wwceramics. org/ACT Cracking Resistance of Silicon Carbide Composites dense NITE-SiC/SiC composites would provide an ad- would impact the failure behavior. Of particular impor ditional advantage to apply this to gas-cooled systems. tance is then to separately discuss micro-and macrocrack- Accordingly, several fusion blanket designs propose to ing behaviors. A simple guess indicates that, as the failure utilize this class of SiC/SiC composites. With the behavior generally depends on the type and distribution of completion of the "proof-of-principle"phase, the R&d the internal Aaws as a crack origin, there would be an ap- on SiC/SiC composites is now shifting to the more parent difference between dense NITE-SiC/SiC compos pragmatic phase like material data generation and en- ites and conventional porous composites such as CVI- gineering design. SiC/SiC composites. The failure initiation behavior would One of key features of nuclear-grade SiC/SiC com- also vary from the choice of fabric architecture and applied osites is high stiffness due to the utilization of high-crys- loading directions due to the inherent anisotropy of the talline SiC for both reinforcing fibers and matrix. When composites. Therefore, careful discussion is necessary applying this class of composite materials, crack propaga This study aims to identify crack resistance and tion behavior, that is, matrix crack pop-in and crack ex- damage tolerance of a new class of SiC/SiC composites, tension, therefore, needs to be clarified first and the failure that is, NITE-SiC/SiC composites. For this purpose, the criterion needs to be properly determined to deliver in- existing testing methodology for fracture toughness eval formation for the engineering design activities, as it has uation was first applied. Second, the developmental frac- been addressed that the inherent "brittle-like "fracture ture resistance method based on nonlinear fracture (so-called"quasi-ductile"fracture) gives significant diffi- mechanics was applied and the damage accumulation be lty in engineering component design. Note that this havior of composites was evaluated using various notched metals as this quasi-ductility occurs as a result of cumu- lative accumulation of irreversible permanent damages Historically, many efforts have been devoted for the crack propagation analysis by fracture toughness testing Experimental Procedure methodology using notched specimens. -14 Rice devel- Materials oped a path-independent integral method, so-called J integral, and demonstrated the validity of this method Two types of pilot-grade NITE-SiC/SiC for approximate estimates of strain concentrations at ites were produced by the Institute of Energy Science smooth-ended notch tips in elastic and elastic-plastic and Technology(Ube, Japan)(Table I). For the first Hashida composite, a -250-nm-thick pyrolytic carbon(PyC et al. further developed the fracture testing technic interphase as a form of fiber/matrix(F/M) interface was for ceramic matrix composites, which have matrix chemically vapor deposited(CVD)on the fiber surface toughness comparable with bridging toughness induced prior matrix densification (hereafter"NITE-Thick by fibers in the fracture process zone. One drawback of Coat"). In contrast, for the other type, a thin PyC in uate the postpeak part of the stress-displacement relation- case, the F/M interfacial coating was not successful.r the Hashida's approach is the limited applicability to eval- terphase(<50 nm)was designed. However, in the latter ship. Another attempt for the cracking energy evaluation formed, as clearly shown in micrographs in Table I. The has been carried out using unloading/reloading hysteresis major parts of fibers were uncoated, although thin PyC curves of the load-displacement relationship. 12-14 This was formed for limited number of fibers. In this study, technique separately distinguishes irreversible energies we designate this composite"NITE-Thin-Coat " For from elastic energy and crack formation energy. Similarl both types, highly crystalline and near-stoichiometric for fast fracture property evaluation, such hysteresis ap- Tyranno-SA third-grade SiC fibers were uni-direc- nportant composite tionally reinforced ith a fiber volume fraction of parameters can be obtained successfully. However, due 0.4-0.45. Typical micrographs show well-densified ma- to the complex failure behaviors of ites, it is some- trix indicating that the porosity of this material was very how questionable whether these techniques are fully ap- low(<5%). A secondary phase(white contrast in Table plicable to composites. Indeed, there is a general D), which was reportedly an oxide phase composed of understanding that damage accumulation by microcrack sintering additives such as Al2O3, SiO2, and Y20 nitiates prior the complete fracture of composites and this was localized in the matrix, specifically within intrabun-dense NITE–SiC/SiC composites would provide an ad￾ditional advantage to apply this to gas-cooled systems.5 