Air-coupled Ultrasonic Measurements Research Articles

OsciCheck: A Novel Fluidic Transducer for Air-Coupled Ultrasonic Measurements

Ultrasonic measurement technology has become indispensable in NDT-CE. Air-coupled ultrasonic (ACU) measurement techniques promise to reduce measurement time. However, the signal quality suffers from large specific impedance mismatch at the transducer-air and air-specimen interface. Additionally, large pressure amplitudes are necessary for the penetration depth required in NDT-CE applications. To address the specific requirements of ultrasonic testing in NDT-CE, a robust ACU transducer was developed, that generates ultrasound by quickly switching a pressurized air flow. The simple design of the fluidic transducer makes the device maintenance free and resilient against harsh environmental conditions. Since the signal is generated by aeroacoustics, there is no specific impedance mismatch between the transducer and the surrounding air. The ultrasonic signal exhibits frequencies in the 30-60 kHz range and is therefore well suited to penetrate heterogenous materials such as concrete. This contribution gives an introduction in the working principle and signal characteristics of the fluidic transducer. Its application on different materials is validated. A detailed outlook is given to discuss the future potential of fluidic ultrasonic actuators.

1Pb3-3 Fundamental study of 2-2 ceramic-air composite transducers for air-coupled ultrasonic measurement

1Pb3-3 Fundamental study of 2-2 ceramic-air composite transducers for air-coupled ultrasonic measurement

3J3-2 Development of 1-3 ceramic-air composite transducers for air-coupled ultrasonic measurement

3J3-2 Development of 1-3 ceramic-air composite transducers for air-coupled ultrasonic measurement

Evaluation of surface cracks of bending concrete using a fully non-contact air-coupled nonlinear ultrasonic technique

In this paper, a fully non-contact second harmonic generation (SHG) technique using a pair of air-coupled ultrasonic transducers is developed and the feasibility of the technique is investigated through the evaluation of surface cracks of concrete beams subjected to the bending load. The reliability of developed technique is subsequently validated by comparing the non-contact nonlinear ultrasonic measurements with measurement results based on contact-type sensors, where the coefficient of variation of non-contact measurements is averagely about 46% of the contact measurements. The defined nonlinear parameter is found to have a monotonically increasing trend with the growth of concrete crack, and the nonlinear parameter corresponding to the largest crack increases about 7 times from its initial value corresponding to the sample in intact state. In contrast, the increase of linear parameter namely the time-of-flight of surface wave is only about 40%. The difference as high as one order of magnitude verifies the high sensitivity of developed air-coupled SHG technique. With consideration of the easily removable characteristic of air-coupled ultrasonic measurements, the developed SHG technique could be promising for the large scale quality control of concrete structures in engineering practice.

Air-coupled ultrasonic tomography of solids: 1 Fundamental development

Ultrasonic tomography is a powerful tool for identifying defects within an object or structure. But practical application of ultrasonic tomography to solids is often limited by time consuming transducer coupling. Air-coupled ultrasonic measurements may eliminate the coupling problem and allow for more rapid data collection and tomographic image construction. This research aims to integrate recent developments in air-coupled ultrasonic measurements with current tomography reconstruction routines to improve testing capability. The goal is to identify low velocity inclusions (air-filled voids and notches) within solids using constructed velocity images. Finite element analysis is used to simulate the experiment in order to determine efficient data collection schemes. Comparable air-coupled ultrasonic signals are then collected through homogeneous and isotropic solid (PVC polymer) samples. Volumetric (void) and planar (notch) inclusions within the samples are identified in the constructed velocity tomograms for a variety of transducer configurations. Although there is some distortion of the inclusions, the experimentally obtained tomograms accurately indicate their size and location. Reconstruction error values, defined as misidentification of the inclusion size and position, were in the range of 1.5-1.7%. Part 2 of this paper set will describe the application of this imaging technique to concrete that contains inclusions.

Air-coupled ultrasonic tomography of solids: 2 Application to concrete elements

Applications of ultrasonic tomography to concrete structures have been reported for many years. However, practical and effective application of this tool for nondestructive assessment of internal concrete condition is hampered by time consuming transducer coupling that limits the amount of ultrasonic data that can be collected. This research aims to deploy recent developments in air-coupled ultrasonic measurements of solids, described in Part 1 of this paper set, to concrete in order to image internal inclusions. Ultrasonic signals are collected from concrete samples using a fully air-coupled (contactless) test configuration. These air coupled data are compared to those collected using partial semi-contact and full-contact test configurations. Two samples are considered: a 150 mm diameter cylinder with an internal circular void and a prism with 300 mm x 300 mm square cross-section that contains internal damaged regions and embedded reinforcement. The heterogeneous nature of concrete material structure complicates the application and interpretation of ultrasonic measurements and imaging. Volumetric inclusions within the concrete specimens are identified in the constructed velocity tomograms, but wave scattering at internal interfaces of the concrete disrupts the images. This disruption reduces defect detection accuracy as compared with tomograms built up of data collected from homogeneous solid samples (PVC) that are described in Part 1 of this paper set. Semi-contact measurements provide some improvement in accuracy through higher signal-to-noise ratio while still allowing for reasonably rapid data collection.

