M. Shioya, M. Nakatani/Composites Science and Technology 60(2000)219-229 Results of axial bending test of composite strands and properties of fibres Fibre Composite strand Properties of fibre Diameter Fibre volume bending Reduced be fraction GPa strength/GPa strength/GPa strength"/GPa 19±0.10 0.42 18±0.0 Compressive T4 Estimated with micro-compression test of single fibres [9 During the bending test of the composite strand of the has a circular cross-section. Thus, the volume of the fibres having a compressive strength lower than the specimen where the stress at this level can be applied is tensile strength, fracture initiates from the compressive roughly calculated to be 0.20.007x 1/2=0.07% of the side of the composite strand. The composite strand, entire specimen volume. This value is the maximum however, does not split into pieces immediately because estimate and the effective specimen volume decreases the tensile side of the composite strand is not fractured with increasing total specimen length and with decreas- yet, and the compressive load can be transmitted ing specimen diameter [11]. Therefore, an extremely through the damaged region of the compressive side of smaller volume of the material is involved in the axial the composite strand. This is presumably responsible for compression bending test as compared with the simple the result of the observation that the final fracture of axial compression test even though longer specimens are some composite strands initiated from the tensile side of used in the former test. The difference in the volume of the composite strand. It is considered that the compressive the tested material together with the size dependence of racture or microbuckling of the fibres inside the com- the strength of the material cause inconsistency of the posite strand commences earlier than the final fracture strength values determined with different test method of the composite strand. Therefore, in order to estimate The difference between the effective specimen volum the accurate strength of the component fibres from the and the entire specimen volume should also be take bending test of the composite strand, it is necessary to into account in the compression tests of single fibres. It detect the initial fracture of the fibres inside the composite has been pointed out that in the micro-compression test strand by using some method such as the acoustic of single fibres, the uniform stress distribution along the emission technique fibre length is provided in the region apart from the In discussing inconsistency of the strength values clamps by a certain fibre length [6, 7, 14]. In the recoil determined with different test methods, difference of the test of single fibres, bending-induced stress causes frac- volume of the tested material should be addressed ture near the fixed end of the fibre and in such a case oppressive stresses reach maximum, respectively, at the the limited volume of the fibre resents the strength of convex and concave sides of the specimen in the central crOSs-section. and decrease with increasing distance from these points in both longitudinal and transverse 4. Conclusions directions. The detailed calculation of the size of the effectively stressed region of the specimen during the The fracture surfaces of the carbon fibres and com- axial compression bending test has been given in a pre- posite strands produced by various compression tests vious paper [11]. By calculating similarly, it can be showed different features depending on the types of obtained that the tensile and compressive stresses aris- fibres and matrix resins. An inclined fracture surface ing at the convex and concave sides of the cross-section was typically produced for the pitch-based carbon fibres lecay less than 95% of the maximum value if the cross- by the micro-compression and recoil tests and for their section is apart from the central cross-section more than composite strands by the axial compression and bend 10% of the entire specimen length. Thus, the effective ing tests. A fracture surface characteristic to the flexural specimen length is 20% of the entire specimen length. fracture was produced by the recoil test of the PAN On the other hand, the tensile and compressive stresses based carbon fibres. A transverse fracture surface, in in the cross-section change linearly with the distance addition to the inclined fracture surface, was produced from the neutral plane of the cross-section. In the cen- for the PAN-based carbon fibres by the micro-com- tral cross-section, the tensile and compressive stresses pression test and for their composite strands by the no less than 95% of the maximum value can be applied axial compression and bending tests. Since segmented only to 0.7% of the cross-section area if the specimen fibre bundles inclined from the longitudinal directionDuring the bending test of the composite strand of the ®bres having a compressive strength lower than the