Y Li et al./ Materials Science and Engineering A 507 (2009)6-12 30° Imm (b)10 6 0 Imm Imm (c)1.1×10°1/s 103 Fig. 6. Fractures of the 3D needling-punched C/siC composites under static and dynamic compression. in 3D C/c composite tested at compression load by Xiong et al. 3.4. Comparison with 2D-C/Sic composites [16]. An increase of shear fracture angle of the failed specimens from 35 at the strain rate of 10-4 1/s to about 45 at 10-2 1/s was In this section, in order to display the advantages of the 3D also observed in Fig. 6(a)and(b). Garland et al. [17)and Bi et al. needle-punched C/Sic composite tested in this study, the mechan- [18 proposed that the strain rate effect of the interface strength ical behavior of the material is also compared with that of 2D-C/Sic may contribute to the magnitude of shear fracture angle in fibrous composites tested by liu et al. [15]. The true stress vs strain curves composites. In other words, high compression strength of the 3d of both two kinds of 2D composites with different densification needle-punched C/SiC composite at high strain rates results in a and the 3d composite tested in this study are shown in Fig. 8. It large shear fracture angle. Fig. 6(c) and (d) shows the fracture sur- can be seen that at the same strain rate the compression strength faces of the material tested at the strain rates of 1.1 x 10 and of 3D needle-punched c/Sic composite is higher than that of low 3x 10 1/s respectively. Differing from the failure pattern under densification(LD)2D-C/Sic composite but lower than that of high juasi-static loading condition, the failure mode under dynamic densification(HD) counterpart. It is also interesting to find the ading displays a split pattern the same failure mode was al 3D needle-punched C/Sic composite shows larger failure strain bserved in 3D needle-punched C/C composite loaded at high strain than both kinds of 2D-C/Sic composite As has been proposed in ates [19]. It is also interesting to find one end of the specimens of [15]. the relatively low interface strength of LD 2D-C/SiC compos 3D composite tested at high strain rate broke into pieces. The exact ite resulted in a weak load-bearing capacity. Thus an increase in eason for such phenomenon is not clear at present time. Shear the strength of 2D-C/SiC composite necessitates an increase of its fracture angle of the 3D needle-punched C/Sic composite are not intensity. But it should be pointed out that the increase of inter defined either. sity can also lead to a reduction in inhere low toughness of the The SEM images of the ruptured surface of the specimens tested material [15 By contrast, the carbon fibers needle-punched into at strain rates of 10-4, 10-2. 1.1 x 10 and 3 x 10 1/s are shown in the laminates in thickness direction of 3D composite can lead Fig. 7 respectively. It can be observed from Fig. 7(a)and (b )that a higher interface shear strength. At the same time, the inserted ge number of fibers were pulled out during deformation at strain carbon fibers can also play a toughening effect on the 3D needle- ates of 10-4 and 10-2 1/s, the length of which decreases with the punched C/SiC composites. As the result, the 3D needle-punched increase of strain rate. In contrast, the specimens tested at stra C/Sic composite shows not only high compression strength but rel- rates of 1.1 x 10 and 3 x 10 1/s display smoother fracture surfaces atively high toughness even after the stress reaches its compression (see in Fig. 7(c)and (d). This indicates the number of fragmental strength. Hence it can be concluded that the 3D needle-punched fibers in the 3D needle-punched C/Sic composite under dynamic C/Sic composite possess advantages of both LD and HD 2D-C/Sic ading increases with the strain rate.Y. Li et al. / Materials Science and Engineering A 507 (2009) 6–12 9 Fig. 6. Fractures of the 3D needling-punched C/SiC composites under static and dynamic compression. in 3D C/C composite tested at compression load by Xiong et al. [16]. An increase of shear fracture angle of the failed specimens from 35◦ at the strain rate of 10−4 1/s to about 45◦ at 10−2 1/s was also observed in Fig. 6(a) and (b). Garland et al. [17] and Bi et al. [18] proposed that the strain rate effect of the interface strength may contribute to the magnitude of shear fracture angle in fibrous composites. In other words, high compression strength of the 3D needle-punched C/SiC composite at high strain rates results in a large shear fracture angle. Fig. 6(c) and (d) shows the fracture sur￾faces of the material tested at the strain rates of 1.1 × 103 and 3 × 103 1/s respectively. Differing from the failure pattern under quasi-static loading condition, the failure mode under dynamic loading displays a split pattern. The same failure mode was also observed in 3D needle-punched C/C composite loaded at high strain rates [19]. It is also interesting to find one end of the specimens of 3D composite tested at high strain rate broke into pieces. The exact reason for such phenomenon is not clear at present time. Shear fracture angle of the 3D needle-punched C/SiC composite are not defined either. The SEM images of the ruptured surface of the specimens tested at strain rates of 10−4, 10−2, 1.1 × 103 and 3 × 103 1/s are shown in Fig. 7 respectively. It can be observed from Fig. 7(a) and (b) that a large number of fibers were pulled out during deformation at strain rates of 10−4 and 10−2 1/s, the length of which decreases with the increase of strain rate. In contrast, the specimens tested at strain rates of 1.1 × 103 and 3 × 103 1/s display smoother fracture surfaces (see in Fig. 7(c) and (d)). This indicates the number of fragmental fibers in the 3D needle-punched C/SiC composite under dynamic loading increases with the strain rate. 3.4. Comparison with 2D-C/SiC composites In this section, in order to display the advantages of the 3D needle-punched C/SiC composite tested in this study, the mechan￾ical behavior of the material is also compared with that of 2D-C/SiC composites tested by Liu et al. [15]. The true stress vs. strain curves of both two kinds of 2D composites with different densification and the 3D composite tested in this study are shown in Fig. 8. It can be seen that at the same strain rate, the compression strength of 3D needle-punched C/SiC composite is higher than that of low densification (LD) 2D-C/SiC composite but lower than that of high densification (HD) counterpart. It is also interesting to find the 3D needle-punched C/SiC composite shows larger failure strain than both kinds of 2D-C/SiC composite. As has been proposed in [15], the relatively low interface strength of LD 2D-C/SiC compos￾ite resulted in a weak load-bearing capacity. Thus an increase in the strength of 2D-C/SiC composite necessitates an increase of its intensity. But it should be pointed out that the increase of inten￾sity can also lead to a reduction in inhere low toughness of the material [15]. By contrast, the carbon fibers needle-punched into the laminates in thickness direction of 3D composite can lead to higher interface shear strength. At the same time, the inserted carbon fibers can also play a toughening effect on the 3D needle￾punched C/SiC composites. As the result, the 3D needle-punched C/SiC composite shows not only high compression strength but rel￾atively high toughness even after the stress reaches its compression strength. Hence it can be concluded that the 3D needle-punched C/SiC composite possess advantages of both LD and HD 2D-C/SiC composites