Defect characterization in the short-wavelength regime

The scattering of acoustic waves by different types of spherical defects in a silicon nitride matrix is calculated by using Ying and Truell's scattering theory. The theoretical scattering results are interpreted using a ray tracing approach. Experiments were carried out at a high frequency (150–450 MHz) to characterize defects in silicon nitride. Time and space averaging, Wiener filtering, diffraction, and propagation loss corrections were used to remove the effect of the transducer response and propagation loss in the material from the scattered signal. Our experimental results indicate the presence of a new type of defect in silicon nitride. They give the type and size of voids, cracks, and Si inclusions in good agreement with measurements obtained after sectioning.     

Ultrasonic evaluation of geometrical and surface parameters of rough defects in solids

Ultrasonic measurements from rough cracks were carried out using both broad-band and narrow-band methods. An analysis is suggested to determine parameters of the crack quantitatively such as size, shape, rms surface roughness, and distribution function of the surface roughness. Ultrasonic measurements of the parameters compare very well with the actual parameters of the defect.     

A new ultrasonic technique for detection and sizing of small cracks in studs and bolts

If small cracks in stud bolts are not detected early enough, they can grow rapidly and cause catastrophic disasters in industrial facilities such as nuclear power plants. Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts. This study shows a method of detecting the existence and determining the size of a small crack in a root between two crests in the bolt threads using ultrasound. The Rayleigh wave propagating from the tip of a crack to the opening of the same crack is utilized. A delayed pulse, due to the Rayleigh wave, is detected between regularly spaced pulses from the threads, with the delay time being proportional to the size of the crack. Theoretical explanation is presented and experimental results demonstrating detection of cracks as small as 0.5 mm are shown.     

Self-calibrating ultrasonic technique for crack depth measurement

A configuration of transducers together with a self-calibrating measurement technique is proposed to investigate the reflection and transmission of surface waves by a surface-breaking or near surface defect. By means of this technique, the ratio of the reflection and transmission coefficients (R/T and/orT/R) can be obtained in a reliable and accurate manner. The reflection and transmission of surface waves for oblique incidence on a surface breaking crack is investigated in detail. Information onT/R for the latter case can be used to determine the depth of the crack. The experimental measurements ofT/R show excellent agreement with theoretical results.     

Scattering of ultrasound in solids as visualized by the photoelastic technique

Our efforts in the past few years to visualize the scattering of ultrasound in solids by the photoelastic technique are briefly reviewed. Photoelastic photographs are presented showing the dynamic processes of scattering in glass of plane longitudinal or transverse waves by a cylindrical cavity or cavities as well as by two-dimensional surface-breaking or internal cracks. Phenomena like creeping and multiple scattering are clearly seen, in addition to some details which are not predicted by known theories.     

Ultrasonic diffraction technique for characterization of fatigue cracks

This paper describes an ultrasonic diffraction technique for characterizing fatigue cracks. The angular field of energy scattered from a crack tip was computed. Using the theoretically predicted and experimentally verified optimum range of angles, we measured the crack profiles by the ultrasonic diffraction technique. Ultrasonic measurements agreed very well with direct destructive measurements. In addition, fatigue crack closure was detected and information on crack surfaces was obtained.Oak Ridge National Laboratory; Oak Ridge National Laboratory is operated by Union Carbide Corporation under contract W-7405-eng-26 with the U.S. Department of Energy.     

Ultrasonic Pulse-Echo Subwavelength Defect Detection Mechanism: Experiment and Simulation

The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique [IEEE Trans. UFFC, 45:30 (1998)] has demonstrated sensitive detection of subwavelength channel defects (38-m diameter reliably and 6-m diameter occasionally) in flexible 220-m-thick food package seals (17.3 MHz, 86 m). However, the underlying subwavelength defect detection mechanism is poorly understood. In this contribution, a theoretical modeling study was undertaken to elucidate the mechanism. The subwavelength diameter channel was fused in-between two plastic package films by applying heat from one side of the films. The sample cross-section microstructure was characterized from both optical and acoustic images. The cross-section impedance profiles along sample thickness dimension were determined. Although identical in nominal impedance properties before sealing, the two binding films showed an asymmetric impedance profile after sealing. Transient finite-element heat conduction analysis and impedance profiles of multiple-sealed package samples showed that the single-sided heating process caused an asymmetric impedance profile. A generalized impedance model was proposed based on these observations. An efficient two-dimensional simulation tool using a finite-difference time-domain method and the perfectly matched layer numerically evaluated the defect detection behavior of the radio-frequency (rf) echo waveforms. The normalized correlation coefficients between the simulated and the measured rf echo waveforms were greater than 95% for this generalized model, which suggested the validity of the proposed impedance model.     

