Piezoelectric Materials used in Aircraft SHM
Aircraft Structural Monitoring
During the service life of an aircraft, regular monitoring is required to detect and assess structural defects such as cracks, corrosions and misalignments within the materials and components of the aircraft. These assessments are required to be carried out at regular intervals to ensure the safety and airworthiness of the aircraft, and since these are carried out during its service life, the testing methods must be non-destructive techniques (NDT).
Some tests check for surface defects and include methods such as visual inspection, liquid penetration and tap testing, whereas other more modern high-tech tests check for sub-surface defects. Examples of sub-surface testing methods include radiography, ultrasonic and eddy-current testing.
Whilst the technology is effective, NDT techniques are often labour intensive and time consuming, and also require that the aircraft is taken out of service at scheduled time or flight hour intervals for testing. The major drawback of these techniques is therefore in terms of the time taken to carry out the tests, for which there is a major financial cost to the airline in lost service hours. Since the tests are carried out at scheduled intervals, there is also the likelihood of a time lag between structural defects occurring and being detected. This can lead to the reduction of safety and airworthiness of an aircraft in service with as yet undetected structural defects.
More modern structural monitoring techniques currently being developed are called Structural Health Monitoring (SHM) systems. It is proposed that these systems use sensors, installed within the aircraft permanently, to monitor the structural health of the aircraft throughout its entire service life. The sensors will therefore be required to be microscopic, and their technology in the realms of nanotechnology. They would be capable of relaying information back to a processor to enable structural health information to be available continuously.
The main benefit to real-time monitoring of aircraft structural health is increased safety. Damage can be detected as it occurs, long before it may lead to failure. As an added benefit, SHM systems do not require the aircraft to be taken out of service for inspection, thus saving financial losses otherwise incurred with traditional NDI inspection methods.
SHM systems can provide active or passive monitoring and can be used with both metallic and composite airframes. Those already in development use a variety of technologies, some of which include eddy current foil sensors, piezoelectric materials, optical fibres, nanomaterials and air/vacuum galleries. The benefit to using piezoelectric materials is that sensors can be used as actuators and vice versa due to the piezoelectric effect. Therefore sensors based on piezoelectric materials are of particular interest for aircraft SHM systems.
The crystalline structure of piezoelectric materials gives them the ability to mechanically deform by up to 4% of their original size when an electrical current is applied to them, and with the effect being vice versa, an applied mechanical stress will induce electrical current within the material. Certain biological matter such as bone, DNA, proteins and viruses are piezoelectric materials, as well as some ceramics and crystals.
The effect is enabled in certain materials by an unsymmetrical unit cell which exists in an electrically neutral state under normal unstressed conditions. When a stress is applied, the rearrangement of the crystal structure allows an electrical current to flow through. Conversely, when the material is mechanically unstressed and an electrical current is passed through, the unsymmetrical nature of the crystalline structure allows the atoms to rearrange themselves to enable the current to flow, hence the change in shape.
Piezoelectricity in materials was first discovered by Jacques and Pierre Currie in the 1880s. Despite the ability of the materials to generate electricity, there has been little useful application for them in powering electrical devices due to the relatively small quantity of electricity produced. They have been used in small gadgets such as lighters, but very large quantities of the materials would be required to power modern small to medium electrical devices around the home. Further obstacles to widespread usage are that once the mechanical stress is released the electrical current stops, and that many of the substances required to produce piezoelectric materials are toxic.
Piezoelectric SHM technology
A typical SHM system consists of a network of sensors attached to the surface of, or embedded within a structure. The quantity of sensors the system uses, along with their type and location, define the sensitivity of the system. The sensors can detect damage, load and temperature conditions that the structure is subject to and communicate with diagnostics hardware and analysis software to convert the sensor signals into useful data. The systems may be used for either active sensing, in that they are constantly generating data to detect possible damage, or passive sensing, in that they produce data as a result of damage occurring. Due to the piezoelectric effect, sensors made from piezoelectric materials can act as actuators, i.e. transmitting a signal through a structure, or sensors to receive the resulting signals after passing through a structure.
Two piezoelectric SHM technologies that are capable of providing active and passive damage detection use wave propagation and electromechanical impedance (EMI) as methods of damage detection.
Wave Propagation SHM
Wave propagation SHM systems can be used with both metallic and composite structures. They consist of a large network of piezoelectric actuators which constantly query the structure to actively seek damage to the structure, as well as information on environmental conditions. To be effective, they must therefore query as large an area of the structure as possible. The actuators transmit guided Rayleigh or Lamb waves through the structure before being sensed by piezoelectric sensors, which is known as a pitch/catch system. Alternatively a 'pitch-echo' system uses the same transducers as both actuators and sensors. They have the ability to generate a specific waveform and collect sensor data at a high sampling rate and resolution. The advantage of piezos for wave propagation is that they can monitor larger areas of the structure using fewer sensors due to their dual use capability, as well as the large areas over which they can emit signals.
