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Health monitoring of aircraft by Nonlinear Elastic Wave Spectroscopy

Final Report Summary - AERO-NEWS (Health monitoring of aircraft by Nonlinear Elastic Wave Spectroscopy)

The primary objective of the AERONEWS project was to enhance and implement new experimental and simulation tools necessary to measure, characterise, predict, quantify, and locate early stage damage in aircraft components and structures, based on the nonlinear response of the material.

Nonlinear elastic wave spectroscopy (NEWS) has proven to be very sensitive and effective in detecting micro-damage in materials at early stages of failure, and long before linear acoustic properties show signs of material degradation. The focus was on the expansion of the present knowledge of the nonlinear behaviour of progressive fatigue damage in aircraft parts and structures (WP1-WP2), on the development of explicit sensor systems and advanced self-monitoring components (WP3), on the formulation of an integrated design for a testing procedure and a unique engineered monitoring system for microdamage inspection (WP4), including remote control and communication tools, and on the validation of the applicability of the system to real time in-situ inspection of a full-scale model on the ground (WP5).

As a first task within the project, the partners of AERONEWS and the user group have concentrated on selecting critical parts and structures of interest to aeronautics NDT and on identifying several most common failure scenarios. This work led to an extensive database of available testing material, with representative samples ranging from steels to CFRP, honeycombs, and glass. Following the selection and distribution of the samples, the AERONEWS activity has mostly concerned with the description and critical evaluation of NEWS techniques for quality control of basic aeronautical components (composites, layered materials and metals) and with their comparison with competing methods. At first, experimental investigations of NEWS techniques were performed on a range of simple components including various metals, polymers, welded joints, adhesive bonds, and composite laminates with various kinds of defects such as corrosion, fatigue cracks, impact damage, thermal damage, etc. In the second and third year, WP1 work concentrated mainly on the feasibility of the NEWS techniques by their application to thin extended structures and components of complex geometries. It has provided an important feedback to the limitations and potentials of the NEWS techniques.

To achieve the goals of WP1 (experiments) and WP5 (application), the aeronautical partners provided several models of extended aircraft structures: a part of a fuselage, a wing lower surface test panel, and a Clevis attachment with two and three lugs. A first special hands-on workshop (the Prague experimental week I) devoted to NEWS tests on the latter two samples took place at VZLU on 22-26 May 2006 in the presence of 26 participants.

The conclusion of the first 'Prague experimental week' (described in the 'Prague experimental week activity report' which has been submitted together with the 30 m intermediate report) was that, although the various techniques were not yet optimised for the use on extended and complex samples, results were overall positive and provided useful guidance as to how the technique can be improved.

In the final stages of the project, WP1 continued to operate as a support for the work that was planned in WP5 on the implementation of the NEWS techniques and the validation of the results. Part of this considered the comparison of the sensitivity of the results to traditional inspection techniques. Apart from the NEWS techniques developed as part of WP1, the consortium members provided eddy current testing, Rayleigh surface acoustic wave inspection and phased array inspection. The different NEWS techniques have been constantly updated based on the experience gained within the project during the fatigue testing of the steering bracket. Most of the techniques that had been applied to single components were attempted on extended structures and complex geometries with varying degrees of success. Based on the experience of all partners using NEWS techniques, a database of the most important characteristics, requirements and limitations of the various techniques has been produced.

Based on the numerical modelling, two methodologies for damage localisation were proposed:
- The main interest of the partners has been on the use of time reversed (TR) acoustics and its modification in order to exploit the potential and sensitivity of NEWS techniques. A nonlinearity based version of TR was developed and the methodology was tested by means of real and numerical experiments with interesting and encouraging results regarding damage localisation. NEWS-TR can be operated in either harmonic filtering mode or using the phase coded pulse sequence filtering (also known as pulse inversion method). An alternative version consists of using TR to focus energy on certain spots (surfaces) and to exploit and investigate the local nonlinear effects that way (TR-NEWS).
- The second NEWS based imaging technique which is proposed is called multi-mode nonlinear resonance acoustic spectroscopy (MUMONRAS). The methodology is based on the interpretation of the amplitude dependent resonance frequency shift for different resonance modes of a given object in terms of the corresponding stress field of the resonance mode. The results suggest an alternative procedure for imaging micro-scale damage with high sensitivity. The methodology and some results are also reported in the internal report WP2/D7.

To focus the wave energy on the damage areas, different filtering techniques have been investigated. The sensitivity obtained in the NEWS based TR simulations in such cases tells us that it is feasible to apply the technique in real situations. As part of WP2, the simulations also concerned an optimisation study regarding the critical position of the receivers, the number/extent of receiver arrays, the robustness of the technique, the influence of experimental system noise on the treated signals, the discrimination of several sources of nonlinearity by iteration, etc.

