Wikipedia - Fibra di carbonio (it) carbon fibres type HM

https://doi.org/10.1351/goldbook.C00833
Carbon fibres type HM (high modulus) are @[email protected] with a value of @[email protected] (tensile modulus) larger than $$300\ \text{GPa}$$ (nearly 30% of the $$C_{11}$$ elastic constant of a @[email protected] single crystal).
Note:
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 @[email protected] single crystals in the direction of the layer planes, is $$1060\ \pm 20\ \text{GPa}$$. In general, the ratio of tensile strength to tensile modulus is smaller than $$1 \times 10^{-2}$$ 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 $$600\ \text{GPa}$$, surpassing 50% of the theoretical $$C_{11}$$ number. Such high values of @[email protected] can be achieved most readily in @[email protected] (@[email protected]).
Source:
PAC, 1995, 67, 473. (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 481 [Terms] [Paper]