IMPACT DETECTION IN A COMPOSITE TAIL-BOOM STRUCTURE WITH ULTRASONIC IMAGING - AND GUIDED WAVES TECHNIQUES

Kurzfassung

Sandwich components with thin skins of GFRP or CFRP and honeycomb cores are high performance components for aerospace structures with high specific stiffness and strength. However these materials are impact sensitive. The DLR part of the EU-project AISHA II (Aircraft integrated structural health assessment) is focused on a full-scale test of a 3.5 m long tail-boom oft the EC 135. This component consists of a honeycomb sandwich structure with thin skins out of CFRP and GFRP layers. For in-field inspections ultrasonic echo-technique has been further developed for one side access. The honeycomb structure only penetrates frequencies below 1 MHz. Therefore a low frequency inspection technique has been developed. The harmonics and the scattering of the material are suppressed on the receiver side by filters. The C-scans clearly show the damaged areas. A scanning technique like ultrasonic imaging is time consuming and therefore expensive. In opposite to longitudinal waves Guided waves provide a global propagation with relatively low attenuation and can easily be excited and received by piezo patches. Such a structural health monitoring system provides in principle a push-bottom inspection. However, Lamb waves are dispersive and for each frequency there are two wave modes in minimum so that the received signals are very complex. In order to get more information about the Lamb wave propagation the wave fields have been scanned using a broadband capacitive sensor as a receiver. The full-wave A-scans are stored in a 3D-file. Out of this data, Lamb wave A-, B-, C- and D-scans and video animations can be calculated. Virtual sensors can be placed in the displayed scanning area and the received signal can be computed. Optimizations of sensor layouts and sensor positions are possible. Video snap-shots show the wave propagation and an interaction with defects like delaminations. The impact causes a mode conversion from the S0-mode (fast mode) into the A0-mode (slower mode). The A0-mode from the excitation piezo-patch and the one from the impact interfere. Therefore the impact detection in the tail-boom structure using Guided waves is a hard challenge. This paper presents first results of the impact detection using ultrasonic echo- and Guided waves technique. After optimization of the pulse parameters the ultrasonic imaging technique delivers a clear indication of impacts. The received signals of Guided Wave techniques are very complex due to mode conversions not only at defects but also at structural stiffness changes.