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Lamb waves have become a focal point in ultrasonic testing owing to their potential for long-range and inaccessible detection. However, accurately estimating the flaws in plates using Lamb waves remains challenging because of scattering, mode conversion, and dispersion effects. Recent advances in laser ultrasonic wave techniques have introduced innovative visualization methods that exploit the dispersion effect of Lamb waves to visualize defects via, for example, acoustic wavenumber spectroscopy. In this study, we developed an interdigital transducer (IDT)-based scanning laser Doppler vibrometer (SLDV) system without a power amplifier using a low-power IDT fabricated from lead magnesium niobate–lead zirconate titanate single crystals. To validate the proposed low-power IDT-based SLDV, four different defective plates were measured for defects. A comparison between a conventional IDT-based SLDV, a dry-coupled IDT-based SLDV, and the proposed method demonstrated that the latter is highly reliable for measuring thin plate defects. [ABSTRACT FROM AUTHOR]

In this study, we investigate the potential of cold atmospheric plasma (CAP) as a non-contact excitation device, comparing its performance with an ultrasound transmitter. Utilizing a scanning Laser Doppler Vibrometer (LDV), we visualize the acoustic wavefront generated by a CAP probe and an ultrasound sensor within a designated 50 mm × 50 mm area in front of each probe. Our focus lies in assessing the applicability of a CAP probe for exciting a small polymethyl methacrylate (PMMA) sample. By adjusting the dimensions of the sample to resonate at the excitation frequency of the probe, we can achieve high vibrational velocities, enabling further mechanical analysis. In contrast with traditional vibration excitation techniques such as electrodynamical shakers and hammer impact excitation, a plasma probe can offer distinct advantages without altering the structure's dynamics since it is contactless. Furthermore, in comparison with laser excitation, plasma excitation provides a higher power level. Additionally, while pressurized air systems are applicable for limited low frequencies, plasma probes can perform at higher frequencies. Our findings in this study suggest that CAP is comparable with acoustic excitation, indicating its potential as an effective mechanical excitation method. [ABSTRACT FROM AUTHOR]

Metal structural plates are extensively used in various engineering structures due to their high strength, high-temperature resistance, toughness, and plasticity. However, they are susceptible to damage from external loads and impacts over time. The current Lamb wave detection methods suffer from dispersion and multimodal effects, leading to ineffective identification of damage information. In this paper, we investigate Lamb wave propagation in steel structure plates with flat-bottomed holes using a sinusoidal modulation five-peak wave signal. Finite element numerical models are developed, and an experimental platform is constructed using steel and aluminum boards. Experimental data is collected using a Scanning Laser Doppler Vibrometer (SLDV, PSV-500, Polytec Inc., Baden-Württemberg, German). The results demonstrate that, under the same frequency, the damage reflection energy for different modes is distinct. By fusing the data from the two modes, more accurate damage imaging results are obtained in the frequency-wavenumber (f-k) domain compared to single-mode imaging. Furthermore, experiments are conducted to locate damage in a steel board with a through hole and an aluminum plate with double flat-bottomed holes, confirming the feasibility of the proposed algorithm in isotropic plates. [ABSTRACT FROM AUTHOR]

Interdigital transducer (IDT)-based scanning laser Doppler vibrometers (SLDVs) have recently been developed to detect shallow defects in plates. An IDT-based SLDV was augmented with Lamb wave frequency fusion capability to enable the imaging of defects across the entire depth of a plate. Dry-coupled IDTs are a type of field-deployable IDT that do not require the attachment of a couplant to the structure. However, this technique is not applicable to the analysis of shallow wall thinning, because IDTs composed of lead zirconium titanate (PZT) cannot be attached to the curved surface of a structure. To address this issue, we developed a new type of IDT—the flexible IDT—that can be bent along the circumferential direction of a pipe. To set the gap spacing of the flexible IDT, the wavenumber sensitivity and minimum distance from the reference mode (MDRM) was calculated, and the S 0 mode is selected to ensure high-wavenumber sensitivity above the threshold of the MDRM. The proposed flexible IDT-based SLDV was tested by using it to visualize the wall thinning on pipes with 4%, 6%, and 20% thinning. The results of this study will contribute toward the development of systems that improve on existing planar laser scanning schemes and enable shallow defect detection in pipes using laser ultrasonics. [ABSTRACT FROM AUTHOR]

