T Nozawa et aL/Journal of Nuclear Materials 384(2009)195-211 dopted due to the perceived applicability to the multilayered interphase and matrix on clamping stresses was first examined. interphase structure. In the following analysis, isotropic swelling Fig. 10 shows typical analytical results of residual radial stresses of Sic and Pyc was assumed. As aforementioned, quantification with respect to PyC interlayer thickness. It is apparent that the of the contribution of irradiation creep is very complicated for sev- residual stress at the interface depends significantly on PyC inter eral reasons, therefore this study did not take this into account for layer thickness. The differential thermal expansion between the fi implicity. ber and the matrix determines the direction of the stress at the F/M Fig 9 shows comparison of calculated thermal residual stresses interface, i.e., residual compression when of <%m or tension when in the radial and axial directions for monolayer and multilayer f>%m. In contrast, the magnitude of the CtE of Pyc imposes stress composites with respect to the radial distance from the fiber cen- deviation with increasing Pyc interlayer thickness. For instance, ter. By taking data listed in Table 2, a residual tensile stress is in- with a higher CTE of carbon assumed, e.g., high-density Pyc. duced in the radial direction. In the axial direction, a residual residual stress becomes much more tensile and vice versa. Consid tensile stress is in the fiber, while a residual compression stress ering the hysteresis analysis[48 for stress-strain curves of non- is in the matrix. However, because the manufacturer-claimed ther- irradiated composites in Fig 3, the thermal residual stress in the mal expansion coefficients of Hi-Nicalon Type-S and PyC in Table radial direction becomes zero or slightly tensile for advanced Sic/ 2 are uncertain, the effect of variation of CTEs of the fiber, F/M SiC composites. It is therefore suggested that the CTE of Hi-N alon Type-S should be very close to or slightly higher than that of CVD-SiC. From these facts, a negative contribution to clamping a200 at the interface is therefore anticipated. In contrast, the huge con- tribution to the interfacial shear strength during the fiber pull-out PyC20nm UD Hi-Nicalon Type-S stage was generally induced by the improved surface roughness of V=03,Vpom=0.15 the advanced Sic fiber. the fiber roughness influences the radial Multilayer stress for debonded interface in a simple assumption [ 20]. Fig. 11 shows an example of analytical results of residual radial stresses y assuming a fiber surface roughness of 10 20 nm. From Fig. 11, the rougher fiber surface can theoretically induce more 810c0m compressive (clamping) stress. Specifically, a radial compressive 切 INTERPHASE stress increases for thinner PyC interlayer thickness. Fig 9, another remark to be addressed is that the stress state in the multilayer composites is nearly identical to that of mono- 50+ PyC500nm layer composites with a Pyc interlayer thickness of the total Pyc sub-layers of the multilayer. incorporating the total Pyc thickness will be fairly reasonable for the analysis of thermal residual stress in multilayer composites. In contrast, there is no doubt that the innermost Pyc layer(adja the fiber)is important in an interface design to contro FIBER MATRIX COMPOSITES tion behavior because of the fact that a critical crack prop- agates along the fiber surface From Fig 8, the primary crack prop agates along the surface of the first Py C layer. For the analysis of multilayer composites, it should therefore be reasonable to adopt Radial distance from the fiber center [um the first Pyc interlayer thickness and the total Pyc interlayer thick- ness for a crack propagation analysis and a residual stress analysis. UD Hi-Nicalon Type-S Under neutron irradiation, residual stresses can be further in- PyC 20nm ultilayer V0.3, Pore =0.15 duced by differential swelling among the fiber, F/M interphase and matrix. Fig. 12 shows an example of the analysis of irradia- tion-induced residual radial and axial stresses at the F/m interface. In Fig. 12, the input data of the swelling and Youngs moduli for 8 200+PyC 100nm irradiated SiC and high-density PyC were taken from the estimates by empirical equations as listed in Table 2. This analysis assumes PyC500nm no change of Cte by irradiation. No contribution from irradiation INTERPHASE creep was also considered for simplicity. Preliminary analysis in Fig. 12 suggests that neutron irradiation enhances more tensile tion. It should be also noted that (1) the radiation-induced residual stresses for thick PyC monolayer composites are potentially larger than that of thin-layered multilayer composites and (2)no signif COMPOSITES cant influence of irradiation temperature would be analytically anticipated. The tensile hysteresis curves for irradiated composite in Fig 3 may support these phenomena From Fig. 12, it is obvious that there exists a stress peak around a few dpa corresponded with turn-around of the swelling behavior of PyC. This result is quite similar to the trend suggested from the preliminary evaluation Radial distance from the fiber center [um by Henager et al. [49 Presently no analysis can be done for the of limited data availability. Ne Fig9. Calculated thermally induced residual stresses: (a)radial stresses and (b) theless, considering many similarities with high-density Pyc, it is axial stresses likely that the