Air-coupled Ultrasonic System Research Articles

Hybrid laser and air-coupled ultrasonic defect detection of aluminium and CFRP plates by means of Lamb mode

This study presents two non-contact systems – hybrid laser with air-coupled ultrasound and pure air-coupled ultrasonic system for the non-destructive inspection and detection of flaws in CFRP and aluminium plates. B-scans are used to identify the size and location of the defects. The defects are detected through changes in amplitude of the waves passing through them. B-scans, represented as intensity graphs, clearly show the changes in the defect profiles. In addition, there is agreement between the experimental values of the diameter of the defect and the measured values. The two systems and configurations are tested and compared in order to determine their effectiveness in defect identification. The results of this study prove the potential of these non-destructive and non-contact systems for defect detection in aluminium and CFRP composites.

Experimental determination of pressure-dependent stiffness of a nonlinear acoustic metamaterial

The field of acoustic metamaterials (AMM) involves the design of subwavelength structures to create material properties exceeding those of traditional composite materials. One such structure developed recently, known as a “Buckliball,” displays strong pressure-dependent stiffness [Proc. Natl. Acad. Sci. USA 109, 5978 (2012)]. The unique response of these structures results from element geometry: spherical elastomeric shells with thin, patterned circular membranes whose skeleton buckles when subjected to a differential pressure. We present measurements of the properties of a nonlinear effective medium consisting of Buckliballs immersed in water. The experimental apparatus consists of four elements: (1) a pressure vessel, which contains (2) a resonance tube filled with Buckliballs suspended in water, (3) an air-coupled ultrasonic system to monitor water height in the resonance tube, and (4) acoustic excitation and measurement instrumentation to obtain the frequency dependent acoustic response of the Buckliball and water-filled resonator. Estimates of the effective stiffness and density are obtained by measuring the fundamental resonance frequency and water column height as a function of overpressure. Experimental results are compared with model estimates and discussed in the context of nonlinear acoustic wave propagation. [Work partially supported by ONR.]

Enhanced electromechanical response of ferroelectret ultrasonic transducers under high voltage excitation

Polypropylene based ferroelectret films exhibit a strong electromechanical activity and provide a promising solution for the air coupled ultrasonic (ACUS) transducers. Ultrasonic transmission between two air coupled ferroelectret transducers in dependence on the amplitude and polarity of the high voltage exciting pulse revealed a strongly non-linear electromechanical response of the ferroelectret transmitter which provides an increase in the transmitter efficiency. The authors present a simple model describing both promotion and competition of the piezoelectric and electrostriction contributions, as well as increase in the transducer constant under high voltage excitation. Enlargement of the inverse transducer constant of the polypropylene ferroelectret film by a factor of 4 was demonstrated. The non-linear properties of the polypropylene ferroelectrets result in a strong increase in their ACUS figure of merit under the high voltage excitation, which exceeds the results of their technological optimisation. Consequently, enhancement of the ACUS system transmission by 12 dB and signal to noise ratio by 32 dB was achieved.

Application of Golay codes to improve the dynamic range in ultrasonic Lamb waves air-coupled systems

Air-coupled ultrasonic systems are usually limited by poor signal-to-noise ratios. The use of pulse compression techniques based on Golay codes can help to improve the ultrasound signal-to-noise ratio. This type of codes allows virtually side lobe free operation and causes the cancellation of self-induced noise. This paper proposes the use of Golay sequences to code Lamb waves excited in thin materials with a NDT system based on air-coupled piezoelectric arrays. Furthermore, a system for the generation of Lamb waves modulated by 16-bits Golay codes is also proposed. Successful results are presented in copper plates. An improvement in signal-to-noise ratio up to 21 dB is achieved when compared with conventional pulse transmission.

