Y Li et al./ Materials Science and Engineering A 507 (2009)6-12 Socket Projectle elocity derganc bar High dynamic Fig. 1. The diagram illustration of the split Hopkinson press bar systems 3D-C/SiC composite with density of 1.90 g/cm and open porosity strain rate. The compression strength of the material increases 1609°C from about 350 MPa at the strain rate of 10-4 1/s to 430 MPa at the strain rate of 6.5 x 10 1/s with a relative increment of about 2.2. Experiment procedure 23%. Although remarkable dispersity exists, a close inspection of the experimental results reveals that the compression strength of The specimens were cut from a 3D needle-punched C/Sic com- the material varies linearly with the logarithm of the strain rate(as posite plate to ensure that the loading direction is parallel to the shown in Fig 3). Such phenomenon was also observed for 2D-C/Sic It is believed that microcracks are contributed to the strain rate cylindrical, with a diameter of 5 mm and a length of 4 mm. Both sensitivity of the material. As we known, such defects as micro the quasi-static and dynamic experiments were performed at the cracks caused by the mismatch of thermal expansion coefficients temperature. The strain rates were controlled in the range between the carbon fibers and the sic matrix are unavoidable dur- from1.0×10-4to6.5×1031/s ing the preparation of the composites Under compression loading. An electronic universal testing machine with a maximum load capacity of 10 kN was used to perform the quasi-static experiments. failure of the composites. It should be pointed out that both the The split Hopkinson pressure bar(SHPB), situated in Northwestern nucleation and extension are processes of time dependence.Under Polytechnical University, was used for the high strain rate experi- quasi-static loading. the microcracks have enough time to nucleate ents. A steel pad was employed to restrain the maximum strain and extend. While under dynamic loading, the applied load pulse ithin certain limits so that the fracture surface of the tested spec- is usually several hundred microseconds, which is insufficient for imens could be preserved. The diagram of the SHPB and the steel the nucleation and extension of microcracks. As the result, higher ad was shown in Fig. 1. According to the one-dimensional elas- compression strength is needed at high strain rates stress wave theory, the strain, stress and strain rate of tested It should be noticed that the catastrophic brittle failure was not specimen can be calculated as observed for the specimens tested at different strain rates. Instead, despite of the decrease of true stress with true strain after the stress reaches its compression strength, the material still possesses a rel- Es = (1) where ER and Er are, respectively, the transmitted and reflected train pulses which can be measured by the strain gages stuck on the input and output bars: Co, E, and a denote the longitudinal elastic wave velocity. Young's modulus and cross-sectional area of 250 the loading bars respectively; Is and As are the length and cross- sectional area of the specimen respectively 3. Experimental results and discussion 10e2 I 100/s 3.1. Experimental results stress vs strain curves of the 3d needle- punched C/Sic at different strain rates. It can be observed 0020.040060080.10.120.140.16 that both the lon strength and failure strain of the 3D True strain needle-punched C/Sic composite increase with applied strain rate, while elastic modulus of the material is almost independent on Fig. 2. The quasi-static and dynamic true stress vs strain curves of the 3D needling-Y. Li et al. / Materials Science and Engineering A 507 (2009) 6–12 7 Fig. 1. The diagram illustration of the split Hopkinson press bar systems. 3D-C/SiC composite with density of 1.90 g/cm3 and open porosity 16.09 ◦C. 2.2. Experiment procedure The specimens were cut from a 3D needle-punched C/SiC com￾posite plate to ensure that the loading direction is parallel to the thickness direction of the material. The specimens tested under quasi-static and dynamic loadings at room temperature were both cylindrical, with a diameter of 5 mm and a length of 4 mm. Both the quasi-static and dynamic experiments were performed at the room temperature. The strain rates were controlled in the range from 1.0 × 10−4 to 6.5 × 103 1/s. An electronic universal testing machine with a maximum load capacity of 10 kN was used to perform the quasi-static experiments. The split Hopkinson pressure bar (SHPB), situated in Northwestern Polytechnical University, was used for the high strain rate experi￾ments. A steel pad was employed to restrain the maximum strain within certain limits so that the fracture surface of the tested spec￾imens could be preserved. The diagram of the SHPB and the steel pad was shown in Fig. 1. According to the one-dimensional elas￾tic stress wave theory, the strain, stress and strain rate of tested specimen can be calculated as S = E A AS εT εS = −2C0 lS  t 0 εR d ε˙ S = −2C0 lS εR (1) where εR and εT are, respectively, the transmitted and reflected strain pulses which can be measured by the strain gages stuck on the input and output bars; C0, E, and A denote the longitudinal elastic wave velocity, Young’s modulus and cross-sectional area of the loading bars respectively; ls and As are the length and cross￾sectional area of the specimen respectively. 3. Experimental results and discussion 3.1. Experimental results Fig. 2 shows the true stress vs. strain curves of the 3D needle￾punched C/SiC composite at different strain rates. It can be observed that both the compression strength and failure strain of the 3D needle-punched C/SiC composite increase with applied strain rate, while elastic modulus of the material is almost independent on strain rate. The compression strength of the material increases from about 350 MPa at the strain rate of 10−4 1/s to 430 MPa at the strain rate of 6.5 × 103 1/s with a relative increment of about 23%. Although remarkable dispersity exists, a close inspection of the experimental results reveals that the compression strength of the material varies linearly with the logarithm of the strain rate (as shown in Fig. 3). Such phenomenon was also observed for 2D-C/SiC composite [2,15]. It is believed that microcracks are contributed to the strain rate sensitivity of the material. As we known, such defects as micro￾cracks caused by the mismatch of thermal expansion coefficients between the carbon fibers and the SiC matrix are unavoidable dur￾ing the preparation of the composites. Under compression loading, the nucleation and extension of the microcracks will cause the failure of the composites. It should be pointed out that both the nucleation and extension are processes of time dependence. Under quasi-static loading, the microcracks have enough time to nucleate and extend. While under dynamic loading, the applied load pulse is usually several hundred microseconds, which is insufficient for the nucleation and extension of microcracks. As the result, higher compression strength is needed at high strain rates. It should be noticed that the catastrophic brittle failure was not observed for the specimens tested at different strain rates. Instead, despite of the decrease of true stress with true strain after the stress reaches its compression strength, the material still possesses a rel￾Fig. 2. The quasi-static and dynamic true stress vs. strain curves of the 3D needling￾punched C/SiC composites.