M. Shioya, M. Nakatani/Composites Science and Technology 60(2000)219-229 2.5. Axial compression test of composite strand 3. Results and discussion Determination of the axial compressive strength of 3. Fracture surface ne fibres from the axial compressive strength of uni- directional composites has been reviewed by Kozey et Fracture surfaces of the carbon fibres and composite aL. [14]. In the present study, the axial compression test strands produced by various compression tests revealed of the composite strands was carried out by supporting different features depending on the types of fibres and, both ends of the specimen with resin blocks as shown in in the case of composite strands, the matrix resins Fig. 2(b)in order to prevent local fracture at the loading During the micro-compression tests of single fibres points. A circular hole was drilled through rectangula two types of fracture surfaces were produced. One is a epoxy resin blocks with cross-section sizes of 20 mm by fracture surface inclined with an angle of about 450 20 mm and with a thickness of 10 mm, in the thickness against the fibre axis as shown in Fig 3(a )and the other direction. Both ends of the specimen were put into the is a fracture surface which runs almost transversely to holes of the resin blocks and bonded with an epoxy the fibre axis. The formation of transverse fracture sur- resin so that the gage length of the specimen between face was only observed by the optical microscopy during two resin blocks became 5 mm. This gage length assured the micro-compression test because fractured specimens compressive fracture of the specimen before buckling. suitable for SEM observation could not be collected The specimen with resin blocks was compressed by successfully. The inclined fracture surface was produced using a mechanical tester with a crosshead speed of 0.5 for X7 and X5 pitch-based carbon fibres In the cases of mm min-l Reduced compressive strength of the com- N3 pitch-based carbon fibre, and H4 and T4 PAN ponent fibres was calculated by dividing the fracture based carbon fibres, both the inclined fracture surface load of the composite strand with the cross-section area and a transverse fracture surface were produced. of the fibres The compressive fracture by the recoil tests took place almost invariably in the zone close to the fixed ends of 2.6. Axial compression bending test of composite strand the fibre. During the recoil tests of single fibres, two types of fracture surfaces were produced. One is a frac- used techniques while these tests are insufficient for the fibre axis as shown in Fig. 3(b)and the other l p The three- and the four-point bending tests are widely ture surface inclined with an angle of about 45 agains advanced composite materials because local fracture fracture surface suggesting flexural fracture as shown in tends to occur at the loading points owing to stress Fig 3(c). In Fig 3(c), two regions with different features concentration [15]. Fukuda has proposed a method and which can be attributed to the tensile and compression a loading zig for the axial compression bending tests to sides of the fibre, appeared in a fibre cross-section. The overcome the disadvantages of the three- and the four- inclined fracture surface was produced for X7, X5 and point bending tests [16] N3 fibres. The fracture surface suggesting flexural fracture In the present study, the axial compression bending was produced for H4 and T4 fibres tests were carried out on the composite strands without The difference between the pitch- and PAN-based using any special loading zig as shown in Fig. 2(c)[11]. carbon fibres in the appearance of the fracture surface The composite strand was axially compressed between produced by the micro-compression and recoil tests is metal bases attached to the mechanical tester. without considered to be related to the fibre cross-section texture applying a bending moment at both ends of the speci- It seems that the inclined fracture surface is produced for men. The metal bases had a dimple in order to prevent the fibres with the pleat-like cross-section texture recoiling of the specimen, and both ends of the speci During the axial compression tests of composite men were ground into a hemispherical shape with strands, two types of fracture surfaces of the composite abrasive. By increasing the axial compressive load, the strands were produced. One is a fracture surface specimen was buckled, bent into an increasing curva- inclined with an angle of about 45 against the fibre axis ture and eventually fractured at either the convex or as shown in Fig 4a)and the other is a fracture surface the concave side of the specimen due to the tensile or which runs almost transversely to the fibre axis as the compressive stress whichever was critical. The axial shown in Fig. 4(b). The inclined fracture surface was displacement was calculated from the loading time and produced for the X7, X5 and N3 fibre composite the crosshead speed. The gage length was 50 mm and strands. The transverse fracture surface was produced the crosshead speed was 0.5 mm min-. Reduced for the H4 fibre composite strands. In the case of the strength of the component fibres was calculated by T4 /epoxy-A and T4/epoxy-B composite strands, both of dividing the bending strength of the composite strand these two types of fracture surfaces were produced with the fibre volume fraction. The axial compression Near the transverse fracture surface, segmented fibre bending test will be simply called a bending test hen- bundles which were inclined from the longitudinal direction ceforth of the composite strand, suggesting