Air-coupled Ultrasonic Testing Research Articles

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.

The Spec-Radiation Method as a Fast Alternative to the Re-Radiation Method for the Detection of Flaws in Wooden Particleboards

For real-time evaluation of non-destructive air-coupled ultrasonic testing of wood-based materials, efficient and reliable calculation methods from ultrasonic holography are essential. Presented here is the spec-radiation method as a fast alternative to the re-radiation method. The spec-radiation method offers a more accurate and up to 88% faster evaluation than the re-radiation method for the determination of flaws in particleboards. Flaws of sub-wavelength sizes can be identified and their shape and location can be determined with this method. The spec-radiation method produces a better reproduction of the sound field than the re-radiation method, especially in the area of the measuring plane.

Air-Coupled Reception of a Slow Ultrasonic A0 Mode Wave Propagating in Thin Plastic Film.

At low frequencies, in thin plates the phase velocity of the guided A0 mode can become slower than that of the ultrasound velocity in air. Such waves do not excite leaky waves in the surrounding air, and therefore, it is impossible to excite and receive them by conventional air-coupled methods. The objective of this research was the development of an air-coupled technique for the reception of slow A0 mode in thin plastic films. This study demonstrates the feasibility of picking up a subsonic A0 mode in plastic films by air-coupled ultrasonic arrays. The air-coupled reception was based on an evanescent wave in air accompanying the propagating A0 mode in a film. The efficiency of the reception was enhanced by using a virtual array which was arranged from the data collected by a single air-coupled receiver. The signals measured at the points corresponding to the positions of the phase-matched array were recorded and processed. The transmitting array excited not only the A0 mode in the film, but also a direct wave in air. This wave propagated at ultrasound velocity in air and was faster than the evanescent wave. For efficient reception of the A0 mode, the additional signal-processing procedure based on the application of the 2D Fourier transform in a spatial–temporal domain. The obtained results can be useful for the development of novel air-coupled ultrasonic non-destructive testing techniques.

Comparison between air-coupled ultrasonic testing and active thermography for defect identification in composite materials

ABSTRACT Glass or carbon fibre-reinforced polymer (FRP) structures have attained a relevant role in modern production thanks to their lightweight, high resistance to corrosion and fatigue, and relative ease of manufacturing complex shapes. Nevertheless, several types of defects, such as voids, matrix fractures, delaminations and impact damages, which might significantly reduce the useful life of composite-made structural components, are often difficult to be identified. Conventional non-destructive methods, such as ultrasonic testing, are routinely used to monitor composite constructions with good results. However, an inspection of large structures requires alternative and faster NDT methods. Among these, air-coupled ultrasonic testing (ACUT) and active infrared thermography (AIRT) are generally considered the most promising ones. In this paper, ACUT and AIRT have been compared by performing experimental tests on two reference composite panels with synthetic defects. Polytetrafluoroethylene (PTFE) and aluminium inserts with different sizes and located at various depths have been used as artificial defects. The relative merits and drawbacks of the two techniques have been highlighted in terms of defect size sensitivity, maximum depth that can be detected, and thermo-physical properties of the reflector. A qualitative and quantitative analysis has been carried out to evaluate the ability of the two techniques in retrieving the actual size of the subsurface defects.

Air-coupled ultrasonic testing to estimate internal defects in composite panels used for boats and luxury yachts

In constructions of boats and luxury yachts, Glass Fiber Reinforced Polymer (GFRP) composite materials are widely employed owing to their high mechanical performance, light weight and flexibility in manufacturing complex shapes. However, GFRP composite materials can be affected by several intrinsic defects (e.g. inclusion, delamination, resin excess or lack), because of both wrong production process and post-production handling. Although conventional ultrasonic testing is the most applied method to enhance the structural safety and quality, it presents a strong limitation in achieving an efficient contact between the transducers and the testing surfaces. Air-coupled ultrasonic testing can be used as a valid alternative, overcoming the previous disadvantage. Aim of this study is to demonstrate the ability of air-coupled ultrasonic testing (ACUT) for qualitatively and quantitatively evaluation of internal defects in GFRP composite panels as those typically used for boats and luxury yachts construction. Simulated delaminations of different sizes have been analysed as well as excess and lack of resin. Results highlighted that a good signal-to-noise ratio can be achieved for delamination greater than 15 mm with correct defect shape recovering, hence making ACUT an appropriate choice for the interactive engineering design and manufacturing of boats.

