Introduction to Structural Health Monitoring
1.1. Definition of Structural Health Monitoring Structural Health Monitoring (SHM) aims to give, at every moment during the life of a structure, a diagnosis of the “state” of the constituent materials, of the different parts, and of the full assembly of these parts constituting the structure as a whole. The state of the structure must remain in the domain specified in the design, although this can be altered by normal aging due to usage, by the action of the environment, and by accidental events. Thanks to the time-dimension of monitoring, which makes it possible to consider the full history database of the structure, and with the help of Usage Monitoring, it can also provide a prognosis (evolution of damage, residual life, etc.). If we consider only the first function, the diagnosis, we could estimate that Structural Health Monitoring is a new and improved way to make a NonDestructive Evaluation. This is partially true, but SHM is much more. It involves the integration of sensors, possibly smart materials, data transmission, computational power, and processing ability inside the structures. It makes it possible to reconsider the design of the structure and the full management of the structure itself and of the structure considered as a part of wider systems. This is schematically presented in Figure 1.1.
Chapter written by Daniel BALAGEAS.
Structural Health Monitoring
Monitored physical phenomenon, depending on the damage
Integrity Monitoring system, defined by a sensed physical phenomenon and an adapted data reduction Damage Sensors Sensors multiplexing fusion and networking
Diagnosis Structural Health Monitoring
Usage Monitoring Prognosis
Monitored structure Monitoring of usage conditions Data cumulative recording Damage and behavior laws
Health and Usage Monitoring Maintenance organization Health Management of the structure Health Management of the full system (fleet, plant...)
Figure 1.1. Principle and organization of a SHM system
In Figure 1.1, the organization of a typical SHM system is given in detail. The first part of the system, which corresponds to the structural integrity monitoring function, can be defined by: i) the type of physical phenomenon, closely related to the damage, which is monitored by the sensor, ii) the type of physical phenomenon that is used by the sensor to produce a signal (generally electric) sent to the acquisition and storage sub-system. Several sensors of the same type, constituting a network, can be multiplexed and their data merged with those from other types of sensors. Possibly, other sensors, monitoring the environmental conditions, make it possible to perform the usage monitoring function. The signal delivered by the integrity monitoring sub-system, in parallel with the previously registered data, is used by the controller to create a diagnostic. Mixing the information of the integrity monitoring sub-system with that of the usage monitoring sub-system and with the knowledge based on damage mechanics and behavior laws makes it possible to determine the prognosis (residual life) and the health management of the structure (organization of maintenance, repair operations, etc.). Finally, similar structure management systems related to other structures which constitute a type of super system (a fleet of aircraft, a group of power stations, etc.) make possible the health management of the super system. Of course, workable systems can be set up even if they are not as comprehensive as described here.
Introduction to SHM
1.2. Motivation for Structural Health Monitoring Knowing the integrity of in-service structures on a continuous real-time basis is a very important objective for manufacturers, end-users and maintenance teams. In effect, SHM: – allows an optimal use of the structure, a minimized downtime, and the avoidance of catastrophic failures, – gives the constructor an...