tensile strength, fracture initiates from the compressive side of the composite strand. The composite strand, however, does not split into pieces immediately because the tensile side of the composite strand is not fractured yet, and the compressive load can be transmitted through the damaged region of the compressive side of the composite strand. This is presumably responsible for the result of the observation that the ®nal fracture of some composite strands initiated from the tensile side of the composite strand. It is considered that the compressive fracture or microbuckling of the ®bres inside the com￾posite strand commences earlier than the ®nal fracture of the composite strand. Therefore, in order to estimate the accurate strength of the component ®bres from the bending test of the composite strand, it is necessary to detect the initial fracture of the ®bres inside the composite strand by using some method such as the acoustic emission technique. In discussing inconsistency of the strength values determined with di€erent test methods, di€erence of the volume of the tested material should be addressed. During the axial bending test, the axial tensile and compressive stresses reach maximum, respectively, at the convex and concave sides of the specimen in the central cross-section, and decrease with increasing distance from these points in both longitudinal and transverse directions. The detailed calculation of the size of the e€ectively stressed region of the specimen during the axial compression bending test has been given in a pre￾vious paper [11]. By calculating similarly, it can be obtained that the tensile and compressive stresses aris￾ing at the convex and concave sides of the cross-section decay less than 95% of the maximum value if the cross￾section is apart from the central cross-section more than 10% of the entire specimen length. Thus, the e€ective specimen length is 20% of the entire specimen length. On the other hand, the tensile and compressive stresses in the cross-section change linearly with the distance from the neutral plane of the cross-section. In the cen￾tral cross-section, the tensile and compressive stresses no less than 95% of the maximum value can be applied only to 0.7% of the cross-section area if the specimen has a circular cross-section. Thus, the volume of the specimen where the stress at this level can be applied is roughly calculated to be 0.20.0071/2=0.07% of the entire specimen volume. This value is the maximum estimate and the e€ective specimen volume decreases with increasing total specimen length and with decreas￾ing specimen diameter [11]. Therefore, an extremely smaller volume of the material is involved in the axial compression bending test as compared with the simple axial compression test even though longer specimens are used in the former test. The di€erence in the volume of the tested material together with the size dependence of the strength of the material cause inconsistency of the strength values determined with di€erent test methods. The di€erence between the e€ective specimen volume and the entire specimen volume should also be taken into account in the compression tests of single ®bres. It has been pointed out that in the micro-compression test of single ®bres, the uniform stress distribution along the ®bre length is provided in the region apart from the clamps by a certain ®bre length [6,7,14]. In the recoil test of single ®bres, bending-induced stress causes frac￾ture near the ®xed end of the ®bre and in such a case, the obtained strength value represents the strength of the limited volume of the ®bre near the ®xed end [4]. 4. Conclusions The fracture surfaces of the carbon ®bres and com￾posite strands produced by various compression tests showed di€erent features depending on the types of ®bres and matrix resins. An inclined fracture surface was typically produced for the pitch-based carbon ®bres by the micro-compression and recoil tests and for their composite strands by the axial compression and bend￾ing tests. A fracture surface characteristic to the ¯exural fracture was produced by the recoil test of the PAN￾based carbon ®bres. A transverse fracture surface, in addition to the inclined fracture surface, was produced for the PAN-based carbon ®bres by the micro-com￾pression test and for their composite strands by the axial compression and bending tests. Since segmented ®bre bundles inclined from the longitudinal direction Table 3 Results of axial bending test of composite strands and properties of ®bres Fibre Composite strand Properties of ®bre Diameter/ mm Fibre volume fraction Bending strength/GPa Reduced bending strength/GPa Fracture mode Tensile strength/GPa Compressive strengtha /GPa X5 1.2 0.50 1.9‹0.10 3.7 Tensile 3.6 0.51 H4 1.0 0.42 1.8‹0.05 4.2 Compressive 4.4 1.6 T4 1.2 0.42 2.4‹0.13 5.7 Tensile 4.9 2.0 a Estimated with micro-compression test of single ®bres [9]. 228 M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229