The present paper comprehensively investigates the complex interaction between ultrasonic guided waves and local structural discontinuities, such as damages, through highly sensitive features: so-called wave damage interaction coefficients (WDICs). These WDICs are unique for each structural discontinuity and depend solely on their characteristics for a given structure and condition. Thus, they can be particularly useful for advanced assessment of lightweight structures in the context of non-destructive evaluation and structural health monitoring. However, the practical application of WDICs entails significant difficulties due to their sensitivity and complex patterns. Therefore, this study attempts to guide researchers and practitioners in the estimation of WDICs from numerical simulations and physical experiments. Detailed investigations are made for an aluminum host plate modified by artificial structural discontinuities, i.e., surface-bonded steel sheets. The numerical simulations are performed to predict WDICs and study sensitivities using a sophisticated finite element model. The experimental setup uses piezoelectric transducers to excite guided waves in the host plate. A single scanning laser Doppler vibrometer measures the scattered guided waves caused by the surface-bonded steel sheets, and the resulting WDICs with possible influences are investigated. In both cases, the orientation and thickness of the attached steel sheets were varied to create 12 different damage scenarios. In general, the comparison between numerical and experimental WDICs show good agreement. This underpins the applicability of the general methodology for simulating and measuring WDICs over all scenarios. Furthermore, the WDIC scattering patterns reveal a clear dependency of the peaks in the back-scattered reflections for both the numerical and experimental amplitude coefficients on the damage orientation, basically following the law of reflection. However, some discrepancies between both studies were observed. Numerical sensitivity analysis identified the adhesive layer as one reason for such differences. Additionally, misalignment errors in the experimental measurements were also found to affect WDICs. Therefore, an improved baseline subtraction method is proposed, which clearly enhances the experimental WDICs. Finally, an experimental sensitivity study of WDICs for selected sensing radii revealed only a minor influence. All these investigations were made for the amplitude as well as the phase representation of WDICs. Thus, these findings may open the way to future research and development of techniques employing WDICs for advanced applications of non-destructive evaluation and structural health monitoring. [ABSTRACT FROM AUTHOR]

Subwavelength defect imaging using guided waves has been known to be a difficult task mainly due to the diffraction limit and dispersion of guided waves. In this article, we present a noncontact super-resolution guided wave array imaging approach based on deep learning to visualize subwavelength defects in plate-like structures. The proposed approach is a novel hierarchical multiscale imaging approach that combines two distinct fully convolutional networks. The first fully convolutional network, the global detection network, globally detects subwavelength defects in a raw low-resolution guided wave beamforming image. Then, the subsequent second fully convolutional network, the local super-resolution network, locally resolves subwavelength-scale fine structural details of the detected defects. We conduct a series of numerical simulations and laboratory-scale experiments using a noncontact guided wave array enabled by a scanning laser Doppler vibrometer on aluminate plates with various subwavelength defects. The results demonstrate that the proposed super-resolution guided wave array imaging approach not only locates subwavelength defects but also visualizes super-resolution fine structural details of these defects, thus enabling further estimation of the size and shape of the detected subwavelength defects. We discuss several key aspects of the performance of our approach, compare with an existing super-resolution algorithm, and make recommendations for its successful implementations. [ABSTRACT FROM AUTHOR]

The interdigital transducer–based scanning laser Doppler vibrometer has recently been introduced to efficiently generate the symmetric mode for damage detection of shallow defects in thick plates. To measure shallow defects in a carbon steel plate, the excitation frequency is optimized based on the analysis of wavenumber sensitivity and degree of separation between modes. Even though the interdigital transducer–based scanning laser Doppler vibrometer method using continuous excitation is a promising method for visualizing defects in plate-like structures, the interdigital transducer has to be coupled to media, such as oil or water, to minimize the variation in acoustic impedance between the lead zirconium titanate and the plate-like structures. In this study, we develop a dry-coupled interdigital transducer–based scanning laser Doppler vibrometer system for the detection of shallow defects in a plate to facilitate easy mounting and demounting from the plate. To verify the proposed dry-coupled interdigital transducer–based scanning laser Doppler vibrometer, plates with four and eight different depth defects are introduced, and it is demonstrated that the defects in thick plates measured by dry-coupled interdigital transducer–based scanning laser Doppler vibrometer are visualized compared to those measured by interdigital transducer–based scanning laser Doppler vibrometer. [ABSTRACT FROM AUTHOR]

