The advantages of AdhFAST

Adhesives have been around a long time but because of prejudices they have not been fully utilised, particularly in the marine sector, despite the fact that they have some obvious advantages. Fire is one of the biggest problems in the ship repair industry and welding is usually to blame, so any process that could cut down the amount of welding needed should be welcome. Many see the application of adhesives as a non-skilled job and that is perhaps where one of the biggest problems lies ? you wouldn?t have an untrained welder carrying out welding ? and the same goes for adhesives. “Reliability and consistency have to be achieved through in-process control from the time the raw materials arrive until the last finishing operation is completed,” says Ewen Kellar, principal project leader for the Polymer Technology Group at The Welding Institute (TWI). “Regard the whole bonding process as a system.” TWI has developed what he describes as a hybrid joining system called AdhFAST, which says Kellar “is one that combines two or more joining processes, most often one of these processes is adhesive bonding”. In the case of AdhFAST, the second is mechanical fastening. The big advantage of the system is that it allows the components to be assembled and fastened together in a dry state and controls the bondline spacing or gap between panels. This means that the injection of adhesive can be carried out in the final stage, so no jigging is required, even for complex joint types. Kellar highlights the advantages of the system as: l Simplification of the bonding process l Increased quality control l Adhesive pressure control l Self-jigging during adhesive cure l Maximising the benefits of hybrid joint technology. TWI has been experimenting with different fastener types to offer all areas access, topside access only, or blind access and this process is continuing through threaded, collapsible and expanding fasteners. AdhFAST also offers repeatable results. Kellar says that the mean variance between the normalised peak failure load for the AdhFAST joint was about 50% less than that for conventionally assembled joint. He explains: “The mean difference for normalised peak failure load between each pair of test-pieces was 50% less for the AdhFAST joint.” The AdhFAST joint also retained a residual shear strength of about 20% peak load through the presence of the fastener within the joint, claims Kellar. He sees that the system could be particularly useful in the ship repair sector where welding is difficult. TWI has also recently looked at modifying the technology to allow studs to be fastened to bulkheads and the like. Kellar sums the advantages of the system up: “The AdhFAST system provides greater control with regards to assembly tolerances and enables the variance between ?identically? produced structures to be reduced.” He told The Motor Ship: “At present AdhFAST is not used commercially, however we believe that the system which AdhFAST defines i.e. injection of an adhesive through a fastener (nut/bolt, rivet etc) into a controlled cavity between the substrates to be bonded, offers great potential. It enables the elimination of external jigging, simplifies the bonding process, reduces operator exposure to adhesives, speeds up the process and enables greater quality control ? all key factors which will attract end users.”