Finite element simulation and modeling of 2-D arrays for 3-D ultrasonic imaging

Issues of modeling and design of 2-D arrays in three dimensions with finite element code are discussed. These ultrasonic arrays are used for real time dynamic imaging of the heart. Topics include optimization, sensitivity, and performance and methods to speed up the run times required for computer simulations of large three-dimensional models. Empirical results from 45×45 2-D arrays are also presented     

Finite element analysis of an inelastic interface in ultrasonic welded metal/fibre-reinforced polymer joints

The ultrasonic welding technology is an innovative method to produce hybrid joints for multi-material components. In this contribution, the behaviour of an interface layer of metal/fibre-reinforced polymer single overlap tensile specimens is considered. The investigations are carried out using the ultrasonic metal welding technique (UMW) for joining carbon fibre reinforced thermoplastic composites (CFRP) with aluminium alloys. An interfacial traction-separation-law based on elastoplasticity with Lemaitre-type damage is applied. The finite element method is used for the analysis of damage evolution. Two-dimensional interface elements are employed for modelling the solid interface in a 3-D problem. Numerical simulations are carried out for three different interface geometries: square, elongated rectangle and cross rectangle. It is shown that damage develops slower in the specimen with square interface than in the specimen with rectangle interface. The damage parameter reaches the maximum value in every loadstep in the specimen with cross-rectangle interface. Comparison with experimental data shows that the damage process and the fractured zone are identical to simulated results for the specimen with square interface.     

Investigation of laser pulse fatigue effect on unpainted and painted CFRP structures

Laser ultrasonic based nondestructive evaluation (NDE) techniques are being increasingly used in aerospace industries. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy in the long-term repetitive maintenance inspection. In this paper, the effect of repeat laser pulse scanning on the mechanical characteristics of unpainted and painted CFRP specimens (USN175BX Carbon UD prepreg, Bisphenol A epoxy resin) is investigated to verify the reliability regarding the use of laser ultrasonic scanning based NDE techniques on the inspection of the CFRP structure. A high-speed laser ultrasonic scanning system is setup to perform repeat scanning of 1300 times on both CFRP specimens at the five laser pulse energy levels using the 532-nm and 1064-nm Q-switched continuous wave lasers. Elastic modulus assessment based on the ultrasonic Lamb wave pitch–catch method is used and the surface condition of the scanned area is investigated by a microscope. As a result, the laser pulse fluences that is shown in this paper are suitable for the long-term repetitive maintenance inspection in unpainted and painted CFRP structure even if it demonstrates an embrittlement phenomenon similar to the modulus measurement resolution in the unpainted CFRP specimen. In addition, the laser pulse fluence for maximum signal-to-noise ratio without any damage is investigated in both unpainted and painted CFRP specimens. As a result, both 102.45 mJ/cm² in unpainted CFRP specimen and 51 mJ/cm² in painted CFRP specimen can be the laser pulse energy for the maximum signal-to-noise ratio without any damage.     

Building three-dimensional images using a time-reversal chaotic cavity

The design of two-dimensional (2-D) arrays for three-dimensional (3-D) ultrasonic imaging is a major challenge in medical and nondestructive applications. Thousands of transducers are typically needed for focusing and steering in a 3-D volume. In this article, we propose a different concept allowing us to obtain electronic 3-D focusing with a small number of transducers. The basic idea is to couple a small number of transducers to a chaotic reverberating cavity with one face in contact with the body of the patient. The reverberations of the ultrasonic waves inside the cavity create at each reflection virtual transducers. The cavity acts as an ultrasonic kaleidoscope multiplying the small number of transducers and creating a much larger virtual transducer array. By exploiting time-reversal processing, it is possible to use collectively all the virtual transducers to focus a pulse everywhere in a 3-D volume. The reception process is based on a nonlinear pulse-inversion technique in order to ensure a good contrast. The feasibility of this concept for the building of 3-D images was demonstrated using a prototype relying only on 31 emission transducers and a single reception transducer.     

Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.     

Element shape design of 2-D CMUT arrays for reducing grating lobes

Fully populated two-dimensional (2-D) arrays are needed to produce high quality ultrasonic volumetric images for real-time applications, but they present many challenges for their physical realization because of the large number of elements. In fact, lambda/2 and lambda minimum spacing between elements is required, respectively, for pyramidal and rectilinear scanning in order to avoid unwanted grating lobes (GLs). However, in past years, capacitive micromachined ultrasonic transducer (CMUT) technology has made possible the production of arrays with large flexibility in element shape and size. In this paper, this property is analyzed, and a new element shape, based on the concept of spatial interpenetration of adjacent elements, is proposed in order to design fully populated 2-D CMUT arrays with a low number of elements, whose beam characteristics are valid for volumetric imaging. Through the use of simulations, it is demonstrated that arrays with pitch larger than lambda (up to 3lambda) used for rectilinear scanning, have notably lower GLs than the equivalent standard arrays designed according to the classical squared element shape. As consequence, the proposed geometry has the advantage of reducing the number of elements (up to a factor of 9) and of enlarging the element size, implying an increase of the SNR relative to the single element. When beam steering is required, arrays can be designed with pitch equal to lambda, reducing the number of elements by a factor of 4 if the maximum steering angle is limited to +/-15 degrees .     

