Much of the strength of fiberglass composites is due to the type, amount and arrangement of the fiber reinforcement. While more than 90% of the reinforcements in use are glass fibers, other reinforcements satisfy the needs of various application niches.
Glass
The most common reinforcement – glass is strong, has good heat resistance and high electrical properties. For more critical needs, S-Glass offers higher heat resistance and about 1/3 tensile strength (at a higher cost).
Carbon Fiber
Carbon fibers (graphite) are available in a wide range of properties and price points. Carbon fibers combine light weight with very high strength and modulus of elasticity (a measure of the stiffness or rigidity). For high stiffness applications these reinforcements are hard to beat, with a modulus of elasticity that can equal steel. They also have excellent fatigue properties. The primary use of carbon fibers is in aerospace parts where weight savings is a major objective. While cost limits use in commercial applications, it is viable where material content is low, such as sporting equipment.
Aramid
Also known as aromatic polyamide fibers (Kevlar® or Twaron®), aramid provides high strength and low density (40% lower than glass) as well as high modulus. These fibers can be incorporated in many polymers and are extensively used in high impact applications, including ballistic resistance.
Natural Fibers
Natural fibers such as sisal, hemp and flax can be used for some applications with low strength requirements. They are limited to applications not requiring resistance to moisture or high humidity.
Arrangement of Fibers
Unidirectional
Reinforcement Types:
Continuous strand roving
Processes:
Continuous pultrusion, compression molding
Bidirectional
Reinforcement Types:
Woven fabrics, Woven roving
Processes:
Hand lay-up
Multidirectional
Reinforcement Types:
Chopped strands, continuous, chopped strand mat, tri axial fabric
Processes:
Compression and injection molding, spray-up pressure bag, preform
Percentage of fiberglass reinforcements increases strength in direction of fiber orientation
The way the individual strands are positioned determines both direction and level of achievable strength. The three basic arrangements are unidirectional, bidirectional and multidirectional.
Reinforcement Forms
Reinforcements are supplied in several basic forms to provide flexibility in cost, strength, compatibility with the resin system, and process requirements.
Continuous Strand Roving
Supplied as untwisted strands wound into a cylindrical package for further processing. Continuous roving is typically chopped for spray-up, preform or sheet molding compounds. In the continuous form, it is used in pultrusion and filament-winding processes.
Woven Roving
This is a heavy, drape-able fabric available in various widths, thicknesses and weights. Woven roving costs less than conventional woven fabric and is used to provide high strength in large structural components such as tanks and boat hulls. Woven roving is used primarily in hand lay-up processing.
Woven Fabrics
Made of fiber yarns, woven fabrics are of a finer texture than woven roving. They are available in a range of sizes and in weights from 2.5-18 ounces/per sq yard, in various strength orientations.
Reinforcing Mat
Made from either continuous strands laid down in a swirl pattern or from chopped strands, reinforcing mat is held together with a resinous binder or mechanically stitched. These mats are used for medium-strength composites. Combination mat, consisting of woven roving and chopped strand mat bonded together, is used to save time in hand lay-up operations. Hybrid mats of glass and carbon and aramid fibers are also available for higher-strength reinforced products.
Surfacing Mat
Surfacing mat or veil is a thin fiber mat made of monofilament and is not considered a reinforcing material. It is used to provide a good surface finish because of its effectiveness in blocking out the fiber pattern of the underlying mat or fabric. Surfacing mat is also used on the inside layer of corrosion-resistant products to produce a smooth, resin-rich surface
Chopped Fibers
Chopped strands or fibers are available in lengths from 1/8” to 2” for blending with resins and additives to prepare molding compounds for compression or injection molding and other processes. Various surface treatments are applied to ensure optimum compatibility with different resin systems.
Resources
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Technical Design Guide for FRP Composite Products and Parts
Fiber glass (FRP) composite materials and processes are explained in detail. This design guide outlines various selection criteria with helpful technical data and comparisons to alternative materials.