T Nozawa et aL /Joumal of Nuclear Materials 384(2009)195-211 Unirradiated Irrad 380C, 1.dpa Irrad 800C, 7.7dpa N/A 2 Fig 8. Typical micrographs of pushed-out fiber surfaces The least-square fits by Eqs. (1)and (4)can provide an interfa- primary crack occurred in the inner most Pyc layer adjacent the fi- cial debond shear strength(ts)and an interfacial friction stress(tr), ber for both cases. For monolayer composites, there was no signif- respectively, from the experimental push-out data in Table 4 Table icant change in cracking path by neutron irradiation. The pushed 5 lists ts and tr of monolayer and multilayer composites and Fig. 7 out fiber surfaces were smooth regardless of the irradiation, indi- plots interfacial shear property data as a function of neutron dose cating crack propagation along the fiber surface. In contrast, the (for the low-density Py C case). Note that the preliminary analysis rough fiber surface for the as-received multilayer composites im- in this study disregarded the contribution from swelling because plies crack propagation within PyC interphase adjacent the fiber. the actual swelling in composite forms depend significantly on The changing crack path: the crack path 'within interphase before irradiation assisted stress relaxation: The importance of irradiation irradiation Vs the crack path along, the fiber/carbon interface after creep was pointed out by the bend stress relaxation test by Katoh irradiation (7.7 dpa at 800C)was identified. et al.[43 The anisotropic swelling of Sic under stress has also een reported [44]. Considering precisely the effect of irradia- 4.Discussion tion-induced dimensional change with combined effects of dy namic irradiation effects is presently very difficult due to many 4.1. Clamping stresses at the fiber/matrix interface uncertain factors and scarcity of data. Meanwhile, simple estima tion by applying a minimum value of omax gives a lower limit (tr>210 MPa)as plotted in the figure with an arrow in Fig. 7, residual atre sig ind a tpe em nin ten although the interfacial friction following irradiation at 740C face. These clamping stresses are generally induced by the thermal to 0.7 dpa could not be quantified due to the load limitation of expansion mismatch among the fiber, F/M interphase and matrix, the test system and by roughness of the fiber surface [20 Additionally, the sec From Fig. 7, it is apparent that shear properties at the F/M inter- ondary stresses induced by swelling and stress relaxation by irra- face depend on neutron dose. They apparently changed after irra- diation creep may impose added complexity under neutron diation,however,the dose dependence is not so significant when irradiation For advanced Sic/SiC composites, the fiber and matrix 1080.C. Both ts and tr decrease by irradiation in low dose are highly-crystalline Sic and it has been believed that their irradi- region and remain nearly unchanged with further increasing neu- ation effects are essentially similar. However, recent study identi tron dose regardless of the interface structure types, e.g. monolayer fied some differences in defect accumulation between fibers and or multilayer. Of particular emphasis is that both ts and tr for mul- purer SiC matrix, implying potential difference in swelling [41]. tilayer composites were most affected when Tirr-1080C. How- which probably gives a residual stress at the F/M interface by irra- ever, it is worth noting that the interfacial shear properties for diation. In spite of the presence of CTE mismatch between the Sic irradiated multilayer composites are still above average for mono- fiber and matrix, dose-dependent dimensional change of PyC layer composites. Even if the interfacial shear stresses decrease by would inevitably impose substantial stresses at the F/M interface irradiation, the high shear strength is still sufficient for the load- [26]. In short, the radiation-induced stresses would undoubtedly sharing at the F/M interface. Presently, no irradiation data is avail- influence the results and ultimately need to be consideree able for monolayer composites when Tirr>1000C and therefore it A clamping stress working at the F/M interface can be qualita- should be noted that it is unclear whether this is specific to the tively predicted under the simple assumption by adopting the multilayer interface case. Another uncertainty to be pointed out four-phase( fiber core/PyC interphase/Sic matrix/composites )cyl- is a reduction of interfacial friction by irradiation at 800C up to inder model [45-47]. This model discusses(1)anisotropy of the 7.7 dpa for multilayer composites. constituents, (2)the effect of interlayer thickness, (3)contributionsThe least-square fits by Eqs. (1) and (4) can provide an interfa￾cial debond shear strength (ss) and an interfacial friction stress (sf), respectively, from the experimental push-out