microbuckling of the2.5. Axial compression test of composite strand Determination of the axial compressive strength of the ®bres from the axial compressive strength of uni￾directional composites has been reviewed by Kozey et al. [14]. In the present study, the axial compression test of the composite strands was carried out by supporting both ends of the specimen with resin blocks as shown in Fig. 2(b) in order to prevent local fracture at the loading points. A circular hole was drilled through rectangular epoxy resin blocks with cross-section sizes of 20 mm by 20 mm and with a thickness of 10 mm, in the thickness direction. Both ends of the specimen were put into the holes of the resin blocks and bonded with an epoxy resin so that the gage length of the specimen between two resin blocks became 5 mm. This gage length assured compressive fracture of the specimen before buckling. The specimen with resin blocks was compressed by using a mechanical tester with a crosshead speed of 0.5 mm minÿ1 . Reduced compressive strength of the com￾ponent ®bres was calculated by dividing the fracture load of the composite strand with the cross-section area of the ®bres. 2.6. Axial compression bending test of composite strand The three- and the four-point bending tests are widely used techniques while these tests are insucient for advanced composite materials because local fracture tends to occur at the loading points owing to stress concentration [15]. Fukuda has proposed a method and a loading zig for the axial compression bending tests to overcome the disadvantages of the three- and the four￾point bending tests [16]. In the present study, the axial compression bending tests were carried out on the composite strands without using any special loading zig as shown in Fig. 2(c) [11]. The composite strand was axially compressed between metal bases attached to the mechanical tester, without applying a bending moment at both ends of the speci￾men. The metal bases had a dimple in order to prevent recoiling of the specimen, and both ends of the speci￾men were ground into a hemispherical shape with abrasive. By increasing the axial compressive load, the specimen was buckled, bent into an increasing curva￾ture and eventually fractured at either the convex or the concave side of the specimen due to the tensile or the compressive stress whichever was critical. The axial displacement was calculated from the loading time and the crosshead speed. The gage length was 50 mm and the crosshead speed was 0.5 mm min-1. Reduced strength of the component ®bres was calculated by dividing the bending strength of the composite strand with the ®bre volume fraction. The axial compression bending test will be simply called a bending test hen￾ceforth. 3. Results and discussion 3.1. Fracture surface Fracture surfaces of the carbon ®bres and composite strands produced by various compression tests revealed di€erent features depending on the types of ®bres and, in the case of composite strands, the matrix resins. During the micro-compression tests of single ®bres, two types of fracture surfaces were produced. One is a fracture surface inclined with an angle of about 45 against the ®bre axis as shown in Fig. 3(a) and the other is a fracture surface which runs almost transversely to the ®bre axis. The formation of transverse fracture sur￾face was only observed by the optical microscopy during the micro-compression test because fractured specimens suitable for SEM observation could not be collected successfully. The inclined fracture surface was produced for X7 and X5 pitch-based carbon ®bres. In the cases of N3 pitch-based carbon ®bre, and H4 and T4 PAN￾based carbon ®bres, both the inclined fracture surface and a transverse fracture surface were produced. The compressive fracture by the recoil tests took place almost invariably in the zone close to the ®xed ends of the ®bre. During the recoil tests of single ®bres, two types of fracture surfaces were produced. One is a frac￾ture surface inclined with an angle of about 45o against the ®bre axis as shown in Fig. 3(b) and the other is a fracture surface suggesting ¯exural fracture as shown in Fig. 3(c). In Fig. 3(c), two regions with di€erent features, which can be attributed to the tensile and compression sides of the ®bre, appeared in a ®bre cross-section. The inclined fracture surface was produced for X7, X5 and N3 ®bres. The fracture surface suggesting ¯exural fracture was produced for H4 and T4 ®bres. The di€erence between the pitch- and PAN-based carbon ®bres in the appearance of the fracture surface produced by the micro-compression and recoil tests is considered to be related to the ®bre cross-section texture. It seems that the inclined fracture surface is produced for the ®bres with the pleat-like cross-section texture. During the axial compression tests of composite strands, two types of fracture surfaces of the composite strands were produced. One is a fracture surface inclined with an angle of about 45o against the ®bre axis as shown in Fig. 4(a) and the other is a fracture surface which runs almost transversely to the ®bre axis as shown in Fig. 4(b). The inclined fracture surface was produced for the X7, X5 and N3 ®bre composite strands. The transverse fracture surface was produced for the H4 ®bre composite strands. In the case of the T4/epoxy-A and T4/epoxy-B composite strands, both of these two types of fracture surfaces were produced. Near the transverse fracture surface, segmented ®bre bundles which were inclined from the longitudinal direction of the composite strand, suggesting microbuckling of the 222 M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229