Air-coupled Ultrasonic Testing Research Articles

A generic time-frequency analysis-based signal processing and imaging approach for air-coupled ultrasonic testing

The utilization of air-coupled ultrasonic technology is crucial for examining objects that are challenging to establish contact with or cannot endure coupling. Nevertheless, air-coupled ultrasonic is susceptible to various factors such as impedance mismatch, sound wave attenuation, and environmental noise, consequently leading to a diminished signal-to-noise ratio of the received signal. This reduction in signal quality amplifies the complexity of analysis and imaging processes. In this study, a generic air-coupled ultrasonic signal processing and imaging procedure for ultrasonic inspection is proposed, which capitalizes on time-frequency analysis. This methodology attains a heightened level of stability and superior signal-to-noise ratio imaging through a series of procedural steps, encompassing feature extraction, denoising, and the reconstruction of the A-scan signals. The efficacy of the proposed approach is quantitatively assessed in terms of signal-to-noise ratio, probability of detection, robustness, and generality. This innovative method presents a newfangled resolution for automated air-coupled ultrasonic testing.

Fluidic Ultrasound Generation for Non-Destructive Testing.

Air-coupled ultrasonic testing (ACU) is a pioneering technique in non-destructive testing (NDT). While contact testing and fluid immersion testing are standard methods in many applications, the adoption of ACU is progressing slowly, especially in the low ultrasonic frequency range. A main reason for this development is the difficulty of generating high amplitude ultrasonic bursts with equipment that is robust enough to be applied outside a laboratory environment. This paper presents the fluidic ultrasonic transducer as a solution to this challenge. This novel aeroacoustic source uses the flow instability of a sonic jet in a bistable fluidic switch to generate ultrasonic bursts up to 60kHz with a mean peak pressure of 320Pa. The robust design allows operation in adverse environments, independent of the operating fluid. Non-contact through-transmission experiments are conducted on four materials and compared with the results of conventional transducers. For the first time, it is shown that the novel fluidic ultrasonic transducer provides a suitable acoustic signal for NDT tasks and has potential of furthering the implementation of ACU in industrialapplications.

Spatial and Temporal Deep Learning in Air-Coupled Ultrasonic Testing for Enabling NDE 4.0

Air-coupled ultrasonic (ACU) testing has been used for several years to detect defects in plate-like structures. Especially, for automated testing procedures, ACU testing is advantageous in comparison to conventional testing. However, the evaluation of the measurement data is usually done in a manual manner, which is an obstruction to the application of ACU testing. The goal of this study is to automate and improve defect characterization and NDE 4.0 accordingly with deep learning. In conventional ACU testing the measurement data contains temporal (A-scans) and spatial (C-scans) information. Both data types are investigated in this study. For the A-scans, which represent time series data, neural network architectures tailored to such data types are applied. In addition, it is evaluated if further adaptions of the training procedure increase the performance. The C-scans are segmented by applying different U-net similar architectures and training strategies. In order to use spatial and temporal information, a further approach is taken. The prediction of the time series models is segmented with image models. The performance of all trained models and training strategies is compared with the F1-score and benchmarked against the conventional evaluation, which is thresholding of the C-scans. As specimens, artificial defects in acrylic and carbon fiber-reinforced polymer plates are investigated.

Phased array probes for air-coupled ultrasonic testing based on cellular polymer

Air-coupled ultrasonic testing is an established alternative to conventional ultrasonic methods for testing of light-weight components in aerospace and automotive industry. Compared to direct coupled contact technique or immersion technique it is less invasive, so that it offers numerous financial incentives. While phased arrays are already an industrial standard in fluid-coupled mechanized ultrasonic testing, air-coupled testing is usually performed using two single-element probes and a transmission technique. Therefore, advantages of phased arrays such as active focussing and manipulation of the incidence angle are rarely used with air-coupled testing. This paper presents a newly developed air-coupled phased array device with the emphasis on the characterization of the probes. The device consists of a linear array probe, a transmitter unit and a receiver unit. The main novelty is that the array probe is based on high-voltage excitation of charged cellular polypropylene. Charged cellular polymers are referred to as ferroelectrets or as piezoelectrets. They exhibit an extremely low stiffness and acoustic impedance, so that a matching layer is not necessary. Extremely low mechanical cross-coupling enables independent excitation of the elements of the array. The development of the phased array probes is supported by computations of the sound field using point source synthesis. The main intended applications are transmission with focussing, transmission with an electronic sweep and testing with air-coupled guided waves by using beam steering, applied to carbon-fibre-reinforced plates and similar materials. Air-coupled guided waves enable testing of objects with one-sided access. This work was funded from the “Technologietransfer-Programm Leichtbau” by the German Federal Ministry for Economic Affairs and Climate Action, project 03LB1001A.

