Non-contact Ultrasonic Imaging Research Articles

Suppressing artifacts in the total focusing method using the directivity of laser ultrasound

Based on a synthesized laser ultrasonic array, full matrix capture can be used to acquire data, which can then be post-processed using the total focusing method. However, this noncontact ultrasonic imaging technique has not been widely used because of the numerous artifacts in ultrasonic images and time-consuming data acquisition. To address these issues, this study proposes a post-processing algorithm, which uses the laser ultrasound directivity information to suppress the artifacts in the total focusing method’s images. In particular, a weight factor is defined using the directivity information. By multiplying the image intensity of the total focusing method with this factor, the algorithm uses not only the amplitude and phase information of laser ultrasound but also its directivity information. The experimental results indicate that four types of artifacts are suppressed. Because the grating lobe artifacts can be suppressed, a larger element spacing can be used to reduce the data acquisition time.

Shedding Damage Detection of Metal Underwater Pipeline External Anticorrosive Coating by Ultrasonic Imaging Based on HOG + SVM

Underwater pipelines are the channels for oil transportation in the sea. In the course of pipeline operation, leakage accidents occur from time to time for natural and man-made reasons which result in economic losses and environmental pollution. To avoid economic losses and environmental pollution, damage detection of underwater pipelines must be carried out. In this paper, based on the histogram of oriented gradient (HOG) and support vector machine (SVM), a non-contact ultrasonic imaging method is proposed to detect the shedding damage of the metal underwater pipeline external anti-corrosion layer. Firstly, the principle of acoustic scattering characteristics for detecting the metal underwater pipelines is introduced. Following this, a HOG+SVM image-extracting algorithm is used to extract the pipeline area from the underwater ultrasonic image. According to the difference of mean gray value in the horizontal direction of the pipeline project area, the shedding damage parts are identified. Subsequently, taking the metal underwater pipelines with three layers of polyethylene outer anti-corrosive coatings as the detection object, an Autonomous Surface Vehicle (ASV) for underwater pipelines defect detection is developed to verify the detection effect of the method. Finally, the underwater ultrasonic image which used to detect the metal underwater pipeline shedding damage is obtained by acoustic sensor. The results show that the shedding damage can be detected by the proposed method. With the increase of shedding damage width, the effect of pipeline defect location detection is better.

振動速度情報を用いた超音波イメージングの一方法

A new ultrasonic imaging method by detecting the surface vibration velocity of a sample placed on the piezoelectric transducer is described. Surface vibration velocity of a sample is optically observed and their simulation is also performed to confirm the necessity of constant voltage drive in generating arbitral velocity waveforms on the surface of piezoelectric transducer. To demonstrate the possibilities of ultrasonic imaging, characters on an object was imaged by measuring the difference in surface vibration velocity between the object and the piezoelectric transducer. An experiment for an aluminum sample shows the effectiveness of our method in non-contact ultrasonic imaging in air. The experimental results suggested that the spatial resolution of the proposed system depend on the spot size of laser light of Doppler vibrometer is also described.

Ferroelectret non-contact ultrasonic transducers

Dielectric and electromechanical properties of the cellular polypropylene ferroelectret films (EMFIT), combining strong piezoelectric response with a low density and softness, evidence their high potential for the air-coupled ultrasonic applications. The disadvantage of the low coupling factor is compensated by the extremely low acoustic impedance, which provides excellent matching to air and promises efficient sound transmission through the air–transducer interface. The influence of the electrodes on the electromechanical properties was investigated. Electron beam evaporation technology was adapted to the EMFIT films, and films with both-sided Au and Al electrodes were prepared without reducing or suppressing of the electromechanical properties. Finally, prototype transducers based on the EMFIT films were developed. In spite of the simple construction and absence of matching layers, high sensitivity of the EMFIT transducers was proved in the air-coupled ultrasonic experiment. Amplitude and delay time scanned images of the polyethylene step wedge with holes, obtained in both pulse-echo and transmission modes, demonstrate that non-contact ultrasonic imaging and testing with EMFIT transducers is possible.

Air-Coupled Ultrasonic Transducers Based on Cellular Polypropylene Ferroelectret Films

Analysis of the dielectric and electromechanical properties of the cellular polypropylene ferroelectret films (EMFIT) evidences their high potential for the application in the air-coupled ultrasonic transducers. Simple prototype transducers, based on the EMFIT films, were developed and their high sensitivity was proved in the air-coupled ultrasonic experiment. Amplitude and delay time scanned images of the polyethylene step wedge with holes, obtained in both pulse-echo and transmission modes, demonstrate that non-contact ultrasonic imaging with EMFIT transducers is possible.

