Structures with attitude

In the future, buildings, planes and ships will not only be able to let us know when their structural integrity is deteriorating but will also address the problem and fix it.

Researchers tell us that in the not too distant future we will own cars that tell us when their bodywork is damaged – even if the fracture is invisible to the naked eye – and live and work in buildings that will let us know when their structural integrity is deteriorating. Looking even further ahead, structures will not only tell us when they are damaged but also fix themselves.

The next generation of structures will contain both sensors (to detect the need for change) and actuators (to implement that change). Keith Worden, reader in the dynamics research group at Sheffield University, says: ‘Smart structures are going to be endemic, and the applications will be numerous. For example, if a plane has a cracked wing, rather than the pilot having to take action the plane itself will automatically change the wing loading.’

However, that is still some way off. The first step towards a truly smart structure is to create materials that can sense their surroundings. This is close to reality, with likely applications in sectors such as aerospace or yacht building where composite materials are widely used. Sensor networks can be embedded within the composite to provide feedback about the condition of the structure.

Embedding sensors has two benefits: you can place them in the most critical areas of the structure, and they are protected from the external environment, so the potential for interference and damage is greatly reduced. But the downside is that integrity of the material may be slightly weakened Embedded Sensor Networks (ESNs) are influential in structural health monitoring, and research into this is taking place at several universities and commercial centres throughout the UK.

Worden, Wieslaw Staszewski and colleagues at Sheffield University department of engineering have been using embedded piezoceramic sensors as monitoring devices in aircraft. These are cheap and simple and can be placed between composite layers. When the signals produced and received by the sensor are no longer homogenous, it shows that damage has occurred.

Researchers at Sheffield are also investigating acoustic emissions for damage sensing. The eventual goal is to be able to identify several different types of damage, with low-frequency emissions indicating large-scale composite cracks and high-frequency signals much smaller-scale fractures.

Companies such as Smartfibres, a spin-off of Carbospars, use fibre optics for embedded sensing in carbon-fibre yacht masts to find the points of maximum strain and improve the design. The technology was developed in a collaborative research project involving Smartfibres, Pendennis Shipyard, Aston University and BAE Systems. The system has been used in a 38m mast for the yacht Jaquelina, and Carbospars is building Erika, which will have a 60m mast, the biggest ever spar according to the company’s managing director Damon Roberts. It will also be adopted by at least one contender in the forthcoming America’s Cup.

Though Carbospars has employed the system to optimise design, it can collect data over the life of the mast and can be programmed to act as a black box, automatically downloading data for a fixed time if triggered at a threshold stress level.

Smartfibres has also been using this technology for strain measurement in aircraft, wind turbines and offshore oil rigs. The turbines are like the masts, each blade essentially a cantilevered mast 1.08m in diameter and 50m long with embedded fibre optics.

On oil rigs the optical fibres measure stress on riser pipes and tethers; being physically stable and immune from electrical interference, they provide a safe method of monitoring structural health.

The technology is calibration-free so less time consuming than traditional techniques.Qinetiq is another firm interested in fibre-optic sensors. One of the company’s latest designs is based on a multi-core fibre (MCF) structure capable of measuring magnitude and direction of bend in a structure, as well as longitudinal strain and temperature. Using standard fibre optics, two sensors would need to be embedded along the plane of a bend to ascertain the differential strain. With an MCF, however, only a single fibre is needed. The cores are also more stable when within a single fibre; they have well-defined spacing, don’t slip with respect to each other and are unaffected by temperature differences.

The sensors can monitor bending cycles and fatigue as well as detect delamination in composites. Dr James Burnett, from the photonics department of Qinetiq, says: ‘An indicator of damage to a composite structure is delamination. If a load-bearing structure begins to show signs of abnormal deformations without loading this may be due to delamination.’

Dr Ian Sage, from the advanced visual concepts department at Qinetiq, has been working on a damage sensor system more focused on impact and brittle failures. In strain gauge monitoring, signal processing is needed to distinguish normal from abnormal outputs. Sage is using triboluminescent sensors which emit light (due to friction) when they are fractured.

So a sensor that is only sensitive to damage can be produced by dissipating the triboluminescent material into a resin that is used in the lay-up of the composite. When the composite is fractured the sensors emit light which is guided by embedded fibre-optic waveguides to a central detector. The extent of the damage is proportional to the amount of light emitted; the location can be pinpointed by embedding different triboluminescent materials that emit different wavelengths of light.

The sensors also add little non-structural weight and cause only minimal disruption to the composite structure. Potential uses include highlighting brittle failure in composite structures in planes, high-performance cars and large-scale civil structures such as bridges.

Steve Elliot, chairman of the Signal Processing and Control Group at the Institute of Sound and Vibration Research, Southampton University is researching a project that not only involves sensing but also actuation, using single loop sensor and piezoelectric actuator pairings to counteract vibration. When vibration is detected, the actuator produces an equal counteracting vibrational force. Vibration cancellation is needed in aircraft fuselages and internal panelling to reduce engine noise.

At Sheffield University researchers led by Jem Rongong , deputy director of the Rolls-Royce university technology centre, are also looking into active vibration control. Passive control, in which visco-elastic materials of specific shapes dissipate energy, is now used in the industry.

Although the active damping system being developed at Sheffield can be up to 10 times as effective as its passive counterparts, so far it is only useful for smaller-scale devices and its complexity makes it less reliable. However, Rongong thinks it will become a priority research area in around five years’ time.

Another actuator-based project is research into the use of piezoelectric and phase-change ceramics in shape control of polymer composite aerospace components and structures. This can be used to improve aerodynamic performance in several ways. Wafer-thin electro-active actuators can be bonded to the surface of or embedded within composites to cause bending and flexing.

These components provide an alternative to hydraulics and have several advantages: hydraulics are expensive in time and money, are comparatively complex, require frequent maintenance and contain a fluid that can leak. The technology also has implications for a specific area of military aerospace: the stealth properties of unmanned aerial vehicles. Traditional hinged and hydraulic control systems can increase the aircraft’s radar profile; piezoelectrics can reduce it. The same shape of technology could be used in other areas, such as in ships or even cars, raising the possibility of cars automatically altering their body shape to be more aerodynamic when travelling at high speeds.

Although progress in both sensor and actuator technologies is impressive, it will be some time before the two become integrated to produce truly smart systems. Alan Hooper, technical manger of structural and materials centre at Qinetiq, says: ‘The ability of a structure to sense its own environment and monitor its own health will have a significant impact on critical issues such as safety and cost of ownership.

‘The integration of active elements, to produce truly smart structures, is the ultimate goal, but the timescales for this will be strongly application dependent. However, I think we can expect to see such structures becoming a reality over the next decade.’