COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 60(2000)219-229 Compressive strengths of single carbon fibres and composite strands M. Shioya*, M. Nakatani Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 18 September 1998; received in revised form 7 June 1999: accepted 9 August 1999 y. Micro-compression and recoil tests have been carried out on single filaments of pitch- and polyacrylonitrile-based carbon fibres xial compression and bending tests were also carried out on unidirectional composite strands containing these fibres and a reduced compressive strength was calculated by dividing the fracture load of the composite strand by the cross-sectional area of the fibres. The fracture surfaces produced by different test methods were compared and a correlation between the compressive strength values determined by these test methods was investigated. The fracture surfaces of the fibres and composite strands showed different features depending on the type of fibre and matrix resin. The compressive strength of the composite strands increased with increasing matrix modulus. The compressive strengths of the fibres determined by the recoil test and from the axial compression test on the composite strand with a stiff matrix resin were almost in proportion to the strength determined with the micro-compression test. 2000 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibres; A Polymer-matrix composites; B Fracture; B Strength; D Scanning electron m 1. Introduction In the loop test, a compressive stress is produced by bending the fibre into a loop. Thus, the compressive The axial compressive strength of carbon fibres is inferior stress is not uniformly distributed in the fibre cross-sec- to the tensile strength and decreases with increasing tensile tion and a tensile stress arises in the convex side of the modulus [1]. Thus, in the structural application of carbon- fibre. With the recoil test, a compressive stress is pro- fibre-reinforced composites, the superior tensile properties duced in the recoil process which takes place after a of the fibres are often not utilized to the maximum possible pretensioned fibre is cut between fixed ends. Lateral extent since the compressive strength of the fibres limits the displacements imposed on the fibre when the recoil loading capacity of the composites process is initiated cause flexural fracture [4], and vis- A considerable effort has been devoted to under- cous damping in the fibre and at the fixed ends of the standing the relationship between the compressive fibre affects the results [3]. In addition, the tested fibre strength and microstructure of the carbon fibres in should be sufficiently stronger in tension than in com- order to improve the compressive strength. To address pression. In a compression test gle-fibre com- these subjects, it is imperative that the compressive posite, residual stresses imposed on the fibre due to strength of the fibres be accurately determined. The matrix shrinkage affect the results. In the axial com small diameter of the carbon fibres, which is usually less pression test of a unidirectional composite, the stress than 10 um, causes difficulty in measuring the axial fields are much more complicated and the precise frac compressive strength. Several techniques have been ture mechanism should be elucidated in order to relate proposed including the loop test [2], the recoil test [3-5, the compressive strength of the composite to that of the the micro-compression test [6-9], compression test of a component fibres. It is possible that even if the compo- single-fibre composite [1, 10] and estimation from the site appears to fracture in compression, the component compressive strength of a unidirectional composite fibres are fractured in flexure microscopically. Thus, in view of applying a true axial compressive stress to a Corresponding author. Tel. +81-3-5734-2434: fax: +81-3-5734- single fibre, the micro-compression test, in which the fibre is directly compressed, is the most suitable. Such a test, however, requires laborious procedures for preparing 0266-3538/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0266-3538(99)00123-2
Compressive strengths of single carbon ®bres and composite strands M. Shioya*, M. Nakatani Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 18 September 1998; received in revised form 7 June 1999; accepted 9 August 1999 Abstract Micro-compression and recoil tests have been carried out on single ®laments of pitch- and polyacrylonitrile-based carbon ®bres. Axial compression and bending tests were also carried out on unidirectional composite strands containing these ®bres and a reduced compressive strength was calculated by dividing the fracture load of the composite strand by the cross-sectional area of the ®bres. The fracture surfaces produced by dierent test methods were compared and a correlation between the compressive strength values determined by these test methods was investigated. The fracture surfaces of the ®bres and composite strands showed dierent features depending on the type of ®bre and matrix resin. The compressive strength of the composite strands increased with increasing matrix modulus. The compressive strengths of the ®bres determined by the recoil test and from the axial compression test on the composite strand with a sti matrix resin were almost in proportion to the strength determined with the micro-compression test. