H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 3247 4-R85 185 Fig. 1. Geometry of the as received composite specimen(all dimensions in mm). electrical resistance monitoring are presented in this paper. (c) After acquisition for 10 S, data was saved in the Much of analysis and discussion will then focus on the harddisk as the real-time resistance ys. time effects of thermal cycling on the composites in the wet xygen and on how resistances change with the specific Repeat the above step(a)-(c)till the specified durations damage mechanisms of acquisition were completed or till the failure of the specimen whichever was earlier. Specimen rate was set to 8 kHz and interval of a frame was 10 ms. The calculation 2. Experimental formula of the resistance is given by 2/. System of damage information acquisition for C/iC R(=20C As we know, the voltage signals changing with electrical resistance of the vibratile carbon particles in R(i)·di microphones can be transformed into the digital signals by the sound card in the multimedia computer. Similarly, the where, R() is the real-time resistance of the frame i, C a voltage signals on both ends of the composite specimens transformation coefficient, ui (n) voltage of the specimen in testing collected by the sound card can directly reflect point n in the frame i, and Rtotal an accumulated resistance the changes in electrical resistance. For this purpose, a which is an integral of R(i) vs. frame di System of Damage Information Acquisition(SDIA)was designed to in situ acquire the electrical resistance of the 2. 1.2.(2) Data analyzer C/SiC composites by using a sound card of personal This programme module could resume and analyze the computer. It comprised of the following parts: recorded data by using the functions in Matlab@, then drew diagrams in the windows or created reports directly 2.1.1.(1) Data acquisition The Data Acquisition module was developed according 2.2. Preparation of 2D C/SiC composite to the following steps 2D C/SiC composites were prepared by CVI technology (a)Voltage signals of both ends of the composite using a laminated cross-ply carbon-cloth [0/9 specimen were detected and sent to a sound card volume fraction of fibers was about 40%. The approximate by a sensor and Shielded Twisted Pair (STP) with 50 um SiC coating was deposited on the surfaces of the lifier specimens. Geometry of the as received composite speci (b) Analog signals were acquired in real time, trans- men is shown in Fig. I and the dimensions are 3 x 185 formed into the digital signals, and then stored in data mm. The virgin properties of the 2D C/SiC composites are engine of Matla listed in Table 1 erties of the as-received 2D-C/SiC composites rty Density(x 10 kg/km) Modulus(GPa) Strength(MPa) Poisson's ratio Porosity (% CTE(x 10-/C) 600°C800°C1000°C1200°C value 0.32electrical resistance monitoring are presented in this paper. Much of analysis and discussion will then focus on the effects of thermal cycling on the composites in the wet oxygen and on how resistances change with the specific damage mechanisms. 2. Experimental 2.1. System of damage information acquisition for C/SiC composites As we know, the voltage signals changing with electrical resistance of the vibratile carbon particles in microphones can be transformed into the digital signals by the sound card in the multimedia computer. Similarly, the voltage signals on both ends of the composite specimens in testing collected by the sound card can directly reflect the changes in electrical resistance. For this purpose, a System of Damage Information Acquisition (SDIA) was designed to in situ acquire the electrical resistance of the C/SiC composites by using a sound card of personal computer. It comprised of the following parts: 2.1.1. (1) Data acquisition The Data Acquisition module was developed according to the following steps: (a) Voltage signals of both ends of the composite specimen were detected and sent to a sound card by a sensor and Shielded Twisted Pair (STP) with amplifier. (b) Analog signals were acquired in real time, trans￾formed into the digital signals, and then stored in data acquisition engine of Matlab\ transitorily. (c) After acquisition for 10 s, data was saved in the harddisk as the real-time resistance vs. time. Repeat the above step (a) – (c) till the specified durations of acquisition were completed or till the failure of the specimen whichever was earlier. Specimen rate was set to 8 kHz and interval of a frame was 10 ms. The calculation formula of the resistance is given by, R iðÞ¼ 20C X N n¼1 log10u2 i ðÞ ð n 1Þ Rtotal ¼ Z V 0 R iðÞ di ð2Þ where, R(i) is the real-time resistance of the frame i, C a transformation coefficient, ui(n) voltage of the specimen point n in the frame i, and Rtotal an accumulated resistance which is an integral of R(i) vs. frame di. 2.1.2. (2) Data analyzer This programme module could resume and analyze the recorded data by using the functions in Matlab\, then drew diagrams in the windows or created reports directly. 2.2. Preparation of 2D C/SiC composite 2D C/SiC composites were prepared by CVI technology using a laminated cross-ply carbon-cloth [0/90-]. The volume fraction of fibers was about 40%. The approximate 50 Am SiC coating was deposited on the surfaces of the specimens. Geometry of the as received composite speci￾men is shown in Fig. 1 and the dimensions are 33185 mm. The virgin properties of the 2D C/SiC composites are listed in Table 1. Fig. 1. Geometry of the as received composite specimen (all dimensions in mm). Table 1 Properties of the as-received 2D-C/SiC composites Property Density (103 kg/km3 ) Modulus (GPa) Strength (MPa) Poisson’s ratio Porosity (%) CTE (106 / -C) 600 -C 800 -C 1000 -C 1200 -C Value 2.0 70 248 0.32 13 4.6 6.1 5.2 5.4 H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3247