What role does fiber reinforcement play in a composite?
Reinforcing fibers increase the mechanical properties of the resin matrix because the mechanical properties of fibers are higher than the un-reinforced resin. They are designed to carry tensile and compressive loads along their length, not across their width. This 'anistropic' feature is advantageous as fibers can be orientated to load paths, minimizing weight and material usage. Four factors influence a fiber's contribution; its mechanical properties, the resin/fiber interface, fiber orientation, and the Fiber Volume Fraction (FVF).

What form do reinforcing fibers come in?
Fibers come in numerous forms to suit a wide variety of applications and include rovings, chopped strands, milled fiber, continuous, chopped and thermo-formable mat. Unidirectional reinforcements feature long fibers aligned in one direction, and include rovings, tapes, tows and fabrics. Strictly speaking, fibers are loose strands and are actually rarely used, as they are difficult to handle. 'Chopper guns' use single fibers, however these are mixed with resin as they exit the gun making them easy to position in a mold. Reinforcing fabrics differ as they feature long fibers woven into a textile. Multi-directional reinforcing fiber is produced by creating woven fabrics from long, continuous fibers.

Why do short fibers produce weaker laminates?
Short fibers provide the lowest performance laminates because tensile and compressive loads are carried along the fiber's length, and short fibers cannot transfer loads to adjacent fibers. Laminates produced with 'chopper guns' feature very short fibers.

Is Chopped Strand Matt a fiber or a fabric?
'Chopped Strand Mat' (CSM) is composed of loose glass fibers bundled together with a 'sizing' or binder that promotes adhesion to the resin. As it is not woven it cannot be classed as a fabric.

How does the performance of CSM compare to a reinforcing fabric?
CSM features relatively short, randomly orientated fibers and low fiber density. Although the fibers are longer than those poduced by a chopper gun they are short compared to those in woven fabrics. As loads are carried along a fiber's length, random orientation compromises the ability to carry and transmit loads in an optimum direction. CSM's low fiber density results in a resin-rich laminate and less than optimum Fiber Volume Fraction - again impacting on a laminate's strength and stiffness. Like chopper gun laminates, CSM can be made strong, but at the expense of heavy weight.

What's the difference between reinforcing fibers and reinforcing fabrics?
'Chopper guns' chop long glass fibers into very short ones and mix them with resin. With slightly longer fibers, 'CSM' features randomly deposited glass fibers held together with a 'binder' or 'sizing'. Uni-diirectional fibers are loose strands or bundles of fibers, however they are continuous and are typically long. Woven reinforcing fabrics differ as they feature long, continuous fibers woven into a textile. They are used exclusively in high performance composites because they have higher load bearing capacity.

Why do long fibers produce stronger, stiffer laminates?
In any FRP composite, fibers carry tensile and compressive loads along their length. Compared to short randomly orientated fibers, long continuous fibers aligned in the direction of loads carry and transfer these better.

What type of woven fabrics are there?
'Uni-directional' fabrics have fibers running in one direction only. 0/90 degree fabrics have continuous, interlaced fibers running perpendicular to each other the length and width of the fabric. Multi-axial fabrics feature multiple layers or plies of uni-directional fibers.

Woven 0/90 fabrics
These are the most common form of woven fabric and provide good all-around performance carrying loads in two directions. Ultimate performance is compromised slightly because the fibers do not lie perfectly flat, being 'crimped' by the weaving process that interleaves one fiber over another.

What style of woven 0/90 fabrics are there?
There are numerous patterns that woven textiles use, each enabling different performance and handling characteristics. Commonly used fabrics include Plain Weave, Twill Weave and Satin Weave.

   • Click here to see the properties of plain, satin & twill weave fabrics

What are the characteristics of Plain Weave?
Plain Weave is stable and difficult to distort and while it produces laminates of predictable thickness it can be difficult to use in highly contoured composites, as it doesn't drape well. Patterns greatly influence the mechanical properties of a textile and the numerous interlacings in Plain Weave lower its mechanical properties. It is widely used in general applications as its high porosity ensures it is easily wetted-out by resin, and entrapped air is easily removed.

