N. Oya, D.J. Johnson /Carbon 39(2001)635-645 strength in HM fibres. Therefore, the apparent porosity is energy. Also, because the small crystallite higher not well related to the compressive strength in the case of capacity to deform, these fibres are comp highly graphitized fibres Nakatani et al. [4] also reported degree of deformation until the final cat failure that longitudinal compressive strength is determined by the without any indication of crystallite buckling on the ore size rather than the number of pores per unit fibre sur volume Considering the failure mechanism which involves The effect of large needle-like pores in HM fibres can be crystallite buckling due to insufficient lateral support of he figure del pores, it might be possible to say that compressive mplified skin-core model of longitudinal cross-section in properties in carbon fibres can be improved by infiltrating HM fibre structure, highly oriented large crystallites a substance into the pores by means of a post-treatment to enclose large needle-like pores in the outer layers, carbon fibres. Dobb et al. [24] attempted to deposit silver relatively random structure with many small pores exists in sulphide into aramid fibres to infiltrate the internal pores the core region [17. There may be misoriented crystallites using a conventional method, which was originally de- surrounding the pores, and such crystallites suffer from veloped to utilise the silver sulphide as an indicator to concentrated shear stress. Moreover, crystallite layer reveal the pore distributions when sections were examined planes oriented along the fibre axis also contribute to the the transmission electron microscope (TEM)[25]. Very failure when they have insufficient lateral supports nearby interestingly, such treated fibres exhibited a remarkable the elongated pores. Therefore, the initiation of failure inprovement in longitudinal compressive strengths by mechanism is a crystallite buckling near the localised 0% compared to untreated fibres when tested by the pores, and consequently, shear between basal planes. With tensile recoil measurement [26]. It was concluded that such increase of the modulus in HM fibres, the local crystallite an improvement may be due to either an increase in the buckling is more likely to occur at lower stress level effective cross-sectional area caused by infiltration or an because of the longer and larger pores, and the failure actual reinforcing effect of the epi ially grown silver epeatedly occurs and propagates through the large struc- sulphide crystals. If such a technique is applied for PAN- tural continuity as multiple kinks, they eventually become based carbon fibres, especially for HM fibres, the large ible on the lateral surface at final failure as seen in the needle-like pore may be effectively filled to prevent any in-situ microscopic studies local buckling of adjacent crystallites; which hopefully A Also in HS fibres, compressive failure initiates when a produces high performance carbon fibres with high com- lure occurs from crystallites near small pores. However, pressive strength and modulus these fibres have almost the same level of modulus. and there is no significant difference in pore size between fibre variants. Therefore, the mean porosity well refects the 5. Conclusions difference of longitudinal compressive strength in these fibres. HS fibres generally show higher resistibility to A direct compression device was developed to be compressive deformation owing to the high amount of applied to single carbon fibres in this study. The measured disorder which effectively dissipates the crack propagation longitudinal compressive strength was generally higher than that from the tensile recoil technique. The compres- sive strength was <30 to 50% of the tensile strength mpression Longitudinal compressive modulus was also estimated using the Euler buckling formula, the compressive modulus was -50% of the tensile modulus. The in-situ Fibre arge crystallites SEM observation was useful to observe compressive behaviour of carbon fibres HM fibres indicated distinct kink bands that usually developed into splitting failure along the fibre Structure-compressive property relationships revealed that compressive strength of Hs PAN-based carbon fibre is mainly influenced by the mean porosity. In HM fibres. 