radiation-enhanced densification of the low-densityadopted due to the perceived applicability to the multilayered interphase structure. In the following analysis, isotropic swelling of SiC and PyC was assumed. As aforementioned, quantification of the contribution of irradiation creep is very complicated for sev￾eral reasons, therefore this study did not take this into account for simplicity. Fig. 9 shows comparison of calculated thermal residual stresses in the radial and axial directions for monolayer and multilayer composites with respect to the radial distance from the fiber cen￾ter. By taking data listed in Table 2, a residual tensile stress is in￾duced in the radial direction. In the axial direction, a residual tensile stress is in the fiber, while a residual compression stress is in the matrix. However, because the manufacturer-claimed ther￾mal expansion coefficients of Hi-NicalonTM Type-S and PyC in Table 2 are uncertain, the effect of variation of CTEs of the fiber, F/M interphase and matrix on clamping stresses was first examined. Fig. 10 shows typical analytical results of residual radial stresses with respect to PyC interlayer thickness. It is apparent that the residual stress at the interface depends significantly on PyC inter￾layer thickness. The differential thermal expansion between the fi- ber and the matrix determines the direction of the stress at the F/M interface, i.e., residual compression when af < am or tension when af > am. In contrast, the magnitude of the CTE of PyC imposes stress deviation with increasing PyC interlayer thickness. For instance, with a higher CTE of carbon assumed, e.g., high-density PyC, a residual stress becomes much more tensile and vice versa. Consid￾ering the hysteresis analysis [48] for stress–strain curves of non￾irradiated composites in Fig. 3, the thermal residual stress in the radial direction becomes zero or slightly tensile for advanced SiC/ SiC composites. It is therefore suggested that the CTE of Hi-Nic￾alonTM Type-S should be very close to or slightly higher than that of CVD-SiC. From these facts, a negative contribution to clamping at the interface is therefore anticipated. In contrast, the huge con￾tribution to the interfacial shear strength during the fiber pull-out stage was generally induced by the improved surface roughness of the advanced SiC fiber. The fiber roughness influences the radial stress for debonded interface in a simple assumption [20]. Fig. 11 shows an example of analytical results of residual radial stresses by assuming a fiber surface roughness of 10 20 nm. From Fig. 11, the rougher fiber surface can theoretically induce more compressive (clamping) stress. Specifically, a radial compressive stress increases for thinner PyC interlayer thickness. In Fig. 9, another remark to be addressed is that the stress state in the multilayer composites is nearly identical to that of mono￾layer composites with a PyC interlayer thickness of the total PyC sub-layers of the multilayer. From this fact, the approximation incorporating the total PyC thickness will be fairly reasonable for the analysis of thermal residual stress in multilayer composites. In contrast, there is no doubt that the innermost PyC layer (adja￾cent the fiber) is important in an interface design to control crack propagation behavior because of the fact that a critical crack prop￾agates along the fiber surface. From Fig. 8, the primary crack prop￾agates along the surface of the first PyC layer. For the analysis of multilayer composites, it should therefore be reasonable to adopt the first PyC interlayer thickness and the total PyC interlayer thick￾ness for a crack propagation analysis and a residual stress analysis, respectively. Under neutron irradiation, residual stresses can be further in￾duced by differential swelling among the fiber, F/M interphase and matrix. Fig. 12 shows an example of the analysis of irradia￾tion-induced residual radial and axial stresses at the F/M interface. In Fig. 12, the input data of the swelling and Young’s moduli for irradiated SiC and high-density PyC were taken from the estimates by empirical equations as listed in Table 2. This analysis assumes no change of CTE by irradiation. No contribution from irradiation creep was also considered for simplicity. Preliminary analysis in Fig. 12 suggests that neutron irradiation enhances more tensile residual stress at the F/M interface in the radial (clamping) direc￾tion. It should be also noted that (1) the radiation-induced residual stresses for thick PyC monolayer composites are potentially larger than that of thin-layered multilayer composites and (2) no signifi- cant influence of irradiation temperature would be analytically anticipated. The tensile hysteresis curves for irradiated composites in Fig. 3 may support these phenomena. From Fig. 12, it is obvious that there exists a stress peak around a few dpa corresponded with ‘turn-around’ of the swelling behavior of PyC. This result is quite similar to the trend suggested from the preliminary evaluation by Henager et al. [49]. Presently no analysis can be done for the low-density PyC case because of limited data availability. Never￾theless, considering many similarities with high-density PyC, it is likely that the radiation-enhanced densification of the low-density Fig. 9. Calculated thermally induced residual stresses: (a) radial stresses and (b) axial stresses. T. Nozawa et al. / Journal of Nuclear Materials 384 (2009) 195–211 205