C. Sauder et al./ Composites Science and Technology 62(2002)499-504 eached, and a uniform temperature along the fibre to be apparatus. Fibre section was assumed to be circular. generated. Temperature in the fibre is measured by using This is true for the PAN-based fibre bichromatic pyrometer in the 1000-3000C range. In practice, temperatures in the 1200-3000oC range only 2.3. Experimental procedure are detected on fibres having a 7 um diameter. Since tests are performed under secondary vacuum (residual pres- 2.3.1. Measurement of electrical conductivity sure <10-3 Pa)there is a relationship between fibre The calibration curve established for temperature temperature and the electric power supplied as shown in determination(Fig. 1)allows determination of electrical Fig. l. As a consequence, temperature of fibre can be conductivity(a)at various temperatures easily derived from the electric power. placement of grips are measured by whose sensitivity is smaller than 0. 1 um. Fibre strain is o(Q2-1m-=I-L a sensor derived from grip displacement by using a compliance p RS calibration technique that allows deformation of load strain to be taken into account [4]. a technique of Where p=R S/L(resistivity), where R is the resistance direct measurement of fibre elongation is under devel- (R-UlL, U=tension, I=intensity ), S=sectional area opment. It is based upon optical extensometry and and L=fibre length. image analys A schematic diagram of the high temperature fibre 2.3.2. Measurement of thermal expansion testing apparatus is shown in Fig. 2. Alignment of grips Very few data on thermal expansion of carbon fibres is improved using internal and external devices. The are available at high temperatures [2, 5, 6]. The technique all the experimental data are recorded by a computer. onto the fibre (a few MPa). This stress is maintained constant during fibre heating through control of grip 2. 2. Materials and specimen preparation displacement. The displacement which is applied bal ances the longitudinal expansion of fibre induced by Two different carbon fibres were investigated: a heating. Such thermal expansion measurements are very rayon-base and a polyacrylonitrile-based fibre. Fibres easy and very accurate under computer controlled test- were tested either as-received or after treatment at 1600 ing conditions or 2200C. Fibres were fixed to graphite grips using a cement( C-34, UCAR). Batches of 20 specimens were 2.3.3. Mechanical behavior prepared, except for the tests at 1200, 1400 and 1800C Tensile tests were performed on those PAN-based and on the rayon-based fibres. rayon-based fibres which had been treated at 2200C Fibre diameter was measured before tests using laser The range of test temperatures could thus be increased diffractometry. The fibre was mounted on the testing from 1000 to 2200C, since mechanical properties of carbon fibres depend on the heating rate when the test temperature is higher than the treatment temperature Gauge length was 50 mm. The Youngs modulus was 口 estimated data 2400 3000 Fig. 1. Correlation between electrical power applied to the fibre and Fig. 2. Schematic diagram of the high temperature fibre testing appa- the temperature measured by using a pyromete atusreached, and a uniform temperature along the fibre to be generated. Temperature in the fibre is measured by using a bichromatic pyrometer in the 1000–3000
C range. In practice, temperatures in the 1200–3000
C range only are detected on fibres having a 7 mm diameter. Since tests are performed under secondary vacuum (residual pres￾sure <103 Pa) there is a relationship between fibre temperature and the electric power supplied as shown in Fig. 1. As a consequence, temperature of fibre can be easily derived from the electric power. Displacement of grips are measured by a sensor whose sensitivity is smaller than 0.1 mm. Fibre strain is derived from grip displacement by using a compliance calibration technique that allows deformation of load strain to be taken into account [4]. A technique of direct measurement of fibre elongation is under devel￾opment. It is based upon optical extensometry and image analysis. A schematic diagram of the high temperature fibre testing apparatus is shown in Fig. 2. Alignment of grips is improved using internal and external devices. The load cells are located within the vacuum chamber and all the experimental data are recorded by a computer. 2.2. Materials and specimen preparation Two different carbon fibres were investigated: a rayon-base and a polyacrylonitrile-based fibre. Fibres were tested either as-received or after treatment at 1600 or 2200
C. Fibres were fixed to graphite grips using a cement (C-34, UCAR). Batches of 20 specimens were prepared, except for the tests at 1200, 1400 and 1800
C on the rayon-based fibres. Fibre diameter was measured before tests using laser diffractometry. The fibre was mounted on the testing apparatus. Fibre section was assumed to be circular. This is true for the PAN-based fibre. 2.3. Experimental procedure 2.3.1. Measurement of electrical conductivity The calibration curve established for temperature determination (Fig. 1) allows determination of electrical conductivity (
) at various temperatures:
ð
1 m1 Þ ¼ 1 ¼ L R:S Where =R.S/L (resistivity), where R is the resistance (R=U/I, U=tension, I=intensity), S=sectional area and L=fibre length. 2.3.2. Measurement of thermal expansion Very few data on thermal expansion of carbon fibres are available at high temperatures [2,5,6]. The technique used here is very simple. A very low stress is applied onto the fibre (a few MPa). This stress is maintained constant during fibre heating through control of grip displacement. The displacement which is applied bal￾ances the longitudinal expansion of fibre induced by heating. Such thermal expansion measurements are very easy and very accurate under computer controlled test￾ing conditions. 2.3.3. Mechanical behavior Tensile tests were performed on those PAN-based and rayon-based fibres which had been treated at 2200
C. The range of test temperatures could thus be increased from 1000 to 2200
C, since mechanical properties of carbon fibres depend on the heating rate when the test temperature is higher than the treatment temperature. Gauge length was 50 mm. The Young’s modulus was Fig. 1. Correlation between electrical power applied to the fibre and the temperature measured by using a pyrometer. Fig. 2. Schematic diagram of the high temperature fibre testing appa￾ratus. 500 C. Sauder et al. / Composites Science and Technology 62 (2002) 499–504