A refined finite element (FE) model of the steering actuator bracket has been built and successfully tested in NEWS-TR simulations. Promising results were obtained with synthetic forward-propagation data. Complementing the localisation technique using NEWS based time reversal, an extension of the NEWS methods to nonlinear tomographic techniques has been proposed. The main conclusion of this WP is that the numerical simulations have indeed supported the developments on the experimental level for NEWS based imaging techniques of micro-damage The techniques NEWS-TR and TR-NEWS with their added features such as advanced filtering have been implemented as a result in the full scale testing in WP5 after the robustness and effectiveness of the methods has been studied numerically.

In WP3, the main efforts were dedicated to the development of new transducers and to the adaption of existing ones according to the specific requirements of NEWS techniques. The main achievements by the AERONEWS partners participating in WP3 were:
- development of paired piezoelectric ceramic elements for selectively receiving specific harmonics of a given excitation frequency;
- development of cheap optical transducers which use lasers for excitation and detection of ultrasound;
- experiments with dual ultrasonic sources (low frequency and high frequency) using surface acoustic waves and laser excitation for the high frequency;
- development of transducers with low higher harmonic output, i.e. extremely narrowband transducers, using piezoelectric single crystal materials such as ZnO, LiNbO3, PMN-PT;
- development of high-frequency wideband and air-coupled piezoelectric transducers (0.75 MHz centre frequency, planar or focused) operating in pulse-echo mode for surface vibration measurements of the sample;
- investigation of the nonlinearity of transducers showing the influence of the spot position on the emitting surface of the transducer and the influence of the mechanical load on the second harmonic quantity generated;
- optimisation of excitation applied to the NL-TRA experiments with the application of the classical pulse inversion method and with the development of an advanced pulse inversion method;
- investigation of piezoelectric actuators and sensors integration and testing of technologies for bonding and / or embedding of devices in fibre-glass panels using piezoelectric and Bragg Grant devices leading to the realisation of 'smart' components with self diagnostic features with a relative low number of actuating / sensing devices;
- design and construction of smart surface contact transducers and systems with an integrated pre-amplifier and power amplifier to work with wide band PZT / composite transducers for incorporation into a NEWS inspection system (developed under WP4);
- design and testing of system concepts using SAW for specific alloy air plane components (slat tracks) with limited available space on fully equipped slat tracks.

The initial work in WP4 concerned the inventory of details to enable a hardware and software requirements specification to be made, based on the interaction between WP1-2 and WP3. From the analysis of excitation and sensing systems, together with current experimental tests and mathematical modelling of acoustic properties within damaged materials, a hardware and software specification for an integrated NEWS analysis system has been made. The specification considered operational frequencies up to 10 MHz. Some initial prototype electronics, called 'NBOX', were developed during the first year of operation, following these hard- and software requirements, and were tested to evaluate the enabling technology.

WP5 started off at month 18 with the subtask about the 'choice of an appropriate site and testing object' for validation of the efficiency of the proposed NEWS system. At an early stage in the project, the decision was made to use the facilities of VZLU in Prague for the ultimate full scale tests. Ten different partners tested conventional and NEWS based techniques on gradually fatigued components to validate the sensitivity of each technique with regards to the detection of early micro-damage. During the experimental week, the fatigue loading (0-20 000 cycles in steps of roughly 5 000 cycles) was not enough to create noticeable micro-damage, even though AE monitoring and some of the NEWS based techniques gave an indication of incipient damage during the last stage of fatiguing. High frequency and high amplitude nonlinear techniques such as NEWS-TR and impact-modulation gave a slight indication of some damage whereas no linear technique could detect the miniature cracks. Additional fatiguing and NEWS testing have been continued over the last six months as part of the final validation of the techniques. The subsequent testing was performed at the individual laboratories of the partners involved in WP5. At the final stage (123 000 cycles), a 2 mm crack was observed near one of the corners of the sample. It was detected by eddy current, by the linear Rayleigh wave technique, and by the high frequency nonlinear techniques. This proves the initial hypothesis that the nonlinearity based techniques are more sensitive than the linear techniques.

The experimental results obtained in WP1 and WP5 gave a good foundation to analyse the possible applicability of nonlinear methods in the aeronautical industry as a tool for non destructive testing. Achievements regarding dissemination over the four years include the implementation of a dedicated web-based portal for communication and file exchange between AERONEWS partners, the organisation of more than 25 special sessions at several international conferences, the publication of about 50 scientific papers in international reviewed journals, and the advertisement of the project and project results through magazine publications, folders, brochures, and posters.