Bonding fasteners suitable for composite panels

Paul Boughton

Bonding fasteners require no depth of material to provide a very strong and reliable fastening. Matthew Stevens reports.

Manufacturers everywhere, from the makers of cars, trains, boats and buses to the construction industry are increasingly using composite panels. The desire to reduce weight while maintaining strength continues to drive demand. However, users of composite panels are often faced with the problem of how to fasten or fix these panels in place.

Steel, stone and concrete

Fastening solutions that worked for solid panels of steel, wood, stone or concrete are often ineffective with composite panels made with thin sheets of carbon fibre, aluminium or plastic.

Traditional fastening methods such as rivets, spot welds, inserts, screws and bolts are sometimes no good. For a panel with skins only 3mm thick, an insert requiring solid 15mm material thickness is no use. For a lightweight cladding tile of 15mm depth, an anchor bolt designed for 75mm is useless.

Bonding fasteners are increasingly seen as the answer. When surface bonded with adhesive, bonding fasteners require no depth of material to provide a very strong and reliable fastening.

When embedded in the panel, bonding fasteners with heads as thin as 1.5mm need very little panel depth to provide a very strong and reliable fastening. Bonding fasteners are also discrete and highly versatile.

The automotive sector contains many good examples for the use of bonding fasteners.

One example is a supercar manufacturer's use of bonding fasteners to attach a carbon fibre diffuser panel. Twelve bigHead Bonding Fasteners are surface mounted to the 2-3mm thick carbon fibre plate with adhesive. The fastener provides simple threaded screw fixings with which the plate is securely bolted to the car structure.

Fig.1. shows how the adhesive flows through the holes in the bonding fastener head to secure the bonding fastener in position.

Building renovation

An interesting example from the construction sector is the renovation of an old building in central London. The project included the cladding of an interior atrium with stone panels.

Due to the age of the building lightweight thin panels had to be used rather than traditional 75mm thick stone. Consequently, traditional anchors could not be used to secure the panels because there was insufficient material depth.

After much research and testing, bigHead bonding fasteners were embedded in the composite stone panels. Invisible from the facing side, threaded bolts extended from bonding fastener to be bolted to the building structure.

Before installation the tensile loading was tested to make sure the fastening solution was strong enough.

Incredibly, the bigHead Bonding Fasteners withstood 330kg tensile load before being ripped out of the panel.

Composite materials

BigHead fasteners were invented in 1966 to solve a fastening problem that most traditional fasteners could not; How to achieve a strong and efficient fastening in a composite material.

BigHead came up with a unique design and the first ever bonding fastener: a flat perforated head which locks the product securely into position and spreads the load. They are used by manufacturers and designers in the automotive, composites, construction, marine, utility, petrochemical and general manufacturing industries. These include Sunseeker Yachts and Aston Martin. bigHead is a BMW approved supplier to Rolls Royce Motors.

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Matthew Stevens is Managing Director, bigHead Bonding Fasteners Ltd, Bournemouth, Dorset. UK.

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