Zdenek Prevorovsky

Nonlinear Time Reversal Tomography of Structural Defects

Nonlinear Elastic Wave Spectroscopy (NEWS) comprises a new class of NDT techniques providing sensitive detecting and imaging of damages like micro-cracks, etc. The main advantage of nonlinear methods is their high sensitivity and ability to detect small defects relatively far from ultrasonic actuators and sensors. Some limitations of their use are due to difficulties with defect localization when global excitation of complicated structures is considered. To overcome these constraints, a modified TR-NEWS procedure is proposed in this paper. Procedure uses transducer arrays providing improvement of damaged zone location. One array transmits and another array records signals, which are then processed as to obtain nonlinear parameters corresponding to wave-paths between transducers. DORT method is used to derive Greens function of defects and TR-NEWS procedure is completed by the Excitation Symmetry Analysis Method (ESAM) as to extract nonlinear terms from acoustic responses along different wave-paths, appro...

Expert AE signal arrival detection

Accurate acoustic emission (AE) source location is the primary goal of the defect analysis following the AE signal detection. The source localisation is mostly based on arrival time differences of signals detected by several transducers. Considerable signal distortion occurs during the wave propagation through the body. Inaccurate determination of signal onsets and arrival time differences, consequently, are the greatest sources of localisation errors. Especially, in a case of higher requirements on accuracy and robustness (e.g., small sensor distances or elevated background noise), the results of currently used localisation methods are unsatisfactory. In this paper, an improved version of the wave arrival detection algorithm emulating an expert system is introduced. The new algorithm is based on signal waveform analysis taking into account the signal energy and trend of local gravity centre. The method has been proved in many applications as reliable enough, fast and easily implemented.

Time reversal transfer: Exploring the robustness of time reversed acoustics in media with geometry perturbations

In this letter, fundamentals of transferring a time reversal experiment between similar objects are discussed. The time reversal experiment consists of two steps: forward propagation, when a source excites the medium and a complex wave field is created, and back propagation, resulting in time reversal focusing. Here the procedure of performing the first step on one specimen and the second step on another is investigated. The theory of time reversal transfer is explained on an example of object shape variations. However, conclusions of the theoretical analysis are applicable universally. The feasibility of the proposed procedure is validated in experiments modeling conditions in practice.

Nonlinear wave modulation spectroscopy: Quasistatic solution and experimental evidence

Nonlinear Wave Modulation Spectroscopy (NWMS) is one of Nonlinear Elastic Wave Spectroscopy (NEWS) methods proven as very sensitive to the variety of material damage ranging from distributed microdamage to single cracks. The experimental procedure consists of simultaneous excitation of a specimen by two harmonic waves with frequencies f0 resp. f1. Nonlinear parameters can be estimated by analysis of harmonic and intermodulation component dependencies in the response signal. The current methodology of NWMS evaluation is based on dynamic solution of classical nonlinear response and quadratic hysteretic model. This describes a case with distributed microdamage, however in case of a localized damage e.g. a single crack, a quasistatic solution is more appropriate. The quasistatic responses of the same model were computed for varying input parameters. To properly describe hysteretic response an amplitude ratio dependence of nonlinear spectral components was found to be the most fitting. By using the scaling rel...

Echodentography based on nonlinear time reversal tomography: Ultrasonic nonlinear signature identification

This paper examines possibilities of using Nonlinear Elastic Wave Spectroscopy (NEWS) methods in dental investigations. Themain task consisted in imaging cracks or other degradation signatures located in dentin close to the Enamel-Dentine Junction (EDJ). NEWS approach was investigated experimentally with a new bi-modal acousto-optic set-up based on the chirp-coded nonlinear ultrasonic time reversal (TR) concepts. Complex internal structure of the tooth is analyzed by the TR-NEWS procedure adapted to tomography-like imaging of the tooth damages. Ultrasonic instrumentation with 10 MHz bandwidth has been set together including laser vibrometer used to detect responses of the tooth on its excitation carried out by a contact piezoelectric transducer. Bi-modal TR-NEWS images of the tooth were created before and after focusing, which resulted from the time compression. The polar B-scan of the tooth realized with TR-NEWS procedure is suggested to be applied as a new echodentography imaging.

Non-linear time reversal ultrasonic pseudo-tomography

Non-linear elastic wave spectroscopy (NEWS) became an effective methodology for NDT of complex structures, providing high sensitivity of damage detection. One of the most promising NEWS procedures is based on non-linear time reversal mirrors (NLTRM). General NLTRM procedure is mostly global, which brings difficulties with defect localisation. In this paper, a procedure modification is proposed, based on similar arrangement as ultrasonic tomography. Array of transducers is spaced on a structure with defect. Individual piezoelectric transducers subsequently transmit direct signals, which are detected by receivers, then time-reversed and sent back. High-voltage relay multiplexer is used to switch between transmitters and receivers. Matrices of back-received signals and their spectra are analysed, and their non-linear features are extracted along all wave-paths. Procedure application is illustrated by NDT of aircraft parts where NLTRM pseudo-tomography is used to allocate defective zones.

Inverse Problem Solution in Acoustic Emission Source Analysis: Classical and Artificial Neural Network Approaches

Solution of nonlinear Inverse Problems (IPs) is a frequent task in nondestructive testing of materials and structures when structural defects or imperfections must be recognized. One among the most promising ultrasonic NDT techniques is the Acoustic Emission (AE) method, which can reveal a dangerous defect (e.g., cracks) growth in realtime. Two practical IP examples of AE source analysis are presented in this chapter: AE source location and identification. As a comparison, a classical approach to the identification IP is shown, whereas the source location and AE signal parameter correction IPs are treated by the use of the soft computing method based on Artificial Neural Networks (ANNs). A short introduction to the ANN approach is presented for that purpose.