Thermoacoustic phased-array radiators – Theory, characteristics, and applications

Standard non-invasive acoustic non-destructive testing (NDT) methods like testing in contact or using immersion tank arrangements are limited since the probes need to be in direct contact with the test piece or using a fluid as coupling media. Air-coupled ultrasonic testing avoids these limitations and enables new testing methods. Two up-and-coming transducer technologies are recognized as promising: transducers using ferroelectrets and transmitters based on the thermoacoustic principle. Thermoacoustic radiators based on thin-film technology offer a resonant-free generation of ultrasound. The resonance-free operation enables the wideband generation of acoustics waves and promises advantages for existing NDT methods. Using specially shaped substrates and electrodes enables the fabrication of planar and focused thermoacoustic radiators. This contribution presents the latest results of our ongoing research regarding thermoacoustic radiators, which is the thermoacoustic phased-array. Phased-array transducers offer the on-the-fly change of the steering angle and the position of the focal point without moving or modifying the transducer itself. Combining both technologies, thermoacoustic emitter and the phasedarray principle broaden the spectrum of classical NDT applications and modern approaches like the excitation of guided waves or structural health monitoring. In detail, we modelled the sound field of a self-fabricated thermoacoustic phased-array radiator, characterised the emitter elementwise and tested several application scenarios (e.g., transmission, beam steering, focusing and the excitation of guided waves).

Simultaneous nondestructive estimation of thickness and longitudinal wave velocity of adhesive layers in adhesive joints through air-coupled ultrasonic testing

This paper proposes a nondestructive method for the simultaneous estimation of the thickness and longitudinal wave velocity of an adhesive layer using a transmission coefficient spectrum, including amplitude and phase spectra measured via air-coupled ultrasonic testing to confirm adhesive joint reliability. The method correctly estimated the values of the adhesive layer from the transmission coefficient spectrum experimentally, unlike the conventional estimation using only the amplitude spectra. The absolute values of the estimation errors of thickness and longitudinal wave velocity obtained using this method were 1.3–12.1% and 0.4–6.1%, respectively. The proposed method can be useful for the quality assurance of adhesive joints.

A novel two-level approach to defect detection in braided CFRP using air-coupled ultrasonic testing

Air-coupled ultrasonic testing and C-scan technique has been increasingly applied to the braided CFRP structures owing to its non-destruction, non-contact and high visualization characteristics. Due to the noise, structural vibration, and airflow in the process of detection, the accuracy of defect identification is easily deteriorated. To address this issue and further determine the relationship between the ultrasonic acoustical pressure attenuation and structural parameters, a novel two-level identification method based on the modified two-dimensional variational mode decomposition (2D-VMD) has been proposed. In the first level, C-scan images have been sparsely decomposed into ensembles of modes by 2D-VMD method. Then, the modes have been screened by mutual information method to realize the reconstruction of new image in the second level. Experimental results have shown that the proposed method has the good ability to identify defects with a minimum detectable diameter of 1–2 mm. It has been noted that the ultrasonic acoustical pressure attenuation has become remarkably higher in the twill weave CFRP than the plain weave CFRP and the ratio of pressure attenuation between two weave types of CFRP has decreased with the defect depth increase. Meanwhile, shadows around defects in C-scan images have been suppressed to a great extent. It has been demonstrated that the capability of denoising has enabled the developed method with the accurate detection in terms of the shape, size, depth and weave type. With these advantages, the proposed method has provided valuable insights into the development of an effective method for defect detection of braided CFRP structures.

Air-Coupled Ultrasound Sealing Integrity Inspection Using Leaky Lamb Waves in a Simplified Model of a Lithium-Ion Pouch Battery: Feasibility Study.

