N. Oya, D.J. Johnson /Carbon 39(2001)635-645 was randomly chosen from a tow and vertically cut by a sharp razor blade to obtain a fat end-surface. The sample X Pgra (2) was then placed on a metallic stage by means of yanoacrylate adhesive, with some portion sticking out. where per is the density of pure graphite (por=2.268 This portion was regarded as the sample gauge length, and /cm). The total pore volume pore was defined as was accurately adjustable from 20 to 550 um with the aid of an optical microscope. The sample stage was then connected to micrometer screw threads that rotate with a C minimotor and reduction gear(68, 200: 1). By slowly rotating the motor, the sample stage moved linearly where Pobs is the density of observed carbon fibre. The forward at 0. 13 m/s, and eventually, the fibre sample total porosity p was then calculated from made contact with a thin cantilever beam causing it to be p=pore X Pobs X 100 deflected. The cantilever had strain gauges attached in order to detect the small deflection. With a suitable calibration for the deflection and force applied to the 3. Experimental results cantilever established, longitudinal compressive force on the fibre sample was monitored during the course of 3.1. Longitudinal compressive behaviour testing. In addition, this device was able to operate not only under an optical microscope, but also in a scanning 3.1.1.Lon opressIve sireng electron-microscope chamber in order to observe longi Fig. 2 shows typical stress-time curves obtained with tudinal compressive behaviour of carbon fibres in detail different sample gauge lengths. Compressive stress linearly In this study, tensile recoil tests were also carried out to increased until the final failure with short gauge lengths measure longitudinal compressive strength in carbon fibres whereas a reduction of stress was evident with long gauge for comparative purposes. Details of the recoil technique ths for each carbon fibre such a difference was due to can be found elsewhere [ 3, 5-10 different compression modes of samples, either axial compression or buckling instability. Therefore, it was thought that the buckling effect can be avoided by making 2.3. Wide-angle x-ray diffraction the gauge length short enough, and longitudinal compres- sive strength must be estimated with such a gauge length X-ray diffraction patterns of aligned fibres Fig. 3 shows a typical plot of longitudinal compressive orded using a position-sensitive detector(PSD) with Cu strength as a function of sample gauge length with ka radiation (A=0.154 nm)operating at 40 kV and 16 mA. indications of compression modes. Compressive strength he diffraction pattern was normalised, corrected and generally declined because of the buckling effect with eaks resolved using standard computational methods [11] uge length i However. each carbon fibre showed recently adapted for PC analysis. The interlayer spacing the true axial compression mode at sufficiently short gauge looz and the crystallite size in the c-axis direction Le were lengths. As long as the axial compression mode appeared, estimated from the position and half-height width of the compressive strength was almost constant for any gauge esolved(002) refection, position being calibrated from length. Therefore, as a criterion for longitudinal co standard silver peak positions. The crystallite sizes in the sive strength, a gauge length of 20 um, the -axis directions, Lal and lai, were determined from the practical length with this method, was adopted (100) reflections on the meridian and equator, respectively parison purpose As a measure of disorder, the relative degree of in- It is noted that the buckling mode did not appear with tracrystallite imperfection D was calculated from the gauge lengths up to around 100 um with th simple relation compression device(see Fig 3 in Ref. 3)). The difference with the present device may be due to the rate of loading D 100 (1) appropriate compression mode was not detected with the former device due to the high strain rate whereas it is in a turbostratic carbon, perfect graphite, and the observed nd the obss oo clearly identified with the present device owing to the low where durb, dgra and dobs are the interlayer spacings door strain rate and direct microscope observations Table 2 lists the direct longitudinal compressive carbon fibre, respectively; it may be noted that durb was strengths together with the recoil and tensile strengths in taken as 0.350 nm. and d=0.335 nm each carbon fibre. In general, the direct compressive The microporosity p was estimated from doo? and strength was higher than the recoil strength. It is also seen density values in carbon fibres [12]. Firstly, the X- that the compressive strength was much lower than the density p was calculated from the interlayer spacing do tensile strength. The scatter of compressive strength was comparable to that of tensile strength. It is not possible toN. Oya, D.J. Johnson / Carbon 39 (2001) 635 –645 637 was randomly chosen from a tow and vertically cut by a dgra sharp