Y Li et al/ Materials Science and Engineering A 507(2009)6-12 三易 1 2 y=10.053x+37258 A Needlimg-punched C/SiC 一7# Trend(Needling-punched C/SiC) 012 te) Fig 4. The true stress vs strain curves of the 3D needling-punched C/SiC composites at the same strain rates under Fig 3. Compression strength vs strain rate curves of the 3D needling-punched C/Sic dispersity of the material. Larger value of m indicates low den- atively high load-bearing capacity. The failure strain of the material sity of defects and low dispersity of the compression strength. believe that the fracture of the SiC matrix may cause a drop in the when o equals ao. It can be seen from Fig. 5 that the dynamic loading capacity of the material after the peak stress; yet pullout compression strength obeys the Weibull distribution. The calcu of the needle-punched carbon fibers demands additional energy lated Weibull modulus m and the characteristic strength ao which in a way results in a higher failure strain of the 3D needle- the 3D needle-punched C/Sic composite are 8.19 and 426. MPa espectively. not occur in Fig. 2, in place of which an obvious strain softening Because of the remarkable dispersity of the compression is observed. While for 2D-C/SiC composites tested by Liu [15]. the strength of the 3D composites, at least three tests were conducted materials fracture quickly after the stress reaches its compress or each experimental condition. The stress-strain curve with the strength. That is to say, the needle failure strength closing to the trend line in Fig. 4 was picked to tive contribution toward the toughness of the 3D needle-punched be presented in Fig. 2 as a typical stress-strain curve of the 3D C/SiC composite As the results, the 3D needle-punched C/Sic com- composites at corresponding experimental condition posite have improved fracture toughness compared with 2D-C 3.3. Observation of failure surface 3. 2. Weibull distribution of the dynamic compression strength The fractures of the failure specimens were observed by using an As the material was prepared using the CVI process, defects optical microscope and JSM6460 SEM Optical images of the frac- as gas-holes are inevitable and the heterogeneous distribution ture surface of the 3D needle-punched C/SiC composite tested at of them may lead to the dispersity of the material strength. (b)respectively. It can be seen that shear failure and delamina- Thus, remarkable dispersity of the compression strength of the 3d tion feature the fracture of the material, which was also observed eedle-punched C/Sic composites tested at the same strain rate under dynamic loading conditions can be observed in Fig. 4.It should be explained that as steel pads were employed to restrain strain within certain limits during experiments on some samples, the failure strain of three samples(4#, 5# and 7#) is only 0.04, while five of other samples have failure strains more than 0.16 strength of the material, the Weibull distribution was applied this study. In the case of 3D needle-punched C/Sic composites, the s three basic constituents, namely, fiber, interphase, and Sic matrix 2-0.5 re essentially brittle and the cracking involves defect-induced random failures. It thus follows that the statistical distribution of the strength of the 3D needle-punched C/Sic composite can be described by Weibull equation as A Needling-punched C/SIC 6.2 where Fo)is a probability function, o the stress applied, oo the scale parameter or characteristic strength, and m is the Weibul Fig. 5. The Weibull distributions of dynamic failure strengths of the 3D needling modulus, the most important parameter which characterizes the punched C/SiC composites8 Y. Li et al. / Materials Science and Engineering A 507 (2009) 6–12 Fig. 3. Compression strength vs. strain rate curves of the 3D needling-punched C/SiC composites. atively high load-bearing capacity. The failure strain of the material tested at different strain rates even exceeds 8%. It is reasonable to believe that the fracture of the SiC matrix may cause a drop in the loading capacity of the material after the peak stress; yet pullout of the needle-punched carbon fibers demands additional energy which in a way results in a higher failure strain of the 3D needle￾punched C/SiC composite. Thus, catastrophic brittle failure does not occur in Fig. 2, in place of which an obvious strain softening is observed. While for 2D-C/SiC composites tested by Liu [15], the materials fracture quickly after the stress reaches its compression strength. That is to say, the needle-punched fibers may make a pos￾itive contribution toward the toughness of the 3D needle-punched C/SiC composite. As the results, the 3D needle-punched C/SiC com￾posite have improved fracture toughness compared with 2D-C/SiC composite. 3.2. Weibull distribution of the dynamic compression strength As the material was prepared using the CVI process, defects as gas-holes are inevitable and the heterogeneous distribution of them may lead to the dispersity of the material strength. Thus, remarkable dispersity of the compression strength of the 3D needle-punched C/SiC composites tested at the same strain rate under dynamic loading conditions can be observed in Fig. 4. It should be explained that as steel pads were employed to restrain the maximum strain within certain limits during experiments on some samples, the failure strain of three samples (4#, 5# and 7#) is only 0.04, while five of other samples have failure strains more than 0.16. To quantify the degree of dispersity of the dynamic compression strength of the material, the Weibull distribution was applied in this study. In the case of 3D needle-punched C/SiC composites, the three basic constituents, namely, fiber, interphase, and SiC matrix are essentially brittle and the cracking involves defect-induced random failures. It thus follows that the statistical distribution of the strength of the 3D needle-punched C/SiC composite can be described by Weibull equation as F() = 1 − exp  − 0 m (2) where F() is a probability function, the stress applied, 0 the scale parameter or characteristic strength, and m is the Weibull modulus, the most important parameter which characterizes the Fig. 4. The true stress vs. strain curves of the 3D needling-punched C/SiC composites at the same strain rates under dynamic compression. dispersity of the material. Larger value of m indicates low den￾sity of defects and low dispersity of the compression strength. The maximum density of the Weibull distribution can be achieved when equals 0. It can be seen from Fig. 5 that the dynamic compression strength obeys the Weibull distribution. The calcu￾lated Weibull modulus m and the characteristic strength 0 of the 3D needle-punched C/SiC composite are 8.19 and 426.4 MPa respectively. Because of the remarkable dispersity of the compression strength of the 3D composites, at least three tests were conducted for each experimental condition. The stress–strain curve with the failure strength closing to the trend line in Fig. 4 was picked to be presented in Fig. 2 as a typical stress–strain curve of the 3D composites at corresponding experimental condition. 3.3. Observation of failure surface The fractures of the failure specimens were observed by using an optical microscope and JSM6460 SEM. Optical images of the frac￾ture surface of the 3D needle-punched C/SiC composite tested at the strain rates of 10−4 and 10−2 1/s are shown in Fig. 7(a) and (b) respectively. It can be seen that shear failure and delamina￾tion feature the fracture of the material, which was also observed Fig. 5. The Weibull distributions of dynamic failure strengths of the 3D needling￾punched C/SiC composites.