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon ®bres; A. Polymer-matrix composites; B. Fracture; B. Strength; D. Scanning electron microscopy 1. Introduction The axial compressive strength of carbon ®bres is inferior to the tensile strength and decreases with increasing tensile modulus [1]. Thus, in the structural application of carbon- ®bre-reinforced composites, the superior tensile properties of the ®bres are often not utilized to the maximum possible extent since the compressive strength of the ®bres limits the loading capacity of the composites. A considerable eort has been devoted to understanding the relationship between the compressive strength and microstructure of the carbon ®bres in order to improve the compressive strength. To address these subjects, it is imperative that the compressive strength of the ®bres be accurately determined. The small diameter of the carbon ®bres, which is usually less than 10 mm, causes diculty in measuring the axial compressive strength. Several techniques have been proposed including the loop test [2], the recoil test [3±5], the micro-compression test [6±9], compression test of a single-®bre composite [1,10] and estimation from the compressive strength of a unidirectional composite. In the loop test, a compressive stress is produced by bending the ®bre into a loop. Thus, the compressive stress is not uniformly distributed in the ®bre cross-section and a tensile stress arises in the convex side of the ®bre. With the recoil test, a compressive stress is produced in the recoil process which takes place after a pretensioned ®bre is cut between ®xed ends. Lateral displacements imposed on the ®bre when the recoil process is initiated cause ¯exural fracture [4], and viscous damping in the ®bre and at the ®xed ends of the ®bre aects the results [3]. In addition, the tested ®bre should be suciently stronger in tension than in compression. In a compression test on a single-®bre composite, residual stresses imposed on the ®bre due to matrix shrinkage aect the results. In the axial compression test of a unidirectional composite, the stress ®elds are much more complicated and the precise fracture mechanism should be elucidated in order to relate the compressive strength of the composite to that of the component ®bres. It is possible that even if the composite appears to fracture in compression, the component ®bres are fractured in ¯exure microscopically. Thus, in view of applying a true axial compressive stress to a single ®bre, the micro-compression test, in which the ®bre is directly compressed, is the most suitable. Such a test, however, requires laborious procedures for preparing 0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00123-2 Composites Science and Technology 60 (2000) 219±229 * Corresponding author. Tel.: +81-3-5734-2434; fax: +81-3-5734- 2434. E-mail address: mshioya@o.cc.titech.ac.jp (M. Shioya)
M. Shioya, M. Nakatani/Composites Science and Technology 60(2000)219-229 specimens with a very small gage length and for adjust- plates prepared by curing these epoxy resins are shown ing the fibre and loading axes. It is beneficial, therefore, in Table 2. These values were determined according to to know the compatibility of various test results so that JIS K7113 [12] using a strain rate of 0.01 min-l.The a relatively simple method can be selected as a supple- tensile modulus was determined in the strain range from mental method, if a large number of fibres are to be 0.001 to 0.02 evaluated The composite strands with a circular cross-section In the present study, axial compressive strengths of vere prepared as follows. A carbon fibre tow was pitch- and polyacrylonitrile(PAN)-based carbon fibres soaked in liquid epoxy resin and passed through a die were determined by means of the micro-compression with an appropriate diameter to adjust the fibre volume and recoil tests on single fibres Axial compression and fraction For X7, X5 and N3 fibres, two fibre tows were axial compression bending tests [ll] were also carried combined before passing through the die. The resin out on unidirectional composite strands. A comparison impregnated carbon fibre tow was wound on a frame was made between the compressive strength values and cured in an air circulating oven. For the epoxy-A determined with these test methods resin, the resin impregnated tow was left for 18 h before curing in order to evaporate methyl ethyl ketone 2. Experimental 2. Materials Pitch- and polyacrylonitrile(PAN)-based carbon fibres with characteristics shown in Table I were used for the experiments. The tensile properties in this table are the values shown by the producers. These pitch-and PAN-based carbon fibres revealed different textures in the cross-section cut with a blade. The X7.X5 and n3 fibres had a pleat-like texture extending radially from the center of the cross-section as shown in Fig. 1 (a) This pleat-like texture was more distinctively developed a 5 um for the X7 and X5 fibres as compared with the N3 fibre which had lower tensile modulus. on the other hand he H4 and T4 fibres had no characteristic cross-section texture as shown in Fig. I(b) As the matrix of the unidirectional composite strand of carbon fibres, three types of epoxy resins named epoxy-A, B and C were used. These were the mixtures of diglycidyl ether of bisphenol A-type epoxy resin(Epi kote 828, Yuka Shell Epoxy), difunctional diluent (YED 205, Yuka Shell Epoxy), methylnadic acid anh dride, benzyldimethylamine and methyl ethyl ketone by the weight ratios of 100: 0: 90: 2.5: 15 for epoxy-A 20:80:100:4.75:0 for epoxy-Band0:100:100:4.75:0for epoxy-C. The cure conditions were 2 h at 110.C and additionally I h at 150C for epoxy-A, and 3 h at 140oC 5 um for epoxy-B and C. The tensile properties of the resin Fig 1. SEM images of cross-section of (a)X5 and (b) T4 fibres Table I Characteristics of carbon fibres Fibre Precursor material Number of filaments per tow Density/g cm Tensile properties Petroleum pitch 9.8 Petroleum pitch 1200 14 PAN 12.000