Nonlinear electromechanical response of the ferroelectret ultrasonic transducers

The ultrasonic transmission between two air-coupled polypropylene (PP) ferroelectret (FE) transducers in dependence on the amplitude of the high-voltage exciting pulse revealed a strongly nonlinear electromechanical response of the FE transmitter. This phenomenon is described by a linear increase of the inverse electromechanical transducer constant \(t_{33}^{(1)}\) of the PP FE film with an increase of the exciting electrical pulse amplitude. Enlargement of \(t_{33}^{(1)}\) by a factor of 4 was achieved by application of 3500 V exciting pulses. The electrostriction contribution to \(t_{33}^{(1)}\) can be attributed to the electrostatic force between electrodes and the Maxwell stress effect. The nonlinear electromechanical properties of the PP FE result in a strong increase of its air-coupled ultrasonic (ACUS) figure of merit (FOM) under the high-voltage excitation, which exceeds results of the PP FE technological optimization. The FOM increase can be related to the increase of PP FE coupling factor and/or to the decrease of its acoustic impedance. A significant enhancement of the ACUS system transmission (12 dB) and signal-to-noise ratio (32 dB) was demonstrated by the increase of excitation voltage up to 3500 V. The nonlinear electromechanical properties of the PP FEs seem to be very important for their future applications.

Precise Displacement Measurements Using Phase Information of 40 kHz Ultrasonic Waves in Pinhole-Based Air-Coupled Ultrasonic System

Noncontact precise displacement measurements, including surface profiling, were demonstrated using the phase information of narrow-band 40 kHz continuous waves (wavelength λ=9 mm) in a pinhole-based air-coupled ultrasonic system beyond the diffraction limit. Real-time displacement measurements with a distance resolution of λ/900,000 (10 nm) were accomplished using a highly sensitive phase variation when the distance between the pinhole and the surface of an object was in the range from approximately 5 to 10 µm. To extend the working distance of our system while maintaining a high resolution, an automatic distance control system was constructed and introduced in our system. Using this advanced system, a surface topographical profile of 16 µm height was successfully measured, while maintaining maximum phase sensitivity. These results demonstrate the usefulness of our system when applied to noncontact precise displacement measurements.

Low-Frequency Air-Coupled Ultrasonic System beyond Diffraction Limit Using Pinhole

A novel noncontact air-coupled ultrasonic system introducing a conical acoustic probe (CAP) with a pinhole is constructed at low frequency. A 40 kHz ultrasonic wave in air goes through the pinhole, which has a diameter much smaller than the ultrasonic wavelength λ (≃9 mm), and forms a unique sound field. The lateral -3 dB widths of both theoretical and experimental sound fields approximately coincide with the pinhole diameter. A lateral resolution of λ/30 (0.3 mm) beyond the diffraction limit has been achieved using this system. As an application of the system, the surface profile for a 0.5 mmφ (λ/18) drilled hole in an aluminum plate was successfully imaged. The surface topographical profile of a step difference of 1 µm (λ/9000) was also measured.

Detection of Defects on Reverse Side of Metal Plate Using MHz-Range Air-Coupled Lamb Wave

A novel noncontact air-coupled ultrasonic system using MHz-range Lamb waves was constructed. In both the S0- and A0-modes, dispersion curves agreed well with the k– f image, which was obtained by applying a two-dimensional Fourier transform of the received waveforms using our system. The detection of square, flat-bottomed defects on the reverse side of a metal plate was demonstrated using two types of experimental arrangement. The position and width of the defects were estimated using the arrival time of the Lamb wave reflected from the defects. The profiling of the defect using the change in the amplitude of the Lamb wave confirms the usefulness of this system in the field of noncontact nondestructive evaluation.

Ultra-high distance resolution using phase information of 40 kHz air-coupled ultrasonic wave

An ultra-high distance resolution of a pinhole-based low-frequency air-coupled ultrasonic system beyond the diffraction limit is described. The high phase sensitivity of 40 kHz air-coupled ultrasonic wave signals is obtained when a distance between the pinhole and an object is small. Distance resolution of λ/900 000 (10 nm) with noncontact has been achieved using phase information of narrowband continuous ultrasonic waves.