Volumetric ultrasound imaging using 2-D CMUT arrays

Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128 x 128-element 2-D CMUT array with through-wafer via interconnects and a 420-microm element pitch. As an experimental prototype, a 32 x 64-element portion of the 128 x 128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16 x 16-element portion of the array to surrounding bondpads. An 8 x 16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4 x 4 group of elements in the middle of the 8 x 16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.     

High-spatial-resolution sub-surface imaging using a laser-based acoustic microscopy technique

Scanning acoustic microscopy techniques operating at frequencies in the gigahertz range are suitable for the elastic characterization and interior imaging of solid media with micrometer-scale spatial resolution. Acoustic wave propagation at these frequencies is strongly limited by energy losses, particularly from attenuation in the coupling media used to transmit ultrasound to a specimen, leading to a decrease in the depth in a specimen that can be interrogated. In this work, a laser-based acoustic microscopy technique is presented that uses a pulsed laser source for the generation of broadband acoustic waves and an optical interferometer for detection. The use of a 900-ps microchip pulsed laser facilitates the generation of acoustic waves with frequencies extending up to 1 GHz which allows for the resolution of micrometer-scale features in a specimen. Furthermore, the combination of optical generation and detection approaches eliminates the use of an ultrasonic coupling medium, and allows for elastic characterization and interior imaging at penetration depths on the order of several hundred micrometers. Experimental results illustrating the use of the laser-based acoustic microscopy technique for imaging micrometer-scale subsurface geometrical features in a 70-μm-thick single-crystal silicon wafer with a (100) orientation are presented.     

Adaptive imaging on a diagnostic ultrasound scanner at quasi real-time rates

Constructing an ultrasonic imaging system capable of compensating for phase errors in real-time is a significant challenge in adaptive imaging. We present a versatile adaptive imaging system capable of updating arrival time profiles at frame rates of approximately 2 frames per second (fps) with 1-D arrays and up to 0.81 fps for 1.75-D arrays, depending on the desired near-field phase correction algorithm. A novel feature included in this system is the ability to update the aberration profile at multiple beam locations for 1-D arrays. The features of this real-time adaptive imaging system are illustrated in tissue-mimicking phantoms with physical near-field phase screens and evaluated in clinical breast tissue with a 1.75-D array. The contrast-to-noise ratio (CNR) of anechoic cysts was shown to improve dramatically in the tissue-mimicking phantoms. In breast tissue, the width of point-like targets showed significant improvement: a reduction of 26.2% on average. Brightness of these targets, however, marginally decreased by 3.9%. For larger structures such as cysts, little improvement in features and CNR were observed, which is likely a result of the system assuming an infinite isoplanatic patch size for the 1.75-D arrays. The necessary requirements for constructing a real-time adaptive imaging system are also discussed.     

The Use of Ultrasonic Arrays to Characterize Crack-Like Defects

The use of ultrasonic arrays to image and size crack-like defects is an important area in non-destructive evaluation. In this paper, two approaches are considered for extracting the key parameters of crack length and orientation angle. The parameters for small cracks are inferred from the scattering matrix. For larger cracks, the parameters are extracted directly from a high resolution image. The performance of both techniques is first assessed using a hybrid model to generate the full matrix of array data for a specified inspection configuration. The model combines a ray-based propagation model with scattering matrices to describe wave interaction with a defect. Good agreement is achieved between simulated and experimental results, hence validating this model-based approach. This model is then used to assess the impact of time-domain noise on the characterisation of crack-like defects. For the low-noise case, the scattering matrix approach shows good performance if the crack length is greater than 0.2 wavelengths and the image-based approach is good for crack lengths greater than 0.5 wavelengths. The scattering-matrix-based approach is found to be more sensitive to the addition of noise than the image-based approach. Finally, both techniques are demonstrated experimentally on samples containing more realistic defects.     