Featured Application: This work presents In-plane Heatwave Thermography as a new in-service qualification method of fasteners in aircraft fuselage panels. By performing feasibility tests on a serviceable part of an aircraft, the results of this work aim towards the application in an industrial environment like the aircraft hangar. A benchmark and comparison with Ultrasound Lock-in Thermography and Scanning Laser Doppler Vibrometry ranks this technology with respect to industrial applications. The inspection of fasteners in aluminium joints in the aviation industry is a time consuming and costly but mandatory task. Until today, the manual procedure with the bare eye does not allow the temporal tracking of a damaging behavior or the objective comparison between different inspections. A digital inspection method addresses both aspects while resulting in a significant inspection time reduction. The purpose of this work is to develop a digital and automated inspection method based on In-plane Heatwave Thermography and the analysis of the disturbances due to thermal irregularities in the plate-like structure. For this, a comparison study with Ultrasound Lock-in Thermography and Scanning Laser Doppler Vibrometry as well as a benchmarking of all three methods on one serviceable aircraft fuselage panel is performed. The presented data confirm the feasibility to detect and to qualify countersunk rivets and screws in aluminium aircraft fuselage panels with the discussed methods. The results suggest a fully automated inspection procedure which combines the different approaches and a study with more samples to establish thresholds indicating intact and damaged fasteners. [ABSTRACT FROM AUTHOR]

In this paper, a new fully automated scanning laser Doppler vibrometer (LDV) measurement technique is presented. In contrast to existing scanning LDV techniques which use a 2D camera for the manual selection of sample points, we use a 3D Time-of-Flight camera in combination with a CAD file of the test object to automatically obtain measurements at pre-defined locations. The proposed procedure allows users to test prototypes in a shorter time because physical measurement locations are determined without user interaction. Another benefit from this methodology is that it incorporates automatic mapping between a CAD model and the vibration measurements. This mapping can be used to visualize measurements directly on a 3D CAD model. The proposed method is illustrated with vibration measurements of an unmanned aerial vehicle [ABSTRACT FROM AUTHOR]

This paper presents a method of measuring the vibration patterns on facial surfaces by using a scanning laser Doppler vibrometer (LDV). The surfaces of the face, neck, and body vibrate during phonation and, according to Titze (2001), these vibrations occur when aerodynamic energy is efficiently converted into acoustic energy at the glottis. A vocalist's vibration velocity patterns may therefore indicate his or her phonatory status or singing skills. LDVs enable laser-based non-contact measurement of the vibration velocity and displacement of a certain point on a vibrating object, and scanning LDVs permit multipoint measurements. The benefits of scanning LDVs originate from the facts that they do not affect the vibrations of measured objects and that they can rapidly measure the vibration patterns across planes. A case study is presented herein to demonstrate the method of measuring vibration velocity patterns with a scanning LDV. The objective of the experiment was to measure the vibration velocity differences between the modal and falsetto registers while three professional soprano singers sang sustained vowels at four pitch frequencies. The results suggest that there is a possibility that pitch frequency are correlated with vibration velocity. However, further investigations are necessary to clarify the relationships between vibration velocity patterns and phonation status and singing skills. [ABSTRACT FROM AUTHOR]

The analysis of acoustic wave fields is important for a large number of engineering designs, communication and health-related reasons. The visualization of wavefronts gives valuable information about the type of transducers and excitation signals more suitable for the test itself. This article is dedicated to the development of a fast procedure for acoustic fields visualization in underwater conditions, by means of laser Doppler vibrometer measurements. The ultrasonic probe is a focused transducer excited by a chirp signal. The scope of this work is to evaluate experimentally the properties of the sound beam in order to get reliable information about the transducer itself to be used in many kinds of engineering tests and transducer design. [ABSTRACT FROM AUTHOR]

This paper presents a machine learning approach to localizing a five-peak narrow-band modulated sinusoidal signal excitation source within a composite panel. In particular, the Back Propagation (BP) neural network is used. The idea is to use the arrival time of the first wave packet in a five-peak wave signal to locate their source. Specifically, this paper divides the composite material board into multiple regions, designs 8 receiving points to receive the signal from the excitation source, and finds the region where each source is located. The COMSOL numerical simulation platform is used to build a composite plate model and simulate the propagation of five-peak waves to train and test the machine learning network. Correspondingly, carry out experimental verification and use a scanning laser Doppler vibrometer (SLDV) to build a non-contact experimental platform to obtain the wave field information in the composite material plate. The results show that BP neural networks can learn to map signal features to their sources in both contexts. • The arrival time was used to locate acoustic source based on BP neural network. • The COMSOL numerical simulation platform is used to build composite plate model. • The propagation of five-peak waves was used to train and test the machine learning network. • A SLDV is used to obtain the wavefield information in composite plat. • BP neural networks can learn to map signal features to their sources. [ABSTRACT FROM AUTHOR]