specimens with a very small gage length and for adjusting the ®bre and loading axes. It is bene®cial, therefore, to know the compatibility of various test results so that a relatively simple method can be selected as a supplemental method, if a large number of ®bres are to be evaluated. In the present study, axial compressive strengths of pitch- and polyacrylonitrile(PAN)-based carbon ®bres were determined by means of the micro-compression and recoil tests on single ®bres. Axial compression and axial compression bending tests [11] were also carried out on unidirectional composite strands. A comparison was made between the compressive strength values determined with these test methods. 2. Experimental 2.1. Materials Pitch- and polyacrylonitrile(PAN)-based carbon ®bres with characteristics shown in Table 1 were used for the experiments. The tensile properties in this table are the values shown by the producers. These pitch- and PAN-based carbon ®bres revealed dierent textures in the cross-section cut with a blade. The X7, X5 and N3 ®bres had a pleat-like texture extending radially from the center of the cross-section as shown in Fig. 1(a). This pleat-like texture was more distinctively developed for the X7 and X5 ®bres as compared with the N3 ®bre which had lower tensile modulus. On the other hand, the H4 and T4 ®bres had no characteristic cross-section texture as shown in Fig. 1(b). As the matrix of the unidirectional composite strands of carbon ®bres, three types of epoxy resins named epoxy-A, B and C were used. These were the mixtures of diglycidyl ether of bisphenol A-type epoxy resin (Epikote 828, Yuka Shell Epoxy), difunctional diluent (YED 205, Yuka Shell Epoxy), methylnadic acid anhydride, benzyldimethylamine and methyl ethyl ketone by the weight ratios of 100:0:90:2.5:15 for epoxy-A, 20:80:100:4.75:0 for epoxy-B and 0:100:100:4.75:0 for epoxy-C. The cure conditions were 2 h at 110C and additionally 1 h at 150C for epoxy-A, and 3 h at 140C for epoxy-B and C. The tensile properties of the resin plates prepared by curing these epoxy resins are shown in Table 2. These values were determined according to JIS K7113 [12] using a strain rate of 0.01 minÿ1 . The tensile modulus was determined in the strain range from 0.001 to 0.02. The composite strands with a circular cross-section were prepared as follows. A carbon ®bre tow was soaked in liquid epoxy resin and passed through a die with an appropriate diameter to adjust the ®bre volume fraction. For X7, X5 and N3 ®bres, two ®bre tows were combined before passing through the die. The resin impregnated carbon ®bre tow was wound on a frame and cured in an air circulating oven. For the epoxy-A resin, the resin impregnated tow was left for 18 h before curing in order to evaporate methyl ethyl ketone. Table 1 Characteristics of carbon ®bres Fibre Precursor material Number of ®laments per tow Diameter/mm Density/g cmÿ3 Tensile properties Modulus/GPa Strength/GPa X7 Petroleum pitch 4,000 9.8 2.16 720 3.6 X5 Petroleum pitch 4,000 10.1 2.14 520 3.6 N3 Coal pitch 3,000 10.3 2.00 296 3.4 H4 PAN 12,000 6.4 1.80 392 4.4 T4 PAN 12,000 6.8 1.80 235 4.9 Fig. 1. SEM images of cross-section of (a) X5 and (b) T4 ®bres. 220 M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229
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 tested
2.2. Measurements of ®bre volume fraction and crosssection 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 compressive 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 diraction 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 optical 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 diraction of He± Ne laser beam. The cardboard with the ®bre was gripped 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 surgical 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
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 the
2.5. Axial compression test of composite strand Determination of the axial compressive strength of the ®bres from the axial compressive strength of unidirectional 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 component ®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 insucient 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 fourpoint 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 specimen. The metal bases had a dimple in order to prevent recoiling of the specimen, and both ends of the specimen were ground into a hemispherical shape with abrasive. By increasing the axial compressive load, the specimen was buckled, bent into an increasing curvature 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 henceforth. 3. Results and discussion 3.1. Fracture surface Fracture surfaces of the carbon ®bres and composite strands produced by various compression tests revealed dierent 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 surface 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 PANbased 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 fracture 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 dierent 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 dierence 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
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 transverse fracture surface, ®bre cross-sections characteristic to ¯exural fracture, and debonding of the ®bre±matrix interfaces were also revealed as shown in Fig. 4(d). In the case of the T4/epoxy-C composite strand, fracture of the ®bres and matrix resin did not take place even at the maximum compression load but the composite 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 composite 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 dicult. 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 produced. 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 produced 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 microcompression 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 3Eofd2 002 48o 2S3 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 microcompression test and (b) N3 and (c) T4 ®bres after recoil test. M. Shioya, M. Nakatani / Composites Science and Technology 60 (2000) 219±229 223