Piezoelectric Sensors and Self Monitoring Planes
Key text
This topic is sponsored by the Defence Science and Technology Organisation.
Metal fatigue has been the cause of many airline, ship and bridge disasters. New piezoelectric sensors are now helping to detect, report and even fix such problems as they occur.
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In 1954 the world’s first passenger jet-plane, the de Havilland Comet, suffered a mid-air disaster over the Atlantic. The plane broke up in flight with total loss of life and aircraft. Some months later a second Comet suffered the same fate. Beyond the human cost, the tragedies set the development of jet airliners back by several years, though the Comet itself, once redesigned, went on to serve successfully for decades.
The cause of the disaster was later diagnosed as metal fatigue, the build-up of stresses and strains in the aircraft hull in the form of minute cracks. These grow and coalesce until the accumulated damage causes calamitous failure in some part of the structure. The Comets were the first airliners to have pressurised cabins, so they could fly much higher than older aircraft. The two Comets that crashed were torn apart by the greater internal air pressure once a small segment of the hull suddenly gave way, weakened by the repeated raising and lowering of the cabin pressure in hundreds of successive flights.Other aircraft have come to grief in the same way, as have many other metal structures subjected to repetitive back and forward movements and stresses, such as drill rigs, bridges and the hulls of ships and submarines.
(Image: Defence Science and Technology Organisation.)
Various forms of non-destructive testing and monitoring can identify the early signs of metal fatigue. However, current methods are labourious, time consuming and costly – often it costs more to keep a plane in the air than it cost to buy in the first place. Australian researchers, including those working within the Defence Science and Technology Organisation (DSTO) are leading the world in the search for ‘self-monitoring’ planes. These aircraft can check their own ‘vital signs’ and give warning when trouble is looming (known technically as 'structural health monitoring'). The idea is to provide planes with a network of permanent in-built sensors that act something like the network of nerves in our own nervous system. If a problem is detected, it can be conveyed as a signal to the ‘brain’, letting us know that something may be amiss.
To achieve this goal, DSTO researchers are exploiting a phenomenon that has been known about for more than a century and is now widely used, even though in its early days scientists were unsure as to what possible applications it might have.
The secret is ‘piezoelectricity’
Certain man-made and natural materials have the amazing ability to convert a mechanical stress, vibration or change of shape into a tiny electric current and vice versa. This phenomenon is known as piezoelectricity. The science of piezoelectricity is complicated, and as yet it is not entirely understood.
However, gadgets based on this science are widely used every day. Push-button barbecue lighters, digital quartz watches, phonographic needles, pressure sensors in touch pads, Wii balance boards, industrial ink-jet printers and musical greeting cards all make use of piezoelectricity. For example, when the trigger of a barbecue lighter is squeezed, a spring-loaded hammer hits a piezoelectric crystal, creating an electrical voltage which jumps between two wires in the form of a spark and sets fire to the gas. In a musical greeting card a current is applied to the electrodes of a tiny inbuilt diaphragm causing it to vibrate back and forth, generating sound. (To learn more about piezoelectricity and other piezoelectric devices see Box 1 and Box 2.)
Image: Defence Science and Technology Organisation.
So how does piezoelectricity help to prevent planes from crashing?
One of main challenges for aircraft operators is to find and deal with metal fatigue before it becomes catastrophic. Australian defence scientists are using piezoelectricity to help detect tiny metal fatigue cracks as they occur. They have developed piezoelectric sensors that can be laid across surfaces or structures that are prone to cracking as a result of metal fatigue. If a crack opens even to a very small extent, various parts of the sensor move relative to each other and an electric current is generated to signal the event. By placing such sensors in those parts of the aircraft where stresses tend to accumulate, such as near sharp corners, where holes have been made for windows or antennas, or where different sorts of materials have been bonded together, they can continuously monitor the likely trouble spots.
With such technology it is possible to know in advance where metal fatigue is accumulating so that appropriate action can be taken. This is particularly applicable to equipment such as military aircraft that are put through many stressful manoeuvres in training and combat and are therefore more likely to become fatigued.
The ‘Smart Patch’
To extend the experimental life of expensive military aircraft, maintenance crews are able to cover parts of the aircraft skin or inner structures where fatigue is beginning to show with synthetic materials called composites bonded to the underlying metal. But such patches need constant monitoring to ensure the aircraft remains certified and safe to operate. With the addition of piezoelectric sensors and associated electronics the patch of composite becomes a kind of smart sensor, or ‘Smart Patch’. In early trials by Australian defence scientists Smart Patches successfully detected and recorded the moment when the adhesive bond between a piece of composite material and a piece of underlying aluminium failed. The bond had ‘fatigued’ by being flexed several times a second over a long period of time.