Air-Coupled Ultrasonic Transducers Based on an Application of the PMN-32%PT Single Crystals

Application of air-coupled ultrasonic measurements is problematic due big acoustic transduction losses of conventional ultrasonic air-coupled transducers. One of the ways to solve this problem is application of more efficient ultrasonic transducers made of PMN-32 %PT crystals. The objective of this work was development of novel effective air-coupled ultrasonic transducer with PMN-32% PT crystal plates. The high electromechanical factor of the transverse extension mode enables to achieve a good performance of air-coupled transducers. The developed and investigated ultrasonic transducer possesses in a transmission mode 10 times better efficiency than a PZT composite ultrasonic transducer.

A Fully Non-contact, Air-Coupled Ultrasonic Measurement of Surface Breaking Cracks in Concrete

Previous researchers have shown that the surface wave transmission coefficient across a surface breaking crack in concrete can be used to estimate the depth of the crack. This study is the first to investigate a fully non-contact ultrasonic technique using air-coupled transducers which can reliably measure the surface wave transmission coefficient across a surface breaking crack in concrete. Using this setup, the transmission coefficient of Rayleigh surface waves is measured for three notches with different depths (0.5, 1, and 2 cm). Then, the relationship between the transmission coefficient and the normalized notch depth is experimentally reconstructed and compared with analytical and numerical results. It is demonstrated that the proposed fully non-contact technique is efficient and promising as a field application in civil infrastructure. The advantages of the proposed non-contact technique compared to existing methods are also discussed.

Air-coupled ultrasonic through-transmission thickness measurements of steel plates

Non-destructive ultrasonic testing of steel structures provide valuable information in e.g. inspection of pipes, ships and offshore structures. In many practical applications, contact measurements are cumbersome or not possible, and air-coupled ultrasound can provide a solution.This paper presents air-coupled ultrasonic through-transmission measurements on a steel plate with thicknesses 10.15mm; 10.0mm; 9.8mm. Ultrasound pulses were transmitted from a piezoelectric transducer at normal incidence, through the steel plate, and were received at the opposite side. The S1,A2 and A3 modes of the plate are excited, with resonance frequencies that depend on the material properties and the thickness of the plate.The results show that the resonances could be clearly identified after transmission through the steel plate, and that the frequencies of the resonances could be used to distinguish between the three plate thicknesses.The S1-mode resonance was observed to be shifted 10% down compared to a simple plane wave half-wave resonance model, while the A2 and S2 modes were found approximately at the corresponding plane-wave resonance frequencies.A model based on the angular spectrum method was used to predict the response of the through-transmission setup. This model included the finite aperture of the transmitter and receiver, and compressional and shear waves in the solid. The model predicts the frequencies of the observed modes of the plate to within 1%, including the down-shift of the S1-mode.

Noncontact ultrasonic guided wave inspection of rails

This article describes a new system for high-speed and noncontact rail integrity evaluation being developed at the University of California at San Diego. A prototype using an ultrasonic air-coupled guided wave signal generation and air-coupled signal detection has been tested at the University of California at San Diego Rail Defect Farm. In addition to a real-time statistical analysis algorithm, the prototype uses a specialized filtering approach due to the inherently poor signal-to-noise ratio of the air-coupled ultrasonic measurements in rail steel. The laboratory results indicate that the prototype is able to detect internal rail defects with a high reliability. Extensions of the system are planned to add rail surface characterization to the internal rail defect detection. In addition to the description of the prototype and test results, numerical analyses of ultrasonic guided wave propagation in rails have been performed using a Local Interaction Simulation Approach algorithm and some of these results are shown. The numerical analysis has helped designing various aspects of the prototype for maximizing its sensitivity to defects.

Advances in the ultrasound characterization of dry-cured meat products

In this work, the feasibility of using non-contact ultrasonic techniques (air-coupled and scanning acoustic microscopy, SAM) for characterizing different dry-cured meat products was assessed. Air-coupled ultrasonic measurements were performed on vacuum packaged sliced dry-cured ham, and compared with contact measurements. The average ultrasonic velocity in dry-cured ham was 1846±49m/s and 1842±42m/s for air-coupled and contact measurements, respectively. The deviation (1% relative error) between both techniques was related to the influence of the heterogeneous structure and composition of dry-cured ham and the transducer focusing. The SAM was used to characterize dry-cured ham and chorizo samples. B-scan images for dry-cured ham and chorizo showed two dominant reflections from the sample, linked to reflections in the lean and fatty tissues. The results indicate that contact ultrasonic measurements could be replaced by the air-coupled technique, reducing the measuring time and the material handling. On the other hand, SAM technique allows the microscopic characterization of dry-cured meat products.

