(nearly 30% of the 
elastic constant of a graphite 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. 
, the elastic constant of graphite single crystals in the direction of the layer planes, is
. In general, the ratio of tensile strength to tensile modulus is smaller than
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
,
surpassing 50% of the theoretical 
number. Such high values of Young's modulus can be achieved most readily in mesophase pitch-based carbon fibres (MPP-based carbon fibres).
, the elastic constant of graphite single crystals in the direction of the layer planes, is
. In general, the ratio of tensile strength to tensile modulus is smaller than
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
,
surpassing 50% of the theoretical number. Such high values of Young's modulus can be achieved most readily in mesophase pitch-based carbon fibres (MPP-based carbon fibres).
Source:
PAC, 1995, 67, 473
(Recommended terminology for the description of carbon as a solid (IUPAC Recommendations
1995))
on page 481