Wall thickness evaluation of single-crystal hollow blades by eddy current sensor

Advanced high-pressure turbine blades of jet engines are hollow and monocrystalline. The external wall thickness of these blades has to be checked systematically and quickly after manufacturing in order to guarantee the blade strength. Thickness evaluation is made difficult by the presence of internal partitions and by the crystalline anisotropy of the superalloy used in blade manufacturing. In this paper, the authors present the advantages of the eddy current (EC) technique in comparison to other non-destructive evaluation (NDE) techniques for wall thickness evaluation. A dedicated EC sensor was developed and implemented. The thickness evaluation was carried out with a neural network inverse model, and the results show the high accuracy and reliability of the proposed method.     

焊缝跟踪用电磁传感器的理论分析

本文以磁路理论为基础,分析了各有关因素对用于焊缝跟踪的差动变压器式电磁传感器输出的影响,建立了该种传感器的计算公式.该公式简单,物理意义清楚,利用它进行设计,可以获得性能良好的电磁传感器。     

Hall sensor array based validation of estimation of crack size in metals using magnetic dipole models

Nondestructive evaluation (NDE) is an important methodology for quantifying cracks in engineering structures. In this paper, we propose a dipole model method (DMM) for NDE. The method is used to simulate an alternating magnetic field around a crack on a paramagnetic metal specimen and to estimate the shape and volume of the crack. This method enables faster and simpler evaluation of crack size than the traditional analytical methods. The DMM performance was verified by comparing the simulation results with the experimental results obtained using an AC-type magnetic camera.     

3-D movement simulation techniques using FE methods: Application to eddy current non-destructive testing

In this paper, sample calculations were performed with a three-dimensional (3-D) numerical method to describe the response of an eddy current (EC) probe to defects in a conducting plate. The governing field equations are given in terms of coupled magnetic vector and electrical scalar potentials and solved using a 3-D finite element method implemented in Matlab environment. The displacement of the sensor operating in absolute or differential mode is simulated using three methods: The first and second methods known, respectively, as the nodal interpolation technique (NIT) and the Lagrange multiplier technique (LMT) consist in ensuring the connection between fixed and moved 3-D independent meshes. The third method known as the properties assignment technique (PAT) is based on the geometrical band.In this work, these methods are used to compute the real and imaginary parts of the impedance correlation at every position of the coil(s). This allows the characterisation of the presence of a defect through the EC distribution.     

Thin-skin analysis technique for interaction of arbitrary-shape inducer field with long cracks in ferromagnetic metals

In using the AC field measurement (ACFM) technique for non-destructive evaluation (NDE) of metals, a current-carrying wire structure is used to induce eddy current within a thin layer of the metal and a magnetic field sensor to measure the field perturbations in the vicinity of the metal. The sensitivity of ACFM crack detection and sizing relies on an appropriate design of the wire structure geometry together with a dully placement of the sensor. This paper presents an analytical modeling technique for evaluating the electromagnetic field interaction of an ACFM probe with a long uniform crack in a ferromagnetic metallic slab. The probe in the proposed model can have an arbitrary-shape wire inducer with no restrictions on its relative sensor position. The technique is accurate and very efficient computationally. It first uses the two-dimensional Fourier transform to obtain the field distribution at the metal surface. The Laplacian field distribution above the metal is then determined by satisfying the so-obtained boundary condition at air–metal interface. To demonstrate the accuracy of the model, we consider the special case of a rhombic wire inducer. The comparison of our results with those obtained using the conventional algorithm in the literature validates the accuracy of the model introduced in this paper. To show the generality of the model, we also present theoretical and experimental results associated with a solenoid inducer with a three-dimensional geometry for which no analytical solution is available in the literature. The theoretical prediction of crack signal supported by experimental results is used to develop a model-based method for inverting crack signal into crack depth.     