Abstract: This study explores the feasibility of using a non-contact guided wave imaging system to detect hidden delamination in multi-layer composites. The study is conducted in two phases. In the first phase, Lamb waves are excited by a lead (Pb) Zirconate Titanate transducer (PZT) mounted on the surface of a composite plate, and the out-of-plane velocity field is measured using a one-dimensional (1D) scanning laser Doppler vibrometer (LDV). From the scanned time signals, wavefield images are constructed and processed to study the interaction of Lamb waves with delamination. The paper presents additional signal and image processing techniques used to highlight the defect in the scanned area. The techniques are demonstrated using experimental data collected from a 1.8mm thick multi-layer composite. In the second phase, a completely non-contact system is described to excite and measure guided waves. A modulated continuous wave (CW) laser source in conjunction with a photodiode is used to wirelessly excite an attached PZT and the resulting waves are again sensed using the vibrometer. The non-contact system is used to excite and measure elastic waves in a composite channel test article. The elastic wave propagation image sequences are created from the non-contact excitation system. [Copyright &y& Elsevier]

Operating Deflection Shapes (ODS) has emerged as one of the powerful techniques in vibration analysis to understand and to evaluate the absolute dynamic behaviour of a machine, component or an entire structure. Traditionally, accelerometers have been used to get the ODS of a structure. However, recent development shows that certain situation may not allow direct contact with the structure under investigation. Therefore, Scanning Laser Doppler Vibrometer (SLDV) has become popular in the investigation. In this paper, a new ODS Frequency Response Function (ODS FRF) for investigations using SLDV is formulated. The ODS FRF is used to construct the ODS of the structure. A new form of scale factor for the ODS FRF is also introduced to normalize the effects from variable excitation force. The importance of this scale factor is demonstrated on a beam and plate under the excitation of varying forces. It is found that the suggested ODS FRF and the scale factor give the desired result in comparison with theory. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents measurement procedures for the evaluation of velocity/displacement components of surface elastic waves through a single beam Laser Vibrometer. The developed technique is applied for the estimation of Rayleigh wave polarization in aluminum specimens. The estimated polarization orbits are compared with the analytical ones to demonstrate the effectiveness of the measurement technique. The estimations rely on the preliminary analysis of the recorded signals in the frequency/wavenumber domain, as enabled by the fine spatial resolution provided by a scanning Laser vibrometer, which allows identification and separation of Rayleigh waves from the recorded wavefield. In addition, comparison of polarization measurements performed on untreated and shot-peened samples suggests the measurement technique as a tool for the estimation of the effect of residual stresses on the polarization properties of surface waves. [Copyright &y& Elsevier]

Abstract: When using optical measurements of the sound fields inside a glass tube, near the material under test, to estimate the reflection and absorption coefficients, not only these acoustical parameters but also confidence intervals can be determined. The sound fields are visualized using a scanning laser Doppler vibrometer (SLDV). In this paper the influence of different test signals on the quality of the results, obtained with this technique, is examined. The amount of data gathered during one measurement scan makes a thorough statistical analysis possible leading to the knowledge of confidence intervals. The use of a multi-sine, constructed on the resonance frequencies of the test tube, shows to be a very good alternative for the traditional periodic chirp. This signal offers the ability to obtain data for multiple frequencies in one measurement, without the danger of a low signal-to-noise ratio. The variability analysis in this paper clearly shows the advantages of the proposed multi-sine compared to the periodic chirp. The measurement procedure and the statistical analysis are validated by measuring the reflection ratio at a closed end and comparing the results with the theoretical value. Results of the testing of two building materials (an acoustic ceiling tile and linoleum) are presented and compared to supplier data. [Copyright &y& Elsevier]

• We propose an accelerated noncontact guided wave array imaging method with sparse scanning measurements; • Our method using sparse array data provides defect imaging performance comparable to the dense scanning case; • We demonstrate the proposed approach on laboratory experiments on composite plate specimens with multiple defects with encouraging results. Noncontact guided wave array imaging with a scanning laser Doppler vibrometer (SLDV) is an effective tool to detect and locate defects within a plate-like structure, as it obviates the need for installing, calibrating, and maintaining a transducer array. However, it requires collecting guided wave signals through scanning across dense spatial grid points to avoid non-defect artifacts in the array image, which is time-consuming. In this paper, we present an accelerated noncontact guided wave array imaging method that does not require dense scanning array while providing defect imaging performance comparable to the dense scanning case. In our approach, sparse scanning measurements at only a small number of points are carried out first for fast guide wave data acquisition. Then, dense guided wave array data is reconstructed from these sparse array measurements using a sparsity-promoting optimization technique, followed by delay-and-sum (DAS) beamforming to image defects within a test structure. We validate this method with laboratory experiments on composite plate specimens with multiple defects. The results demonstrate that defects within a composite plate can be successfully detected and located using sparsely sampled guided wave array measurement data. Such a significant reduction in the number of required measurement points enables accelerated noncontact guided wave array imaging. [ABSTRACT FROM AUTHOR]