Electromechanical Impedance (EMI) SHM
The EMI system exploits the ability of piezoelectric transducers to transmit electrical signal, specifically by using the electrical impedance of the transducers to indicate damage to a structure. As damage occurs to a structure, the stiffness of the structure is altered, which in turn alters its resonant characteristic. Damage can be identified when an actuator-sensor couple picks up on the change in electrical impedance. The sensor data from a damaged structure can be compared to that of the structure before damage occurred, and analysis of the data can provide information as to the location and extent of damage.
Combined Active and Passive Monitoring
There are drawbacks to purely active or passive, systems such as the inability of passive systems to detect damage not caused by impact, and the energy wasted by active systems in querying a whole structure to detect impact damage in a small localised area. Therefore an SHM which uses a combination of the two systems is ideal. Used in combination, a system can switch to active mode upon detection of impact damage occurring to ensure that only a small relevant area is actively queried.
Requirements for Practical SHM Implementation and Challenges
The practicalities of integrating SHM systems into real life aircraft structures are somewhat more complicated than developing the technologies in a test environment. The overall objectives of the systems are to monitor structural state such as strain, temperature and impact energy, as well as to monitor structural damage such as cracks, delamination and corrosion. Aircraft structures are complicated, and different sensing requirements are required for different zones. For example, impact damage detection is required over large areas, crack detection in hotspot areas and strain distribution monitoring is required in some critical areas. For this reason it is often necessary to integrate several different types of sensor into the system. Airworthiness compliance regulations such as safety, installation and weight requirements must also be adhered to.
Despite the obvious advantages, SHM is not currently in use in commercial aircraft due to the challenges in practical implementation. The main challenges surround implementing a senor network that is large and complex enough to be effective, as well as the ability to detect damage to, or detachment of a senor or associated wiring from the host structure (self-diagnostics). Other main challenges are concerned with the environment; the sensor must be sufficiently robust to withstand high strain conditions and also have the ability to perform consistently in varying temperatures. Resolution of the system is also a concern, particularly the ability to provide probability of detection (POD), as well as providing accurate quantitative results in terms of the size and location of damage detected.
The Future of Piezoelectric SHM Technology
As sensor and network capabilities become more sophisticated, demand increases for larger and more complex networks with the ability to measure a wider variety of parameters. SMART layer technology, developed at Stanford University, offers a solution which comprises a tiny dielectric film with a series of layers; one incorporating an embedded network of piezoelectric transducers, with further layers incorporating the circuit and insulation. Different types of sensor may be integrated into in a single layer, and the benefit to the multilayer design is that all parts of the system are incorporated into a single film for attachment to the host structure as one. The benefits to an increasingly intelligent system with advanced damage tolerance, reduced maintenance costs and increased safety will continue to drive investment in this area.
In terms of the future of integrating sensors into aircraft structures, as a step further to SMART Layer technology, developments in nanotechnology and materials science have enabled the full integration of sensing technology within composite materials at the manufacture stage, as opposed to the current arrangement of mounting sensors onto the structure subsequent to independent manufacture. This way the sensors become entirely homogenised with the aircraft structural material. Materials such as carbon nanotubes have been incorporated into composite materials to function as sensors, as well as constitute part of the structural material.
As composite materials become more common and complex, SHM has the potential to be utilised not only in aircraft maintenance, but during the whole lifecycle of aircraft materials, including design, development and manufacture. This is particularly useful for composite materials due to their complexity in comparison to traditionally used metals. Non-linear monitoring such as the use of non-linear guided waves is replacing previously employed linear methods, the main benefits to this being the increased amount of data produced which greater enables the identification of micro-damage and its characterisation. Diagnosis results are therefore developing from qualitative to quantitative, and the advanced data has the potential to aid the design process of composites.
Sensors such as strain gauges have long been used to monitor the environmental conditions in which aircraft operate such as temperature and strain. The benefit to SHM is the ability to provide a more diverse set of data, including the presence of damage and quantifying its extent. There is a long way to go in the development of SHM systems due to the complexity of both sensor networks and the structures they are monitoring. There are some promising ideas in terms of sensor integration such as SMART layer technology, as well as developments in nanotechnology and materials science which have enabled certain materials to have dual use as aircraft structural composites as well as performing as sensors. There will be much investment required to bring SHM to the level of sophistication required to reach their full multidimensional potential. The payoff will be in terms of increased damage tolerance, which will in turn dramatically reduce aircraft operational and maintenance costs, as well as to improve safety and reliability.