多频涡流裂纹重构方法及其在金属夹芯板焊部裂纹定量检测中的应用

金属栅格夹芯板是近年来出现的一种新型多功能材料,在航天航空、舰船和铁路运输等方面具有广阔应用前景。夹芯板焊接部位若出现裂纹损伤将会对材料的力学性能和结构的安全性产生极大影响。目前尚无完善的金属栅格夹芯板定量无损检测方法。利用涡流检测技术,对夹芯板焊接部位裂纹定量检测进行了理论和试验研究,提出了一种基于多频检测信号进行裂纹重构的反问题方法。通过对十字涡流检测探头所得焊部检测信号进行反演,证实了多频涡流检测裂纹重构方法对夹芯板焊部裂纹定量检测的有效性。     

An adaptive channel equalization algorithm for MFL signal

Magnetic Flux Leakage (MFL) method is one of the most robust non destructive evaluation (NDE) techniques employed by in-line inspection (ILI) tools such as an instrumented pipeline inspection gauge (IPIG), to assess the health of buried cross country pipelines, carrying petroleum products. An array of hall-effect sensors disposed ideally at the magnetic neutral plane of a permanent magnet resident in IPIG, cling to the inner surface of the pipe wall, measuring the change in leakage flux density. However, ideal sensor disposition is almost never satisfied and each sensor measures differently. In this paper, a new scheme of channel equalization is proposed for MFL signal so as to correct sensor misalignments, which eventually improves accuracy of defect characterization. Also, for the above, a baseline estimator is developed, which estimates the baseline of MFL sensor output signal under pipeline defects and key features (wields, sleeves, valves). The scheme proposed is adaptive to the effects of error in the disposition of the sensor due to manufacturing imperfections and sensor movements (bounce/axial shift near a weld). The algorithm is tested on field data acquired by an IPIG running in a commercial pipeline. The final result obtained shows excellent signal recovery and noise cancellation.     

Detection of thermally grown oxides in thermal barrier coatings by nondestructive evaluation

The thermal-barrier coatings (TBC) sprayed on hot-section components of aircraft turbine engines commonly consist of a partially stabilized zirconia top-coat and an intermediate bond-coat applied on the metallic substrate. The bond-coat is made of an aluminide alloy that at high engine temperatures forms thermally grown oxides (TGO). Although formation of a thin layer of aluminum oxide at the interface between the ceramic top-coat and the bond-coat has the beneficial effect of protecting the metallic substrate from hot gases, oxide formation at splat boundaries or pores within the bond-coat is a source of weakness. In this study, plasma-sprayed TBC specimens are manufactured from two types of bond-coat powders and exposed to elevated temperatures to form oxides at the ceramic-bond-coat boundary and within the bond-coat. The specimens are then tested using nondestructive evaluation (NDE) and destructive metallography and compared with the as-manufactured samples. The objective is to determine if NDE can identify the oxidation within the bond-coat and give indication of its severity. While ultrasonic testing can provide some indication of the degree of bond-coat oxidation, the eddy current (EC) technique clearly identifies severe oxide formation within the bond-coat. Imaging of the EC signals as the function of probe location provides information on the spatial variations in the degree of oxidation, and thereby identifies which components or areas are prone to premature damage.     

Assortment of ultrasonic nondestructive evaluation signals using the centroid searching method with the LMS algorithm and SAFT

NDE using ultrasonic signals is a very useful technique for the assessment of solid materials, construction, food, and biomedicine. Among many NDE methods, the ultrasonic inspections may involve the extraction of an appropriate set of features or a neural network for the classification of the signals in the feature space. This paper presents an approach that uses a geometric method and the LMS (Least Mean Square) algorithm to determine the coordinates of the ultrasonic probe followed by the SAFT (Synthetic Aperture Focusing Technique) and centroid searching technique to estimate the location of the ultrasonic reflector. The proposed method is employed to classify ultrasound NDE signals from cracks and deposits within steam generator tubes in a nuclear power plant.     