   • Click here to see illustration of plain weave fabric

What are the advantages of Satin Weave?
Satin Weave is similar to Twill Weave but with very high drapability, so it's well suited to use in highly contoured laminates. Its flat surface makes it ideal for surface finishes, however due to its design Satin Weave must have plies inverted to achieve a balanced laminate.

   • Click here to see illustration of satin weave fabric

What are the benefits of Twill Weave?
Due to its design Twill Weave features a distinct herringbone pattern that is common in high performance carbon fiber composites. It drapes more easily than Plain Weave and is easily wetted-out. Due to its design, Twill Weave doesn't require plies to be inverted either side of a laminate's mid-plane to achieve a balanced laminate.

   • Click here to see illustration of twill weave fabric

Uni-directional fibres
In 'uni-directional' fabrics various different methods are used to hold fibers in orientation to each other. Because they are not woven together, the fibers lie flat which optimizes their load carrying ability. However as strength and stiffness is optimized in one direction, 'unidirectional' fibers cannot effectively carry loads perpendicular to the main fiber orientation. Optimizing reinforcement in one direction limits their use in general applications such as automotive body panels.

Stitched 0/90 fabrics
Combining the flat fiber properties of 'unidirectional' fabrics and the ability to carry loads in two directions, 'Stitched 0/90 fabrics' feature a layer of uni-directional fabric stitched to a second layer of unidirectional fabric with fibres orientated perpendicular to the first.

Multi-axial fabrics
Gaining in popularity because they combine the best properties of all fabric designs, 'Multi-axial' fabrics are similar to 'Stitched 0/90' ones in that they feature layers of 'unidirectional' fibers. 'Multi-axial' fabrics take this a step further, offering many combinations of layers in 0, 45, 90 and -45 degree fiber orientation.

   • Click here to see illustration of multi-axial fabrics

Why are some fabrics that appear to be the same, more expensive?
High performance reinforcements are expensive because they use smaller diameter fibers that are more costly to manufacture. Reinforcements that appear to be of the same type (i.e. carbon) and weight (200 gsm) can vary in price because although they weigh the same per square metre, the more expensive reinforcement fabric features smaller diameter (tex) fibers that result in a stronger component with the same weight.

What is a 'hybrid fabric' ?
A 'hybrid' is a fabric that combines different types of fiber, usually with one running in the 0 degree orientation (warp) and the other in the 90 degree orientation (weft). For example, a glass/aramid hybrid will provide lighter weight, higher stiffness and greater abrasion resistance than a 100% glass fiber fabric. A carbon/glass hybrid provides the light weight, strength and stiffness of carbon with the improved impact resistance of glass.

What does 'isotropic' mean?
Woven reinforcing textiles feature directional fibers that help create 'isotropic' composites that demonstrate the same physical or mechanical properties in all directions. Woven textiles may feature 0-90 degree fiber orientations, 45/-45 degree orientations, or textiles with plies that feature a combination of these.

What is a 'pre-preg' ?
'Pre-pregs' are sophisticated reinforcing fabrics that have a pre-catalyzed resin applied in controlled quantities during manufacture to result in an optimum Fiber Volume Fraction. Due to the resin being pre-catalyzed, 'pre-pregs' must be stored at temperatures below zero to slow the curing process almost to a stop. At -18 degrees Celsius a 'pre-preg' can be stored for approximately one year. Used extensively in aerospace and Formula 1 cars, 'pre-pregs' are arguably the most sophisticated form of reinforcement fabric and bring numerous benefits to high performance laminates.

Do 'pre-pregs' need a different type of mold?

As 'pre-pregs' require curing in an oven, the molds must be manufactured with a resin that's capable of withstanding high temperatures. Molds used with 'pre-pregs' should be made from the same materials as the finished component (i.e. the reinforcing fiber and resin.) This is because both the mold and component must have the same co-efficient of thermal expansion. If the material used for the mold is different from the 'pre-preg', when heated to cure the 'pre-preg' the different rates of thermal expansion between mold and component will result in poor quality in terms of physical dimensions and surface finish. The need to use the same type of material for mold and 'pre-preg' contributes to the cost of high performance composites such as carbon fiber. The molds must be made from the same material, and as molds are typically much thicker than finished components substantially more carbon fiber and resin is required..........read more

 

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