It is extremely important that the level of fatigue, imposed on an aircraft structure and associated components can be monitored and recorded so that the respective fatigue lives are not exceeded. Several methods have been developed to assist in the vital tasks involved with SHM
Fatigue meters are used to check overall stress levels on aircraft and to monitor the fatigue history of the aircraft. Fatigue meters also allow a check to be made on the moment in time when the aircraft exceeds the design limits imposed on it.
Strain gauges may be used to monitor stress levels on specific aircraft structures. Strain gauges are thin-foil electrical resistor elements that have been bonded to the aircraft structure during manufacture. Their resistance varies proportional to the applied stress loading.
Fatigue fuses are metallic fuses, which are bonded to the structure and which fail at different fatigue stresses. The electrical current flowing through the fuse, will vary and so provide an indication of the stress level.
Modern developments in aircraft structures will allow the structures to be designed and built with a variety of sensors and systems embedded into the structure and skin. This would mainly be restricted to structures manufactured from composite materials. One major benefit of this is to allow the structure to monitor it's own loads and fatigue life.