wwceramics. org/ACT Cracking Resistance of Silicon Carbide Composites 311 sensitivity. In short, the averaged stress applied on the [ No ligament area then determines the composite strength This result was quite consistent with that of SENB test results. In contrast, the limited number of dara indicate the apparent notch sensitivity of NITE-Thin-Coat, whereby the strength of NITE-Thin-Coat exhibits non linear parabolic relation to 1-do/W. This would be rea- interfile Biven that the strong bonding at the F/M eventually produces brittleness of a material like ceram- w/ crack branching ics. In general, brittle ceramics are subject to surface Haws. she 田SENB-2 sion,the unique PLS of 133 MPa and the ultimate tensile strength(UTS)of 143 MPa were determined for NITE-Thick-Coat, regardless of the presence of notches. The same is true for the PIP-Coat, yielding 0.4 0.5 0.6 the unique PLS of 59 MPa and UTS of 234 MPa. Fig. 7. Crack extension to width ratio versus crack opening Discussion displacement for NITE- Thick-Coat Fracture Toughness by Linear Fracture Mechanics initial ligament area as stress forms were plotted as a Approach function of the initial notch depth to specimen width One classical approach to discuss fracture energy is normalized microcracking load and macrocracking load the fracture toughness, defined by the linear elastic frac were proportional to 1-do/W for the cases of NITE- ture mechanics (LEFM) and integral that has been hick-Coat and PIP-Coat, clearly indicating notch in- widely applied. According to the literature, the integral can be defined as a simplified form 20 80 J 口Bend.PLs where u is total work obtained from the load versus SENB-1, PLS load line displacement curve. Generally, the use of SENB-2 PLS ou=615±85 LEFM in complex composite materials particularly in NB-3 UFS the in-plane direction is inappropriate. However, as- V SENB-4 suming that the present SiC/SiC composites follow this 乏Na V SENB-4, UFS equation consistently, integral for NITE-Thick-Coat was expressed as a function of Aa. Specifically, by plot ting as a function of Aa/W, a unique trend was obtained (Fig. 11). The value of/ integral for NITE-Thick-Coat gradually increased at pus=244±45MPa first with increasing Aa/W and the inclement of the/ in- tegral was accelerated when Aa/W>0. 2(in the third stag in Fig. 7). One possible explanation is that the integral considers entire energy consumption during the test in- 0.6 cluding microcrack formation, friction at the F/M inter- face thermal strain relief, fiber breaking, et As the lateinitial ligament area as stress forms were plotted as a function of the initial notch depth to specimen width ratio, 1-a0/W (Fig. 10). It is worth noting that both normalized microcracking load and macrocracking load were proportional to 1a0/W for the cases of NITE￾Thick-Coat and PIP-Coat, clearly indicating notch in￾sensitivity. In short, the averaged stress applied on the ligament area then determines the composite strength. This result was quite consistent with that of SENB test results. In contrast, the limited number of data indicate the apparent notch sensitivity of NITE-Thin-Coat, whereby the strength of NITE-Thin-Coat exhibits non￾linear parabolic relation to 1a0/W. This would be rea￾sonable given that the strong bonding at the F/M interface prohibits crack branching at the interface, eventually produces brittleness of a material like ceram￾ics. In general, brittle ceramics are subject to surface flaws, showing significant notch sensitivity. In conclu￾sion, the unique PLS of B133 MPa and the ultimate tensile strength (UTS) of B143 MPa were determined for NITE-Thick-Coat, regardless of the presence of notches. The same is true for the PIP-Coat, yielding the unique PLS of B59 MPa and UTS of B234 MPa. Discussion Fracture Toughness by Linear Fracture Mechanics Approach One classical approach to discuss fracture energy is the fracture toughness, defined by the linear elastic frac￾ture mechanics (LEFM) and J integral that has been widely applied. According to the literature, the J integral can be defined as a simplified form10: J ¼ 2U t Wð Þ  a ð2Þ where U is total work obtained from the load versus load line displacement curve. Generally, the use of LEFM in complex composite materials particularly in the in-plane direction is inappropriate. However, as￾suming that the present SiC/SiC composites follow this equation consistently, J integral for NITE-Thick-Coat was expressed as a function of Da. Specifically, by plot￾ting as a function of Da/W, a unique trend was obtained regardless of the specimen size (Fig. 11). The value of J integral for NITE-Thick-Coat gradually increased at first with increasing Da/W and the inclement of the J in￾tegral was accelerated when Da/W40.2 (in the third stage in Fig. 7). One possible explanation is that the J integral considers entire energy consumption during the test in￾cluding microcrack formation, friction at the F/M inter￾face, thermal strain relief, fiber breaking, etc. As the latest SiC/SiC composites possesses strong bonding and im￾proved friction resistance due to the rough fiber surface, Fig. 7. Crack extension to width ratio versus crack opening displacement for NITE-Thick-Coat. Fig. 8. Strengths as a function of the notch depth to width ratio in single-edge notched bend (SENB) tests. www.ceramics.org/ACT Cracking Resistance of Silicon Carbide Composites 311