N. Oya, D.J. Johnson /Carbon 39(2001)635-645 compressive strength and total P. HS fibres T1000 showed lower compressive strength increased po- a25 rosity. However, HM fibres showed compressive strength with more porosity. The inverse relationships seen 三2 in HS and HM fibres are probably due to different types of pores in both categories M60J Y 4. Discussion O0.5 It was revealed that longitudinal compressive strength in PAN-based carbon fibres can only be estimated with sufficiently short gauge lengths of around 20 um. Accord- Intracrystalline disorder DC (% ng to the previous paper, it should be noted that the clar Fig. 9. Relationship between longitudinal compressive streng effect on samples must be taken into account in the case and intracrystalline disorder in PAN-based carbon fibres anisotropic carbon fibres [3, 13-16]. The results obtained this work may well include the effect of adhesive used fix the samples to some extent. It is not certain how the clamp effect infuences the compressive properties in the carbon fibres measured; however, it should be enough to discuss compressive properties of different fibres with the same condition as a comparative stud The structure-compressive property relations in PAN- based carbon fibres suggested that crystallite disorder and T crystallite stacking size appreciably affected longitudina compressive strength only when the fibres were produced with higher graphitization temperature to attain higher modulus as seen in HM fibres. HS fibres had almost the same disorder amount and crystallite size. however. thei 605 compressive strength decreased with increase of porosity t is thought that increased pores became stress concen- trators to induce compressive failure at lower stress level in Hs fibres Crystallite thickness Lc(nm) HM fibres exhibited lower compressive strength with Fig. 10. Relationship between longitudinal compressive streng increase of fibre modulus. because of reduced disorder and and crystallite thickness in PAN-based carbon fibres enlarged crystallites. The compressive strength also de- creased with reduced porosity; it was a completely differ ent trend compared with HS fibres. This result implies that 30 different types of pores must be considered in HS and HM fibres PAN-based carbon fibre basically contains a turbostratic carbon structure in which interlinking crystallites exist between crystallite ribbons enclosing pores [17] However. the structure of carbon fibres moves from a system consisting of many small pores to one with fe larger needle-like pores as the modulus increases [12, 18- 23]. HM PAN-based carbon fibres used in this study may also have such a structural change, these fibres are produced with higher graphitization temperature, and the internal pores are much larger and longer as if small pores are coalesced and oriented along the fibre direction 15 Consequently, the number of pores is smaller than in HS Porosity p(%) fibres, even though the pore size is much larger. It is Fig.I1. Relationship between longitudinal compressive strength thought that such few and large needle-like pores cause the and porosity in PAN-based carbon fibres low porosity and significantly lower the compressiN. Oya, D.J. Johnson / Carbon 39 (2001) 635 –645 643 compressive strength and total % porosity p. HS fibres showed lower compressive strength with increased po￾rosity. However, HM fibres showed higher compressive strength with more porosity. The inverse relationships seen in HS and HM fibres are probably due to different types of pores in both categories. 4. Discussion It was revealed that longitudinal compressive strength in PAN-based carbon fibres can only be estimated with sufficiently short gauge lengths of around 20 mm. Accord￾ing to the previous paper, it should be noted that the clamp effect on samples must be taken into account in the case of Fig. 9. Relationship between longitudinal compressive strength anisotropic carbon fibres [3,13–16]. The results obtained in and intracrystallite disorder in PAN-based carbon fibres. this work may well include the effect of adhesive used to fix the samples to some extent. It is not certain how the clamp effect influences the compressive properties in the carbon fibres measured; however, it should be enough to discuss compressive properties of different fibres with the same condition as a comparative study. The structure–compressive property relations in PAN￾based carbon fibres suggested that crystallite disorder and crystallite stacking size appreciably affected longitudinal compressive strength only when the fibres were produced with higher graphitization temperature to attain higher modulus as seen in HM fibres. HS fibres had almost the same disorder amount and crystallite size, however, their compressive strength decreased with increase of porosity. It is thought that increased pores became stress concen￾trators to induce compressive failure at lower stress level in HS fibres. HM fibres exhibited lower compressive strength with Fig. 10. Relationship between longitudinal compressive strength increase of fibre modulus, because of reduced disorder and and crystallite thickness in PAN-based carbon fibres. enlarged crystallites. The compressive strength also de￾creased with reduced porosity; it was a completely differ￾ent trend compared with HS fibres. This result implies that different types of pores must be considered in HS and HM fibres. PAN-based carbon fibre basically contains a turbostratic carbon structure in which many interlinking crystallites exist between crystallite ribbons enclosing pores [17]. However, the structure of carbon fibres moves from a system consisting of many small pores to one with few larger needle-like pores as the modulus increases [12,18– 23]. HM PAN-based carbon fibres used in this study may also have such a structural change; these fibres are produced with higher graphitization temperature, and the internal pores are much larger and longer as if small pores are coalesced and oriented along the fibre direction. Consequently, the number of pores is smaller than in HS fibres, even though the pore size is much larger. It is Fig. 11. Relationship between longitudinal compressive strength thought that such few and large needle-like pores cause the and porosity in PAN-based carbon fibres. low porosity and significantly lower the compressive