Monitoring of structures and systems of aircraft by highly non-linear sensing devices

Sensing  systems,  especially  when  used  for  interrogating  the  structural  integrity of aircraft in traditional inspections, are frequently working in a quasi-linear  mode.  This  approach  however  creates  difficulties  in  some  cases,  especially  when  applications for structural health monitoring (SHM) are concerned. One of the major  problems are base-line deviations interfering with damage-related signals that are in  some cases moreover complicated due to interferences at complex aircraft structures  creating  major  obstacles  for  a  broad-scale  implementation  of  SHM  in  aircraft.  Advanced data processing and dedicated high-end hardware components are certainly  capable  of  tackling  part  of  these  problems,  but  additional  hardware  requirements  require extra power supply, together with advanced data transmission and processing  facilities, also additional maintenance needs emerge.     In some cases, an interesting alternative is offered by highly non-linear sensing  devices. They produce a sharp sensor response that ideally only depends on a certain  damage-related outer parameter representing in this way a material-based threshold.  Moreover, this sharp sensor response ideally ranges over many orders of magnitude  above the baseline variations and in this manner, an ideal tool would be established  to finally enormously increase the probability of detection, at least within the range  of damage to detect.    After a short review on examples taken from the literature, that are in some cases  successfully applied in operations, new developments are presented, such as water  leakage sensors based on the percolation effect, dedicated optical sensors for reliably  detecting damage in hydraulic pipes as well as defects in bleed air ducts. Most of them  are able to cover bigger surfaces and are, when appropriate, also partially equipped  with  dedicated  facilities  for  reliable  damage  localisation.  Finally,  a  couple  of  examples are given that were already implemented in operational airliners, such as  devices for the detection of corrosive liquids in aircraft (Boeing 737-500, Boeing 747- 400).