, it would have been deflected in the coating. The proof of this statement is provided by the treated-fiber materials, in which the cracks deflect in the coating before reaching the fiber/coating interface. Provided that the debonding in the weak-interface (untreated-fiber) composites is truly at or very near the coating/fiber interface, this observation seems to pro￾vide a definitive sequence of events for the deflection process in weak-interface composites. Additional consideration of the failure process tempers this conclusion somewhat and provides interesting speculation on the degree to which oxide coatings can be expected to provide protection against fiber oxidation. Consider the tensile failure process that is illustrated in Fig. 5, which shows a matrix crack that impinges on a coated fiber and is deflected in the coating. The consequent debonding crack propagates some distance in the coating away from the matrix crack plane. With subsequent loading, the matrix crack bypasses the fiber and the debonding crack propagates somewhat farther within the coating. At that point, the crack is bridged by a fiber that is still coated by the remaining intact portion of the coating. This remaining coating could provide some protection; however, unless the coating is extremely strain tolerant, it must fail in tension with subsequent loading, which introduces another mode I crack. That crack may deflect into another mode II crack that has traversed par￾allel to the fiber axis; however, the process must repeat until the coating is completely cracked and there is a debonding crack in the fiber/coating interface. The only possibility for retaining the coating is if it is as strain tolerant (to axial tensile strains) as the fiber (>1%)—i.e., much more strain tolerant than would be expected from equivalent bulk materials. Although thin coatings can be expected to exhibit increasing strain-to￾failure with decreasing thickness (for example, see Hu et al.27), such high values may be problematic for approaches such as those which use porous oxides. This scenario implies two con￾sequences: (i) even though crack deflection occurred in the coating, post-failure analysis may show cracks in the coating/ Fig. 4. Predicted curved of bridging-fiber stress versus displacement for composite tension tests (Type II boundary conditions24), assuming the same crack characteristics as the push-out curves of Fig. 3. Transition to the secondary crack is assumed (somewhat arbitrarily) to occur at the same crack progression as in pushout: slightly after the Region II/III transition, which is ∼2.2 GPa in tension. Fiber strengths in composite tension tests are typically 1.8–2 GPa; hence, composite failure can be expected to occur before transition to the secondary crack. July 1998 Interface Properties in High-Strength Nicalon/C/SiC Composites 1885