Many military platforms such as fighter aircraft are nowadays operated for several decades under sometimes varying missions. Additional requirements resulting from more severe fatigue spectra or extended life for these platforms may require additional means of ensuring structural integrity. It is then important to gain the maximum usage (fatigue life) of aircraft components most efficiently still ensuring structural integrity at all times. Conventional structural health monitoring systems are typically based on loads and usage monitoring. Together with modern non destructive damage detection techniques it could be possible to safely operate even aged platforms. This goal is achieved by periodic examinations in order to ensure that a structural item is free of damage. However, the dismantling of structures for the purpose of non destructive testing can be very costly, time intensive and sometimes harmful to the surrounding structure itself. Therefore integrated, reliable and affordable damage detection techniques are needed to avoid disassembly where economically or technically justified. Especially for well known hot spots an integrated damage sensor could provide an alternative solution to conventional procedures. SWISS (Smart Wide area Imaging Sensor System) is an ultrasonic imaging approach. A small sensor is permanently surface mounted on the component that is to be monitored. Typically the sensor is activated on ground and interrogated via cables that are built into the platform. These sensors facilitate the examination of the internal structure of a subcomponent. The ultrasonic beam is electronically controlled in order to scan the most critical areas from a fixed position. Functionality aspects as well as practicability issues of such a technology had to be addressed and solved. As a result of this study, simulated fatigue tests on a real complex fitting structure have proven the reliability of the imaging ultrasonic sensor under laboratory conditions for more than 60000 simulated flight hours without problems and that the high volume coverage proved to be beneficial for detecting even unexpected damages.
For some structural components conventional methods of loads and usage monitoring, supplemented by on ground NDT measurements to detect damages can be unsatisfying due to various reasons: low reliability of sensing technique or low accessibility and sometimes high cost of dismantling for regular NDT. Sensor methodologies having evolved form conventional ultrasonic inspection in principle have the potential to detect damages in various types of structures: 2D, 3D, metal, composite - provided significant interaction of ultrasonic waves with local damage is given. SWISS (Smart Wide-area Imaging Sensor System) requires a physical interaction model of ultrasonic excitation with structure and its potential damage and merges sensor data to determine spatial distribution of impedance by imaging which performs localization and sizing of damage in one. Second: use of signal and image processing techniques to alert when damage becomes critical. SWISS has been designed by EADS to image various types of damage from considerate distance by means of permanently installed piezo-elements. SWISS features a sensor minimization to achieve low weight and still high reliability and the use of cheap electronics but clever testing and analyzing to achieve low cost for in-service application. SIEMENS NDT has been able to demonstrate SWISS functionality on complex components by appropriate use of Phased Arrays.
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