Eddy current nondestructive testing with giant magneto-impedance sensor

Technologies based on magnetic sensors with high sensitivity such as magneto-resistance (AMR, GMR), fluxgate or squid sensors have demonstrated their capability to improve the performances of the classical eddy current (EC) probes.In this paper, a new kind of magnetic sensor based on giant magneto-impedance (GMI) effect has been evaluated. This sensor combines good sensitivity performances at low frequencies and small size. The design of a probe using this new technology has been optimized with fast semi-analytical models. The performances of this GMI based probe have then been successfully evaluated on a 304 L stainless steel mock up for the detection of embedded flaws.     

Design of a wide-angle spectrum source for a non-destructive material characterization: low-frequency ultrasonic point sensor

To develop further instrumentation for the industrial monitoring in test and control, low-frequency ultrasonic techniques with highly sharpened end sensors that behave as point sources were examined. Due to their particular shape, these sensors eliminate coupling problems during the analysis of media. The vibration behavior of these sensors was studied using a simplified analytical approach to obtain the longitudinal vibration mode on the basis of the resonant frequency and the amplification coefficient of velocity at the sensor ends. A numerical study, based on the finite elements method, was also developed and confirmed the validity of the simple analytical approach to define the main parameters that are useful for sensor design. All the vibration modes of the sensor, that were also found experimentally by electrical impedance analysis, were calculated.     

A noncontact NDE method using a laser generated focused-Lamb wave with enhanced defect-detection ability and spatial resolution

The laser generation method of a focused Lamb wave is expected to have high defect-detection abilities and spatial resolution, with the added advantage of noncontact testing. In this method, the laser beam is illuminated on the surface of an object through an arrayed arc slit. The energy of the generated Lamb wave is then concentrated at a focus of an arc. This focusing effect improves the NDE (nondestructive evaluation) performance, which is dependent on the geometries of an arrayed arc slit. In this paper, the relationships between the parameters determining the slit geometry and the focusing effect of the generated Lamb wave was investigated using a parametric study, and appropriate values of the parameters were obtained to maximize the focusing ability. In order to validate the performance of this method, an NDE system was constructed and experiments were performed to detect through-hole defects in a plate. The results showed that the proposed system could clearly detect defects as small as 0.3 mm in diameter, while the conventional line array method failed to detect defects smaller than 1 mm in diameter. Moreover, this method showed possible detection of defects much smaller than 0.3 mm, and great improvements in the spatial resolution as compared with the line array method.     

Confidence metric for signal classification in Non Destructive Evaluation

Quantitative assessment of the reliability of defect classification is critical in non-destructive evaluation (NDE) applications. Particularly in automated data analysis systems, such a measure enables the system to monitor its own performance and automatically flag indications where operator intervention is required. Apart from inherent ambiguity of non-discriminative features and inadequate training samples, noisy measurement is a primary reason underlying the classifier's unreliable decisions. In this paper, we have developed a framework to incorporate the major sources of classification errors into a single quantitative measure. By bootstrapping and weighting Bayes posterior probability with estimated noise distribution, effect of noise in NDE measurements is embedded in the resultant confidence measure. The effectiveness of the proposed method is first demonstrated on synthetic dataset from an eddy current simulation model. It is then used to analyze confidence of classifying experimental data from eddy current inspection of defects in steam generator tubes.     

Friction Stir Welding assisted by electrical Joule effect

This paper presents a variant of Friction Stir Welding (FSW) aiming to minimize or eliminate the root defects that still constitute a major constrain to a wider dissemination of FSW into industrial applications. The concept is based on the use of an external electrical energy source, delivering a high intensity current, passing through a thin layer of material between the back plate and the lower tip of the tool probe. Heat generated by Joule effect improves material viscoplasticity in this region, minimizing the root defects. The concept was validated by analytical and experimental analysis. For the later, a new dedicated tool was designed, manufactured and tested. Numerical simulations were performed to study the electrical current flow pattern and its effect on the material below the probe tip. The potential use of this variant was shown by reducing the size of the weld root defect, even for significant levels of lack of penetration, without affecting overall metallurgical characteristics of the welded joints.     

