Structural Health Monitoring
All man-made structures and machines have finite lifespans and begin to degrade as soon as they are put into service. Processes such as corrosion, fatigue, erosion, wear and overloads degrade them until they are no longer fit for their intended use. Depending on the value of a structure, the cost of repairing it and the consequences of it failing, a number of actions can be taken

1) Wait until it breaks and throw it away. (Low sticker price relative to repair cost, low criticality)

2) Wait until it breaks and repair it. (High sticker price relative to repair cost, low criticality)

3) Examine it at intervals and decide whether or not remedial action is needed. (High criticality).
This is Structural Health Monitoring (SHM)

Many engineering structures; ships, bridges, aircraft, buildings, fall in to the latter category. The consequences of a critical aircraft component failing in flight or of a bridge collapsing are such that regular inspections are performed by skilled engineers to assess the health of structures and systems. This inspection process is necessary, costly and usually finds no faults. It is also subject to human error, meaning that some unnecessary maintenance is performed and some faults go undetected. Examples of on-line machine health monitoring exist for rotating machinery (e.g., the FAA has endorsed the effectiveness of helicopter gearbox monitoring systems), and for machine tools (to assess and compensate for tool wear) but there are no equivalents for evaluating the health of most engineering structures.

A refinement of this approach could involve fitting the structure of interest with its own sensing and analysis systems to enable monitoring and evaluation to be carried out continuously and autonomously. Certain parameters are measured, giving the location and severity of damage in the structure as it occurs. Real-time structural health monitoring offers increased safety, since faults cannot grow to a dangerous level between inspection intervals and it is not subject to the vagaries of human behaviour, and lower ownership costs, by replacing pre-planned precautionary servicing with targeted, responsive maintenance. Because the potential benefits of this embodiment of SHM are so great, a great amount of research is in progress worldwide into developing and improving systems that bring some rudimentary degree of 'self-awareness' to man-made structures.

A SHM system consists of sensing and processing elements. A network of sensors measures relevant parameters, which could be loads on the structure and records of the corresponding strains, data from ultrasonic transducers as used currently for NDE, vibration, corrosion, temperature, acoustic emission and so on. Signal processing and analysis routines relate sensor data to the health of the structure and present this to the owners and maintainers. This could simply be in the form of global on/off sensor data (There is corrosion), or localised information (There is corrosion at location X) which could be further quantified (There is Y amount of corrosion) and its implications presented (This means Z for the health of your structure). It may be possible in the future to remove the man further from the loop by having the system make recommendations as to what remedial action to take.

The requirements placed on on-line SHM are strict if they are to replace current inspection methods performed by skilled labour. Considering the demands placed on the sensors, they must be sufficiently sensitive to detect indicators of damage. They must be stable, durable and reliable for the lifetime of the structure they are monitoring. Reliability is crucial. If automatic systems are to replace human inspections we must have absolute confidence in them. They should not produce too many false positives (cry 'wolf') and certainly should not miss damage features they are supposed to detect. Many initial installations of integrated SHM will have to prove their worth alongside conventional inspection techniques. They must also be cost-effective. That is, the cost of installing the system must be less than an equivalent inspection regime (unless the safety or performance benefits outweigh the cost) and the sticker price should not deter potential users. Conventional measurement systems simply cannot satisfy all these requirements simultaneously.

Considering the sensitive elements of our SHM system, strain sensors of one form or another are at the heart of many proposed schemes. They can be used for continuous load monitoring, allowing the structure's owner to build up an accurate picture of its usage. This information can be used to verify design assumptions about loading patterns or combined with fatigue models to predict residual life. An immediate application could be to indicate any exceptional load cases (buildings in earthquakes, airliners that have suffered hard landings) which may have caused damage, suggesting that the structure requires further detailed inspection.

Furthermore, strain gauge elements are used in pressure, load, vibration and acceleration transducers.

Electrical foil strain gauges are not suited to use over many years because they are prone to failure by disbonding, creep or fatigue and the measurement electronics drift with time. Certain types of gauge, such as the vibrating wire devices employed by civil engineers are free from drift but are larger and much more expensive. Even if the sensors themselves are cheap, the cost of labour and cabling of installing a significant number of gauges on a structure as large as a bridge or dam, or the weight of cabling in an aircraft or spacecraft, often rules out their use.

However, fiber-optic sensors have none of these disadvantages and are ideally suited to use in long-term continuous SHM. Although practical fiber-optic sensors are relatively new, it is beginning to be realised that they are going to be crucial for some SHM applications. Research institutions and companies, Smart Fibres among them, are building up an extensive portfolio of applications and experience. This will enable forward-looking owners, users and maintainers of all manner of structures to benefit early from this exciting new technology.