Local resonances, formed by zero-group velocity (ZGV) and cutoff frequency points, have been extensively studied using impulse-based approaches, such as pulse laser and impact echo. In this work, we showcase the electromechanical impedance (EMI) technique as an option to extract and promote zero-group velocity and cutoff frequency resonances in a waveguide structure. We identify the mechanisms of multiple resonances in the EMI spectra via a wave propagation perspective. Furthermore, we extract the dynamic response profiles at a cutoff frequency and a ZGV frequency to confirm the localized minimum frequency behavior within corresponding branches.
Local resonances formed by zero-group velocity (ZGV) and cutoff frequency points usually demonstrate sharp resonance peaks in frequency spectra, which can be utilized for nondestructive evaluation (NDE) and Structural Health Monitoring (SHM). The existence and application of those local resonances have been extensively reported in plate and pipe structures. However, local resonances in rails are rarely studied. The team recently reported that impulse dynamic tests can promote the local resonances in rails up to 40 kHz, and the results were verified using both semi-analytical finite element (SAFE) analysis and frequency-domain fully discretized finite element analysis. In this work, we present the discovery of ZGV modes and cutoff frequency resonances in free rails up to 80 kHz using piezoelectric elements. A miniature low-cost PZT patch works as a consistent excitation source compared with the impulse dynamic testing method. First, we implement the SAFE analysis to compute dispersion curves of a standard AREMA 115RE rail and to identify potential ZGV and cutoff frequency points up to 80 kHz. Then, to understand the existence and detectability of identified ZGV and cutoff points in a free rail, we install one PZT patch on the side of the rail head. A chirp signal covering 20 to 120 kHz is selected as the excitation to cover the desired frequency range. Finally, we perform a spatial sampling of wave propagation using three receivers along the wave propagation direction to calculate the dispersion relations experimentally via two-dimensional Fourier Transforms (2D-FFT). This study verifies the existence of ZGV modes in free rail up to 80 kHz and demonstrates the feasibility of using piezoelectric elements to generate local resonances.
Inspection of railway tracks using ultrasonic techniques has been growing in importance since the last few years. Most of the existing technologies, however, operate at low speeds (~30 mph) using specialized test vehicles. This paper is based on a new technology utilizing non-contact air-coupled ultrasonic transducers for high-speed (up to 80 mph) rail inspection through the extraction of the acoustic Green’s function of a rail segment between a pair of sensors. The Green’s function is extracted passively using an output-only approach with the wheels of the locomotive acting as the source of excitation. The paper will focus on the results of various field tests conducted at the Transportation Technology Center in Pueblo, CO. Specifically, the detection performance of the “passive” prototype will be determined based on Receiver Operating Characteristic (ROC) curves that are obtained for various rail discontinuities (joints, welds, defects) and with varying operational parameters (speed, length of baseline distribution, number of runs, etc.). The optimum selection of these parameters will be determined based on these curves.
The elastic property identification of composite materials is of great interest for the conditional monitoring of lightweight structures, such as modern aircrafts (e.g., B737, A380, etc…). Because of the uncertainties in manufacturing processes for composite materials, it is important to develop a way to measure the properties of the final part in a manner that is non-destructive and can be applied in-situ on an existing structure. The first part of this paper examines the possibility of property characterization of quasi-isotropic multi-layered composite laminates using pseudo-experimental data (phase velocity dispersion curves with true elastic constants). An inversion scheme is proposed by minimizing the discrepancy between phase velocity dispersion curves for the ‘true’ elastic constants and those for “trial” constants, determined from the Semi-Analytical Finite Element (SAFE) method. Of interest are the three fundamental Lamb modes (the axial mode -S0, the flexural mode- A0, and the shear horizontal mode -SH0). A simulated annealing optimization (SA) is exploited to fast locate the global optimum with the smallest discrepancy. The second part of this paper implements data-driven matched field processing to locate the defects on the same type of composite with complicated geometry, including skin and stringer. Using a full matrix data acquisition system, two defect locations are investigated, one is on the skin, the other is the stringer.
Ultrasonic guided-wave testing can greatly benefit from (1) an ability to provide quantitative information on the damage that is being detected, and (2) an ability to select the best mode-frequency combination for maximum sensitivity to a given type of damage. Achieving these capabilities in complex structures (e.g. nonprismatic structures such as a stiffened panel in aerospace fuselages) is a nontrivial task. This paper will discuss an improved Global-Local (GL) method where the geometrical “local” discontinuity (e.g. the stiffener) is modelled by traditional FE discretization and the rest of the structure (“global” part) is modelled by Semi-Analytical Finite Element (SAFE) cross-sectional discretization. The boundaries of the “local” domain and the “global” domain are then matched in terms of wave displacement and stresses. GL models have been proposed in the past using theoretical (Lamb) wave solutions that only apply to isotropic plates. The authors have also previously studied GL methods using the SAFE approach for application to multi-layered anisotropic plates for which theoretical solutions are either not existent or hard to obtain. This work will extend recent research on these methods by optimizing the Matlab routine that is used to run the GL code, correcting some formulation errors that were present in the previous edition, and studying the specific case of a composite panel stiffened with cocured stringers that is representative of modern commercial aircraft construction (e.g. Boeing 787). The newlyformulated GL method will be shown to provide excellent results that can help designing a guided-wave test on these aircraft components for optimum detection of relevant damage that can be induced by impacts (including skin delaminations, stringer heel cracks, and stringer to skin disbonds). Other applications of the GL methods beyond stiffened aircraft panels will be discussed.
Composite materials make up an increasing portion of today’s aerospace structures (see, e.g. Boeing 787 and Airbus 380). These aircrafts’ fuselage, for example, is composed of a laminated composite skin connected to composite stringers and C-frames. Of primary importance is the detection of damage in these built-up structures, whether caused by the manufacturing process or in service (e.g. impacts). A related issue is the characterization of the composite elastic mechanical properties, that can also be related to the quantification of potential damage. Guided elastic waves propagating in the ~100s kHz regime lend themselves to provide the necessary sensitivity to these two conditions (damage and mechanical properties). This presentation will discuss the use of these waves to provide information on both damage and mechanical properties of composite structures that are typically used in modern commercial aircraft fuselages. In particular, a scanning system using air-coupled ultrasonic transducers and transfer function reconstruction will be presented for the detection and the quantification of impact-induced damage in laboratory test panels representative of fuselage construction. An optimization scheme that uses Simulated Annealing and the Semi-Analytical Finite Element (SAFE) technique as the forward model will be used to identify the layer-by-layer elastic properties of the composite skin laminate by observation of the guided wave phase velocity dispersive behavior.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.