Inspecting the sealing integrity of lead tabs is an important means of ensuring the reliability and safety of pouch-type lithium-ion (Li-ion) batteries with a thin multi-layered aluminum (Al) laminated film. This paper presents a new air-coupled ultrasonic non-destructive testing (NDT) inspection method based on leaky Lamb wave transmission; and reception for evaluating the sealing integrity between the lead tab and the Al pouch film. The proposed method uses the critical incidence angle between the air and the layer with the fastest Lamb wave velocity to maximize the signal-to-noise ratio in the through-transmission mode. To determine the critical incidence angle, phantom experiments with two test pieces (i.e., an Al tab and an Al tab sealed with an Al pouch film) are conducted. In addition, 2D scans are performed at various incidence angles for an inhouse pouch-type Li-ion battery with a 1-mm-wide foreign material inserted as a defect. At the critical incidence angle (i.e., 22°), the proposed air-coupled ultrasonic NDT method in through-transmission mode successfully identifies the shape and location of the defect through c-scan image reconstruction. These preliminary results indicate that the proposed air-coupled ultrasonic NDT method with leaky Lamb waves can be used to inspect the sealing integrity of Li-ion pouch batteries in dry test conditions.

Ferroelectrets: Recent developments

Ferroelectrets (also called piezoelectrets) are relatively young members in the family of piezo-, pyro- and ferroelectric materials. They exhibit ferroic behaviour phenomenologically undistinguishable from that of traditional ferroelectrics, although the materials per se are essentially non-polar space-charge electrets with artificial macroscopic dipoles (i.e. internally charged cavities). Since ferroelectrets not only represent a scientific curiosity but also have great application potential, they have attracted tremendous attention from science and industry. The research and development of ferroelectrets has witnessed significant progress in the past few years. New ferroelectrets with large transverse piezoelectric activity, biodegradable ferroelectrets as well as 3-D printed ferroelectrets are reported. Charging methods of high efficiency are proposed based on better understanding of the physico-chemical processes during charging. New insights into the piezoelectricity of ferroelectrets are provided. The development of ferroelectret-based piezoelectric-magnetic multimodal transducer films opens up new avenues for the research of ferroelectrets. Particularly, more and more novel applications of ferroelectrets in flexible pressure sensors, health monitoring, energy harvesting, air-coupled ultrasonic non-destructive testing etc. are reported. Here, these exciting recent advancements in the field of ferroelectret research are reviewed and discussed.

The Spec-Radiation Method for Layered Fluid Media

The real-time evaluation for non-destructive air-coupled ultrasonic testing of panel materials is a big task for several industries. To make these tests more and more accurate, efficient and reliable calculation methods from ultrasonic holography are essential. In the past, we presented the spec-radiation method as a fast and accurate method for such tasks. The spec-radiation method calculates the sound field utilizing data from a measurement plane at another parallel or tilted plane, especially the sound field at the surface of a panel. This can be used to detect flaws. There is a limitation of the current method: using the data on the panel surface limits the accuracy of the detected flaws. A big step forward could be expected if the sound field in the material were known. As a first step, we developed the spec-radiation method forward to consider multiple material layers. For now, we made the major assumption that all layers have fluid-like properties. Hence, transversal waves were neglected. This extension of the spec-radiation method was validated utilizing an experiment. We present that flaws in the panel material can be detected with higher accuracy at a similar speed compared to our former approach.

Investigation of real delamination detection in composite structure using air-coupled ultrasonic testing

Air-coupled ultrasonic testing (ACUT) is a non-destructive testing method that has been used to investigate composite defects. In this research, a new technique of creating a delamination defect is presented to produce a thin air gap for measuring realistic ultrasonic responses. Composite samples have been made of fiberglass and epoxy resin with six delamination defects in different sizes. A proper setup test has been established for the through-transmission ultrasonic inspection of the composite samples. For fault detection, statistical parameters are derived from ultrasonic response signals. Following dimensionality reduction, a support vector machine (SVM) classifier has been used to improve delamination detection accuracy. Also, the numerical simulation of ultrasonic testing on the composite specimen with delamination defects has been performed to obtain the test parameters. Although the ultrasonic responses have been significantly weakened by the delamination defects (air gaps) yet applying signal processing techniques following by an SVM classifier has proven to improve delamination detection in the composite.

