Structural healing

Intelligent structures that can sense damage, actively cancel out vibrations and even ‘heal’ themselves are usually associated with advanced military or aerospace projects. But, thanks to a number of highly promising research projects around the world, such systems are on the verge of finding their way into big civil structures such as buildings and bridges.

Later this month, under its Smart Materials and Related Structures initiative, the DTI will announce the recipients of £7m of funding. Project manager Dr Robert Quarshie, says that a large portion of this money will be aimed at technology that holds great promise for big civil structures. It could, he hopes, help kick-start a profitable new area for the UK’s economy.

‘The field of smart materials has hitherto only been seen in weapons and defence programmes,’ he said. ‘This funding will help build UK commercial capability and enable us to reap the benefits in more down-to-earth applications. If we are going to have an economy looking at the higher value end of products this is exactly the kind of technology we should be developing.’

Activity on the international stage certainly suggests that the climate is right for a new era in construction technology. Following the tsunami in south-east Asia last year, fears of the destructive power of earthquakes are at a high, and technologies offering improved seismic detection and mitigation properties are more attractive to construction firms.

And it’s not just geologically unstable areas that stand to benefit. Just two weeks ago the long-awaited federal inquiry into the collapse of New York’s World Trade Centre recommended a number of changes in safety specifications for US skyscrapers. Among these the report’s authors at the National Institute of Standards & Technology called for improved methods of monitoring the structural stability of tall buildings.

Prof Jerome Lynch, who heads the University of Michigan’ s Laboratory for Intelligent Structural Technology, agrees that now is an exciting time for the technology. Lynch’s team has been developing low-cost wireless sensors equipped with their own microprocessors that can be embedded in buildings in large numbers and used to monitor structural health.

He said the technology is becoming more realistic thanks to the growth of wireless communications, which has driven down the cost of the embedded systems components at the heart of his system.

‘We’re riding on the coat-tails of Moore’s Law,’ he said.

According to Lynch, the big advantage of the technology, which has already been trialled on the Alamosa Canyon Bridge in New Mexico and Geumdang Bridge in South Korea, is that the sensors can actually perform some analysis before transmitting the recorded data. This makes the sensors much more autonomous and potentially gives users far quicker access to results than is possible with traditional structural monitoring systems.

Existing systems, he explained, produce too much data for anyone to be able to sift through. Equipping the sensor with some degree of computing power is one possible solution to this. Lynch said that it is currently possible to encode algorithms that are sufficiently accurate to flag important bits of data for the urgent attention of human operators. However, as these algorithms improve it will be possible to remove humans completely from the loop, he said.

A further advantage of co-locating computing power with the sensor is that it opens the opportunity to tack other services on to the system in the future. ‘All kinds of different services could pop up on the fringes of this technology — just imagine the consumer services you could piggy-back on to an infrastructure that already exists.’

Meanwhile, Bracknell company Smart Fibres is leading the way in the development of a very different structural monitoring system. Smart Fibres’ technology is based on the ‘fibre bragg grating’, a sensor technology in which refractive index gratings, or ripples, are written into the core of optical fibres and act as strain sensors.

The sensor network is illuminated using an optoelectronic unit and the optical reflection from each grating is then measured and used to determine stress or compression. Chief engineer Dr Crispin Doyle explained that the main advantage of the technology is that it is immune to electromagnetic interference and so requires less cabling. The sensors are also smaller than existing technology such as strain gauges so they can be contained within materials to check for internal stresses and strains. The technology has been used to monitor the body of a composite Swedish naval vessel.

But Doyle says that the company is also investigating its potential in a number of other civil applications. For instance, it recently installed an optical fibre sensing network in the roof of Terminal 3 of Singapore’s Changi International Airport.

But structural monitoring can only help up to a certain point. If the building you’re in is close to collapse, it can give you a bit of advance warning, but it won’t stop the inevitable. So engineers around the world are also busy developing active damping systems that can react in real time to the kinds of extreme conditions that would otherwise cause damage.

The US company Lord Corporation is working on the development of dampers based on the properties of magnetorheological (MR) fluid, a free-flowing suspension of micron-sized iron particles, which when exposed to a magnetic field, thickens within milliseconds to a semi-solid state.

Lord’s global marketing manager Jim Toscano said the technology has great potential in seismic mitigation. He explained that during an earthquake tremors will activate a magnetic force inside the damper, causing the fluid to change from solid to liquid and back again in time with each tremor.

He claimed that the technique could significantly limit the amount of damage caused. However, while a damper based on the technology has already been installed at Tokyo’s Science and Technology Museum, Toscano said that widespread use of MR dampers for this purpose is still at least five years away. In the shorter term the company is turning its attention to the use of its dampers in cable stay bridges.

These are particularly attractive to engineers because they cost less to build than suspension bridges and don’t require the huge structural anchors. However, under certain wind and rain conditions the steel cables tend to ‘gallop’ or vibrate, significantly reducing the life of the bridge.

