Fastener solution for composites

Paul Boughton

As composite materials grow in both range and volume of applications, the challenge to find the right fastening solution continues. Matthew Stevens explores some of the trusted fastening solutions.

Composites are materials engineered from two or more constituent materials with different physical or chemical properties. The two main categories of constituent materials making up the composite are the matrix and the reinforcement.

Examples of matrices are resins, ceramics, polymers or cements. Examples of reinforcements are fibres, sandwich cores or aggregate. The matrix material surrounds and supports the reinforcement material and the reinforcement material imparts its special mechanical and physical properties to enhance the matrix properties.

Composite materials are formed into a shape during their manufacture, typically in a mould cavity or on a mould surface. The synergy between the two materials results in a composite material with properties unavailable from the individual constituent materials. The wide variety of matrix and reinforcing materials available provides engineers with significant design freedom and greater potential to optimise their product’s form and function.

The ability to combine different constituent materials to create superior new composite materials allows engineers to target improvements in cost, weight, strength and handling as their product applications and manufacturing processes demand.

The growth of composites

The development of composites and plastics and their migration into applications habitually dominated by traditional materials such as wood and metal is a significant trend. Lighter, stronger, versatile and more mechanically stable, modern composite materials have found a home in almost every industrial sector. The issue of reducing weight has become central to innovation in the automotive sector, accentuated by high oil prices and regulatory pressure to reduce carbon emissions.

The expansion of lightweight composite materials presents challenges as well as opportunities. One of the key challenges is how to securely fasten to such materials. Traditional fastening systems designed for sheet metal, such as rivets, bolts and clinched fixings designed for metal are often incompatible with composites or require too many compromises to work. Weld studs designed for metal cannot be effectively welded to plastic, for example. Lightweight panel composites need to be kept thin and not thickened to hold a traditional fastener in position. Thin carbon panels do not improve with piercing or drilling. 

In short, design and process engineers liberated by composite materials do not want to be limited by fastening solutions not designed for use with composites. 

Embedding fasteners in composites is a common requirement and can provide a secure and discrete fastening solution. Integrated into the manufacturing process, the fasteners become integral to the composite product, which supports efficient final assembly. 

However, embedding a fastener that is not designed to be embedded can lead to design compromises and process inefficiencies. These compromises can weaken the composite product or increase its thickness and weight. Fig.1 shows how plastic is built up on this floor pan around a traditional bolt for no other reason than to hold it securely in position.

Seen in cross section (Fig.2) it is clear to see how the traditional bolt requires significant composite material to anchor it securely in position. 

Alternatively, a bonding fastener such as a bigHead can be used.

The bigHead is designed to be embedded in the composite material without the need for the material to be thickened. The thin, flat head is perforated to allow the flow of composite material and helps transfer the load efficiently into the composite structure. Different levels of tensile and torsional strengths can be achieved by using different size and shape heads.

Surface bonding on composites

Many composite applications rely on the use of thin structures. Carbon-fibre reinforced plastics, used for automotive body panels, are a good example of this. Such composite panels are often only a few mm thick.

In these cases, without sufficient material to embed a fastener, a surface-bonded fastener can be an effective and discrete solution. Bonded to the surface with adhesive, a bonding fastener can provide a secure fixing than does not pierce or damage the composite panel. 

Fig.3 shows a carbon fibre diffuser for a car, attached with bigHead bonding fasteners. This bonded solution is completely invisible from the ‘A-side’ as there are no rivets, drill holes or visible ‘shadows’. Even more important from an end-user perspective, the structural integrity of the single composite panel is fully maintained. 

Due to the unique design of the perforated ‘head’, the glue flows through the holes and locks the fastener into position, achieving high stability and strength. Bonded in position by the Tier 1 supplier, the diffuser is ready for final assembly at the OEM. 

The surface-bonded solution does rely on the adhesive for the critical bond between composite structure and fastener. The performance and reliability of adhesives has continued to grow and they are now commonly used and relied upon in numerous applications. Almost any structural adhesive will be compatible with a bonding fastener such as the bigHead. 

It is no longer necessary for design engineers working in composites to compromise their product design or function by using fasteners designed for non-composite applications. In doing so, they risk their product quality and customer satisfaction. 

Bossard’s range of fastening solutions designed for composites, offering design flexibility and functional reliability, has become well established over many years in applications across a wide range of industries. The quality has been fully tested by leading companies in the automotive, marine, construction, energy and general manufacturing industries.l

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Matthew Stevens is managing director, bigHead Bonding Fasteners.

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