Enhancing the spectral signatures of ultrasonic fluidic transducer pulses for improved time-of-flight measurements

Air-coupled ultrasonic (ACU) testing has proven to be a valuable method for increasing the speed in non-destructive ultrasonic testing and the investigation of sensitive specimens. A major obstacle to implementing ACU methods is the significant signal power loss at the air–specimen and transducer–air interfaces. The loss between transducer and air can be eliminated by using recently developed fluidic transducers. These transducers use pressurized air and a natural flow instability to generate high sound power signals. Due to this self-excited flow instability, the individual pulses are dissimilar in length, amplitude, and phase. These amplitude and angle modulated pulses offer the great opportunity to further increase the signal-to-noise ratio with pulse compression methods.In practice, multi-input multi-output (MIMO) setups reduce the time required to scan the specimen surface, but demand high pulse discriminability. By applying envelope removal techniques to the individual pulses, the pulse discriminability is increased allowing only the remaining phase information to be targeted for analysis. Finally, semi-synthetic experiments are presented to verify the applicability of the envelope removal method and highlight the suitability of the fluidic transducer for MIMO setups.

Application of Air-Coupled Ultrasonic Nondestructive Testing in the Measurement of Elastic Modulus of Materials

It is difficult to measure elastic modulus simply and accurately in the testing of mechanical properties of materials. Combined with static tensile method, this paper presents a method for measuring elastic modulus of materials based on air-coupled ultrasonic nondestructive testing. Firstly, the 1–3 piezoelectric composite material and the matching material of low acoustic impedance are self-made, and 400 kHz air-coupled ultrasonic transducer is fabricated. Then, the performance of the transducer is tested, and the insertion loss and bandwidth of −6 dB are −33.5 dB and 23.4%, respectively. Compared with the traditional instrument for measuring elastic modulus, the measurement of elastic modulus of carbon steel rod material is realized in this paper, and the measured results are in agreement with the accepted value. In addition, from the angle of relative uncertainty, how to reduce the measurement error by improving the device is analyzed. It can be shown that the method has high linearity, high symmetry, and good stability and repeatability. This paper provides a new way for the selection and design of measuring instrument components.

Non-contact phase coded excitation of ultrasonic Lamb wave for blind hole inspection

The combination of air-coupled ultrasonic testing (ACUT) and ultrasonic Lamb wave is featured with long-distance propagation and high sensitivity to discontinuities, which is a promising method for rapid and accurate inspection of plate-like materials and lightweighted structures. However, dispersive nature of Lamb wave, signal attenuation plus inevitable noises would lead to low signal-to-noise ratio (SNR). To address this problem, phase coded excitation and pulse compression technique are proposed in this paper to achieve higher SNR by over 10 dB in received signals. 13-bit and 1-carrier-period Barker code is employed as both main lobe peak and Peak Side-lobe Level (PSL) are relatively high. It is demonstrated that A0 mode Lamb wave has good localization ability for defects based on these SNR-enhanced signals. Furthermore, Damage Index (DI) and modified Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) are applied to realize ultrasonic imaging based defect evaluation. Results show that the imaging results agree well with the actual artificial defects in terms of size and shape. Lamb-wave-based air-coupled ultrasonic testing, combined with DI and ultrasonic imaging algorithm, could be a potential way in the NDT of lightweighted structures.

Vibrometric ranging for air-coupled ultrasonic testing

Ultrasonic testing is a well-established method to assess characteristics of a specimen non-destructively via the time of flight (ToF) of the acoustic signal. While traditional methods require dry or liquid coupling of transducer and specimen, air-coupled ultrasound (ACU) transducers allow contact-free measurements. The signal is passing through air before it interacts with the specimen and, finally, is received by a sensor. Knowledge of the acoustic travel time between transducer and specimen is imperative to calculate the correct ToF through the specimen. In this contribution, we propose the use of a laser Doppler vibrometer to obtain this delay time through air. Exploiting the opto-acoustic effect, an ACU signal is observable before and after reflection at the specimen surface. By auto-correlating the signal, the delay time through air can then be found non-invasively, calibration-free, resonance-free, and on-line. Furthermore, this method provides a reference signal for pulse compression approaches. The benefits of vibrometric ranging are demonstrated experimentally using a piezoelectric and a fluidic transducer.

High-resolution air-coupled laser ultrasound imaging of microstructure and defects in braided CFRP

Defect inspection of braided carbon fiber reinforced polymer (CFRP) is very difficult due to its nonhomogeneity, anisotropy, and sensitivity to coupling agents. This paper presents a hybrid system for detecting microstructure and defects in braided CFRP that combines the advantages of laser-ultrasound and air-coupled ultrasonic testing. Through the finite element method, the ultrasonic field propagating on the braided CFRP was simulated during the laser-induced ultrasound, and the influences of laser parameters and surface braided structure were analyzed on the laser ultrasonic signal. This detection method based on air-coupled laser ultrasound can provide the near-surface microstructure characterization of the resin pockets and braided fiber bundles, and also can achieve both shallow and deep defect detection of CFRP sheets which is comparable to that of the contact-type high-frequency phased array with higher contrast and less distortion. These results indicate that it has the potential to develop a non-contact, high-resolution, and low-cost method for the detection and repairment of advanced composite materials.

