The structures of means of transport, using the specific example of aircraft, are submitted to mechanical, environmental and/or vibratory stresses when in operation. These structures are inspected on a particularly regular basis so as to assess any emerging cracks: a short inspection is performed at each stop, and another, more in depth inspection is performed during maintenance operations referred to as C-check operations after 2,400 hours of flight. The latter inspections are long and costly as they require the removal of access panels to inspect the state of the metallic structures of the aircraft, which are inaccessible in normal time. Performed periodically, these inspections do not therefore enable cracks and their development to be continuously monitored. The same applies, in the railway sector, for some train parts, in particular for high-speed trains.
Systems for monitoring structures are developed in order to optimise and reduce these maintenance operations and therefore their costs and duration. For example, eddy current sensors are used to detect cracks appearing on metallic structures as illustrated in documents U.S. Pat. No. 6,952,095, GB-A-2 400 445 and GB-A-2 396 427.
In order to interpret the information transmitted by these sensors, several electronic interfaces have been produced. One of these interfaces interprets the impedance, more particularly the phase transmitted by the sensor. This interface is used to form a two-dimensional image of the surface to be inspected, as described in document U.S. Pat. No. 5,006,800. The disadvantages of this method include its bulk and high electricity consumption. Therefore, this method cannot be placed onboard, and the results from this method are not provided in real time.
Other interfaces interpret the conductivity transmitted by a sensor and are also used during maintenance operations. Crack monitoring cannot therefore be performed in a continuous manner.
Other systems, using acoustic sensors, have also been improved for studying, in a continuous manner, the state of the structures, as described in patent WO 2006 111679. These methods often consume large quantities of energy. The equipment must therefore be connected to the electrical network onboard the aircraft. Moreover, this equipment is very bulky.
Finally, onboard systems are known to list the number of stresses to which the aircraft is subjected, as described in document EP-A-1 018 641. These stresses: take-offs, landings, turbulences, pressures, etc. are listed by a system of logic gates and then recorded in a memory. Readings are then taken during maintenance operations, and thus help to guide operators during their inspections. However, these aforementioned methods do not provide direct information regarding the state of the structure.