N. Oya, D.J. Johnson /Carbon 39(2001)635-645 confirmed that the com proved with sFF 20mic increase of the tensile Stress (GPa ver, the improve- 7.01m ment of tensile strens nore remarkable 20 especially for HS fibres, than that of compressive strength This may be reflected by the fact that the highly oriented 1.8 structure of carbon fibre is more convenient to improve the tensile properties 12 3.1.2. Longitudinal compressive modulus 0.8 The direct evaluation of modulus vas not very successful in the present device of large 0.4 1.92GPa experimental errors which were mainly difficulty of measuring the microscopic machine com- pliance at very small force level However, the longitudinal compressive modulus may be Time (sec) evaluated by making use of the Euler buckling formula pplied for buckled long samples. This (a)20um derived to account for the critical buckling load of an axially compressed straight column just like a carbon filament under longitudinal compression. If the straight Sample: T300 column has a length L. cross-sectional moment of inertia I Stress (GPa) FL= 100mic and elastic modulus E, and is subjected to a centrally FD= 7. 01mic applied compressive load P, the critical buckling load P 2.0 for the column to show elastic buckling can be expressed P=o tet The factor a depends on the boundary condition of the 0.8 column. For the particular case seen in this work, the clamped-simple support condition which gives a=2.04 0. 68GPa may be used. The elastic modulus E can be obtained if the 47 89sec critical buckling load Per is experimentally measured from 0.0 kled long samples with any particular gauge lengt 020406080100 However, it must be noted that the formula is derived by Time sec) applying the equilibrium equations to the isotropic column in a slightly deformed state. Assuming that the neutral plane of the fiexure exists in the middle of the cross- Fig. 2. Stress-time curves with different sample gauge lengths; section, the apparent elastic modulus E must be considered (b)100 as the average of tensile and compressive moduli for anisotropic carbon fibres obtain the scatter of data in the recoil method. in which e E+E only one representative value is available as a critical rength from several samples. where Et and ec are tensile and compressive modul Fig. 4 presents a comparison of the direct and recoil respectively. As the tensile modulus e, is already ki compressive strengths for each carbon fibre. It is apparent the compressive modulus E can be readily calculated if that both strengths showed different values but in some- the average modulus E is obtained experimentally what similar trends. The reason why the tensile recoil Fig. 6 shows a typical plot of average modulus as a method resulted in lower strengths is probably associated gth for T300 fibre. The with the dynamic loading [6 and the buckling problem modulus generally showed a constant value with long [7, 8] of long samples gauge lengths, but declined with short lengths. The reduc- Fig 5 compares the longitudinal compressive and tensile tion of modulus may be attributed to a change of buckling ngths The compressive strengths were definitely lower; mode from a clamped-simple support(a=2.04)to a from about 30 to 50% of the tensile strengths. It was also 'clamped-free(a=0.25)condition. By reducing the gauge638 N. Oya, D.J. Johnson / Carbon 39 (2001) 635 –645 confirmed that the compressive strengths improved with increase of the tensile strengths. However, the improve￾ment of tensile strength was much more remarkable, especially for HS fibres, than that of compressive strength. This may be reflected by the fact that the highly oriented structure of carbon fibre is more convenient to improve the tensile properties. 3.1.2. Longitudinal compressive modulus The direct evaluation of modulus and strain was not very successful in the present device because of large experimental errors which were mainly associated with the difficulty of measuring the microscopic machine com￾pliance at very small force level. However, the longitudinal compressive modulus may be evaluated by making use of the Euler buckling formula applied for buckled long samples. This formula was derived to account for the critical buckling load of an axially compressed straight column just like a carbon filament under longitudinal compression. If the straight column has a length L, cross-sectional moment of inertia I, and elastic modulus E, and is subjected to a centrally applied compressive load P, the critical buckling load Pcr for the column to show elastic buckling can be expressed by 2 p EI P 5 a]]. (5) cr 2 L The factor a depends on the boundary condition of the column. For the particular case seen in this work, the ‘clamped-simple support’ condition which gives a 52.04 may be used. The elastic modulus E can be obtained if the critical buckling load Pcr is experimentally measured from buckled long samples with any particular gauge length. However, it must be noted that the formula is derived by applying the equilibrium equations to the isotropic column in a slightly deformed state. Assuming that the neutral plane of the flexure exists in the middle of the cross￾section, the apparent elastic modulus E must be considered Fig. 2. Stress–time curves with different sample gauge lengths; as the average of tensile and compressive moduli for (a) 20 mm and (b) 100 mm. anisotropic carbon fibres as follows; Et c 1 E obtain the scatter of data in the recoil method, in which E 5 ]] (6) 2 only one representative value is available as a critical strength from several samples. where E and E are tensile and compressive moduli, t c Fig. 4 presents a comparison of the direct and recoil respectively. As the tensile modulus E is already known, t compressive strengths for each carbon fibre. It is apparent the compressive modulus E can be readily calculated if c that both strengths showed different values but in some- the average modulus E is obtained experimentally. what similar trends. The reason why the tensile recoil Fig. 6 shows a typical plot of average modulus as a method resulted in lower strengths is probably associated function of sample length for T300 fibre. The average with the dynamic loading [6] and the buckling problem modulus generally showed a constant value with long [7,8] of long samples. gauge lengths, but declined with short lengths. The reduc￾Fig. 5 compares the longitudinal compressive and tensile tion of modulus may be attributed to a change of buckling strengths. The compressive strengths were definitely lower; mode from a ‘clamped-simple support’ (a 52.04) to a from about 30 to 50% of the tensile strengths. It was also ‘clamped-free’ (a 50.25) condition. By reducing the gauge