M. Shioya, M. Nakatani/Composites Science and Technology 60(2000)219-229 Table 2 Tensile properties of matrix resins Test fiber Resin Modulus/ Strength/ Strain at Strain at Epoxy resin maximum stress failure EEE Microscope lens Loading piece 2. 2. Measurements of fibre volume fraction and cross- Mechanical stage Load cell section area The volume fractions of the fibres and voids in the Microscope stage composite strand were determined according to JIS K7075 [13] from the densities of the carbon fibre, epoxy (a)Micro-compression test resin and composite strand and the change of the mass of the composite strand when the matrix resin was burned off. The density was measured at 30C by a sink float method using a n-heptane, carbon tetrachloride and ethylene dibromide mixture. The volume fraction of Epoxy resin block the voids in the composite strands estimated in this way was less than 0.015 Composite strand The cross-section area of the fibres in the composit strand was determined from the linear density and the density of the fibres used for the composite strand Metal base 2.3. Micro-compression test Macturk et al. have measured the axial compressive (b)Axial compression test (c) Axial compression bending test strength of single carbon fibres by using a miniature loading apparatus [7]. In their apparatus, the compres- Fig. 2. Schematics illustrations of (a) micro-compression, (b)axial compression and(c) axial compression bending tests. sive load is applied to the fibre through a piezoelectric element while displacement is detected by an optical probe. The compressive load is calculated from the applied voltage and the displacement of the piezoelectric 2. 4. Recoil test element. Thus, the value of the compressive load relies on the accurate correction of the test fixture compliance A single carbon fibre was bonded to a cardboard In the present study, the axial compressive strength of across a rectangular window 25 mm long cut out from single carbon fibres was measured using a miniature the cardboard. To bond the fibre to the cardboard, a loading apparatus where the fibre was compressed by mixture of epoxy resin(Epikote 828, Yuka Shell Epoxy moving a mechanical stage and the compressive load and triethylenetetramin by the weight ratio of 10: I was was directly detected with a load cell as shown in Fig. applied and cured for 120 min at 60C. The diameter of 2(a)[9]. A carbon fibre which was cut perpendicularly to each fibre was determined from the diffraction of He- ne fibre axis and having a flat cross-section was bonded Ne laser beam. The cardboard with the fibre was grip to a carbon tool steel piece so that the gage length ped with the clamps of a mechanical tester and both became from two to three times fibre diameter. The sides of the window were scissored before testing. The diameter of each fibre was determined from the diffrac- fibre was extended to a desired tensile stress level and tion of He-Ne laser beam from the fibre. The steel piece, cut at the center of the gage length with very sharp sur- with the fibre, was mounted on the mechanical stage of gical scissors. Both halves of the fibre were carefully the loading apparatus under observation using an opti- collected from the clamps and observed to ascertain cal microscope. The mechanical stage was moved at a whether or not compressive fracture occurred during the constant velocity of 4.46 mm min- and the fibre was recoil process. Several fibres were tested at each stress axially compressed between the mechanical stage and a level and by changing the stress levels, the fracture loading piece. In the following, quoted values of the probability versus pretensioning stress curve was compressive strength are the averages of at least 6 obtained. For each type of carbon fibre, more than 25 determinations on individual fibres filaments were tested2.2. Measurements of ®bre volume fraction and cross￾section area The volume fractions of the ®bres and voids in the composite strand were determined according to JIS K7075 [13] from the densities of the carbon ®bre, epoxy resin and composite strand and the change of the mass of the composite strand when the matrix resin was burned o€. The density was measured at 30C by a sink- ¯oat method using a n-heptane, carbon tetrachloride and ethylene dibromide mixture. The volume fraction of the voids in the composite strands estimated in this way was less than 0.015. The cross-section area of the ®bres in the composite strand was determined from the linear density and the density of the ®bres used for the composite strand. 2.3. Micro-compression test Macturk et al. have measured the axial compressive strength of single carbon ®bres by using a miniature loading apparatus [7]. In their apparatus, the compres￾sive load is applied to the ®bre through a piezoelectric element while displacement is detected by an optical probe. The compressive load is calculated from the applied voltage and the displacement of the piezoelectric element. Thus, the value of the compressive load relies on the accurate correction of the test ®xture compliance. In the present study, the axial compressive strength of single carbon ®bres was measured using a miniature loading apparatus where the ®bre was compressed by moving a mechanical stage and the compressive load was directly detected with a load cell as shown in Fig. 2(a) [9]. A carbon ®bre which was cut perpendicularly to the ®bre axis and having a ¯at cross-section was bonded to a carbon tool steel piece so that the gage length became from two to three times ®bre diameter. The diameter of each ®bre was determined from the di€rac￾tion of He±Ne laser beam from the ®bre. The steel piece, with the ®bre, was mounted on the mechanical stage of the loading apparatus under observation using an opti￾cal microscope. The mechanical stage was moved at a constant velocity of 4.46 mm minÿ1 and the ®bre was axially compressed between the mechanical stage and a loading piece. In the following, quoted values of the compressive strength are the averages of at least 6 determinations on individual ®bres. 2.4. Recoil test A single carbon ®bre was bonded to a cardboard across a rectangular window 25 mm long cut out from the cardboard. To bond the ®bre to the cardboard, a mixture of epoxy resin (Epikote 828, Yuka Shell Epoxy) and triethylenetetramin by the weight ratio of 10:1 was applied and cured for 120 min at 60C. The diameter of each ®bre was determined from the di€raction of He± Ne laser beam. The cardboard with the ®bre was grip￾ped with the clamps of a mechanical tester and both sides of the window were scissored before testing. The ®bre was extended to a desired tensile stress level and cut at the center of the gage length with very sharp sur￾gical scissors. Both halves of the ®bre were carefully collected from the clamps and observed to ascertain whether or not compressive fracture occurred during the recoil process. Several ®bres were tested at each stress level and by changing the stress levels, the fracture probability versus pretensioning stress curve was obtained. For each type of carbon ®bre, more than 25 ®laments were tested. Table 2 Tensile properties of matrix resins Resin Modulus/ GPa Strength/ GPa Strain at maximum stress Strain at failure Epoxy-A 3.0 75 0.027 0.027 Epoxy-B 2.7 64 0.035 0.088 Epoxy-C 2.4 51 0.028 0.15< Fig. 2. Schematics illustrations of (a) micro-compression, (b) axial compression and (c) axial compression bending tests. M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229 221