Analysis of ultrasonic wave scattering for characterization of defects in solids: I. Spherical inclusions and reciprocity

This paper, as part of a series on elastic wave scattering, presents results of measurements and calculations on scattering of ultrasonic waves by a solid spherical inclusion (tungsten carbide) embedded in titanium alloy by the diffusion bonding process. Both direct scattering and mode-converted scattering angular distributions are reported for shear and compressional incident waves. The consequences upon the signals when transmitter and receiver were interchanged are explored in a reciprocity rule.     

Automatic ultrasonic defect locating and sizing by a probabilistic approach with high accuracy

Based on dynamic Time-Of-Flight Measurement with scanning transducers, a probabilistic approach has been proposed for automatic defect locating and sizing. This approach significantly reduces the inconsistent effects of human factors on results. The probabilistic approach suppresses the effects of random noises as well as measurement errors and makes possible the identification and elimination of spurious indications from mode-converted waves. No prior knowledge of an unknown defect is required to obtain a sizing image for the distribution of the unknown defects under a scan line. The probabilistic approach has been applied to the inspection of three testblocks, which include different types of defects, with the location and sizing accuracy as expected.     

卷积神经网络在印刷品缺陷检测的应用

目的 为了改善传统机器检测印刷产品缺陷存在误费率高的不足。方法 提出以卷积神经网络为控制核心的印刷品缺陷检测系统。设计可在实际检测中应用的卷积神经网络,设计在线印刷质量检测系统的硬件结构。结果 对结构相同而训练次数、学习率不同的卷积神经网络进行了缺陷检测的性能对比,验证了该卷积神经网络在学习率小于0.01时,可以获得较好的识别效果;在学习率大于0.05时,网络不容易收敛。网络训练次数越多,精度越高,相应的训练时间也较长。在满足快速性和精确度的条件下,确定了适应某印刷品的缺陷检验网络训练次数为50,学习率为0.005,此时的识别率为90%。结论 经过实验证明,该检测系统具有良好的缺陷识别能力,缺陷类型的分类准确率较高。该系统具有一定的实用价值。     

Interactive Methodology for Optimized Defect Characterization by Quantitative Pulsed Phase Thermography

ABSTRACT

Pulsed phase thermography is a nondestructive evaluation processing technique based on the discrete Fourier Transform. The time-frequency duality plays a critical role in the selection of the sampling and truncation parameters and has to be addressed experimentally as a function of the inspected depth. To characterize a wide range of depths in a single test, the ideal solution is to sample at the maximum available frame rate for the longest possible time and to process all this data at once. Nevertheless, two factors restrict this operation. First, the maximum frame rate and storage capacity in any acquisition system are limited and so is the span of potentially detected depths. Second, although limited, the storage capacity generally exceeds the ordinary PC's capabilities to handle simultaneously all the collected information. As a result, a compromise between sampling and storage capacity, processing capabilities and range of potentially detected depth needs to be made. A four-step interactive methodology is proposed to deal with this problem. The idea is to perform a first partial processing with a fraction of the recorded data for visualization purposes only and then to individually manage selected (defective) areas, now visible, without repeating any test.     

非均匀软组织声散射模型的研究概况

本文综述了非均匀软组织声散射模型的研究概况及其在超声组织定征中的应用。将声散射模型进行了系统的分类介绍。并对在建立软组织声散射模型中存在的问题以及研究声散射模型的重要意义进行了分析和讨论。     

Application of the boundary element method to elastic wave scattering by irregular defects

A time-harmonic boundary element formulation for elastic wave scattering in 3D is adapted to ultrasonic NDE. Defect classes addressed are volumetric voids and inclusions, and crack-like elliptical voids. For axisymmetric flaws, comparisons are made with method of optimal truncation (MOOT) and transition-matrix calculations. Comparison to experiment is made for more general shapes. For crack-like voids, comparisons are made with the Kirchhoff, geometric theory of diffraction (GTD), and quasistatic asymptotic approximations. The efficiency and usefulness of the boundary element method (BEM) in finding the bounds of applicability of these approximate theories are demonstrated. An example of a flaw characterization technique based on intermediate frequency scattering data simulated by BEM is given. The ability of BEM to handle nonplanar incident fields, as described by a transducer beam model, is shown. Other computational and modeling efficiencies of the BEM are noted.     