00 reduced disorder and enlarged crystallites appeared with Pores 000 Kink bands increase of the fibre modulus. Consequently, the pores tended to be large and needle -like which increases the possibility of localised crystallite buckling with insufficient lateral support. The result suggests that the effect of pore size Fig.12.compressivefailuremechanisminHmPaN-bAsedparticularlyimportantforcompressivestrengthinPan- carbon fibre based carbon fibres. It will be necessary to investigate the644 N. Oya, D.J. Johnson / Carbon 39 (2001) 635 –645 strength in HM fibres. Therefore, the apparent porosity is energy. Also, because the small crystallites have a higher not well related to the compressive strength in the case of capacity to deform, these fibres are compressed to a higher highly graphitized fibres. Nakatani et al. [4] also reported degree of deformation until the final catastrophic failure that longitudinal compressive strength is determined by the without any indication of crystallite buckling on the pore size rather than the number of pores per unit fibre surface. volume. Considering the failure mechanism which involves The effect of large needle-like pores in HM fibres can be crystallite buckling due to insufficient lateral support of envisaged as illustrated in Fig. 12. The figure depicts a pores, it might be possible to say that compressive simplified skin-core model of longitudinal cross-section in properties in carbon fibres can be improved by infiltrating HM fibre structure; highly oriented large crystallites a substance into the pores by means of a post-treatment to enclose large needle-like pores in the outer layers, a carbon fibres. Dobb et al. [24] attempted to deposit silver relatively random structure with many small pores exists in sulphide into aramid fibres to infiltrate the internal pores the core region [17]. There may be misoriented crystallites using a conventional method, which was originally de￾surrounding the pores, and such crystallites suffer from veloped to utilise the silver sulphide as an indicator to concentrated shear stress. Moreover, crystallite layer reveal the pore distributions when sections were examined planes oriented along the fibre axis also contribute to the in the transmission electron microscope (TEM) [25]. Very failure when they have insufficient lateral supports nearby interestingly, such treated fibres exhibited a remarkable the elongated pores. Therefore, the initiation of failure improvement in longitudinal compressive strengths by mechanism is a crystallite buckling near the localised 50% compared to untreated fibres when tested by the pores, and consequently, shear between basal planes. With tensile recoil measurement [26]. It was concluded that such increase of the modulus in HM fibres, the local crystallite an improvement may be due to either an increase in the buckling is more likely to occur at lower stress level effective cross-sectional area caused by infiltration or an because of the longer and larger pores, and the failure actual reinforcing effect of the epitaxially grown silver repeatedly occurs and propagates through the large struc- sulphide crystals. If such a technique is applied for PAN￾tural continuity as multiple kinks; they eventually become based carbon fibres, especially for HM fibres, the large visible on the lateral surface at final failure as seen in the needle-like pore may be effectively filled to prevent any in-situ microscopic studies. local buckling of adjacent crystallites; which hopefully Also in HS fibres, compressive failure initiates when a produces high performance carbon fibres with high com￾failure occurs from crystallites near small pores. However, pressive strength and modulus. these fibres have almost the same level of modulus, and there is no significant difference in pore size between fibre variants. Therefore, the mean porosity well reflects the 5. Conclusions difference of longitudinal compressive strength in these fibres. HS fibres generally show higher resistibility to A direct compression device was developed to be compressive deformation owing to the high amount of applied to single carbon fibres in this study. The measured disorder which effectively dissipates the crack propagation longitudinal compressive strength was generally higher than that from the tensile recoil technique. The compres￾sive strength was |30 to 50% of the tensile strength. Longitudinal compressive modulus was also estimated using the Euler buckling formula; the compressive modulus was |50% of the tensile modulus. The in-situ SEM observation was useful to observe compressive behaviour of carbon fibres; HM fibres indicated distinct kink bands that usually developed into splitting failure along the fibre axis. Structure–compressive property relationships revealed that compressive strength of HS PAN-based carbon fibre is mainly influenced by the mean porosity. In HM fibres, reduced disorder and enlarged crystallites appeared with increase of the fibre modulus. Consequently, the pores tended to be large and needle-like which increases the possibility of localised crystallite buckling with insufficient lateral support. The result suggests that the effect of pore size is Fig. 12. Compressive failure mechanism in HM PAN-based particularly important for compressive strength in PAN￾carbon fibre. based carbon fibres. It will be necessary to investigate the