Application of pulse compression signal processing techniques to electromagnetic acoustic transducers for noncontact thickness measurements and imaging

A pair of noncontact Electromagnetic Acoustic Transducers (EMATs) has been used for thickness measurements and imaging of metallic plates. This was performed using wide bandwidth EMATs and pulse-compression signal processing techniques, using chirp excitation. This gives a greatly improved signal-to-noise ratio for air-coupled experiments, increasing the speed of data acquisition. A numerical simulation of the technique has confirmed the performance. Experimental results indicate that it is possible to perform noncontact ultrasonic imaging and thickness gauging in a wide range of metal plates. An accuracy of up to 99% has been obtained for aluminum, brass, and copper samples. The resolution of the image obtained using the pulse compression approach was also improved compared to a transient pulse signal from conventional pulser/receiver. It is thus suggested that the combination of EMATs and pulse compression can lead to a wide range of online applications where fast time acquisition is required.

Inspection of CFRP components by ultrasonic imaging with air-coupling

This paper reports the investigations of air coupled ultrasonic imaging techniques which are part of the German research project MaTech. Project partners are Airbus, Bremen, Ingenieurburo Dr. Hillger, Braunschweig, and intelligeNDT, Erlangen, with DLR, Braunschweig, as subcontractor. One task is the development and the construction of a non-contact ultrasonic imaging system („Demonstrator III“) for basic researches. This system provides fast scanning with a single-shot data acquisition (without signal averaging). The dynamic range in one shot reaches up to 50 dB. The demonstrator allows the application of different types of air coupled transducers and therefore it allows the comparison of their performances. The modular design of the system provides further developments and adaptations to special inspection tasks. A power transmitter with burst-signals ( peak power of 1.2 kW) and a free programmable transmitter can be used for the excitation of the transducers. The user-friendly software records C-scans and full wave scans and provides C-scan evaluations. The development and construction of special transducers for air coupling and the development of a system for echo-technique are further research tasks of the MaTech project. Usually, the air coupled ultrasound is used in through-transmission technique with separate receiver and transmitter transducers on opposite sides of the component under test. In order to get a high acoustic power and a low noise in the receiver amplifier usually the bandwidth of the system is smaller than 10% of the centre (test) frequency. Therefore, the effects of (*/2)-resonances in the components are investigated. Also the application of broadband transducers will be investigated.

Non-contact ultrasonic imaging for the evaluation of burns and thermal injury

Non-contact ultrasonic imaging for the evaluation of burns and thermal injury

Noncontact ultrasonic imaging for the evaluation of thermal injury

Although conventional wisdom suggests that ultrasonic imaging of the body cannot be accomplished without direct contact (or at least via water coupling), we have shown that noncontact imaging through air is possible, certainly for superficial body regions, provided judicious choices of piezoelectric materials and matching layers are made. In preliminary experiments and clinical studies reported here, noncontact imaging is demonstrated for the evaluation of thermal injury (including the quantitative measurement of burn-depth), for the assessment of wound healing, and for the examination of assorted skin lesions. Specifically, in the case of thermal injury, reflections from the dermal/fat interface in human skin is clearly seen using a noncontact 5-MHz transducer. Such measurements are sufficient to determine burn-depth which, in turn, are sufficient to provide, for the first time, a quantitative and noninvasive method for burn evaluation and treatment specification. Evaluating over 500 burn sites in some 100 patients, noncontact ultrasound showed a much greater accuracy and sensitivity than standard clinical assessment. Our method is applicable to a conventional clinical environment as well as a battlefield situation and should prove particularly effective for large-scale medical triage.

Noncontact Ultrasonic Imaging of Subsurface Defects Using a Laser-Ultrasonic Technique

Noncontact ultrasonic imaging of subsurface defects using a laser-ultrasonic technique was demonstrated. Pulsed laser irradiation was used for ultrasonic generation, and optical heterodyne interferometry was used for detection of ultrasonic vibration on the specimen surface. Bonded thin plates with character-shaped hallmarks with 1.5 mm width on bonding layer were used as specimens. Imagings in ultrasonic transmission and reflection modes were performed, and imagings using not only XY-scanning of specimens but also a laser beam scanner were demonstrated. Ultrasonic images of hallmarks located 0.2 mm below the specimen surface were obtained through the experiments.