Composites and metals combine for strength

Tom Shelley reports on how composite patches can strengthen metal structures and thick metal coatings can enhance composites.

Composite patches can be used to joint metals, while composite tubes and other constructions can be thick metal coated to produce very lightweight fabrications that in other ways, perform like metal components.

The original intention of the EU-wide 'Co-Patch' project was to come up with a low-cost, quick and reliable method of repairing and/or reinforcing marine and civil engineering structures, particularly when they have become cracked or corroded. However, it has also shown an alternative way in which metal elements can be joined or reinforced at critical points. The reverse process of applying thick metal coatings to composites, is appropriate for making very lightweight and low inertia mechanical parts for automotive and possibly automation applications.

Both developments come from TWI, which is leader of one of the Co-Patch project tasks as well as running the website, although the overall project coordinator is the National Technical University of Athens. There are six SMEs in the consortium, three industrial partners, and two other academic institutions. At the same time, TWI has been making advances in the thick metal spray coating of composites and other materials for many years, and offers an extreme depth of expertise with an exceptionally wide range of additive surface engineering techniques, including arc spraying, cold spraying, plasma methods, chemical methods, and just about every other technique anyone has ever thought of.

Dr Farshad Salamat-Zadeh, senior project leader with the advanced materials and processes group at TWI, says that progress with the three year Co-Patch project has been going well, with goals being met and deliverables being produced on time. He says that there was already a composite wrap-round system for undertaking pipeline repairs that has been used in the oil and gas industries for some time, but that the patches need to be replaced every five to seven years. So far, there have been no leaks. However, industry wanted systems that could be applied to a wider variety of constructions, including ships and aircraft and would last 25 years. As a means of stopping crack growth in aircraft structures, it has to be preferred to drilling a hole at the crack tip, although there would have to be some means of inspecting the crack, perhaps from the other side, to ensure that it is not growing underneath the patch.

One of the target uses is repairing GRP-hulled lifeboats for the RNLI, but Dr Salamat-Zadeh remarked that this is actually easier to accomplish than some of the others tasks such as wrapping steel or iron columns, since a GRP hull closely matches the materials in a carbon fibre or GRP-based patch, whereas a steel column requires an anti-galvanic corrosion layer, typically based on glass fibre, laminated to upper layers based on carbon fibres for strength. Furthermore, the thermal expansion coefficient of carbon and glass are different from each other, and both have thermal expansion coefficients that are lower than those of metals which are lower than those of polymer resins.

Asked if it would be possible to adapt the technique for fastening and joining steel or other metallic elements to make new constructions, he replied that it most certainly was possible, but to design the best possible joint, it would be necessary to first calculate all relevant stresses, and then choose the most appropriate kind of resin to cope with thermal stresses. He thought that there could be many potential advantages to be obtained by this technology, all of which TWI has expertise in, but making joints in new things, was of course, not part of the Co-Patch brief.

The advantages of using composite patches for repair or joining are that they can be applied quickly, after suitable surface preparation, and using a completely cold process, so there is no potential problem that might arise associated with heat-induced damage, differential thermal expansion, or setting something in the vicinity on fire. Such patches also avoid stress concentrations, which is not true of bolts, rivets, spot welds and more traditional joining techniques. In addition, they can also easily be added after something has been built or put in service, if it is required to carry greater loads than was originally intended.

The effect is not easy to calculate, but TWI has a whole department devoted to computer modelling structures, particularly for the offshore oil industry, which can assess whether they are still good for service after suffering corrosion or damage, or if not, what can be done to keep them serviceable.

One of the keys to successful bonding of composites to metal is to make use of a new FTIR (Fourier Transform Infra Red) hand-held, point-and-shoot sensor devised by A2 Technologies in Danbury Connecticut and called the 'Exoscan'. Dr Salamat-Zadeh says that he found Exoscan particularly useful as a means of assessing surfaces to ensure they had been prepared well enough to take a patch. It has interchangeable external reflectance and single reflection diamond tips – diamond is transparent to infra red – and can either be touched on a surface, held 5mm to 10mm away from it, or work with a drop of liquid on its tip. The sensing head is initiated against the inside of a gold-plated cap to establish a baseline and then takes a reading in less than a second, but the longer it is allowed to take readings, the more it can take. It works with a PDA control panel that wirelessly links to a laptop for uploading and downloading data and methods. It has a wide range of quality assurance applications including assessing state of cure and whether spots, streaks or blemishes on a surface are of concern or not.

Dr Salamat-Zadeh's colleague Paul Burling, who is responsible for business development in the aerospace and construction and engineering sectors, described some of the laboratory's achievements in the reverse process, applying thick layers of metal to composite. He explained that laying down thick metal coatings on composite is very difficult because of the high stresses that result from different coefficients of thermal expansion. One solution is to spray with Invar, which has a very low thermal coefficient, but he also showed thick coatings on composites made of bronze, brass, aluminium and ceramics.

He did not explain how these coatings had been achieved, other than to reveal that the substrates were carbon fibre in a BMI (Bismaleimide polyimide) resin, with applications that ranged from moulding tools, through aircraft parts, to transmission shafts and pistons for internal combustion engines. The use of carbon fibre composite greatly reduces inertia and has obvious applications in motorsport. The metal surfaces can resist high temperatures, provide wear resistance or resist point loading, or if required, carry large electric currents or provide EMC protection.

Design Pointers
• Composite patches can be applied to metal structures to strengthen them with reliability, extending life of existing structures or improving load-carrying capacity
• Joining or reinforcing with composites is a cold process which produces no stress concentrations
• It is also now possible to lay down thick coatings of metal on light weight, low inertia composites to enhance surface load carrying capacity, resist heat or conduct large electric currents
• The wheel motors can easily be scaled up to deliver more torque or higher speed

Author
Tom Shelley

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