CARBON PERGAMON Carbon39(2001)635-645 Longitudinal compressive behaviour and microstructure of PAN based carbon fibres Naoyuki Oya", David J. Johnson Textile Physics Laboratory, School of Textile Industries, University of Leeds, Leeds, West Yorkshire LS2 91, UK 13 January 2000, accepted 30 May 200 Abstract The longitudinal compressive behaviour of HS and HM PAN-based carbon fibres has been examined by means of a direct ompression method which can be applied to single carbon fibres. Longitudinal compressive strength was found to be 30 to 50% of tensile strength depending on the modulus levels in carbon fibres used. Longitudinal compressive modulus was estimated making use of the Euler buckling formula applied to buckled samples; compressive modulus was about 50% of tensile modulus. SEM observations in-situ revealed distinct kink bands in HM fibres, which usually develop by splitting failure, this implies local crystallite buckling and the existence of needle-like pores in HM fibres. The compressive strength in HS fibres decreases with increased apparent porosity. HM fibres exhibit smaller amounts of disorder and larger crystallites, and have increased modulus; it may be deduced that large needle-like pores also exist. Therefore, a compressive failure mechanism which involves local buckling failure of large crystallites with insufficient lateral support of the large needle-like pores may be suggested in the case of HM fibres. 2001 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibres, C. Scanning electron microscopy (SEM); X-ray diffraction; D. Mechanical properties; Microstructure 1. Introduction stress in tension and compression. However, there has been Carbon fibres are widely used as reinforcement in tudinal compressive properties and their relationships to carbon fibre reinforced plastics(CFRP), metals(CFRM), structural parameters. In particular, it is very difficult to ceramics(CFRC) and C-C composites. Recent develop- measure the compressive properties in carbon fibres at the ments have provided rapid growth in mechanical properties single filament level of polyacrylonitrile-(PAN)and mesophase pitch-(MP)- In our previous work [ 3], we endeavoured to develop a based carbon fibres; now tensile strength and modulus up direct compression technique which can be applied to to 7 GPa and 600 GPa are available in PAN-based carbon single carbon fibres. That method suggested the possibility fibres [1]. However, general concern still exists about their for longitudinal compressive properties to be successfully elatively poor compressive properties along the fibre axis, measured by means of a strain gauge and a minimotor since most materials reinforced by carbon fibres frequently based compression device. In this study, we further fail in compression during practical applications modified the previous technique to achieve a very slow fibres are most probably attributed to their unique micro- nation and direct observations for compressed fibrous structure which consists of carbon crystallite layers, crys- samples with light and scanning electron microscope tallite disorder regions and needle-like pores oriented along (SEM). Furthermore, longitudinal compressive properti the fibre axis [2]. It is understood that such a highly ave been correlated to structural parameters obtained by oriented structure is responsible for the relatively poor wide-angle X-ray scattering (WAXS) study in order to compressive properties, by showing different responses to discuss structure-compressive property relations in carbon fibres. It may be noted that the device shows an advantage Corresponding author for in-situ microscopic studies compared to the work of E-mail address: oya( @ipc. osaka-pct ac Jp(N. Oya Nakatani et al. [4], who recently reported the measurement 0008-6223/01/S-see front matter 2001 Elsevier Science Ltd. All rights reserved PII:S0008-6223(00)00147-0PERGAMON Carbon 39 (2001) 635–645 Longitudinal compressive behaviour and microstructure of PAN￾based carbon fibres Naoyuki Oya , David J. Johnson * Textile Physics Laboratory, School of Textile Industries, University of Leeds, Leeds, West Yorkshire LS2 9JT, UK Received 13 January 2000; accepted 30 May 2000 Abstract The longitudinal compressive behaviour of HS and HM PAN-based carbon fibres has been examined by means of a direct compression method which can be applied to single carbon fibres. Longitudinal compressive strength was found to be |30 to 50% of tensile strength depending on the modulus levels in carbon fibres used. Longitudinal compressive modulus was estimated making use of the Euler buckling formula applied to buckled samples; compressive modulus was about 50% of tensile modulus. SEM observations in-situ revealed distinct kink bands in HM fibres, which usually develop by splitting failure; this implies local crystallite buckling and the existence of needle-like pores in HM fibres. The compressive strength in HS fibres decreases with increased apparent porosity. HM fibres exhibit smaller amounts of disorder and larger crystallites, and have increased modulus; it may be deduced that large needle-like pores also exist. Therefore, a compressive failure mechanism which involves local buckling failure of large crystallites with insufficient lateral support of the large needle-like pores may be suggested in the case of HM fibres.  2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibres; C. Scanning electron microscopy (SEM); X-ray diffraction; D. Mechanical properties; Microstructure 1. Introduction stress in tension and compression. However, there has been little support from quantitative measurement of longi￾Carbon fibres are widely used as reinforcement in tudinal compressive properties and their relationships to carbon fibre reinforced plastics (CFRP), metals (CFRM), structural parameters. In particular, it is very difficult to ceramics (CFRC) and C–C composites. Recent develop- measure the compressive properties in carbon fibres at the ments have provided rapid growth in mechanical properties single filament level. of polyacrylonitrile- (PAN) and mesophase pitch-(MP)- In our previous work [3], we endeavoured to develop a based carbon fibres; now tensile strength and modulus up direct compression technique which can be applied to to 7 GPa and 600 GPa are available in PAN-based carbon single carbon fibres. That method suggested the possibility fibres [1]. However, general concern still exists about their for longitudinal compressive properties to be successfully relatively poor compressive properties along the fibre axis, measured by means of a strain gauge and a minimotor￾since most materials reinforced by carbon fibres frequently based compression device. In this study, we further fail in compression during practical applications. modified the previous technique to achieve a very slow Poor longitudinal compressive properties in carbon rate of loading, accurate sample gauge-length determi- fibres are most probably attributed to their unique micro- nation and direct observations for compressed fibrous structure which consists of carbon crystallite layers, crys- samples with light and scanning electron microscope tallite disorder regions and needle-like pores oriented along (SEM). Furthermore, longitudinal compressive properties the fibre axis [2]. It is understood that such a highly have been correlated to structural parameters obtained by oriented structure is responsible for the relatively poor wide-angle X-ray scattering (WAXS) study in order to compressive properties, by showing different responses to discuss structure–compressive property relations in carbon fibres. It may be noted that the device shows an advantage *Corresponding author. for in-situ microscopic studies compared to the work of E-mail address: oya@ipc.osaka-pct.ac.jp (N. Oya). Nakatani et al. [4], who recently reported the measurement 0008-6223/01/$ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00147-0