The sticking point: Adhesives replace more conventional fasteners

If performance is a driving factor, it is more than likely that there is a tendency toward using composites in conjunction with other materials to bring about an overall improvement to a system.

The trouble with all this is that it adds another variable to the mix: complexity. More materials means having to cope with differing thermal expansion, fatigue properties, failure modes... the list goes on. Bringing materials together used to about over-engineering, using a steel M12, M6 or M4 bolt that you knew was going to deliver.

However, this perhaps crude practice is getting less and less acceptable in many cases and whether it is to reduce weight, remove the visual appearance of a bolt-head or to spread a load, many engineers are exploring the possibilities of adhesives for increasingly rugged and harsh environments.

Adhesives have been used on a recent novel vertical axis wind turbine system (VAWTS). Nova is a two year renewable energy project that aims to investigate the cost and feasibility of manufacturing a radical VAWTS design.

A key milestone was the successful construction of the sails for the fully working 50kW prototype demonstrator of the offshore 'double arm' concept.

The 50KW prototype was manufactured at Cranfield University's Composites Centre, with extensive advice and support of the design and manufacture provided by key material suppliers. This included Scott Bader, which specified its Crystic Crestomer 1152PA urethane acrylate structural adhesive for the lightweight structural bonding of the various carbon fibre and glass fibre epoxy composite parts which make up the two 10m by 1.9m rotor sails. For the planned scale up to an offshore wind turbine to be viable it requires the rotor blades to last at least 20 years without maintenance.

By first developing a 50KW prototype scaled down demonstrator, the Nova Project team will be able to gain an understanding of the engineering performance and aerodynamic behaviour of the design and extensively test the offshore operational conditions of the composite materials and associated joining methods.

The structural approach used for the rotor sails is similar to a large commercial aircraft wing. The sail has a central box section, designed with tapering thickness skins, two C-spars with ribs and 'omega' shaped hat stringers to provide resistance to buckling. The sail central box components were manufactured from multi-directional carbon fibre fabrics and epoxy resin using a vacuum infusion moulding process. The central box has added glass fibre leading and trailing edge components.

Bearing in mind that the scaled up 10MW wind turbine sails would be 80m long, finding ways to reduce weight in the overall sail design was critical. For the prototype, the rotor sail weight was significantly reduced by using a structural adhesive, which had the added benefit of also providing lower overall manufacturing costs compared to a bolted joint section and mechanical assembly design.

The carbon fibre epoxy ribs, spars and skins of the box design were bonded together with Crystic Crestomer 1152PA structural adhesive. The leading and trailing edge components, separately fabricated from glass fibre epoxy resin, were then bonded onto the central box of the sail using Crystic Crestomer 1152PA. All the bonded joints used a simple joggle design and shim plates to allow for rapid assembly with minimal fixtures and smooth joints for aerdynamic performance.

Andrew Mills, Nova project leader at Cranfield University Composites Centre, says: "The design and scale of the sail rotor structure means that there are large bond surface areas and wide bond lines. The fully cured structural adhesive must, therefore, have long term performance properties which meet a number of key requirements. These include gap filling and providing outstanding peel resistance, while at the same time being strong and very tough, with exceptional flexural strength properties."

The adhesive performance comparative testing carried out by the Cranfield project team was carried out on a range of bond-line thicknesses from 30mm down to 2mm. It showed excellent results for bonding both carbon fibre and glass fibre epoxy laminate substrates. The findings were supported by the quality and long term performance of Crestomer adhesives which has been used for over 30 years in fibreglass marine applications such as deck and hull stringers and in bulkhead bonding, endorsed by both Lloyds and RINA which have certified Crestomer adhesives for composite bonding on a variety marine applications.

For the 10m sail prototype, each component and mould tool was designed with joggled part fit, scored fitting positions and 1mm adhesive shims. This provided accurate final sail dimensions without the use of complex assembly jigs. As part of the assembly process to ensure joint accuracy, after positioning without adhesive, each part was drilled and the holes were used to either pin or rivet each part together after adhesive application. All of the adhesive joints were post cured in a portable taped foam board oven for 24h at 50°C and the joint gaps were subsequently filled with a body filler and abraded using a circular sander to eliminate evidence of the component joints.

Fighting complexity
However, for most engineers it is a daunting, and often, brave step in to the world of adhesives, especially if it is for the first time.

"Selecting the right adhesive for a job can be quite difficult," says Bob Goss, senior technology specialist at Henkel. "If you're trying to bond glass to metal, for example, it is very difficult to select the right adhesive. Are you trying to bond a windscreen in to a car, a glass lens on to a microscope, or a shower door catch. Each of those is glass to metal but would need a different adhesive for each of those applications."

There is quite a bewildering array of adhesives out there so how do you select the right one? The simple answer is ask Bob, or more specifically Henkel, which receives some 100 emails a day requesting such clarification.

A recent application saw it advise a manufacturer of a piece of climbing equipment. There was a spindle and nut which needed to be thread locked. The bond needed to be so strong that no one could tamper with it in the field. If you tried the M4 head it would deform before it would turn, which is exactly what is needed with something so safety critical.

There a few rules of thumb when dealing with adhesives that every engineer should know. While initial solidification might occur in as little as a few minutes, it can take 24 hours or more for full curing to occur. During this time handling strength can more than double. The other thing is that while adhesives have pretty good strength in direct tension, peel strength is its weakness and can be a factor of 10 to 100 times lower than its tensile strength. Mix ratio is also really important in terms of the final cure properties of epoxy, and for the best durability a slower curing epoxy is usually better.

"Typically adhesives work up to a maximum temperature of 100-120°C but some go to 300°C," says Goss. "Above that you will struggle with an adhesive.

"For a good bond joint design is very important and generally the bigger the better. Surface preparation is also important and if you can make the surface rougher then that will significantly help to key in to the material. Though generally cleaner material is better, many adhesives now have oil tolerant chemicals built in to them so they will absorb a certain amount of impurity on the surface and carry on with the bonding."

Adhesive tape manufacturer Tesa has also been keen to highlight where adhesives can be used as an alternative to a bolted joint. It recently launched the ACXplus range of double-sided tapes for constructive bonding.

Constructive bonding is based on the viscoelastic properties of ACXplus, defined as both the viscous and elastic performance, can cope with uneven or irregular surfaces.

Tesa says the benefits of using a high-performance tape against traditional mechanical methods include faster application, a seamless finish (better aesthetics), equal distribution of the stress areas of the bond (so no weak points are created), less likely to rattle, corrosion prevention, flexibility of design and compensating for different rates of thermal expansion and contraction of different substrates as well as absorb shock and vibration.

Regardless of the application, adhesives are offering increasing advantages and long term propects to engineers and do not have add complexity. Help is at hand, so don't be afraid to ask the questions.

Justin Cunningham

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