Body to Rayleigh wave mode-conversion at steps and slots

The efficient excitation of Rayleigh waves at surface discontinuities due to body wave incidence has potential as a method for the detection and sizing of surface breaking defects. In this paper, the mode-conversion phenomenon at several types of surface features is studied in detail, using numerical models which employ finite-difference methods. The emphasis is on examining the spectral content of the Rayleigh wave field in order to arrive at a method for relating the spectral information to the defect dimensions. The numerical results are backed up by experimental observations.     

Characterization of the Critically Refracted Longitudinal (LCR) Waves and Their Use in Defect Detection

The critically refracted longitudinal (L_CR) waves are used in various domains of nondestructive evaluation, especially for residual stress measurements. However, few works have characterized the associated ultrasonic beam. In this article, the characterization of the L_CR ultrasonic beam, both numerically and experimentally, is first clarified in order to provide some answers to questions that arise about its behavior in elastic solids. The aim of the second part of this work is to investigate the use of the L_CR waves for the detection of surface defects of different sizes in aluminum. For that, the effect of defects at fixed depth (5 mm) with various diameters (2, 4, 6, and 8 mm) and fixed diameter (8 mm) with various depths (5, 7, and 10 mm) in an aluminum sample have been investigated at frequencies around 1 MHz. It has been experimentally found that the amplitude of the (L_CR) wave decreases and its frequency spectrum changes with a given defect. The rate of the decrease of the amplitude and the change in the spectrum is related to the increase of the defect depth. The study shows that the effect on the propagating of the L_CR waves is larger as the defect depth increases.     

基于嵌入式系统和图像识别的拉索表面缺陷检测技术

以关系桥梁安全健康的拉索表面缺陷检测作为研究内容,针对国内对拉索表面保护材料层的两种检测方法的不足,设计了以嵌入式DSP为核心的扫描采集和缺陷检测系统,包括图像采集机构,DSP硬件平台、爬行运动机构以及损伤定位机构;提出了基于图像的快速识别拉索表面缺陷的检测识别方法,包括图像截取、滤波降噪.图像分割、缺陷识别,并加以算法实现.实验结果表明,该方法不仅实现了拉索表面缺陷的实时检测,同时系统指标达到:最小识别面积为1cm2,图像实时处理速度为10cm/3s.     

Reflection of ultrasonic waves from imperfect interfaces: A combined boundary element method and independent scattering model approach

A numerical technique for obtaining interface reflection coefficients for imperfect bonds between similar materials for a wide range of distributed defects is developed. A numerical boundary element method is utilized to find the far-field scattering amplitudes of a single defect for a normally incident plane wave. Then, the normal incidence reflection coefficient for a planar distribution of such defects is obtained from the independent scattering model. As a validation, the reflection coefficients are compared to the quasi-static model results where the latter are available. This establishes the basis for one application of the new model, the determination of spring constants which are not available. Other applications of the model, including studies of the response at frequencies beyond the quasi-static limit, the ratio of longitudinal to transverse wave reflectivities, and the effects of selected multiple scattering are discussed.     

Trefftz solutions for piezoelectricity by Lekhnitskii's formalism and boundary‐collocation method

In this paper, a solution procedure for plane piezoelectricity is developed by Trefftz boundary‐collocation method. Starting with the general plane piezoelectricity solution derived by Lekhnitskii's formalism, the basic sets of Trefftz functions which satisfy the homogeneous governing equations are derived. Moreover, special sets of Trefftz functions are derived for impermeable elliptical voids, impermeable sharp cracks and permeable sharp cracks with arbitrary orientations with respect to the material poling direction. The functions in the special sets satisfy not only the homogeneous governing equations but also the boundary conditions at the peripheries of the pertinent defects. By adopting Trefftz functions as the trial functions, multi‐domain Trefftz boundary‐collocation method is formulated. Numerical examples are presented to illustrate the efficacy of the formulation. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of Orange Peel Defect in St14 Steel Sheet by Electron Backscattered Diffraction (EBSD)

In this paper, the orange peel defect in the surface range of the st14 steel sheet has been investigated using the electron backscattered diffraction (EBSD) technique. It has been found that the orange peel defect in the st14steel sheet was resulted from the local coarse grains which were produced during hot-rolling due to the critical deformation in dual-phase zone. During deep drawing, the coarse grains with {100}＜001＞ microtexture can slip on the {112}＜111＞ slip system to form bulging and yields orange peel defects, while the coarse grains with {112}＜110＞orientation do not form the defect as the Schmid factor of {112}＜111＞ slip system in it equals zero.