Glass Aramid and Carbon Fiber Reinforcements Compared

Three fibers, three personalities

Most composite parts use one of three reinforcing fibers: glass, aramid or carbon. Each can be paired with the same resin, yet each gives a different balance of stiffness, strength, toughness, weight and cost. There is no best fiber in general. The right one depends on what the part must do and what the budget allows.

This comparison keeps to typical, qualitative differences. Exact property values depend on the specific fiber grade, the form it is supplied in, the resin and the manufacturing process, so any design work should rely on supplier data for the actual material.

Glass fiber

Glass fiber is the workhorse. It is made by drawing molten glass into fine filaments, and it is by far the most widely used reinforcement by volume. Common grades include E-glass, which is general purpose, and S-glass, which is a higher-strength type.

Typical characteristics:

• Cost. It is generally the least expensive of the three, which is why it appears in boats, tanks, pipes, panels and many automotive parts.
• Stiffness. Moderate. Glass composites tend to flex more than carbon composites of similar shape, which can be a drawback in stiffness-critical structures but acceptable in many others.
• Strength. Good in tension, with reasonable impact resistance.
• Weight. Denser than the other two, so a glass part tends to be heavier for the same stiffness.
• Other points. It is electrically insulating and does not corrode. It is also transparent to radio waves, which makes it useful for radomes and electrical housings.

Glass can lose strength under long-term sustained load, particularly in moist or alkaline environments, a behavior often called stress rupture or static fatigue. Designers allow for it with conservative stress limits.

Aramid fiber

Aramid is a family of synthetic organic fibers, best known under trade names. In composites, aramid is valued for toughness and damage tolerance rather than for stiffness alone.

Typical characteristics:

• Toughness. Aramid fibers absorb a lot of energy before failing and resist impact and abrasion. This is why they appear in protective equipment and ballistic products.
• Weight. Lighter than glass, with good tensile strength for the weight.
• Compression. A well-known weakness. Aramid fibers tend to yield and kink under compression, so aramid composites are not usually chosen for compression-dominated parts.
• Moisture. The fibers can absorb moisture, so edges and surfaces generally need protection.
• Processing. The fibers are hard to cut cleanly, since they tend to fray, and they can degrade under prolonged ultraviolet exposure unless protected.

Because of these traits, aramid is often used in hybrid fabrics together with carbon or glass, so that its damage tolerance offsets the brittleness of the other fibers.

Carbon fiber

Carbon fiber is made by heating an organic precursor until it is mostly carbon, producing a fiber that is very stiff and strong for its weight. The production routes and typical uses are covered in the article on carbon fiber. Its structure is related to the layered carbon found in graphite.

Typical characteristics:

• Stiffness. The highest of the three, and the main reason carbon is used in aircraft structures, racing parts and high-end sporting goods.
• Weight. Low density, so stiffness per unit weight is excellent.
• Fatigue. Carbon composites generally resist fatigue well.
• Thermal expansion. Very low, and sometimes slightly negative along the fiber direction, which helps dimensional stability in precision structures. This property ties into the wider topic of matching thermal expansion in assemblies.
• Cost. Higher than glass, and higher still for the stiffest grades.
• Brittleness. Carbon fibers fail with little warning and little energy absorption compared with aramid, so impact can cause internal damage that is hard to see.
• Conductivity. Carbon is electrically conductive, which affects lightning protection design and can cause galvanic corrosion when carbon touches some metals, such as aluminum, in a wet environment.

Side-by-side summary

The table is a rough guide. Specialized grades of each fiber can shift these rankings in particular properties.

Matching fiber to application

• Stiffness-critical, weight-critical parts, such as aircraft structures, drive shafts and bicycle frames, usually point toward carbon.
• Cost-sensitive, large or general-purpose parts, such as boat hulls, tanks and body panels, usually point toward glass.
• Impact, abrasion and ballistic protection usually points toward aramid, often in hybrid form.
• Electrical or radio transparency points toward glass or aramid rather than carbon.

Weight savings are the main reason composites feature in aerospace materials selection, though cost, repairability and inspection requirements weigh on the decision too.

Fiber form and hybrids

The same fiber behaves differently depending on how it is supplied. Unidirectional tapes give the highest properties in one direction. Woven fabrics are easier to drape over curved shapes and give balanced properties in two directions at some cost in peak stiffness. Chopped strand mats and short fibers trade properties for easy molding.

Hybrids combine fiber types, either in the same fabric or in different plies. A glass skin over a carbon core, for example, can cut cost, while aramid layers can add damage tolerance. Hybrid designs need care because different fibers have different stiffness and expansion behavior, which can create internal stress or warping.

The matrix and interface still matter

Fiber choice is only part of the story. How well the resin bonds to a fiber depends on surface chemistry, so each fiber type is usually supplied with a sizing matched to the intended resin. The nature of that bond is the subject of the fiber-matrix interface discussion. In addition, the resin sets the temperature and moisture limits of the part, so even the best fiber can be limited by a poor match with the polymer matrix.