L Mingshuang et al. Materials Science and Engineering A 489(2008)120-126 121 Socket Projectile Laser elocity meter Endergonic bar nput bar _口■■ High dynamic ve collection syste Fig. 1. The schematic illustration of the split Hopkinson pressure bar systems. paper, the layer-directional dynamic compressive behavior of a 2D-C/SiC composites was investigated, and a new constitutive equation including the rate-dependent and damage-softening effects was proposed, which agreed very well with the exper- As imental results 2. Material and experimental techniques es 2. Material where ER and et are the transmitted and reflected strain pulses, respectively; Co = VE/po denotes longitudinal elastic wave The 2D-C/SiC composite materials are supplied by the State velocity in the Hopkinson bars; E Young's modulus of the Hop- Key Laboratory of Solidification Processing in Northwestern kinson bars, po the density of the Hopkinson bars, Is and As Polytechnical University, People's Republic of China. PAN- the length and cross-sectional area of the specimen, A the cross- based carbon fiber clothes were laid up at T300-1K. And then sectional area of the Hopkinson bars, respectively the chemical vapor infiltration process(CVn) was employed Fracture surfaces of the broken specimens were observed to deposit a thin pyrolytic carbon layer and Sic-matrix. The sing a scanning electron microscope(SEM) dimension of the 2D-C/Sic composite specimens is about Φ5mm×4.3mm. 3. Experimental results and discussion 3.. Stress-strain curves 2.2. Experimental techniques The 2D-C/SiC composites were compressed under uniaxial The quasi-static compressive strengths of 2D-C/SiC com- compressive loading at different strain rates. The compressive posites were determined by using a universal test machine. strength increases from 360 MPa at the strain rate of 10-4s" Static tests were performed at strain rates of 10-4 and up to 430 MPa at the strain rate of 2800s-, which is 19.4% 10-2s-. The stiffness of the test machine was calibrated higher compared with quasi-static result. All the compressive and then taken into account in the displacement measure- stress-strain curves are non-linear. Fig. 2 shows the variation ment of failure stress with the logarithm of strain rate. It is clear that The dynamic compressive experiments were performed by the compressive failure strength of the 2D-C/SiC composites the split Hopkinson pressure bar(SHPB)apparatus(as shown increases with an increasing strain rate. As previously observed Fig. 1). Different strain rates were obtained by changing the in the tests of carbon/epoxy matrix composites [7]and C/C com- striker's length and the gas pressure. Based on the theory of posites[8], the elastic modulus for the 2D-C/SiC composites also one-dimensional elastic wave propagation, the average strain increases with loading rate in the present study. But the intrinsic ES, strain rate Es and stress os in the specimen can be evaluated mechanism is not clear.L. Mingshuang et al. / Materials Science and Engineering A 489 (2008) 120–126 121 Fig. 1. The schematic illustration of the split Hopkinson pressure bar systems. paper, the layer-directional dynamic compressive behavior of 2D-C/SiC composites was investigated, and a new constitutive equation including the rate-dependent and damage-softening effects was proposed, which agreed very well with the exper￾imental results. 2. Material and experimental techniques 2.1. Material The 2D-C/SiC composite materials are supplied by the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University, People’s Republic of China. PAN￾based carbon fiber clothes were laid up at T300-1 K. And then the chemical vapor infiltration process (CVI) was employed to deposit a thin pyrolytic carbon layer and SiC-matrix. The dimension of the 2D-C/SiC composite specimens is about 5 mm × 4.3 mm. 2.2. Experimental techniques The quasi-static compressive strengths of 2D-C/SiC com￾posites were determined by using a universal test machine. Static tests were performed at strain rates of 10−4 and 10−2 s−1. The stiffness of the test machine was calibrated and then taken into account in the displacement measure￾ment. The dynamic compressive experiments were performed by the split Hopkinson pressure bar (SHPB) apparatus (as shown in Fig. 1). Different strain rates were obtained by changing the striker’s length and the gas pressure. Based on the theory of one-dimensional elastic wave propagation, the average strain εS, strain rate ε˙S and stress σS in the specimen can be evaluated as: ⎧ ⎪⎪⎪⎪⎪⎪⎨ ⎪⎪⎪⎪⎪⎪⎩ σS = E  A AS  εT εS = −2C0 lS  t 0 εR dτ ε˙S = −2C0 lS εR (1) where εR and εT are the transmitted and reflected strain pulses, respectively; C0 = √E/ρ0 denotes longitudinal elastic wave velocity in the Hopkinson bars; E Young’s modulus of the Hop￾kinson bars, ρ0 the density of the Hopkinson bars, lS and AS the length and cross-sectional area of the specimen, A the cross￾sectional area of the Hopkinson bars, respectively. Fracture surfaces of the broken specimens were observed using a scanning electron microscope (SEM). 3. Experimental results and discussion 3.1. Stress–strain curves The 2D-C/SiC composites were compressed under uniaxial compressive loading at different strain rates. The compressive strength increases from 360 MPa at the strain rate of 10−4 s−1 up to 430 MPa at the strain rate of 2800 s−1, which is 19.4% higher compared with quasi-static result. All the compressive stress–strain curves are non-linear. Fig. 2 shows the variation of failure stress with the logarithm of strain rate. It is clear that the compressive failure strength of the 2D-C/SiC composites increases with an increasing strain rate. As previously observed in the tests of carbon/epoxy matrix composites[7] and C/C com￾posites[8], the elastic modulus for the 2D-C/SiC composites also increases with loading rate in the present study. But the intrinsic mechanism is not clear.