carbon fibres type HM

Carbon fibres type HM (high modulus) are carbon fibres with a value of Young's modulus (tensile modulus) larger than 300 GPa (nearly 30% of the C 11 elastic constant of a graphite single crystal).
The level of the tensile modulus of carbon fibres is controlled by the degree of preferred orientation of the layer planes in the direction parallel to the fibre axis. C 11, the elastic constant of graphite single crystals in the direction of the layer planes, is 1060 ± 20 GPa. In general, the ratio of tensile strength to tensile modulus is smaller than 1 × 10 −2 for carbon fibres type HM (but the tensile strength is influenced by flaws in the fibres and may be improved in the future). Carbon fibres type UHM (ultra-high modulus) have moduli of elasticity in excess of 600 GPa, surpassing 50% of the theoretical C 11 number. Such high values of Young's modulus can be achieved most readily in mesophase pitch-based carbon fibres (MPP-based carbon fibres).
PAC, 1995, 67, 473 (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 481