• A analytical free vibration of long narrow notched plate model is proposed. • A directional continuous wavelet transform of displacement mode shape for line-type damage detection is investigated. • The optimal mother wavelet selection is proposed fully investigated. • The experimental modal test is adopted using non-contact testing. Line-type damage is abundant in engineering structures that have been in service for a long time. Vibration-based nondestructive testing as a global method has been extensively used in structural damage identification. In this study, a two-dimensional directional continuous wavelet transform (2D-DCWT)-based damage identification algorithm using "Morlet2d" wavelet for line-type damage detection in plate structures is presented. An analytical long narrow notch-type damage model of simply-supported plates is proposed to evaluate line-type damage effect on plate vibration performance. Based on the proposed model, the biggest challenge in parameter selection of wavelet analysis is solved. The fundamental aspects of 2D-DCWT algorithm in term of notch location and orientation are assessed. A multi-mode shape fusion approach is presented to make the 2D-DCWT algorithm apply to the high-order mode shapes. A comparative study with isotropic "Mexcan-hat2d" wavelet-based damage identification is performed, and it demonstrates that the proposed 2D-DCWT algorithm is superior in noise immunity. The efficiency and practicability of the 2D-DCWT algorithm are certified by an experimental modal test on damage detection of an artificially-induced notched aluminum alloy clamped plate using excitation-response system of the Piezoelectric Leadzirconate-Titanate (PZT) and Scanning laser Doppler Vibrometers (SLDV). As demonstrated in both the analytical and experimental investigation, the suggested approach may successfully identify line-type damage in plate-type structures. [ABSTRACT FROM AUTHOR]

• Scanning laser Doppler vibrometers sensitivity to sensor head vibrations confirmed. • Correction approach for arbitrary vibration and any scanning configuration derived. • Practical correction schemes successfully demonstrated across range of scan angles. • Optimum correction obtained using a single accelerometer plus geometrical weighting. • Full compensation scheme presented, using three correction measurement locations. • Limitations of schemes described; errors for typical vibration scenarios quantified. Scanning laser Doppler vibrometer (SLDV) measurements are affected by sensor head vibrations as if they are vibrations of the target surface itself. This paper presents practical correction schemes to solve this important problem. The study begins with a theoretical analysis, for arbitrary vibration and any scanning configuration, which shows that the only measurement required is of the vibration velocity at the incident point on the final steering mirror in the direction of the outgoing laser beam and this underpins the two correction options investigated. Correction sensor location is critical; the first scheme uses an accelerometer pair located on the SLDV sensor head front panel, either side of the emitted laser beam, while the second uses a single accelerometer located along the optical axis behind the final steering mirror. Initial experiments with a vibrating sensor head and stationary target confirmed the sensitivity to sensor head vibration together with the effectiveness of the correction schemes which reduced overall error by 17 dB (accelerometer pair) and 27 dB (single accelerometer). In extensive further tests with both sensor head and target vibration, conducted across a range of scan angles, the correction schemes reduced error by typically 14 dB (accelerometer pair) and 20 dB (single accelerometer). RMS phase error was also up to 30% lower for the single accelerometer option, confirming it as the preferred option. The theory suggests a geometrical weighting of the correction measurements and this provides a small additional improvement. Since the direction of the outgoing laser beam and its incident point on the final steering mirror both change as the mirrors scan the laser beam, the use of fixed axis correction transducers mounted in fixed locations makes the correction imperfect. The associated errors are estimated and expected to be generally small, and the theoretical basis for an enhanced three-axis correction is presented. [ABSTRACT FROM AUTHOR]

Laser Doppler Vibrometer (LDV) has been performed as a tool to observe stress fields of ultrasound longitudinal wave inside transparent solids due to its high sensitivity and intuitiveness. Besides, this method is only sensitive to dynamic stress and has no response to static stress, so it has the advantage of not being affected by residual stress in transparent solids. However, this method has limitations. LDV cannot directly detect shear wave, and for longitudinal wave, it can only obtain an approximate value of normal stress. In this paper, we analyze the limitations, and achieve observing shear wave by adding a rotatable linear polarizer to the optical system. Furthermore, the complete photo-elastic tensor can be obtained from the process of measuring longitudinal wave, and then the absolute 2D dynamic stress fields can be calculated. To check the applicability of our method, we compared LDV with the results of numerical analysis and dynamic photoelasticity, respectively. [ABSTRACT FROM AUTHOR]