Stand 26
Comparative vacuum monitoring – a new way to monitor structural integrity
Multi-site fatigue damage and hidden cracks in remote locations are among the major flaws encountered by aircraft fleet managers. Often, maintenance procedures require engineers to gain access to normally inaccessible or hazardous areas to carry out essential NDT inspections. These inspections invariably require components or even major assemblies to be removed, sealant to be disturbed, and fuel tanks to be vented to a safe condition, to provide the required level of access to carry out the inspection reliably and safely.Not only is the inspection regime time-consuming but there is a high risk that the intrusive nature of the inspection may induce damage to the structure at a rate greater than the underlying defect. Also, in an effort to minimise unwelcome 'downtime', the frequency of inspections may not be pitched at the ideal periodicity, and certainly not optimised for the whole cycle of a fault, from defect-free to initiation of a crack and during propagation to limiting length of a crack, before a repair is carried out.
CVM™ sensors, coupled with remote interrogation, can be employed to overcome a myriad of inspection difficulties. The prevention of unexpected fault propagation can be improved, if CVM on-board monitoring is used to monitor the structural integrity more frequently.
These small, unobtrusive and inert sensors incorporate tiny rows of interconnected channels, to which a modest vacuum is applied. When the sensors are adhered to the structure under test, the fine channels and the structure itself form a manifold of galleries alternating at low vacuum and atmospheric pressure. Any propagating crack under the sensor breaches the galleries and the resulting change in pressure is monitored.These sensors can be attached to aircraft structure in areas where crack growth is known to occur. Each time a reading is taken, the system performs a self-test. This inherent fail positive, some say fail-safe, characteristic ensures that the sensor is attached to the structure and working properly. There are no EMI/EMC considerations and no potential hazards in fuel tanks.
The PM200 periodic monitor allows sensors to be monitored from a Test Point (TP) installed at a convenient inspection point. The measurements allow active noise dampening to remove many of the changes that can occur in a pneumatic system due to thermal and permeability changes of the air inside the tubes and of the tubes themselves, and will measure the conductivity (the inverse of impedance) of the sensor system in units of Pascals per second per cubic metre (Pas-1m-3).
The TP is fitted to a convenient location on the aircraft. It has a passive microchip that is only activated when the instrumentation is connected to it. The microchip contains a serial number, and a number of data including the aircraft identification number, the sensor location and a number of baseline values required for the measurement.
Sensors are designed to fit the application and are manufactured from polymers such as polyimide and FEP, to best suit the different environmental and geometrical requirements.
Boeing was the first aircraft manufacturer to accept the Structural Health Monitoring techniques of CVM. The technology is now available to be used as a qualified means for performing in-situ crack detection inspections on Boeing aircraft to address future Service Bulletins and as an Alternative Means of Compliance for existing inspections.
Similarly, Bombardier and Embraer are also conducting validation programmes and Southwest Airlines, Delta Air Lines, and Northwest Airlines are working with Boeing, the FAA, and a Sandia Labs team to approve the use of CVM sensors in a wide range of specific applications on their aircraft. Airbus has for several years been exploiting CVM sensors on its own full-scale fatigue tests (FSFTs) on various structures. The A380 fuselage has some 500 sensors in its FSFT programme.
Structural Monitoring Systems
Stand 26
www.smsystems.com.au