razor blade to obtain a flat end-surface. The sample r 5 3 ] r (2) x gra dobs was then placed on a metallic stage by means of cyanoacrylate adhesive, with some portion sticking out. where rgra gra is the density of pure graphite (r 52.268 3 This portion was regarded as the sample gauge length, and g/cm ). The total pore volume V was defined as pore was accurately adjustable from 20 to 550 mm with the aid 1 1 of an optical microscope. The sample stage was then V 5 2 ] ] (3) pore connected to micrometer screw threads that rotate with a r r obs x DC minimotor and reduction gear (68,200:1). By slowly where r is the density of observed carbon fibre. The obs rotating the motor, the sample stage moved linearly total % porosity p was then calculated from forward at 0.13 mm/s, and eventually, the fibre sample made contact with a thin cantilever beam causing it to be p 5V 3 r 3 100. (4) pore obs deflected. The cantilever had strain gauges attached in order to detect the small deflection. With a suitable calibration for the deflection and force applied to the 3. Experimental results cantilever established, longitudinal compressive force on the fibre sample was monitored during the course of 3.1. Longitudinal compressive behaviour testing. In addition, this device was able to operate not only under an optical microscope, but also in a scanning- 3.1.1. Longitudinal compressive strength electron-microscope chamber in order to observe longi- Fig. 2 shows typical stress–time curves obtained with tudinal compressive behaviour of carbon fibres in detail. different sample gauge lengths. Compressive stress linearly In this study, tensile recoil tests were also carried out to increased until the final failure with short gauge lengths, measure longitudinal compressive strength in carbon fibres whereas a reduction of stress was evident with long gauge for comparative purposes. Details of the recoil technique lengths for each carbon fibre. Such a difference was due to can be found elsewhere [3,5–10]. different compression modes of samples, either axial compression or buckling instability. Therefore, it was thought that the buckling effect can be avoided by making 2.3. Wide-angle X-ray diffraction the gauge length short enough, and longitudinal compres￾sive strength must be estimated with such a gauge length. X-ray diffraction patterns of aligned fibres were re- Fig. 3 shows a typical plot of longitudinal compressive corded using a position-sensitive detector (PSD) with Cu strength as a function of sample gauge length with K radiation (l50.154 nm) operating at 40 kV and 16 mA. a indications of compression modes. Compressive strength The diffraction pattern was normalised, corrected and generally declined because of the buckling effect with peaks resolved using standard computational methods [11] gauge length increase. However, each carbon fibre showed recently adapted for PC analysis. The interlayer spacing the true axial compression mode at sufficiently short gauge d002 and the crystallite size in the c-axis direction Lc were lengths. As long as the axial compression mode appeared, estimated from the position and half-height width of the compressive strength was almost constant for any gauge resolved (002) reflection, position being calibrated from length. Therefore, as a criterion for longitudinal compres￾standard silver peak positions. The crystallite sizes in the sive strength, a gauge length of 20 mm, the shortest a-axis directions, Lai and La', were determined from the practical length with this method, was adopted for com- (100) reflections on the meridian and equator, respectively. parison purposes. As a measure of disorder, the relative degree of in- It is noted that the buckling mode did not appear with tracrystallite imperfection Dc was calculated from the gauge lengths up to around 100 mm with the previous simple relation compression device (see Fig. 3 in Ref. [3]). The difference with the present device may be due to the rate of loading; dturb obs 2 d ]]] the appropriate compression mode was not detected with D 5F G 1 2 3 100 (1) c dturb gra 2 d the former device due to the high strain rate whereas it is clearly identified with the present device owing to the low where d , d and d are the interlayer spacings d strain rate and direct microscope observations. turb gra obs 002 in a turbostratic carbon, perfect graphite, and the observed Table 2 lists the direct longitudinal compressive carbon fibre, respectively; it may be noted that d was strengths together with the recoil and tensile strengths in turb taken as 0.350 nm, and d 50.335 nm. each carbon fibre. In general, the direct compressive gra The microporosity p was estimated from d002 and strength was higher than the recoil strength. It is also seen density values in carbon fibres [12]. Firstly, the X-ray that the compressive strength was much lower than the density r was calculated from the interlayer spacing d tensile strength. The scatter of compressive strength was x 002 from comparable to that of tensile strength. It is not possible to