Fastening & Joining Fastening & Joining
11-99
It's so simple
Fastener combines adhesives with riveting
Problem It is becoming generally accepted that the best means of joining sheet metal, especially in fabrications for upmarket cars, is to combine riveting with adhesive bonding. The resulting joints combine peel strength, rigidity, load spreading, and high energy absorption during impact.
The snag is that applying adhesive between two sheets of metal, followed by riveting, tends to squeeze adhesive out of the joint in a way that is hard to predict.
Solution TWI has developed and patented a joining system called AdhFast. The idea consists of a range of mechanical fasteners that allow adhesive to be injected into a controlled gap between the sheets after mechanical assembly.
Prototype designs have been developed to cope with the three main assembly processes. The first, shown in the illustrations, is where there is access to all sides. The second is where there is access to only the top side, and where there is a continuous bottom surface to which something must be attached. The third is where there are two thin sheets with access only from the top side, as currently addressed by blind rivets.
Advantages of the new processes include: maintenance of a clean uncontaminated surface prior to bonding; reduction of operator contact with the adhesive (especially relevant for epoxies and polyurethane with their known effects on health); elimination of voids and air bubbles; and the facility to automate or semi-automate the process.
Applications TWI’s Dr Ewen Kellar says that the configuration shown in the illustrations is aimed at applications in structural engineering, such as steel framed buildings where traditional nuts and bolts are currently used. The big applications for devices that allow access from only one side, however, are in aerospace applications, where they hope the development will reduce the number of fasteners required, in, say, an aircraft wing. The technique is expected to be suitable for a wide range of joint situations, although Dr Kellar says it might not be appropriate for narrow - 5 to 10mm wide bond lines.
About a dozen prototype fasteners have been made and tested so far. TWI is now looking for development partners to take the work further. TS