Narrowband Ultrasonic Spectroscopy for Nde of Layered Structures

NDE of airspace sandwich structures is often performed using a simple resonant transducer sensing the information in the frequency domain obtained due to the constructive and destructive interference of elastic waves. Application field of ultrasonic narrowband ultrasonic spectroscopy (NBUS) is likely to increase rapidly with the growing application of layered structures in modern aircraft, for example, GLARE. The aim of this paper is to enlighten the potential and the limitations of the NBUS techniques. Operation of the NBUS technique based on the measurements performed using a specially designed resonance transducer with carefully selected narrow frequency band is explained in detail. Novel method based on the use of impedance plane for sensing transducer resonance is presented. Theoretical results illustrate the relation of the impedance plane indications to the adhesive strengths in aluminum sandwich structures. Finally, practical application of the proposed technique on mechanized inspection of carbon fiber reinforced structures is presented. Introduction: Ultrasonic resonance spectroscopy (URS) utilizes information in the frequency domain obtained due to the constructive and destructive interference of elastic waves for nondestructive evaluation of inspected objects. In a resonance test an ultrasonic tone-burst with sweeping frequency is applied to an ultrasonic transducer and a resonance spectrum of the inspected structure is acquired, [1]. The acquired spectrum contains information about the properties of materials used for the structure and flaws that may be present in it. Generally, there are two types of resonance test, global and local. Global tests provide synthetic information about the entire inspected part, and therefore, can be applied to relatively small parts where vibrations can be excited in the whole inspected volume. Local tests are more suitable for aerospace structures, where local structure condition is of interest, and only a selected part of the structure is excited. Ultrasonic resonance spectroscopy has been used for the inspection of aerospace structures and its application field is expected to increase rapidly with the growing application of layered structures in modern aircraft (e.g. GLARE that is a structure material used for the A380 from Airbus). However, the new applications compel increased demands on the RI instruments and transducers in terms of their sensitivity and their ability to characterize defects. Unfortunately, very limited information concerning the operation principles of the commercially available RI instruments is available in the literature. A fundamental limitation of resonance inspection is its sensitivity to factors that are unessential for the test, such as, variations in dimensions or material properties that may mask the effect of smaller defects. This problem can be solved either by modeling or, when modeling is unfeasible by employing sophisticated self-learning algorithms for tuning parameters of the defect detector. In the case of multilayered aerospace structures modeling seems to be the most suitable solution. The purpose of the present paper is to fill this gap and to enlighten the potential and the limitations of the URS techniques. The presented material is a result of our research aimed at development of narrowband ultrasonic spectroscopy (NBUS) capable of coping with new applications and the increased demands. We start from explaining NBUS principles for multilayered structures and presenting a model enabling simulation of the tested structure and transducer. Finally, we present a new solution to the sensor problem based on measuring its electrical impedance. Measurement setup: Narrowband ultrasonic spectroscopy has been used for the NDE of aerospace multi-layered structures since 70-ties. Commercially available instruments (bond testers) used for this test operate on the principle of exciting a mechanical resonance in a multilayered structure. A piezoelectric probe, excited by a swept frequency signal is coupled to the surface of the inspected structure using a coupling agent (see Fig. 1). The instrument acquires probe’s frequency spectrum in the range of some tens of kHz to several hundreds of kHz. A resonance in the layered structure occurs when echoes between two boundaries travel back and forth due to the difference in acoustic impedances at the boundaries. For multi-layered structures, a number of resonance modes can be observed depending on their geometry and condition. A characteristic resonance pattern, an ultrasonic signature, obtained for each particular defect-free structure and given transducer can be used as a reference. It is worth to mention that the probe characteristics are crucial for the NBUS test since shift of its resonance frequency indicates condition of the inspected structure. Therefore highly resonant piezoelectric probes are preferred for this kind of test. Serious difficulties can be encountered when applying NBUS to attenuating materials, like carbon fiber reinforced composites (CFRP). Resonance of a probe coupled to such structure is not only shifted in frequency but also spoiled in the sense that the resonance peak becomes broad and the resonance frequency cannot be detected accurately.