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)00123-2Compressive 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 di€erent 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 di€erent 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 e€ort has been devoted to under￾standing 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 diculty 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-sec￾tion and a tensile stress arises in the convex side of the ®bre. With the recoil test, a compressive stress is pro￾duced 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 vis￾cous damping in the ®bre and at the ®xed ends of the ®bre a€ects the results [3]. In addition, the tested ®bre should be suciently stronger in tension than in com￾pression. In a compression test on a single-®bre com￾posite, residual stresses imposed on the ®bre due to matrix shrinkage a€ect the results. In the axial com￾pression test of a unidirectional composite, the stress ®elds are much more complicated and the precise frac￾ture 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 compo￾site 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)