Lateral resolution beyond diffraction limit using 40 kHz air-coupled ultrasonic systems

A novel air-coupled ultrasonic imaging system introducing a conical acoustic probe (CAP) with a pinhole is constructed at the low frequency of 40 kHz. An ultrasonic wave creeping out of the pinhole with a diameter much smaller than the ultrasonic wavelength λ makes a sound field in front of the tip of the CAP. The −3 dB width of the lateral direction of the sound field coincides with the pinhole diameter (λ/27). Lateral resolution of λ/27 (0.3 mm) beyond the diffraction limit has been achieved using this system.

P3-31 A low-frequency air-coupled ultrasonic system beyond diffraction limit introducing conical acoustic probe

P3-31 A low-frequency air-coupled ultrasonic system beyond diffraction limit introducing conical acoustic probe

2255 空中超音波を用いたC/C複合材の密度分布評価(S26-3 非破壊評価とモニタリング(3),S26 非破壊評価とモニタリング)

Anti-erosion and -ablation performance of C/C composite in the aero-transportation system is reported to depend on the density. We estimated the density distribution of C/C composite utilizing an air-coupled ultrasonic system. Charp-shaped ultrasonic was generated by the system on the market, but both the reflected and transmitted signals were monitored and pulse-compressed by the system developed in the laboratory. Amplitude of transmitted ultrasonic were found to depend on the density or porosity of C/C composite. Amplitude distribution of transmitted ultrasonic by the C-scan method agreed with that of the transmitted X-ray.

Submicrometer-Order Displacement Measurements Using an Air-Coupled Ultrasonic Transducer at Frequencies of 40 and 400 kHz

Noncontact air-coupled ultrasonic systems for measuring micrometer- or submicrometer-order displacements are newly constructed at frequencies of 40 and 400 kHz. The phase detection accuracy of the displacement measurements under several thousandths of ultrasonic wavelengths λ is markedly improved by the introduction of phase differences between the reference waves and the transmitting wave signal. To verify our system, the displacement of the reflector, which is translated by a high-precision mechanical stage, is measured using a single piezoelectric ceramic transducer in the reflection mode. Resolutions of displacement of better than 1 µm or 0.1 µm (λ/8500) using ultrasonic waves of 40 or 400 kHz in air, respectively, are achieved.

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.

Measurement of elastic constants in composite materials using air-coupled ultrasonic bulk waves

This paper presents numerical simulations and experimental measurements of longitudinal and shear bulk waves in composite materials, generated and detected with an air-coupled ultrasonic system. The transfer function of an anisotropic absorbing layered plate immersed in air is computed with the transfer matrix method, and the waveforms produced by transmission through a plate are simulated from the convolution of a reference waveform and the impulse response of the plate. The calculated waveforms compare well to those recorded experimentally, confirming that the waveforms recorded experimentally have the correct characteristics. It is demonstrated that it is possible to identify longitudinal and shear modes. Results are presented showing how this data may be used to recover the stiffness matrix of composite materials, using the angular variation of velocity of both modes.

Air-Coupled Ultrasonic System: A New Technology for Detecting Flaws in Paintings on Wooden Panels

AbstractAir-coupled ultrasound, a noncontact, nondestructive testing technique, has detected splits, checks, delaminations, cleavage, and voids in various materials. The system has inspected highly anisotropic and inhomogeneous materials such as wood and wood products, with surfaces layers of gesso, gesso and linen, paper, and wood veneer. Two paintings were tested; one was an oak-cradled panel painting and the other was illustration board mounted on hardboard. The air-coupled ultrasound technique yielded information additional to that provided by visual examination, xeroradiography, and infrared thermography.

Air-Coupled Ultrasonic System: A New Technology for Detecting Flaws in Paintings on Wooden Panels

Air-coupled ultrasound, a noncontact, nondestructive testing technique, has detected splits, checks, delaminations, cleavage, and voids in various materials. The system has inspected highly anisotropic and inhomogeneous materials such as wood and wood products, with surfaces layers of gesso, gesso and linen, paper, and wood veneer. Two paintings were tested; one was an oak-cradled panel painting and the other was illustration board mounted on hardboard. The air-coupled ultrasound technique yielded information additional to that provided by visual examination, xeroradiography, and infrared thermography.