Analysing Guided Wave Propagation with Ultrasonic Imaging Methods for the Optimisation of a Helicopter Tail Boom SHM System

Abstract

GFRP and CFRP sandwich structures with honeycomb cores are high performance light weight structures for aerospace applications. Their impact sensitivity requires reliable NDT methods after fabrication and in service. Guided waves are able to propagate over large areas in components with a small attenuation and interact with defects. 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 of the helicopter EC 135. Especially for the complex composite tail boom with an asymmetric construction these interactions are difficult to predict and to compute. However their knowledge is important for the development of a structural health monitoring system. At DLR the non-contact ultrasonic imaging techniques are successfully used for a complementary NDT method and for the visualisation of the guided wave propagation. Experimental investigations showed that damage detection in the upper and lower skin is only possible with frequencies below 30 kHz. For S-mode excitation a special actuator based on piezocomposite transducers has been developed. This transducer is a combination of piezoceramic materials with ductile polymers and flexible electrodes to form a reliable and damage tolerant actuator. At delaminations the S-mode with a wavelength of about 160 mm is converted to an A-mode. The paper describes the possibility of a defect selective SHM system and shows the capability of impact detection.