• Raddatz, Florian
• Sinapius, Michael
• Wierach, Peter

Abstract

The current maintenance and inspection strategy for aircraft structures is based on strictly scheduled inspection intervals considering the age and usage of the aircraft. This implies that the structures must be designed in a damage tolerant way, allowingasafeoperationwithundiscovereddamagesuntilthenextmajor inspection.Inaddition,newaircraftaredesignedwithevenlongerinspection intervals to reduce downtime and maintenance costs. For fiber-reinforced composite structures this approach is contradictory to the idea of reducing theweight of the aircraft.Especiallybluntlow-velocityimpactsmaycauselargedamageslike delaminations that are likely to be missed during visual in-service inspection. It is therefore desirable to have an integrated structural health monitoring (SHM) system thatwillmonitorthestructurefordamagesandallowearlydamagedetection between inspection intervals. This does not only pave the way to a more demand-drivenmaintenance,butmaycontributetofurtherexploitingthelight-weight potential of composite materials. In contrast to other monitoring methods the use of Lambwavesallowsdeterminingthelocationofadamage.Thishasbeenshown many times for simple structures like plates or pipelines. However, these methods areoftennotapplicabletocomplexstructuresthatfeaturecurvatures,anisotropic materialpropertiesandchangingmaterialpropertiesthroughoutthestructure. Therefore a damage reconstruction method is proposed, that is similar to methods known from conventional ultrasonic inspection, but takes into account the complex material properties and their influence on the wave propagation. The base for this approach is a time-of-flight calculation that can be applied to structures with many local changes in material properties and allows each material to be anisotropic. This iscombinedwithapulsecompressiontoincreasethetemporalresolutionofthe signalandthereforethespatialresolutionofthereconstruction.Theproposed method can be applied to integrated monitoring systems with a limited number of fixed transducers, high-resolution scans of the wave field obtained with air-coupled ultrasoundsystemsorlaser-vibrometersandcanalsobeadaptedforacoustic emission monitoring.

Topics

• impedance spectroscopy
• anisotropic
• ultrasonic
• fiber-reinforced composite