相控阵兰姆波全聚焦算法的不锈钢薄板分层缺陷检测

航空金属薄板在加工时由于夹杂产生的气孔会随着使用过程中的拉伸和挤压而形成一种闭合型的分层缺陷。针对此类缺陷的检测,提出一种相控阵全聚焦成像算法与兰姆波检测结合的方法。首先从原理上分析并确定激励兰姆波的模态、频率及楔块的角度,其次对3 mm厚度不锈钢板中不同规格、形状的分层缺陷进行试验。结果表明:本研究方法能够对分层缺陷的轮廓成像;兰姆波易受缺陷上端面反射影响,导致缺陷下端面成像出现拖尾图像,且缺陷上端的定位定量精度要高于缺陷下端面;上端面缺陷的定位误差均在4%以内。     

薄壁十字交叉激光焊结构缺陷超声Lamb波检测

为了有效识别薄壁十字激光焊结构中未焊透缺陷,开展了超声Lamb波法检测研究. 采用有限元分析方法模拟了A0和S0模态Lamb波在薄壁中与缺陷的作用结果,对缺陷尺寸与回波反射系数的关系进行了预测. 在此基础上,进行仿真结合试验,确定了不同模态Lamb波的入射点、激励角度,探讨了A0和S0模态Lamb波用于识别1.9 mm厚薄板十字激光焊接结构未焊透缺陷的可行性,并获得了超声Lamb波反射系数与缺陷尺寸的关系. 结果表明,仿真研究和试验实测结果的趋势具有较好的一致性. 采用超声Lamb波检测时,A0模态波能够有效检测十字焊接结构中的未焊透缺陷,且可一定程度上表征缺陷的尺寸;而S0模态波对结构中的未焊透缺陷不易于识别.     

Detection and 3-D positioning of small defects using 3-D point reconstruction,tracking, and the radiographic magnification technique

In this paper the capability of a 3-D point reconstruction algorithm based on multiple hypothesis tracking is experimentally explored on a setup consisting of a microfocus X-ray source and a digital detector array. The algorithm is verified to detect and 3-D position steel particles with diameters 0.16–0.26 mm radiographed behind 4.7 mm Inconel 718 at two to ten times magnification. At ten times magnification the algorithm is verified to detect and 3-D position with an average error of 0.1 mm pore defects with diameters 0.05–0.25 mm in 4.7 mm thick titanium alloy laser welds.     

Time reversal and microwave techniques for solving inverse problem in Non-Destructive Evaluation

Non-Destructive Evaluation (NDE) methods are used to inspect materials without damaging their usefulness. The key problem in NDE is the inverse problem which involves reconstructing materials’ physical profiles, like inner discontinuity etc., using information in the measured NDE signal. Inverse problem solutions in NDE can be classified as model-based and system-based approach. In model-based approach, an accurate forward model is used in an iterative framework. This approach provides a resultant materials' physical profile that minimizes the error between the measured signal and a simulated signal. However, this approach requires repeated calculations of a numerical model in each iteration, making it computationally demanding. This paper presents a model-based method that also provides a direct approach to inversion using principles of time reversal. Time reversal focusing is based on the fact that when a wave solution is reversed in time and back-propagated the wave comes to focus at the source. Using a computational model, this paper applies principles of time reversal to microwave NDE data to solve the inverse problem of defect detection in dielectric materials. A two-dimensional finite difference time domain (FDTD) model, for simulating the propagation of forward and time reversed wave fields, is developed. A dielectric sample with artificial defects, illuminated by a Gaussian modulated pulse, is used in the simulations. The microwave measurements are recorded, time reversed and propagated using the FDTD model to highlight the scatterer/defect. Maxima in the energy image indicate locations of defects. Simulation results demonstrate the feasibility of the technique to detect defects in dielectric materials. The FDTD model is validated using experimental data.     

Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet is used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0–180°, and the setup has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils.     

