The Properties and Applications of the Five Grades of Carbon Fibre

When faced with the bewildering array of carbon fibre grades such as T300, T800 and M40J, do you ever feel at a loss as to where to begin? As a strategic, advanced reinforcing material, carbon fibre exhibits a wide range of properties, and the most fundamental and widely used classification method within the industry is based on mechanical properties—particularly tensile modulus—to categorise these grades. Today, we’ll explain the five main types of carbon fibre categorised by mechanical properties, helping you understand the logic behind these designations.

Modulus and Strength: The ‘Rigidity’ and ‘Strength’ of Carbon Fibre

Before delving into the classifications, it is essential to understand two key indicators. The tensile modulus—commonly referred to simply as the modulus—reflects a material’s ability to resist elastic deformation; the higher the modulus, the more ‘rigid’ the material is, and the less likely it is to elongate and deform when subjected to force. Tensile strength, on the other hand, represents a material’s ultimate ability to resist fracture; the higher the strength, the greater the tensile force it can withstand.

Put simply, modulus relates to ‘resistance to deformation’, whilst strength relates to ‘resistance to breaking’. Carbon fibre consists of countless graphite microcrystals aligned along the fibre axis; the higher the degree of microcrystal orientation and structural perfection, the higher the modulus. However, an excessively high degree of graphitisation increases microcrystal size and raises defect sensitivity, which may conversely lead to a decrease in strength. This inherent trade-off between ‘stiffness’ and ‘strength’ has directly given rise to the classification into five types of mechanical properties.

1 Standard Modulus Type: The Cornerstone of Performance with the Widest Range of Applications

The Standard Modulus (SM) type has a tensile modulus ranging from 200 to 280 GPa, with a tensile strength typically exceeding 2,500 MPa. This is the most widely produced and most extensively used category of PAN-based carbon fibres, offering balanced performance and excellent value for money.

Toray’s T300, an industry benchmark, falls into this category, with a modulus of approximately 230 GPa and a tensile strength of 3,530 MPa; the subsequently developed T700S has the same modulus of 230 GPa, yet its tensile strength has surged to 4,900 MPa, perfectly illustrating the trend towards higher strength within this grade.

Standard modulus carbon fibre can be found in a wide range of applications, from civil aircraft interiors and sports equipment (fishing rods, racquets) to wind turbine blades and pressure vessels. It acts as a universal key, unlocking the door to the large-scale industrialisation of carbon fibre composites.

2 Intermediate Modulus Type: The First-Stage Upgrade in Strength and Stiffness

The Intermediate Modulus (IM) type raises the modulus to 280–350 GPa, whilst requiring a tensile strength of no less than 3,500 MPa. Through optimised drawing processes and high-temperature treatment, this grade enhances stiffness whilst maintaining high strength, thereby achieving superior overall mechanical performance.

Representative products include Toray’s T800H (modulus 294 GPa, strength 5,490 MPa) and T1000G (modulus 294 GPa, strength 6,370 MPa), the latter’s strength approaching the limits of high-strength carbon fibres.

Thanks to their higher stiffness and strength, medium-modulus carbon fibres are widely used in load-bearing structural components of aircraft, such as fuselage frames and wing spars, as well as in high-end golf club shafts, competitive bicycle frames and high-pressure hydrogen storage cylinders; they represent the ideal solution for those seeking the ultimate in lightweight design and safety.

3 High Modulus Type: A Performance-Enhancing Material Designed for Stiffness

When application scenarios impose stringent requirements on structural stiffness, the high modulus (HM) type comes into its own. Its modulus ranges from 350 to 600 GPa, with a tensile strength generally not less than 2,500 MPa. Fibres of this type typically undergo graphitisation at higher temperatures, resulting in a highly oriented graphite flake structure, which significantly increases the modulus but also leads to some compromise in strength.

Typical examples include the PAN-based M40J (modulus 377 GPa, strength 4,400 MPa), M46J (modulus 436 GPa, strength 4,200 MPa) and M55J (modulus 54 GPa, strength 4,020 MPa). Furthermore, some pitch-based carbon fibres also fall within this range.

High-modulus carbon fibres are widely used in satellite structures, optical component mounts, high-precision robotic arms, competition-grade bicycle frames and high-end fishing tackle; it is often within these ‘rock-solid’ components that their rigid backbone is to be found.

4Ultra-high modulus type: Approaching the ultimate stiffness of graphite whiskers

To push the modulus to its absolute limit, an ultra-high modulus (UHM) type is required, with a tensile modulus exceeding 600 GPa whilst maintaining a strength of over 2,500 MPa. This type of carbon fibre is almost exclusively produced using pitch-based precursors, as pitch readily forms highly oriented liquid-crystal structures when molten. Following graphitisation at ultra-high temperatures, its modulus can approach or even exceed 900 GPa, coming very close to the theoretical value of single-crystal graphite.

A prime example is Mitsubishi Chemical’s Dialead K13D2U, which boasts a modulus of up to 935 GPa and a tensile strength of approximately 3,700 MPa. Among PAN-based fibres, Toray’s M60J, with a modulus of approximately 588 GPa, is often regarded as the ‘gatekeeper’ on the threshold of the UHM category.

Ultra-high-modulus carbon fibres have an extremely low coefficient of thermal expansion and unrivalled dimensional stability, making them the sole choice for applications where even the slightest deformation—down to the nanometre level—is unacceptable, such as large space mirrors, satellite antennas and semiconductor manufacturing equipment.

5 Low-modulus types: an overlooked functional role

Finally, there are the low-modulus (LM) types, which have a tensile modulus of 200 GPa or less and a strength not exceeding 3,500 MPa. This category primarily encompasses isotropic bitumen-based carbon fibres, as well as some discontinuous or recycled carbon fibres.

Although their mechanical properties are not outstanding, they are low-cost to produce and excel in terms of thermal insulation, wear resistance, electrical conductivity and chemical stability. As such, they have carved out a niche in functional applications such as aerospace brake discs, high-temperature furnace insulation screens, fuel cell gas diffusion layers and anti-static flooring. With their unassuming nature, they safeguard the safe operation of industrial equipment.

Five Major Grades of Carbon Fibre Mechanical Properties

The Art of Material Selection: Striking a Balance Between Stiffness and Strength

Once this classification system is understood, examining fibre grades no longer leaves one at a loss. Standard-modulus and medium-modulus fibres, which prioritise high strength, are the first choice for load-bearing structures. High-modulus and ultra-high-modulus fibres, which prioritise extreme stiffness, are more commonly used in precision components where dimensional stability is paramount. Low-modulus fibres, meanwhile, excel in terms of functionality and cost-effectiveness.

In fact, PAN-based carbon fibres excel at enhancing strength through microcrystal control and defect suppression, and thus dominate the SM and IM sectors. Asphalt-based carbon fibres, on the other hand, capitalise on their inherent liquid-crystal orientation advantages to more readily achieve ultra-high modulus, reigning supreme in the HM and UHM sectors.

These two technological pathways each have their own strengths across the spectrum of carbon fibre mechanical properties, and together they have shaped the performance legend of modern composite materials. The next time you encounter a string of unfamiliar fibre designations, try interpreting them through the prism of modulus and strength; in doing so, you will have grasped the key to selecting the right carbon fibre.