How Hard Is Carbon Fiber? Strength, Stiffness, and Durability

Carbon fiber is a remarkable material known for its strength and light weight. While often associated with “hardness,” its properties encompass strength, stiffness, and durability. These characteristics explain its widespread use in demanding applications.

Understanding Carbon Fiber’s Key Properties

Carbon fiber excels in mechanical properties like tensile strength, stiffness, and low weight. Its tensile strength, the resistance to being pulled apart, ranges from 3,500 to 7,000 megapascals (MPa), significantly surpassing steel’s 400 to 1,200 MPa. It also exhibits high stiffness, measured by its modulus of elasticity, typically around 228 gigapascals (GPa), with specialized types reaching 900 GPa. This indicates strong resistance to bending or deforming under load.

Carbon fiber has a low density, averaging about 1.6 grams per cubic centimeter (g/cm³). This is considerably lighter than aluminum (2.7 g/cm³) and steel (7.85 g/cm³). Despite its strength and stiffness, carbon fiber composites can be susceptible to localized impact damage or scratching. While generally scratch-resistant, the resin matrix can influence its surface durability.

What Makes Carbon Fiber So Strong?

Carbon fiber’s properties stem from its composite nature. It is created by embedding fine carbon fibers within a resin matrix, often epoxy. Individual carbon fibers, 5 to 10 micrometers in diameter, consist of carbon atoms arranged in a highly ordered crystalline lattice. This atomic alignment, parallel to the fiber’s length, is the source of their strength and stiffness.

The resin matrix plays a role in this composite structure. It binds the carbon fibers, protecting them from external damage and transferring mechanical loads. The arrangement and orientation of fibers within the resin, known as the weave or layup, also influence the material’s performance. Different patterns, such as unidirectional layers, optimize the material for strength and stiffness along a specific axis.

Carbon Fiber Versus Other Materials

Carbon fiber stands out compared to common engineering materials, particularly in its strength-to-weight and stiffness-to-weight ratios. It has a higher strength-to-weight ratio, with tensile strength five to ten times greater than steel or aluminum for the same weight. This allows for the design of lighter components capable of withstanding substantial forces.

In terms of stiffness, carbon fiber also performs well relative to its weight. A plain-weave carbon fiber laminate can achieve a stiffness-to-weight ratio 60% greater than aluminum or steel. For applications requiring rigidity along a single plane, unidirectional carbon fiber can be five to ten times stiffer than an equivalent weight of steel or aluminum. As a result, a carbon fiber component of identical dimensions can be 50% lighter than an aluminum part and over five times lighter than a steel part, while still offering rigidity and strength.

Carbon fiber is also chemically inert and resistant to corrosion, unlike many metals prone to rust. However, galvanic corrosion can occur if carbon fiber composites contact certain metals in the presence of an electrolyte.

Where Carbon Fiber Shines and Where It Doesn’t

Carbon fiber’s properties make it well-suited for specific applications. Its high strength-to-weight ratio and stiffness are valuable in industries like aerospace, high-performance sports equipment, and automotive racing. It is used for structural components like trusses, beams, and chassis frames where minimizing weight while maximizing strength is important. It is also found in items like prosthetics and sports gear, where its lightweight durability offers functional benefits.

Despite its advantages, carbon fiber has limitations. While strong, it can be brittle and prone to failure under sudden impacts or crushing loads. Unlike metals that deform under stress, carbon fiber is more likely to crack.

The manufacturing process for carbon fiber is complex and energy-intensive, contributing to its high cost. Raw materials, such as polyacrylonitrile (PAN), are expensive, making industrial-grade carbon fiber cost more per pound than steel or aluminum. Repairing damaged carbon fiber composites can also be difficult and costly.