冷凝器不锈钢薄壁管的兰姆波检测

小直径不锈钢薄壁管广泛应用于电站机组的冷凝器中,常规涡流检测方法难以发现纵向贯通的条状缺陷。在Ф25mm×1mm和Ф38mm×2mm不锈钢管壁上,沿轴向和周向加工深度分别为0．1mm、0．2mm和0．5mm的人工切槽,并进行检测试验研究,确定了兰姆波检测技术对小直径不锈钢薄壁管上人工缺陷的可检性,探讨了对不锈钢薄壁管材进行兰姆波检测的有效性和可靠性,提出以兰姆波检测技术对小直径不锈钢薄壁管进行检测的建议。     

Self-regulation of the arc in consumable electrode welding

The characteristic features, criteria and algorithm of the currently available method of self-regulation of the arc in mechanised arc welding with a constant consumable electrode feed rate are investigated. The main problem is that when it is necessary to reduce the welding current, for example with an increase of the thickness of the welded edges or the leg length of the fillet welded joint, welding should be carried out with a wire with a smaller diameter, down to 0.5 mm or even 0.3 mm. This complicates welding technology and increases the price of the wire because of multiple drawing of the wire to the small diameter. Therefore, the main task is to improve the algorithm of self-regulation of the arc and, consequently, reduce the minimum value of the current density, lower than 30 A/mm² for a steel wire with a diameter of 5 mm at which self-regulation of the arc is still possible, in order to widen the technological possibilities of the method and reduce the cost of technology. The authors believe that this approach would make it possible to produce efficient tools for understanding physical processes accompanying the formation of permanent joints by different arc welding methods.     

利用频散补偿法消除兰姆波频散效应

兰姆波的多模式及频散特性使得其在激励、传播以及信号处理等方面较为复杂。采用频散补偿方法对在钢板中传播10 cm距离的兰姆波信号进行模拟计算。所得信号明显地减小了因频散而引起的脉冲信号拉长。实验对3 mm厚铝板中传播了10 cm的频散信号进行了处理,也得到较好的结果。     

Lamb wave propagation in composite laminates and its relationship with acousto-ultrasonics

The acousto-ultrasonic technique has many potential applications in the NDE of composite materials. However, problems of poor reproducibility and of the sensitivity of the results to precise instrument settings have restricted its application in industry The waves employed in acousto-ultrasonics are chiefly Lamb waves which propagate in the plane of the laminate. In this paper, Lamb wave propagation in composite laminates both with and without defects is investigated both numerically and experimentally. Acousto-ultrasonic parameters based on both predicted and measured responses are calculated, and tests are carried out using a commercially available instrument. The factors which lead to the poor reproducibility of acousto-ultrasonic results are discussed and possible improvements to the technique are proposed.     

表面不平度对近表面缺陷的超声检测的影响

激光焊接是连接钛合金的一种主要的工艺方法.由于焊缝中气孔形成倾向较大,需要进行严格的无损检测.文中概述了超声自动检测过程中工件表面不平度的不利影响,重点分析了焊缝余高对检测结果的影响.采用不同频率的探头对模拟试件进行超声水浸检测,通过对焊缝的不同位置进行测试得到适用的探头频率及其它检测参数.结果表明,通过选择合适的探头频率和检测工艺,可以减小表面不平度对检测灵敏度的影响,可以发现工件表面以下深度为1.0～2.5 mm、最小直径为0.3 mm的气孔缺陷,从而提高超声自动检测的可靠性.     

Investigation of machine vision assisted automatic resharpening process of micro-drills

The micro-drill used in electronic industry is getting smaller in diameter especially for printed circuit board/ball grid array processing; making it is easier to have many defective drills produced in manufacturing process. Defective drills can be resharpened to become conforming products. Focusing on the planar micro-drills less than 0.5 mm in diameter, the paper proposes using machine vision technique to assist the double-wheel grinding mechanism of moving-drill and fixed wheel to achieve resharpening function. First, we find and adjust the posture of micro-drill from vision system for grinding, then detect the line equation of cutting edge of primary facet grinded micro-drill and rotate cutting edge to be horizontal; final we grind the secondary facet of micro-drill. All grinding parameters for resharpening process are found automatically with machine vision technology. As known from the experimental results, the automatic resharpening method proposed by this paper is feasible.     

