Increase in the use of adhesives is often impeded by practical issues such as H&S worries, jigging requirements, operator training and, crucially, overall quality control of the process.
In an increasing number of applications, the adhesive is employed in primary joints with load-bearing capacity. Such a requirement means that the joints in question meet high degrees of integrity and reliability. At present, inspection of the final product is often the only way of achieving this. This is costly and is not reliable since NDT is unable to detect a range of flaws. Also, it cannot assess durability issues and it is 'after the event'. Reliability and consistency have to be achieved by thorough, in-process control from the time raw materials arrive until the last finishing operation is completed.
Quality assurance can also be achieved through the way in which the bonding operation is carried out by combining adhesive injection with hybrid joints. TWI is at the forefront of innovation with AdhFAST ®, which takes adhesive bonding a stage further.
AdhFAST ® is a hybrid joining system that combines the advantages of adhesives and fasteners with little additional operator training. The primary advantage of such a system is that it enables the bonding process to be controlled by means of a combined 'dry' fit-up (to check for alignment and tolerance mismatch) and a full structure assembly prior to the use of adhesive. The fastener creates a self-jigging mechanism, and operator exposure to the adhesive is minimised. In addition, the volume of adhesive injected into the joint can be monitored and controlled automatically if required.
Experiments in which single lap joints were prepared with and without AdhFAST ® have shown that the statistical variation of peak failure loads and extension to failure for tensile testing were noticeably reduced using the new system. Typical data are shown below, where the 'area' of variation is sketched in for both types of joint (AdhFAST ® - Blue, no AdhFAST ®- Red).
In addition, failure load data obtained from joint pairs (where a single joint had been split into two equal halves along the long axis) showed a 50% reduction in the standard deviation between specimens prepared with and without AdhFAST ® respectively.