Choosing the Right Reinforcement Format for Composites

  • 3rd September 2019
  • Tala Abbasian
  • Reading time: about 6 minutes

Introduction

When deciding upon the right reinforcement material to use in a new composite component, it can be tempting to focus solely on the fibre type (i.e. glass, carbon, natural fibre etc.). However, the way in which the reinforcement fibres are arranged  – the fibre format – can have at least the same impact on the final component properties as the type of fibres chosen. The fibre format can also dictate which manufacturing routes are possible. In this explainer, we’ll briefly describe the most common fibre formats and highlight the applications and manufacturing processes in which they might typically be found.

Unidirectional (UD)

The most efficient use of fibres is to make a “unidirectional” composite out of continuous filaments. In this case, all of the fibres are oriented in the same direction. This results in a composite with maximum strength and stiffness for a given fibre content… but only in the direction of the fibres. At 90 degrees to the fibre direction, there is only the matrix holding the material together and therefore strength and stiffness in this direction will be significantly lower. However, if you need a really lightweight system and have a clear understanding of where strength and stiffness are needed in the component (e.g. as a result of some prior modelling work), then UD reinforcement can provide the optimum solution.

Composites manufacturing methods which use UD fibres (or tows of fibres) include filament winding, automatic fibre placement (AFP) and pultrusion. Alternatively, UD fibres can be transformed into intermediate materials such as UD tapes or UD prepregs (thermoset or thermoplastic), which are then used to produce composite parts.

Roll of carbon fibre unidrectional tape
Carbon fibre unidrectional tape

For infusion processes (RTM, vacuum infusion) or hand layup, “pseudo UD” textiles are often used. These textiles are typically either held together with fine stitching (producing so-called “stitched UD”) or incorporate a thermoplastic binder to hold the fibres in place. It is also possible to weave a material in which a much higher proportion of fibres are in one direction; producing a “biased weave”, which is sometimes considered to be equivalent to UD.

Short and Random Fibres

At the opposite end of the scale is the use of short and random fibres to make composites. These are lower performance than long fibre reinforcements but can be used with a wider range of manufacturing methods. For example, it is possible to compound short fibres into thermoplastic pellets which can then be extruded or injection moulded. In this case, the fibres behave more like a filler although they do provide some increase in properties of the material compared to unfilled materials.

Pellets and moulding

Short fibres can also be used in thermosetting resins, particularly for lower-tech parts. This may be in the form of a dry fibre mat, consisting of short fibres held together by a binder. Chopped strand matt (CSM) of this type, is a common reinforcement used for many traditional “fibreglass” type applications, usually with polyester resins.

Short fibres may also be combined with resins to produce moulding compounds, which are regularly used for automotive applications. While the properties of these types of materials are lower, they remain in heavy use because their cost is low and they still provide a reasonable improvement in properties compared to the unfilled equivalents.

Due to the random nature of the fibres, short fibre composites usually have broadly the same properties in all directions.

Woven textiles

Woven textiles are widely used within composite materials because they provide an effective way of delivering long fibre reinforcements to a variety of processes. Woven materials consist of continuous warp yarns (or tows) with weft (or fill) fibres threaded above and below. They are well suited to a wide range of processes such as RTM, infusion, prepregging, wet layup, pulltrusion and more. More than one type of fibre can be used within a weave. For example, thermoplastic fibres can be combined with fibres such as glass or carbon to produce hybrids which can be used to make thermoplastic composites.

Woven textiles

With woven materials, there is often a need to balance stability (i.e. the ability of the fabric to resist distortion of the warp and weft yarns and remain intact) with drapability (the ability of the fabric to conform to complex shapes). These two properties of the fabric are governed by the weave style.

Even the most drapable fabrics, however, struggle to cope when components has any significant degree of complexity in the z-direction. For parts such as this, additional processing steps usually need to be taken to introduce the desired features, such as adhesive bonding or over moulding.

Another, more general drawback of woven fabrics is the crimp introduced to the fibres – whereby, rather than remaining flat, the fibres become bent as they pass over and under each other. This crimp leads to a reduction in mechanical properties.

Stitched Multiaxials

A compromise between the properties of a pure UD and the convenience of a woven textile is the stitched multiaxial. These materials are made up of multiple layers of UD with different orientations stitched together with a low weight fibre. They can be biaxial (2-directions), triaxial (3-directions), quadraxial (4-directions) etc. Unlike woven materials, the multiaxials are almost always very stable and feature no crimp, for this reason they are sometimes called “non-crimp fabric” and have the potential to have higher performance. Drape is usually worse though making then less suitable for parts with complex features or tight corners.

Biaxial textile
Biaxial textile

These types of materials are especially well suited for RTM or infusion processes as the materials can be delivered in very high areal weights. Prepreg is possible in the case of biaxial materials but the multiple layer structure can make production very difficult for triaxials or higher layer textiles.

Summary

Injection Moulding Compression Moulding Extrusion Pultrusion Wet Layup Prepreg Infusion
UD tows / yarns
UD fabric
Short fibre fillers
Bound short fibres (CSM)
Woven textiles
Stitched multiaxials
= recommended, = not recommended, = not suitable

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About the author

Tala Abbasian

Tala is a Development Engineer at Coventive Composites.

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