data in Table 4. Table 5 lists ss and sf of monolayer and multilayer composites and Fig. 7 plots interfacial shear property data as a function of neutron dose (for the low-density PyC case). Note that the preliminary analysis in this study disregarded the contribution from swelling because the actual swelling in composite forms depend significantly on irradiation assisted stress relaxation; The importance of irradiation creep was pointed out by the bend stress relaxation test by Katoh et al. [43]. The anisotropic swelling of SiC under stress has also been reported [44]. Considering precisely the effect of irradia￾tion-induced dimensional change with combined effects of dy￾namic irradiation effects is presently very difficult due to many uncertain factors and scarcity of data. Meanwhile, simple estima￾tion by applying a minimum value of rmax gives a lower limit (sf > 210 MPa) as plotted in the figure with an arrow in Fig. 7, although the interfacial friction following irradiation at 740 C up to 0.7 dpa could not be quantified due to the load limitation of the test system. From Fig. 7, it is apparent that shear properties at the F/M inter￾face depend on neutron dose. They apparently changed after irra￾diation, however, the dose dependence is not so significant when Tirr = 1080 C. Both ss and sf decrease by irradiation in low dose region and remain nearly unchanged with further increasing neu￾tron dose regardless of the interface structure types, e.g. monolayer or multilayer. Of particular emphasis is that both ss and sf for mul￾tilayer composites were most affected when Tirr = 1080 C. How￾ever, it is worth noting that the interfacial shear properties for irradiated multilayer composites are still above average for mono￾layer composites. Even if the interfacial shear stresses decrease by irradiation, the high shear strength is still sufficient for the load￾sharing at the F/M interface. Presently, no irradiation data is avail￾able for monolayer composites when Tirr > 1000 C and therefore it should be noted that it is unclear whether this is specific to the multilayer interface case. Another uncertainty to be pointed out is a reduction of interfacial friction by irradiation at 800 C up to 7.7 dpa for multilayer composites. Fig. 8 exhibits typical micrographs of pushed-out fiber surfaces of as-received and neutron-irradiated materials. Importantly the primary crack occurred in the inner most PyC layer adjacent the fi- ber for both cases. For monolayer composites, there was no signif￾icant change in cracking path by neutron irradiation. The pushed￾out fiber surfaces were smooth regardless of the irradiation, indi￾cating crack propagation along the fiber surface. In contrast, the rough fiber surface for the as-received multilayer composites im￾plies crack propagation within PyC interphase adjacent the fiber. The changing crack path: the crack path ‘within’ interphase before irradiation vs. the crack path ‘along’ the fiber/carbon interface after irradiation (7.7 dpa at 800 C) was identified. 4. Discussion 4.1. Clamping stresses at the fiber/matrix interface Interfacial shear properties are significantly dependent on residual compressive (clamping) stresses induced at the F/M inter￾face. These clamping stresses are generally induced by the thermal expansion mismatch among the fiber, F/M interphase and matrix, and by roughness of the fiber surface [20]. Additionally, the sec￾ondary stresses induced by swelling and stress relaxation by irra￾diation creep may impose added complexity under neutron irradiation. For advanced SiC/SiC composites, the fiber and matrix are highly-crystalline SiC and it has been believed that their irradi￾ation effects are essentially similar. However, recent study identi- fied some differences in defect accumulation between fibers and purer SiC matrix, implying potential difference in swelling [41], which probably gives a residual stress at the F/M interface by irra￾diation. In spite of the presence of CTE mismatch between the SiC fiber and matrix, dose-dependent dimensional change of PyC would inevitably impose substantial stresses at the F/M interface [26]. In short, the radiation-induced stresses would undoubtedly influence the results and ultimately need to be considered. A clamping stress working at the F/M interface can be qualita￾tively predicted under the simple assumption by adopting the four-phase (fiber core/PyC interphase/SiC matrix/composites) cyl￾inder model [45–47]. This model discusses (1) anisotropy of the constituents, (2) the effect of interlayer thickness, (3) contributions from the thermal expansion mismatch and (4) irradiation effects. Of many models, the analytical method proposed by Oel [47] is Fig. 8. Typical micrographs of pushed-out fiber surfaces under various irradiation conditions. 204 T. Nozawa et al. / Journal of Nuclear Materials 384 (2009) 195–211