Accordingly, several fusion blanket designs propose to utilize this class of SiC/SiC composites.6–8 With the completion of the ‘‘proof-of-principle’’ phase, the R&D on SiC/SiC composites is now shifting to the more pragmatic phase like material data generation and en￾gineering design. One of key features of nuclear-grade SiC/SiC com￾posites is high stiffness due to the utilization of high-crys￾talline SiC for both reinforcing fibers and matrix. When applying this class of composite materials, crack propaga￾tion behavior, that is, matrix crack pop-in and crack ex￾tension, therefore, needs to be clarified first and the failure criterion needs to be properly determined to deliver in￾formation for the engineering design activities, as it has been addressed that the inherent ‘‘brittle-like’’ fracture (so-called ‘‘quasi-ductile’’ fracture) gives significant diffi- culty in engineering component design. Note that this quasi-ductility is totally different from the ductility of metals as this quasi-ductility occurs as a result of cumu￾lative accumulation of irreversible permanent damages. Historically, many efforts have been devoted for the crack propagation analysis by fracture toughness testing methodology using notched specimens.9–14 Rice devel￾oped a path-independent integral method, so-called J integral, and demonstrated the validity of this method for approximate estimates of strain concentrations at smooth-ended notch tips in elastic and elastic–plastic materials.9 Based on this J integral approach, Hashida et al. 11 further developed the fracture testing technique for ceramic matrix composites, which have matrix toughness comparable with bridging toughness induced by fibers in the fracture process zone. One drawback of the Hashida’s approach is the limited applicability to eval￾uate the postpeak part of the stress–displacement relation￾ship. Another attempt for the cracking energy evaluation has been carried out using unloading/reloading hysteresis curves of the load–displacement relationship.12–14 This technique separately distinguishes irreversible energies from elastic energy and crack formation energy. Similarly, for fast fracture property evaluation, such hysteresis ap￾proach has been widely adopted and important composite parameters can be obtained successfully.15 However, due to the complex failure behaviors of composites, it is some￾how questionable whether these techniques are fully ap￾plicable to composites. Indeed, there is a general understanding that damage accumulation by microcracks initiates prior the complete fracture of composites and this would impact the failure behavior. Of particular impor￾tance is then to separately discuss micro- and macrocrack￾ing behaviors. A simple guess indicates that, as the failure behavior generally depends on the type and distribution of the internal flaws as a crack origin, there would be an ap￾parent difference between dense NITE–SiC/SiC compos￾ites and conventional porous composites such as CVI– SiC/SiC composites. The failure initiation behavior would also vary from the choice of fabric architecture and applied loading directions due to the inherent anisotropy of the composites. Therefore, careful discussion is necessary. This study aims to identify crack resistance and damage tolerance of a new class of SiC/SiC composites, that is, NITE–SiC/SiC composites. For this purpose, the existing testing methodology for fracture toughness eval￾uation was first applied. Second, the developmental frac￾ture resistance method based on nonlinear fracture mechanics was applied and the damage accumulation be￾havior of composites was evaluated using various notched specimens. Experimental Procedure Materials Two types of pilot-grade NITE–SiC/SiC compos￾ites were produced by the Institute of Energy Science and Technology (Ube, Japan) (Table I). For the first composite, a B250-nm-thick pyrolytic carbon (PyC) interphase as a form of fiber/matrix (F/M) interface was chemically vapor deposited (CVD) on the fiber surface prior matrix densification (hereafter ‘‘NITE-Thick￾Coat’’). In contrast, for the other type, a thin PyC in￾terphase (o50 nm) was designed. However, in the latter case, the F/M interfacial coating was not successfully formed, as clearly shown in micrographs in Table I. The major parts of fibers were uncoated, although thin PyC was formed for limited number of fibers. In this study, we designate this composite ‘‘NITE-Thin-Coat.’’ For both types, highly crystalline and near-stoichiometric Tyrannot-SA third-grade SiC fibers were uni-direc￾tionally reinforced with a fiber volume fraction of 0.4–0.45. Typical micrographs show well-densified ma￾trix indicating that the porosity of this material was very low (o5%). A secondary phase (white contrast in Table I), which was reportedly an oxide phase composed of sintering additives such as Al2O3, SiO2, and Y2O3, 16 was localized in the matrix, specifically within intrabun￾www.ceramics.org/ACT Cracking Resistance of Silicon Carbide Composites 305