Defect-free joining of zinc-coated steels by bifocal hybrid laser welding

Laser welding of zinc-coated steels in an overlap setup is prone to weld defects and seam expulsion reducing in particular the properties in mechanical loading and in general the deployability of such weldments in industry. Several laser welding process technologies failed to created defect-free welds in zinc-coated steels. This paper renders the welding of zinc-coated steels by the novel technology of bifocal hybrid laser welding. The zinc-coated steels under consideration are DX56D + Z, DC04 + ZE, and HXT700D. The bifocal hybrid laser system is realised by combining an Nd:YAG laser with a high power diode laser, both of 3 kW maximum output power. The beam parameter product (BPP) of the employed Nd:YAG laser of 25 mm mrad translates with an optical system of focal length f = 150 mm into a circular focus of diameter 0.45 mm, whereas the BPP of the HPDL of 85 mm × 200 mm mrad can achieve a rectangular focus of 0.9 mm × 3.7 mm. The optical system allows the respective focal plane and relative position of the foci to be independently vertically and horizontally positioned. This paper presents research into the causes of instabilities in laser welding of zinc-coated steels. Experimental evidence is considered and presented to establish the need for an empirical process model for stable laser welding of zinc-coated steels. The increase of process robustness is discussed.     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

Fabrication of hexagonally arrayed micro-structures with axially symmetrical surface profile by tri-axial excimer laser scanning

This paper presents an excimer laser contour scanning method for achieving hexagonally arrayed micro-structures with an axially symmetrical surface profile. It utilizes a specially designed contour mask for projected laser scanning along three different directions separated by 120° on the surface of a sample. Through rigorous analysis, a new contour mask design method is proposed which guarantees not only the profile accuracy but also the axial-symmetry of machined micro-structures. Experiments have been carried out using a 248 nm KrF excimer laser micromachining system. Hexagonal arrays of micro-lenses with an aperture size of 100 μm in diameter and a maximum sag height from 15 to 30 μm are successfully produced using designed contour masks. The machined profile accuracy is better than ±0.5 μm within most of the central area of lens aperture. The flexibility on machined surface profile and its machining accuracy allow new types of micro-lens arrays with specific optical functions and new applications in the future.     

Using active thermography to inspect pin-hole defects in anti-reflective coating with k-mean clustering

The study here demonstrates the capability of thermography to detect and characterize pinhole defects in a visually transparent anti-reflection (AR) film. The diameter of the pin-holes varies from 0.03 mm to 4 mm. Each inspected area was unevenly heated for 65 s. Thermal images were processed by the following steps: de-trend processing, primary edge detection based on Sobel approximation, and further edge separation based on k-means clustering. Then, the diameter of pinholes was directly measured from binary images. The proposed image processing enables thermography to detect 86 of 90 inspected areas correctly in position.     

基于空气耦合超声的激光焊缝质量评估

随着激光焊接技术在汽车及轨道交通上的广泛应用,对激光焊缝的无损检测技术要求越来越高. 针对3 mm以下的两层金属薄板激光焊缝为对象,探讨了空气耦合超声检测技术对激光焊缝检测的可能性. 利用数值解析和试验分析两种方法,在空气中对铝板激励兰姆(Lamb)波,通过Lamb波在激光焊缝试件中的传播模拟,分析了激光焊缝的宽度以及焊缝的良否对反射率及透射率的影响,解明了Lamb波在激光焊缝试件中的传播规律. 结果表明,可以利用Lamb波A0模式对激光焊缝的质量进行评估.     