The technology has come a long way since then. In addition to the sensors, the total package now contains electronic memory to store the information about the ‘health’ of the patch and a communication device, such as a radio transmitter or a generator of ultra-high-frequency sound waves, to transmit the data on command through the skin of the aircraft to a computer outside the plane for analysis. A prototype Smart Patch has been flown on a F/A-18 aircraft and found to successfully monitor the health of the repair. Whilst the Smart Patch was eventually not retrofitted because the damaged aluminium hinges were replaced with titanium ones instead, developing the Smart Patch provided many valuable lessons for DSTO scientists and engineers. This knowledge is now assisting DSTO to develop the next generation of structural health monitoring systems.
So how are the electronics and the memory chips in the patches provided with power? Connecting up to the plane’s usual power supply can be difficult and costly, so scientists have come up with the ‘vibration energy harvester’. This is a piezoelectric device that converts the vibrations that the plane experiences into electrical power. A tiny mass within the device vibrates many times a second as the plane flies and a couple of attached piezoelectric sensors collect the current. The harvester is able to produce electrical power from the structural vibrations caused by acceleration of an aircraft, such as acceleration during take off and acceleration whilst in flight. The harvester then stores the electrical energy in a rechargeable battery, which can then be used to power the structural health monitoring sensors and electronics. (For more information about energy harvesting, see Box 2).
Using piezoelectricity to counteract fatigue
Most piezoelectric crystals can also convert electrical current to mechanical vibrations, known as the ‘reverse piezoelectric effect’. This is the basis of an exciting branch of technology known as ‘Buffet Load Alleviation’, which provides a way of stopping some forms of metal fatigue from building up. For example, F/A-18s aircraft have a pair of tall tailplanes which are subject to large mechanical stresses when the plane is manoeuvring, especially at high angles of attack. One solution is to fit a number of piezoelectric devices designed to counteract the stress. They first detect the stress using currents that are generated by the piezoelectric sensor and then, by switching to ‘actuator’ mode, they push back against the movement causing stress and so dampen it. Reducing the mechanical stress in an aircraft’s tailplane means less potential for metal fatigue, and hence the formation of fewer cracks.
Composites to the vertical tail of an F/A-18 test specimen for use in a Buffet Load Alleviation test.
This work is the result of an international collaboration between the US Air Force Research Laboratory,
the National Research Council Canada, NASA Langley Research Center and DSTO.
Image: Defence Science and Technology Organisation.
The need to know more
Defence science researchers are also working at a more fundamental level, trying to better understand just how these tiny fatigue cracks form, grow and spread. They have assembled some of the world's most sophisticated scientific instruments to undertake this task, such as an atomic force microscope which can measure structures little larger than individual atoms. It is vital to understand as much as possible about the process of metal fatigue, so that the operators of the aircraft can more accurately determine a plane's operational life.
Research over recent years has greatly increased our understanding. For example we know that fatigue cracks begin from very small defects a few hundredths of a millimetre in size, and remain very small (often less than 1 mm in length) for most of their life. Yet there is still much that remains unknown and as a result the lifespan of military aircraft tends to be set conservatively. For example the F/A 18s are due to be retired from active service from 2018 and replaced with a new generation of fighter aircraft. Will the current fleet continue in service until then? If not how many replacements will be needed?
Research by Australian defence scientists is helping to answer these questions. Recent research has now indicated that the life of the F/A 18 centre-barrel (the part of the airframe that supports the wings and landing gear) is approximately 10% greater than was originally certified. This equates to an additional two years of flying and provides the RAAF Hornet fleet with sufficient fatigue life to safely reach their planned withdrawal date in 2018, saving the Defence budget more than $400 million.
As a result of extensive research by defence scientists into piezoelectric sensor technology, air force planes are now able to fly longer and more cost effectively. We all benefit from that. And we will benefit further when structural health monitoring technology is applied to other metal structures as bridges and civilian aircraft, reducing the possibility that they will fail catastrophically because of metal fatigue. It is not a bad outcome, given that piezoelectricity was once nothing more than a laboratory curiosity.
Boxes
1. More about Piezoelectricity
2. Other piezoelectric devices
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Posted July 2011, edited August 2012