The technology has undergone trials on the Dong Ting Lake Bridge in China’s Hunan province where it is claimed to reduce vibrations by up to 95 per cent. But a far sterner test is just around the corner: Lord is working on MR damper technology for the longest cable stay bridge in the world. Due to open in 2008, the 1,088m Sutong Bridge, over China’s Yangtze River, is expected to contain around 20 miles of steel cable.

Back in the UK, a small technology company in Hull is investigating the potential of a similarly exotic material. Newlands Scientific first hit the headlines a couple of years ago with the invention of the Soundbug, an innovative little audio device that exploits the eccentric behaviour of the metal Terfenol-D to turn your desk, window or any other flat surface into a loudspeaker.

This material, a compound of iron and two rare earth elements, has magnetostrictive properties. This means that when exposed to a magnetic field it will change shape; or, if deformed, it will generate an electrical charge. Thus it can be used either as an extremely sensitive sensor or as an actuator.

While the world of audio gadgets may be a million miles away from bridge and building construction, Dr Kamlesh Prajapati, Newlands’ technology director, says that the technology at the heart of Soundbug has some extremely promising architectural applications.

Prajapati’s team has been investigating the use of active dampers based on Terfenol-D for cancelling out vibrations on bridges. So far the project has been confined to scale bridge models and computer simulations, but the initial test results are promising and Prajapati said that structural control represents a very promising area for his company’s technology.

The advantage of Terfenol-D, he said, is that it has a very high force and high-speed response. ‘There are lots of applications for active control using these materials. Terfenol-D can work at very low frequencies and building vibrations are typically extremely low frequency.’

In the shorter term Newlands is developing engine mounts that use similar real-time damping technology to control the transmission of vibrations through a boat structure. Prajapati said that he is in discussions with a boatbuilder and expects to make an announcement on a project within the next few weeks.

Clearly it is still early days for smart structure technology. One eminent UK civil engineer, who asked to remain nameless, snorted derisively at the very mention of the concept. This reaction, though, probably has more to do with the traditional conservatism of the construction world than any serious objection to the potential of the technology.

This is certainly the view held by Lynch: ‘It’s a tough environment for companies attempting to commercialise such technologies. Civil structures owners are notoriously cost conscious and not keen to take the risks of adopting new technology and seeing where it leads them. They’re looking for something that’s well proven and would then contemplate adoption.’

But Lynch is confident that the huge advantages offered by the technology will prove irresistible.

‘In 20 years’ time this technology will definitely be installed in most large-scale infrastructure systems.’ It will, he claimed, bring about a major change in the way that structures are managed, leading to improved safety and massive savings for owners.

PROVING GROUND FOR SMART IDEAS

While active materials are still poised to make their presence felt in large civil structures, other more traditionally hi-tech areas of engineering are a valuable proving ground for these technologies.

Unconstrained by the cost concerns and the naturally conservative instincts of their construction-minded cousins, engineers working in the military, aerospace and automotive industries are increasingly turning to smart materials.

Crucially, though, there is a strong dialogue between the academics and industrialists working on both sides of the fence. Indeed, a quick glance at a diary of international engineering events reveals a schedule packed with structural monitoring seminars and gatherings attended by engineers specialising in disciplines ranging from rotor blade design to bridge building.

For instance, while it has one eye on the big civil applications, Bracknell’s Smart Fibres is working with researchers at BAE Systems’ advanced technology centre on an embedded sensor system for military jets.

Under the Ahmos-2 project, the two groups are developing a network of piezoelectric sensors to hear and pinpoint cracks in composite and metal structures during flight.

BAE Systems’ Dr Peter Foote says that the company plans to flight test the system on a Hawk jet within the next two to three years.

Meanwhile, Dr Prajapati’s group at Newlands Scientific is involved in an EC-funded project investigating the potential of magnetostrictive materials in aerospace applications. Under the three-year EC-funded Magnetoeleastic Energy Systems for Even More Electric Aircraft (MESEMA) project Prajapati’s team is collaborating with researchers from across Europe on the development of a system for helicopters that will cancel out rotor blade vibrations, and harvest the vibrational energy to power a network of sensors.

Prajapati said that his team is working alongside engineers from Eurocopter and German rotor blade company ZFL and hopes to test a prototype system within six months.

 Clearly when structural damage, or impending damage is detected, it must still be repaired — and there is a significant amount of international research into self-healing structures. For example, researchers at Bristol University (with funding from ESA and the EPSRC) are developing ‘bleeding’ composites for aircraft in which hollow glass fibres within structural composites are filled with resins that enable the material to repair itself.

As the fibre breaks, stored resin and a hardener combine and leak into the damaged areas. The fibres also release an ultraviolet fluorescent dye, making the damaged area easier to spot, said the university’s Dr Ian Bond.