Application of eddy currents induced by permanent magnets for pipeline inspection

This paper presents an alternative to the common concentric coil method to induce low-frequency eddy currents in ferromagnetic pipe and tubes. Pairs of permanent magnets rotating around the central axis of these cylinders in proximity of the surface can be used to induce high current densities in the material that is the object of the inspection. Anomalies and wall thickness variations are detected with an array of sensors that measure local changes in the magnetic field produced by the current flowing in the material. This electromagnetic technology is being developed for pipeline inspection platforms that either crawl slowly inside a pipe to maneuver past physical barriers or are pushed by flexible rods. These devices move down the pipeline independent of the product flow, and potentially stop for detailed defect assessment. Fundamental finite element modeling analysis and experimental investigations performed during this development have led to the derivation of a first-order analytical equation for designing rotating exciters and positioning sensors. The rotating permanent magnet system has the potential for broader application because the sensor configurations can be small in physical size, allowing them to pass obstructions that currently prevent inspection using available NDE implementations.     

三裂解整流变压器改造提高抗短路能力

日照钢铁有限公司1580带钢生产线上选用的三裂解整流变压器频发故障,本文分析了变压器的故障原因,通过提高变压器阻抗值,并对变压器引线夹持和器身压板做了相应的改进,提高了变压器抗短路能力,延长了使用寿命.     

QNDE的核心科学与工程基础进展(英文)

对材料和结构缺陷的无损检测(NDT)已经历了50多年的进化演变。在美国,它已经从一个要求零缺陷的检测策略(NDT),向基于损伤可容度设计的检测和评估技术(NDE)过渡。这里是假设部件始终包含一个缺陷,只有那些比断裂力学确定的临界尺寸更大时,需要通过检查将其消除,以重新确定产品的服务周期。介绍了这些因素对于推动模式转变及转换至定量无损检测方面起到的至关重要的作用。一系列的重大研究项目被启动,用于更新无损检测以满足新的要求。重点介绍了在第一项目发展中的研究重点,以及用于定量缺陷定义的DARPA/AFML跨学科项目。它有三个目的:发展新的核心科学/人员基础,使检测技术满足新的要求,确立发展新的领域,即适当的工程设备的阶段,并继续开展定量无损检测(QNDE)系列会议。从这个和其他方案的进展已导致对所涉及的任何检查和技术的各项测量的基本模型的链接为基础的定量无损评价(QNDE)的科学核心。除了讨论这些模式和它们的联系,还将定义核心结构。利用这些模型,一种新的强大的工程工具集已经开发,包括UT,RT和EC技术的模拟程序。这些工具的应用将成为亮点,在包括结构健康监测和状态检修的工作中将令人注目。最后,讨论了QNDE未来机会、远景和方向。     

电容电感复合桥式焊缝跟踪传感器

针对平板对接焊的错边问题,提出了一种组合电容和电感复合桥式焊缝自动跟踪方法,并设计了传感器的结构.这种复合桥式焊缝跟踪传感器利用电容对极板间距变化的灵敏检测和电感对边缘变化的灵敏检测,可以确定焊枪横向和纵向的位置关系.具有非接触、灵敏度高、抗干扰能力强、响应速度快、工作可靠的特点,并能在一定程度上克服平板对接焊的错边问题.最后,自行搭建了简单的平台进行了焊缝跟踪试验,效果良好,为焊缝自动跟踪技术提供了新的方法.     

一种具有聚焦作用的线圈涡流问题解析解

为检测导体内更深处的缺陷，提出用8字形线圈作为涡流检测线圈，得到了8字形线圈在半无限大线性导体上方时涡流问题的解析解。仿真计算表明，与平行于导体表面放置的圆环形线圈相比，8字形线圈所激励的涡流密度峰值更大，能量分布更集中。试验验证了斜置线圈的阻抗表达式。研究结果可用于指导涡流检测探头的设计。     

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.