Design, fabrication and characterization of Emfi-based ferroelectret air-coupled ultrasonic transducer

Air-coupled ultrasonic testing (ACUT) has many advantages over other non-destructive testing (NDT) methods, among which the most outstanding one is completely no contamination of the sample. Therefore, it can be applied in various conditions where liquid coupling media is not suitable. With the development of ACUT, it is found that mismatch between air and solid is one of the biggest difficulties confronting researchers, and electromechanical film (EMFi) with low acoustic impedance can be a promising solution when used as vibration element in air-coupled ultrasonic transducers. In this contribution, one planar and two spherically focused EMFi-based transducers were designed, fabricated, and their properties were characterized through frequency response test as well as focal length test. Then imaging experiments of PMMA samples were performed to demonstrate the possibility and feasibility of EMFi transducers in non-contact testing.

Numerical Simulation and Experimental Study of Fluid-Solid Coupling-Based Air-Coupled Ultrasonic Detection of Stomata Defect of Lithium-Ion Battery.

Aiming at the characteristics of the periodic stacking structure of a lithium-ion battery core and the corresponding relationship between the air-coupled ultrasonic transmission initial wave and the wave propagation mode in each layer medium of a lithium-ion battery, the homogenized finite element model of a lithium-ion battery was developed based on the theory of pressure acoustics and solid mechanics. This model provided a reliable method and basis for solving the visualization of ultrasonic propagation in a lithium-ion battery and the analysis of ultrasonic time-frequency domain characteristics. The finite element simulation analysis and experimental verification of a lithium-ion battery with a near-surface stomata defect, near-bottom stomata defect and middle-layer stomata defect were performed. The results showed that the air-coupled ultrasonic transmission signal can effectively characterize the stomata defect inside a lithium-ion battery. The energy of an air-coupled ultrasonic transmission signal is concentrated between 350–450 kHz, and the acoustic diffraction effect has an important influence on the effect of the ultrasonic and stomata defect. Based on the amplitude response characteristics of the air-coupled ultrasonic transmission wave in the stomata defect area, a C-scan of the lithium-ion battery was performed. The C-scan result verified that air-coupled ultrasonic testing technology can accurately and effectively detect the pre-embedded stomata defect and natural stomata defect in a lithium-ion battery, which is able to promote and expand the application of the technology in the field of electric energy security.

Trajectory planning method for improving alignment accuracy of probes for dual-robot air-coupled ultrasonic testing system

Composite workpieces, especially the complex-curved surfaces composite workpieces, have been increasingly used in different industries. Non-destructive testing of these parts has become an urgent problem to be addressed. To solve the problem, this article presents a dual-robot air-coupled ultrasonic non-destructive testing scheme and introduces the structure of the system and a general calibration method for the workpiece frame of a dual-robot system in detail. Importantly, this article proposes a tangential constraint method, which makes the probes completely aligned during the inspection process. Verification experiments and ultrasonic testing experiments for a glued multilayered composite workpiece were performed using the dual-robot air-coupled ultrasonic non-destructive testing system. A comparative experiment was also performed using a dual-robot water jet-coupling ultrasonic testing system. Experimental results show that the dual-robot non-destructive testing scheme and the tangential constraint method function well, and all the artificial defects on the sample can be detected by both kinds of testing methods. Vivid 3-D C-scan image based on the test result is provided for convenience of observation. In other words, a kind of flexible versatile testing platform with multiple degrees of freedom is established.

Detection and Characterization of Debonding Defects in Aeronautical Honeycomb Sandwich Composites Using Noncontact Air-Coupled Ultrasonic Testing Technique

The finite models of honeycomb sandwich composite with intact and embedded debonding defects are constructed. The sound pressure in fluid domain and the stress strain problem in solid domain are related by acoustic-structure coupling method, which visually shows the propagation process and modal characteristics of the acoustic wave inside the honeycomb sandwich composite. The simulation results show that the transmission longitudinal wave T1 (transmission initial wave) can effectively characterize debonding defects of honeycomb sandwich composite. However, in the actual detection of honeycomb sandwich composite, there are some problems, such as poor Signal-to-noise ratio (SNR) of received signal, incognizable transmission initial wave. In order to solve these problems, this paper proposes to apply polyphase coded pulse compression technique to air-coupled ultrasonic testing system. The actual test results show that the SNR of received signal is effectively improved, the transmission initial wave can be effectively identified, and the compressed signal has a good response to debonding defect. The air-coupled ultrasonic testing C scan result of honeycomb sandwich composite verifies the rationality and correctness of the theoretical simulation and signal processing technique, which promotes industrial application of air-coupled ultrasonic testing technique in the aerospace field.

