Joined together

Delegates at the Joining Technologies Design Day learnt about lots of new techniques in joining – and the best way to pull their fingers apart after a superglue accident. Lou Reade reports

How do you tell the difference between an engineering student and an arts student? Ask them to describe a joint, of course – and joint design played a key role in Eureka’s recent ‘Joining Technologies’ Design Day.
First, a design problem to ponder: what is the best way to stick two overlapping pieces of flat steel together? To make it easier, there are just two options (see diagram) – should you use a ‘long’ join, or a ‘wide’ join? (Read on, and the answer will be revealed later.)
Bob Goss, product specialist at Loctite, posed the problem during a workshop at the Design Day. He asked for a show of hands – but the lack of certainty among delegates shows a general shaky knowledge of adhesive joint design.
Professor John Watts of Surrey University specialises in working out why certain adhesive bonds fail early.
“Any fool can make a strong adhesive joint,” he said. “The skill is to make a joint where the strength is retained for a long time, under a wide range of conditions. I’ve tried to understand where durability degrades more quickly than you might have expected.”
He pointed to a number of key applications – such as the iconic chassis of the Lotus Elise – that showed the power of adhesive bonding. By understanding the exact nature of adhesive bonds, he can use advanced surface analysis techniques to identify specific problems. In one case, for example, a coil coating process was failing – which he traced to excess levels of a photo-initiator. Cutting levels of this by 80% solved the problem.
And bonding technology is moving further across industry. Recycling directives – such as End of Life Vehicle Directive – put greater restrictions on manufacturers, insisting that they make recycling easier. One way they could do this is to make products easier to disassemble – such as by turning adhesives on and off. A number of methods exist to do this – using, for example, ultrasonics or heat to trigger the ‘collapse’.
Mark Geoghegan, of the University of Sheffield, told delegates about his research that uses pH – acidity – to ‘switch off’ adhesive performance. He has worked on a family of polyelectrolytes – charged polymers – and assessed their adhesion strength.
“The fact that charged polymers stick together at pH6 is not new,” he said. “The fact that they come apart at pH1 is.”
Because the experiments were carried out in water – and these compounds have been used in drug delivery – he initially saw the technology as being of use within medicine. But he now believes that it could also find use in other fields such as circuit manufacturing, labelling, denture fixatives and in ‘recycling applications’ (such as for making products that are easier to disassemble).
“One thing we need to work on is the speed of switching,” he said – referring to the fact that it takes some time to ‘switch off’ the adhesion. “We think we can do this by tuning the polymers.”
In his workshop, Loctite’s Bob Goss showed delegates how to solve a common problem: what to do when you stick your fingers together with superglue.
“If you simply pull your fingers apart, you’re in danger of ‘substrate failure’,” he said. “Instead, you should ‘peel’ your fingers apart.”
The advice highlighted the fact that adhesive bonds have different failure modes – and that apparently identical designs can behave very differently. He showed two overlapping joints, in which the overlap area was identical.
“As you increase overlap length you do not increase strength,” he said. “The wider the joint, the better the peel strength. If you are designing a joint, it’s better to increase width rather than overlap length.”

Design Day

Bonding techniques
The event covered more than adhesives. TWI’s Ajay Kapadia highlighted two breakthrough techniques – Micro Friction Stir Welding, which can join thin parts below 0.3mm; and Co-Meld, which joins dissimilar materials. His colleague Alec Gunner explained some of the difficulties of joining materials.
“Aluminium and steel cannot be welded directly,” he said.
One of the methods used to overcome these problems is diffusion bonding. It joins materials together below their melting points, and has been used to join alumina with copper, for example. It is commonly used in brake pads.
Andrew Fitzpatrick UK sales manager of fastener manufacturer Arnold, said that correct selection of fasteners could cut system cost – even though the individual fastener cost may be more. He estimated that the company’s thread-forming screws are 33% more expensive than conventional metric screws – yet can save up to 80% in production costs.
In his workshop, Arlindo Marques of Spiralock Europe took delegates through the design and analysis of bolted joints – including an explanation of the advantages of a new design that maintains a ‘crest to ramp’ engagement.
“On a standard bolt, 64% of the load is absorbed by the first thread,” he said. “This design spread it out more evenly.”

Tom Shelley

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