Switching Action of a Bistable Fluidic Amplifier for Ultrasonic Testing

Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths required for concrete structures, the use of traditional piezoelectric transducer requires high power electric systems. In this study, a novel fluidic transducer based on a bistable fluidic amplifier is investigated. Previous experiments have shown that the switching action of the device produces a high-power broadband ultrasonic signal. This study will provide further insight into the switching behaviour of the fluidic switch. Therefore, parametric CFD simulations based on compressible supersonic RANS simulations were performed, varying the inlet pressure and velocity profiles for the control flow. Switching times are analyzed with different methods, and it was found that these are mostly independent of the slope of the velocity profile at the control port. Furthermore, it was found that an inversely proportional relationship exists between flow velocity in the throat and the switching time. The results agree with the theoretical background established by experimental studies that can be found in the literature.

Switching Action of a Bistable Fluidic Amplifier for Ultrasonic Testing

Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths required for concrete structures, the use of traditional piezoelectric transducer requires high power electric systems. In this study, a novel fluidic transducer based on a bistable fluidic amplifier is investigated. Previous experiments have shown that the switching action of the device produces a high-power broadband ultrasonic signal. This study will provide further insight into the switching behaviour of the fluidic switch. Therefore, parametric CFD simulations based on compressible supersonic RANS simulations were performed, varying the inlet pressure and velocity profiles for the control flow. Switching times are analyzed with different methods, and it was found that these are mostly independent of the slope of the velocity profile at the control port. Furthermore, it was found that an inversely proportional relationship exists between flow velocity in the throat and the switching time. The results agree with the theoretical background established by experimental studies that can be found in the literature.

Comparison of Ultrasonic Non-Contact Air-Coupled Techniques for Characterization of Impact-Type Defects in Pultruded GFRP Composites.

This article compares different air-coupled ultrasonic testing methods to characterize impact-type defects in a pultruded quasi-isotropic glass fiber-reinforced plastic (GFRP) composite plate. Using the air-coupled transducers, comparisons among three methods were performed, namely, bulk-wave through transmission, single-side access using guided waves, and ultrasonic-guided wave tomography. The air coupled through transmission technique can determine the size and shape of impact-type defects with a higher resolution, but with the consequence of time consumption and, more importantly, the necessity of access to both sides of the sample. The guided wave technique on the other hand, allows a single-side inspection and is relatively fast. It can be used to determine the size of the defect using ultrasonic B-scan, but the exact shape of the defect will be compromised. Thus, in this article, to determine the shape of the defect, application of the parallel beam tomographic reconstruction technique using guided Lamb waves is demonstrated. Furthermore, a numerical finite element simulation was performed to study the effects of guided wave propagation in the composite sample and interaction with the internal defect. Lastly, the results from the experiments of different techniques were compared according to possibilities of defect sizing and determination of its shape.

Air-Coupled, Contact, and Immersion Ultrasonic Non-Destructive Testing: Comparison for Bonding Quality Evaluation

The objective of this study is to compare the performance of different ultrasonic non-destructive testing (NDT) techniques for bonding quality evaluation. Aluminium-epoxy-aluminium single lap joints containing debonding in the form of release film inclusions have been investigated using three types of ultrasonic NDT methods: contact testing, immersion testing, and air-coupled testing. Apart from the traditional bulk wave ultrasound, guided wave testing was also performed using air coupled and contact transducers for the excitation of guided waves. Guided wave propagation within adhesive bond was numerically simulated. A wide range of inspection frequencies causing different ultrasonic wavelengths has been investigated. Average errors in defect sizing per ultrasonic wavelength have been used as a feature to determine the performance of each ultrasonic NDT technique. The best performance is observed with bulk wave investigations. Particularly, the higher frequencies (10–50 MHz) in the immersion testing performed significantly better than air-coupled testing (300 kHz); however, air coupled investigations have other advantages as contactless inspection. Whereas guided wave inspections show relatively lower accuracy in defect sizing, they are good enough to detect the presence of the debonding and enable to inspect long range. Even though each technique has its advantages and limitations, guided wave techniques can be practical for the preliminary in-situ inspection of adhesively bonded specimens.