Crack detection in monocrystalline silicon solar cells using air-coupled ultrasonic lamb waves

In recent years, solar energy becomes one of the most popular renewable energy resources. Among the solar energy systems, solar cells are considered to be one of the most critical components, which are naturally fragile during either fabrication or service. Thus, nondestructive examination of solar cells is required practically. In this research, solar cells made from monocrystalline silicon are selected as the typical samples. The air-coupled ultrasonic nondestructive testing system, based on the generation and reception of the A0 mode Lamb waves, will be adopted for defects detection. Firstly, the dispersion curves of Lamb waves in anisotropic monocrystalline silicon are calculated theoretically, which provides a theoretical foundation for analyzing the behavior of Lamb waves propagating in silicon solar cells. Then, experiments on monocrystalline silicon solar cells are carried out by introducing the air-coupled ultrasonic Lamb waves. The amplitude distributions of Lamb waves propagating along various rotation angles in silicon solar cells are obtained. It shows a strong anisotropic properties of the tested samples. Finally, artificial cracks in solar cells are scanned circularly and laterally. The results of amplitude distributions give the scattering pattern of cracks, which can be used to determine whether the crack breaks through or not. And the extracted amplitude cross-correlation coefficients can help to estimate the length of them.

Real-time monitoring of welding process using air-coupled ultrasonics and acoustic emission

The automated weld quality assurance can improve efficiency and productivity. This paper presents the development of real-time weld quality assurance approach for gas tungsten arc welding (GTAW) using acoustic emission (AE) and air-coupled ultrasonic testing (UT). The major weld defect of interest in this paper is burn through, that is, melting through the base metal during welding that creates a hole/gap. The in situ monitoring system evaluates the changes in weld size leading to burn through by changing the weld heat input. Different categories of burn through are defined that include melting of the back of the plate without any molten metal exiting to formation of a hole in the plate. It is demonstrated that complete air-coupled UT cannot be used simultaneously with welding due to the influence of the magnetic field that develops in the weld torch during welding, which weakens the ultrasonic signal. Consequently, a rolling UT transmitter is combined with air-coupled UT receiver to increase the signal/noise value. Wave dispersion is detected due to the different levels of burn through. While UT method provides quantitative information about the weld state, any localized surface discontinuity causes sudden surges in the AE energy indicating non-uniform welding qualitatively. It is concluded that passive and active nondestructive evaluation methods should be combined to monitor weld quality real time for qualitative and quantitative assessment.

Measurements of the weak bonding interfacial stiffness by using air-coupled ultrasound

An air-coupled ultrasonic method, focusing on the problem that weak bonding interface is difficult to accurately measure using conventional nondestructive testing technique, is proposed to evaluate the bond integrity. Based on the spring model and the potential function theory, a theoretical model is established to predict the through-transmission spectrum in double-layer adhesive structure. The result of a theoretical algorithm shows that all the resonant transmission peaks move towards higher frequency with the increase of the interfacial stiffness. The reason for these movements is related to either the normal stiffness (KN) or the transverse stiffness (KT). A method to optimize the measurement parameters (i.e. the incident angle and testing frequency) is put forward through analyzing the relationship between the resonant transmission peaks and the interfacial spring stiffness at the frequency below 1MHz. The air-coupled ultrasonic testing experiments at the normal and oblique incident angle respectively are carried out to verify the theoretical analysis and to accurately measure the interfacial stiffness of double-layer adhesive composite plate. The experimental results are good agreement with the results from the theoretical algorithm, and the relationship between bonding time and interfacial stiffness is presented at the end of this paper.

Detection of small-flaw in carbon brake disc (C-C) using air-coupled ultrasonic C-scan technique

The purpose of this paper is to conduct the study on detection identification and assessment of internal defects for the C-C brake disc composite for aircrafts by applying non-contact air-coupled ultrasonic testing. The Non-contact Air-coupled Ultrasonic Testing (NAUT) used in this paper is the most advanced ultrasonic testing technique. It is a nondestructive testing that complements the energy loss occurred due to the acoustic impedance difference in the air with two transmitting/receiving non-contact ultrasonic probes and enables the non-contact ultrasonic testing through high-powered ultrasonic focusing by using high-frequency band ultrasonic Pulser- Receiver. In this paper, non-contact air-coupled ultrasonic detection experiment applied the Transmission Method and Reflection Method was conducted as follows; When detecting an internal defect of the carbon brake disc, we compared the C-scan results according to whether masking was applied to the side of the object to be detected, measured at the ultrasonic transmission frequencies of 340 kHz and 400 kHz respectively, to detect internal defects of the carbon brake disc through the measured C-scan images. In this way, we confirmed that there is a significant difference in the internal defect detection performance depending on the masking and the change in the ultrasonic transmission frequency value.