A high-efficiency nondestructive method for remote detection and quantitative evaluation of pipe wall thinning using microwaves

We report an efficient nondestructive evaluation (NDE) method to measure the pipe wall thinning (PWT) remotely using microwaves. A microwave vector network analyzer (VNA) and a self-designed transmitting and receiving (T&R) coaxial-line sensor were employed in the experiment to generate microwave signals propagating in the pipe where the frequency was swept from 14.00 to 14.20 GHz. A brass pipe with inner diameter of 17.03 mm, 1.0 mm wall thickness, 2.0 m length, and connected, respectively, with 6 joints having the length of 17.0 mm and PWT from 0% to 60% of wall thickness was measured. By taking the pipe as a circular waveguide of microwave, after building up a resonance condition and then solving the resonance equations, the evaluation of PWT was realized. By comparing the evaluated results obtained using our suggested method with the nominal inner diameters of the joints, the maximum evaluation error is found to be less than 0.05 mm, which is less than 0.294% of the inner diameter of the pipe, which indicates that a high precision evaluation method is established.     

Optimum eddy current excitation frequency for subsurface defect detection in SQUID based non-destructive evaluation

Detailed experimental studies have been carried out for the determination of optimum eddy current excitation frequencies for the defects located at different depths below the top surface of an aluminum plate. These subsurface defects were detected by using a highly sensitive superconducting quantum interference device (SQUID) based eddy current non-destructive evaluation (NDE) system. The signal to noise ratio was found to be significantly higher at the optimum excitation frequency, which depended on the depth of the defect. The optimum excitation frequencies have been evaluated for defects located at different depths from 2 to 14 mm below the top surface of the plate. The defect depth was varied in steps of 2 mm, while the overall total thickness of the stack of plates was kept constant at 15 mm. Each defect represented a localized loss of conductor volume, which was 60 mm in length, 0.75 mm in width and 1 mm in height. The experimental results show that the square root of the optimum excitation frequency is inversely proportional to the depth of defect.     

Effect of laser offset on microstructure and mechanical properties of laser welding of pure molybdenum to stainless steel

Direct welding of Mo and stainless steel is exceptionally difficult due to intrinsic brittleness of Mo and the formation of brittle Fe-Mo phases. To explore the feasibility of welding of Mo and stainless steel, the laser offset method was used in this study. Experimental results show that the offset of the laser beam toward stainless steel has a positive effect for the quality of Mo/Fe dissimilar joint. As the laser beam shifts from the Mo side to the stainless steel side, the formation of welding defects and Fe-Mo intermetallic compounds (IMCs) are effectively restricted because of the decrease amount of molten Mo. The decrease of Fe-Mo IMCs contributes to the reduction of hardness in the joints. With an increase of laser offset, the thickness of Fe-Mo IMCs layer decreased, consequently the tensile strength of joints increased first and then decreased in the laser offset range of 0.2–0.5 mm. The highest tensile strength of the joints is 290 MPa at the laser offset of 0.3 mm. All joints failed in the Fe-Mo IMCs layer with brittle fracture mode during tensile tests, indicating the weakest zone of the joint was Fe-Mo IMCs layer. A sound weld of Mo and stainless steel can be obtained if an appropriate thickness of Fe-Mo IMCs layer is produced by adjusting the laser offset.     

Preparation and dispersive properties of Ag colloid by electrical explosion of wire

In this work, Ag colloid was prepared by electrical explosion of wire in deionized water with 0.2 mm and 0.3 mm wire diameter. The temperature of water used for medium of explosion process was change from 20 °C to 80 °C. Morphology and particle size of nanoparticles was observed by transmission electron microscope. The particle size and size distribution of nanoparticles was found to shift to a smaller size with a decrease of temperature and smaller wire diameter. Surface plasmon resonance of the silver colloids was studied by UV–vis spectroscopy. Stability of silver colloids was investigated by zeta-potential and Turbiscan techniques. The results indicated that temperature of medium during explosion affects much on the stability of Ag colloid. The silver colloidal stability prepared at lower temperature and smaller wire diameter was more stable.