Investigation of accurate imaging of the defects in composite materials using ultrasonic air-coupled technique

In this paper air-coupled ultrasonic investigation of delamination type defects in GLARE3-3/2 composite material is presented. An air-coupled ultrasonic measurement technique has a great potential for investigation of those materials in manufacturing and maintenance. An important advantage of this technique is that investigations can be performed without couplants and therefore various materials that can absorb liquids (honeycombs, porous) can be tested. The objective of the work was to investigate the interaction of the ultrasonic wave with delamination type defect in GLARE3-3/2 composite material and to determine influence of interaction effects on accuracy of the defect size measurement. Numerical and experimental investigation was carried-out in the through transmission mode. Results of numerical investigation and experimental verification of interaction of the ultrasonic wave are presented. Estimation of accuracy of the measurement of the delamination defect sizes is also presented.

Online Profiling of Nonplanar Objects by High-Resolution Air-Coupled Ultrasonic Distance Measurements

Ultrasonic measurements are used in product sizing in various industrial technologies. The advantage of such techniques is that measurements can be performed contactless on a conveyer using an air-coupled ultrasonic method. Nevertheless, implementation of such systems is complicated due to temperature variations, a nonconstant movement velocity, and geometry of the object necessary to measure. In this paper, an ultrasonic technique for the measurement of geometric parameters of objects transported by a conveyer is presented. Operation of the system is based on precise measurements of the distance between the transducer and the surface of an object. The measurement technique includes such stages as detection of the object, recognition of a typical profile, using a feature extraction algorithm, and measurement of object dimensions at selected positions. As an example of implementation of such a measurement technique, the ultrasonic air-coupled system for the measurement of the thickness of chocolate during the production process is presented.

Air-coupled ultrasonic investigation of multi-layered composite materials

Air-coupled ultrasonics is fine alternative for the immersion testing technique. Usually a through transmission and a pitch–catch arrangement of ultrasonic transducers are used. The pitch–catch arrangement is very attractive for non-destructive testing and evaluation of materials, because it allows one-side access to the object. However, this technique has several disadvantages. It is sensitive to specularly reflected and edge waves. A spatial resolution depends on a distance between the transducers. A new method for detection and visualisation of inhomogeneities in composite materials using one-side access air-coupled ultrasonic measurement technique is described. Numerical predictions of Lamb wave interaction with a defect in a composite material are carried out and the interaction mechanism is explained. Experimental measurements are carried out with different arrangements of the transducers. The proposed method enables detect delamination and impact type defects in honeycomb materials.

Air-coupled ultrasonic measurement of the change in roughness of paper duringwetting

We investigated the change in surface roughness of paper duringwetting. Four kinds of paper were examined by time-dependent attenuation offocused 1.2 MHz airborne ultrasound reflected from a paper sample. The sampleswere wetted with fog from mixtures of water and isopropanol and thetransmitted ultrasonic signal was measured at 15 ms intervals for 16 s afterthe application of the mixture. The results showed that the attenuation of thereflected sound by time differs for various kinds of paper, indicating thatthere are differences in the development of wetting.

The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging

A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-fibre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements.

Air-coupled ultrasonic measurements of adhesively bonded multi-layer structures

An air-coupled ultrasonic system employing wideband micromachined capacitance transducers has been used for non-contact measurements of material properties in adhesively bonded multi-layer aluminum structures. By sweeping the frequency of the ultrasonic toneburst applied to the air-coupled source, while measuring the air-coupled receiver response, through-transmission spectra for normally incident ultrasound were obtained for various multi-layer structures at frequencies below 2 MHz. Resonant transmission peaks, which appeared in the resultant spectra, agree well with the output of a theoretical algorithm that predicts ultrasonic transmission through multi-layer structures. A separate theoretical model is also developed analytically to predict the modal shapes and resonance frequencies of the allowed longitudinal resonances of the structures. Comparison of the results of this model with the experimental data shows that the resonant transmission peaks observed in measured spectra resulted from the excitation of certain longitudinal normal modes of vibration within the layered structures. As such resonant normal modes provide a significant increase in transmission coefficient, they were employed for non-destructive testing purposes by accurately imaging spatial variations in the adhesive layers of the structures (e.g. variations in adhesive thickness, Young’s modulus, density, etc.). In particular, the ability of the air-coupled system to detect and image ∼30 μm thickness variations within the bond-line of an aluminum lap-joint is demonstrated, thus showing the high-accuracy that can result from air-coupled measurements due to the reduced damping of sample vibrations by the air. Such accuracy may prove of interest in developing a non-contact air-coupled ultrasonic bond-tester for multi-layer adhesively bonded structures.