M. Shioya, M. Nakatani/Composites Science and logy60(2000)219229 The SEM images of the X5, T4 and H4 fibre comp site strands using epoxy-A matrix after bending tests are shown in Fig. 5. Final fracture of the X5/epoxy-A composite strand seemed to initiate from the tensile side of the specimen although definite determination of the fracture mode of this specimen from the observation of the fracture process was difficult. Final fracture of the T4/epoxy-A composite strand was observed to initiate from the tensile side of the specimen. On the other hand, final fracture of the H4/epoxy-A composite strand was observed to initiate from the compressive side of the specimen 10m In the compression side of the X5/epoxy-A composite strand, an inclined fracture surface was produced. In the compression side of the T4 epoxy-A composite strand tep wise surfaces transverse to the fibre axis were pro duced. Therefore, the fracture surfaces produced by the bending test, in the compi side of the resemble those of the axial compression It should be noted that the fracture surface is pro duced after the fracture criterion is reached and does not necessarily manifest the fracture criterion 3. 2. Compressive strength determined with the mIcro-compression fest 10 um The tensile and compressive strengths of various carbon fibres are plotted against the tensile modulus in Fig. 6. In this figure, tensile properties referred to Table I and the compressive strength was determined with the micro- compression test. The compressive strength of carbon fibres is lower than the tensile strength and decreases with increasing tensile modulus. It has been reported in a previous paper [9] that the length dependence and distribution of the compressive strength of the carbon fibre are significantly smaller as compared with those of the tensile strength Dobb et al. have discussed that improved compressive strength requires disordered regions in the carbon fibre 10 um homogeneously distributed throughout the fibre and crystallites should have dimensions below about 5 nm in all directions [5]. The present authors have proposed Fig3. SEM images of fracture surfaces of (a) x5 fibre after micro- that the compressive strength is limited by the buckling compression test and(b)N3 and(c)T4 fibres after recoil test. stress of individual carbon layers in the transversely unsupported regions of the crystallites, the length of the fibres, were produced as shown in Fig 4(c). In the trans- unsupported regions being determined by the axial verse fracture surface, fibre cross-sections characteristic to length of the microvoids [9]. By transferring the critical posite strand was deformed into a way shape over a osTE gth or carbe obtained for the axile flexural fracture, and debonding of the fibre-matrix inter- stress to cause buckling of a bar to the faces were also revealed as shown in Fig 4(d) following expression ha In the case of the T4/epoxy-C composite strand, frac- compressive str ture of the fibres and matrix resin did not take place poa S3 When the compression load was removed, the wavy where Eo is the longitudinal modulus of the carbon shape disappeared within a few tens of minutes layer stacks parallel to the layer plane(1020 GPa)[171,®bres, were produced as shown in Fig. 4(c). In the trans￾verse fracture surface, ®bre cross-sections characteristic to ¯exural fracture, and debonding of the ®bre±matrix inter￾faces were also revealed as shown in Fig. 4(d). In the case of the T4/epoxy-C composite strand, frac￾ture of the ®bres and matrix resin did not take place even at the maximum compression load but the com￾posite strand was deformed into a wavy shape over a whole length owing to microbuckling of the ®bres. When the compression load was removed, the wavy shape disappeared within a few tens of minutes. The SEM images of the X5, T4 and H4 ®bre compo￾site strands using epoxy-A matrix after bending tests are shown in Fig. 5. Final fracture of the X5/epoxy-A composite strand seemed to initiate from the tensile side of the specimen although de®nite determination of the fracture mode of this specimen from the observation of the fracture process was dicult. Final fracture of the T4/epoxy-A composite strand was observed to initiate from the tensile side of the specimen. On the other hand, ®nal fracture of the H4/epoxy-A composite strand was observed to initiate from the compressive side of the specimen. In the compression side of the X5/epoxy-A composite strand, an inclined fracture surface was produced. In the compression side of the T4/epoxy-A composite strand, step wise surfaces transverse to the ®bre axis were pro￾duced. Therefore, the fracture surfaces produced by the bending test, in the compression side of the specimen, resemble those of the axial compression test. It should be noted that the fracture surface is pro￾duced after the fracture criterion is reached and does not necessarily manifest the fracture criterion. 3.2. Compressive strength determined with the micro-compression test The tensile and compressive strengths of various carbon ®bres are plotted against the tensile modulus in Fig. 6. In this ®gure, tensile properties referred to Table 1 and the compressive strength was determined with the micro￾compression test. The compressive strength of carbon ®bres is lower than the tensile strength and decreases with increasing tensile modulus. It has been reported in a previous paper [9] that the length dependence and distribution of the compressive strength of the carbon ®bre are signi®cantly smaller as compared with those of the tensile strength. Dobb et al. have discussed that improved compressive strength requires disordered regions in the carbon ®bre homogeneously distributed throughout the ®bre and crystallites should have dimensions below about 5 nm in all directions [5]. The present authors have proposed that the compressive strength is limited by the buckling stress of individual carbon layers in the transversely unsupported regions of the crystallites, the length of the unsupported regions being determined by the axial length of the microvoids [9]. By transferring the critical stress to cause buckling of a bar to the micro scale, the following expression has been obtained for the axial compressive strength of carbon ®bres,c. c ˆ 3Eofd2 002 48o2S3 …1† where Eo is the longitudinal modulus of the carbon layer stacks parallel to the layer plane (1020 GPa) [17], Fig. 3. SEM images of fracture surfaces of (a) X5 ®bre after micro￾compression test and (b) N3 and (c) T4 ®bres after recoil test. M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229 223