SPR imaging measurements of 1-D and 2-D DNA microarrays created from microfluidic channels on gold thin films

Microfluidic channels fabricated from poly(dimethylsiloxane) (PDMS) are employed in surface plasmon resonance imaging experiments for the detection of DNA and RNA adsorption onto chemically modified gold surfaces. The PDMS microchannels are used to (i) fabricate "1-D" single-stranded DNA (ssDNA) line arrays that are used in SPR imaging experiments of oligonucleotide hybridization adsorption and (ii) create "2-D" DNA hybridization arrays in which a second set of PDMS microchannels are placed perpendicular to a 1-D line array in order to deliver target oligonucleotide solutions. In the 1-D line array experiments, the total sample volume is 500 microL; in the 2-D DNA array experiments, this volume is reduced to 1 microL. As a demonstration of the utility of these microfluidic arrays, a 2-D DNA array is used to detect a 20-fmol sample of in vitro transcribed RNA from the uidA gene of a transgenic Arabidopsis thaliana plant. It is also shown that this array fabrication method can be used for fluorescence measurements on chemically modified gold surfaces.     

小截面方管结构CFRP复合材料的超声检测方法

新型飞行器已采用小截面碳纤维增强树脂基(Carbon Fiber Reinforce Polymer, CFRP)复合材料方管结构。针对该类复合材料方管的材料组成和结构特点,分析和对比了常用的超声穿透法、超声反射法和整体超声穿透法对复合材料方管内部质量检测的适用性,提出了超声对面内壁反射法。采用超声对面内壁反射法对试块中的人工缺陷和实际产品进行检测。结果表明,采用该检测方法能够全部检出试样中的10 mm×10 mm的人工分层缺陷,且可有效地检测实际产品的分层和孔洞缺陷。     

Optical inspection of periodic structures using lens arrays and moire magnification

Abstract

This paper describes a technique that uses the phenomenon of moire magnification, developed recently for the measurement and inspection of periodic structures. Moire magnification occurs when an array of lenses is used to view an array of identical objects situated at the focal plane of the lenses. As the lens array is aligned with the object array, a moire pattern is observed in which each moire fringe consists of a magnified image of the repeat element of the object array. As the arrays are rotated with respect to each other, the magnification and orientation of the image changes.

The moire magnifier builds up an image from a large number of components of an array and therefore gives a representation of the average unit. It is a very simple and robust device and may well be more convenient to use, for example in an industrial production environment, than a microscope. A large number of components used in electronic imaging systems are periodic in nature and can conveniently be inspected using this technique. Examples are shown.     

Device for carrying out environmental very high cycle fatigue tests with ultrasonic excitation in asymmetric push–pull mode

The increasing lifetime of many engineering components leads to a growing demand for accelerated testing methods. Fatigue failure of components submitted to cyclic loading at stress levels below the endurance limit occurs even beyond 10⁷ cycles which has been the traditional limit for fatigue testing in most laboratories. Test programs covering this range of cycles on servo-hydraulic or resonance machines are very time consuming. Therefore methods for very high cycle fatigue (VHCF) testing at ultrasonic frequencies have been developed and are now used routinely. These methods rely on the formation of a longitudinal standing ultrasonic wave inside a test specimen. The wave exerts an alternating tensile and compressive stress on the specimen. Because of their origin in a standing wave, the tensile and compressive stresses usually have the same magnitude, i.e. the test is carried out under fully reversed conditions. Several test rigs have been proposed and built to overcome this drawback by coupling an ultrasonic loading device with a classical uniaxial test bench and superposing the ultrasonic stress to a constant or slowly varying stress. We present a different approach for overcoming that limitation where the constant stress is generated by a pressure difference. This approach is especially useful for testing in hazardous environments since all movable parts like pull rods passing through the walls of the test chamber are avoided.We describe the design and the performance of such a VHCF device and present first test results demonstrating the deterioration of the lifetime of Inconel 718 specimens in high pressure gaseous hydrogen compared to argon.     

Beam steering with pulsed two-dimensional transducer arrays

The major problem facing the development of 2-D arrays for imaging is the complexity arising from the large number of elements anticipated in such transducers. The authors have undertaken a theoretical investigation of the focusing and steering properties of pulsed 2-D arrays to characterize the parameters required for medical imaging, such as element size, spacing, and number of elements. Details of the computational methods employed are presented, as well as a discussion of the steered beam properties of wideband 2-D arrays. The effects of apodization and element cross-coupling on the beam properties of a 2-D transducer array are examined. The beam properties of various sparse arrays with elements randomly distributed over the aperture of the transducer are discussed.     