Application of Golay Coded Pulse Compression in Air-coupled Ultrasonic Testing of Flexible Package Seal Defect

Air-coupled ultrasonic testing is a possible implementation method for online seal defect detection of flexible package. However, large acoustic impedance difference between air and solid material leads to difficulties in coupling ultrasonic energy into solid materials and results in a poor signal to noise ratio (SNR). To solve this problem, pulse compression technique based on Golay codes has been applied in air-coupled ultrasonic testing of flexible package seal defect. The effects of different Golay codes (4, 8 and 16 bits) and Golay codes with different bit length (1, 2, and 3 circles) are investigated to obtain a reasonable parameter selection. The results demonstrate that the Golay coded pulse compression technique can improve the SNR ratio of the received signal remarkably, and suppressed the side lobe of the compressed pulse effectively. An improvement in SNR of the seal region image up to 9.4 dB is achieved, compared with the non-modulated excitation method.

Application of P4 Polyphase codes pulse compression method to air-coupled ultrasonic testing systems

Air-coupled ultrasonic testing systems are usually restricted by low signal-to-noise ratios (SNR). The use of pulse compression techniques based on P4 Polyphase codes can improve the ultrasound SNR. This type of codes can generate higher Peak Side Lobe (PSL) ratio and lower noise of compressed signal. This paper proposes the use of P4 Polyphase sequences to code ultrasound with a NDT system based on air-coupled piezoelectric transducer. Furthermore, the principle of selecting parameters of P4 Polyphase sequence for obtaining optimal pulse compression effect is also studied. Successful results are presented in molded composite material. A hybrid signal processing method for improvement in SNR up to 12.11dB and in time domain resolution about 35% are achieved when compared with conventional pulse compression technique.

Ultrasonic Inspection and Data Analysis of Glass- and Carbon-Fibre-Reinforced Plastics

Non-destructive testing (NDT) helps to find material defects without having an influence on the material itself. It is applied as a method of quality control, for online structural health monitoring, and for inspection of safety related components. Due to the ability of automation and a simple test setup ultrasonic testing is one major NDT technique next to several existing options. Whereas contact technique allows the use of higher frequencies of some MHz and phased array focusing, air-coupled ultrasonic testing (ACUT) shows different advantages. Most significant for ACUT is the absence of any coupling fluid and an economical test procedure respective time and costs. Both contact technique and ACUT have been improved and enhanced during the past years. One important enhancement is the development of airborne transducers based on ferroelectrets, like charged cellular polypropylene (cpp), which makes the application of any matching layers being mandatory in conventional piezoelectric transducers unnecessary. In this contribution we show ultrasonic inspection results of specimens made of carbon- and glass-fibre-reinforced plastic. These specimens include defects represented by drill holes and artificial delaminations of various size and depth. We compare inspection results achieved by using contact technique to those achieved by ACUT. For ACUT, conventional piezoelectric transducers and transducers based on cpp were used, both focused as well as non-focused types. Contact inspections were performed with a multi-channel matrix array probe. Once the inspection data is recorded it can be analysed in order to detect and evaluate defects in the specimen. We present different analysing strategies and compare these regarding detection rate and sizing of defects.

Parameter optimization of pulse compression method in air-coupled ultrasonic testing

Parameter optimization of pulse compression method in air-coupled ultrasonic testing

Wavelet Threshold Denoising Experiments of Air-coupled Ultrasonic Signal

In air-coupled ultrasonic testing, the received ultrasonic signal has poor signal to noise ratio(SNR) due to low transmission ratio, noises introduced by transmitting and receiving system, and scattering noises during the propagation of ultrasound in the media. The use of wavelet threshold denoising technique to deal with handicaps above is faced with problems of parameter selection including wavelet base, decomposition level, and threshold function. The wavelet threshold denoising experiments are carried out using the single factor analysis method based on the basic principles of wavelet theory. Different wavelet bases from different wavelet family(Daubechies, Symlet, and Coiflet), decomposition level(4-8 levels), and threshold function(soft threshold function or improved threshold function based on hard and soft threshold) are used to wavelet threshold denoising process for the realistic noisy ultrasonic signal. The impacts of different parameters on noise filtering are analyzed from the comparison of SNR and frequency spectrum of the denoised signal. From the results, denoised signal possesses higher SNR and signal amplitude using wavelet base from coiflet family. The higher the decomposition level, the higher the SNR of the denoised signal, but growth trend gradually stabilized. The impact of threshold function on the performance of denoising is not very significant, and acceptable filtering results can be acquired from soft threshold or improved threshold function.