A Study of Wood Inspection by Infrared Thermography, Part II: Thermography for Wood Defects Detection

Wood poles are among the main components of electrical distribution systems. They have to be replaced every 20—30 years due to wood decay. To reduce costs, utilities need an efficient nondestructive tool to determine the appropriate replacement time. Different techniques exist for this purpose, such as X- or gamma-ray tomography, indentation, and methods based on measurement of electrical conductivity, ultrasonic propagation, or simply bacterial culturing. Since none of these methods satisfy these utilities, it was decided to study in detail infrared thermography (NDT) in this particular context. The hypothesis is that in this particular context, wood decay corresponds to a different moisture content with respect to sound wood. In Part I of the paper the problem of wood pole NDT is analyzed using a dedicated thermal model and three different types of heating: internal through-hole, external, and by microwave. Experiments confirm modeling results: due to large defect depths, low wood thermal diffusivity, and the wood properties dependencies upon temperature, moisture, species, and fiber orientation, infrared thermography (IRT) is not appropriate for this inspection problem unless defects are close to the surface. Discussion of wood thermal properties is also included in Part I. In Part II of the paper, the wood decay inspection problem is revisited in a simpler manner: flat instead of circular geometry and shallower defects. Thermal modeling along with experimental results are presented, and the comparison is encouraging.     

A Study of Wood Inspection by Infrared Thermography, Part I: Wood Pole Inspection by Infrared Thermography

Wood poles are among the main components of electrical distribution systems. They have to be replaced every 20—30 years due to wood decay. To reduce costs, utilities need an efficient nondestructive tool to determine the appropriate replacement time. Different techniques exist for this purpose, such as X- or gamma-ray tomography, indentation, and methods based on measurement of electrical conductivity, ultrasonic propagation, or simply bacterial culturing. Since none of these methods satisfy these utilities, it was decided to study in detail infrared thermography (NDT) in this particular context. The hypothesis is that in this particular context, wood decay corresponds to a different moisture content with respect to sound wood. In Part I of the paper the problem of wood pole NDT is analyzed using a dedicated thermal model and three different types of heating: internal through-hole, external, and by microwave. Experiments confirm modeling results: due to large defect depths, low wood thermal diffusivity, and the wood properties dependencies upon temperature, moisture, species, and fiber orientation, infrared thermography (IRT) is not appropriate for this inspection problem unless defects are close to the surface. Discussion of wood thermal properties is also included in Part I. In Part II of the paper, the wood decay inspection problem is revisited in a simpler manner: flat instead of circular geometry and shallower defects. Thermal modeling along with experimental results are presented, and the comparison is encouraging.     

Performance evaluation of coherence-based adaptive imaging using clinical breast data

Sound-velocity inhomogeneities degrade both the spatial resolution and the contrast in diagnostic ultrasound. We previously proposed an adaptive imaging approach based on the coherence of the data received in the channels of a transducer array, and we tested it on phantom data. In this study, the approach was tested on clinical breast data and compared with a correlation-based method that has been widely reported in the literature. The main limitations of the correlation-based method in ultrasonic breast imaging are the use of a near-field, phase-screen model and the integration errors due to the lack of a two-dimensional (2-D) array. In contrast, the proposed coherence-based method adaptively weights each image pixel based on the coherence of the receive-channel data. It does not make any assumption about the source of the focusing errors and has been shown to be effective using 1-D arrays. This study tested its in vivo performance using clinical breast data acquired by a programmable system with a 5 MHz, 128-channel linear array. Twenty-five cases (6 fibroadenomas, 10 carcinomas, 6 cysts, and 3 abscesses) were investigated. Relative to nonweighted imaging, the average improvements in the contrast ratio and contrast-to-noise ratio for the coherence-based method were 8.57 dB and 23.2%, respectively. The corresponding improvements when using the correlation-based method were only 0.42 dB and 3.35%. In an investigated milk-of-calcium case, the improvement in the contrast was 4.47 dB and the axial and lateral dimensions of the object were reduced from 0.39 to 0.32 mm and from 0.51 to 0.43 mm, respectively. These results demonstrate the efficacy of the coherence-based method for clinical ultrasonic breast imaging using 1-D arrays.     

金属材料缺陷的电磁超声/涡流复合检测技术研究

电磁超声检测和涡流检测因其非接触、检测速度快、对试件表面要求低等优点而被广泛应用于金属材料的缺陷检测中,但电磁超声检测存在近表面的检测盲区,涡流检测对内部深层缺陷灵敏度不高。基于电磁超声和涡流的复合检测方法,设计了能同时满足电磁超声检测和涡流检测的复合式探头,建立了电磁超声和涡流复合检测有限元模型,并对金属试件中不同类型的缺陷进行了检测实验。仿真和实验结果表明,该复合探头不仅能快速检测表面裂纹,而且可激发出具有明显指向性的纵波,一定程度上削弱了波形转换产生的干扰波,可实现对内部缺陷的准确定位、识别,为电磁超声和涡流复合式检测技术在板材的复杂缺陷检测中的应用提供了基础。