飞机结构腐蚀检测中的脉冲涡流无损检测技术

对脉冲涡流无损检测技术的工作原理进行了分析，建立了脉冲涡流的检测系统对加工的模拟飞机多层结构的试件进行了测试，提取其时域瞬态感应信号的峰值和过零时间作为特征量，对出现在飞机多层结构第二层中的腐蚀缺陷进行了定量检测，试验结果和理论分析相一致，证明了脉冲涡流检测方法的有效性，由于其具有快速和定量化的优点，因而在飞机结构的腐蚀检测中具有广阔的应用前景．     

点探头涡流无损检测中磁介质作用机理研究

结合实际涡流无损检测应用中的含磁介质点探头的特点,分析了探头中磁介质引入的各种附加效应及其对缺陷阻抗的影响。通过引入等效电流的概念,讨论了磁介质对涡流场的作用,建立含磁介质点探头涡流无损检测模型,并给出含磁介质探头周围的电磁场方程,求得均匀半无限导体系统的磁矢量势分布和导体中涡流、电场强度分布。通过实例,计算了非铁磁性导体中无缺陷时探头信号与提离高度的关系、存在缺陷时探头阻抗变化的特点。理论计算与实验结果基本吻合。     

Dynamic Piezoresistivity Calibration for Eddy Current Nondestructive Residual Stress Measurements

It has been recently demonstrated [M. P. Blodgett and P. B. Nagy, J. Nondestruct. Eval. 23, 107 (2004)] that eddy current conductivity measurements can be exploited for near-surface residual stress assessment in surface-treated nickel-base superalloy components. To quantitatively assess the prevailing residual stress from eddy current conductivity measurements, the piezoresistivity coefficients of the material must be first determined using known external applied stresses. These calibration measurements are usually conducted on a reference specimen of the same material using cyclic uniaxial loads between 0.1 and 10Hz, which is fast enough to produce adiabatic conditions. Therefore, the question arises whether dynamic calibration measurements can be used or not for accurately assessing the sensitivity of the eddy current method for static residual stress. It is demonstrated in this paper that such dynamic calibration measurements should be corrected for the thermoelastic effect, which is always positive, i.e., it increases the conductivity in tension, when the material cools down, and reduces it in compression, when the material heats up. For low-conductivity titanium and nickel-base engine alloys the thermoelastic corrections are relatively modest at ≈5–10%, but for high-conductivity aluminum alloys the difference between the adiabatic and isothermal properties could be as high as 50%.     

Development of High-T c SQUID System for Nondestructive Evaluation

Nondestructive evaluation (NDE) system with high spatial resolution power based on high-T _c superconducting quantum interference device (SQUID) has been developed. The SQUID is fabricated by YBCO thin film and mounted in vacuum at the end of a cold finger fixed at the inner wall bottom of the dewar, which is filled with liquid nitrogen (77 K) to cool the SQUID sensor. The sensor faces a thin flat sapphire window on the outer wall of the dewar and is thermally isolated from it by a vacuum space at a distance of less than 1 mm. The magnetic field sensitivity of SQUID is about 40 pT/ Hz in magnetically unshielded environment. This NDE system was used to detect the flaws buried into materials. The remnant magnetic field inducted by the local damages in a stainless steel sample was measured. Incorporated a gradiometric double-D excitation coil, the eddy current NDE was carried out. Small cracks in the aluminum below the surface can be detected effectively in unshielded environment.      

Quantitative Fractography for Improved Probability of Detection (POD) Analysis of Bolt Hole Eddy Current

Fatigue cracks are prone to develop around the fasteners found in multilayer aluminum structures such as ageing aircraft. The determination of their probability of detection (POD) using eddy current techniques is critical for risk assessments used in evaluating serviceability. Normally, a large number of samples of both fatigue cracks and electric discharge machined notches are required for such a study. In this study, the use of quantitative fractography (QF) to efficiently gather POD data for bolt hole eddy current is demonstrated. The technique can be readily automated, significantly reducing man-hour requirements. The method is applied to mid-bore cracks, grown without starter notches, in order to simulate naturally occurring fatigue cracks. POD of the mid-bore cracks is examined using a probit analysis. An a _90/95 is the probability that 90% of the cracks of size a or larger will be detected 95% of the time. An a _90/95 of 0.34 mm was obtained for mid-bore crack depth. This value is consistent with skin depth considerations at the 1.6 MHz inspection frequency. An examination of the POD based on crack length found that it was related to that of depth according to the average length-to-depth aspect ratio of 2 to 1.     

Eddy current probe impedance due to a flaw: The influence of phase-shifted fields

An approximate formula for the change in impedance of an eddy current two-coil probe situated above a conducting medium with a volumetric flaw is derived in this paper. The primary field is generated by the superposition of two phase-shifted alternating currents of the same frequency. The Lorentz reciprocity theorem and a layer approximation are used to obtain the change in impedance. Numerical results for different values of the phase difference are presented. These results can be used for developing more selective eddy current testing methods.     

基于脉冲涡流热成像的面内方向性热扩散率测量

针对导电材料面内方向性热扩散率的测量,提出脉冲涡流热成像法这一新方法。该方法采用感应式脉冲线激励源,在导电试件表面形成沿一定方向的感应涡流,实现局部热激励,在非稳态条件下实现了材料面内方向热扩散率的测量;简单调节试件与线感应激励线圈的角度,就可以快速无损非接触地测量试件在垂直线圈方向上的热扩散率值。对感应线激励下面内热传导及高斯温度分布进行了分析,分别对AISI304不锈钢、纯铁、纯镍3种材料的热扩散率进行了测量,测量结果与手册值相符,偏差小于9.0%,相对扩展不确定度分别为3.18%,3.72%,3.70%。     

Recommended NDT Practice and its Model-based Investigation for Eddy Current Inspection of Aerospace Structures

This paper describes a model-based investigation of inspection standards being developed as a recommended practice for eddy current (EC) NDT of aerospace structures for surface cracks. Compared to experimental investigations that can be laborious and expensive, model-based approach provides cost-effective and quantitative verifications of the recommended practice that covers wide ranges of materials, probes, instruments, frequencies, and test conditions. Probe signals for a number of specified standard notches are predicted and the expected amplitudes and phase-angle ranges are determined for various test conditions, in order to examine the influence of material electrical conductivity, frequency, and probe types. For a few selected situations, the model predictions are validated against experimental data. This study verifies the adequacies of the recommended specifications, especially of the reference notch selections and their response amplitudes for various test conditions, as well as confirming the suitability of using vertical amplitude alone for all the test situations. The influence of probe cable and the issue of three-point calibration are also discussed specifically in this investigation.     

新型焊接质量的无损检测技术

焊接是工业生产过程中机械零部件连接的一种常用方法,近些年来对焊接质量的要求日渐提高,而无损检测技术的出现使焊接件的质量得到了有效可靠的保证,该技术不会破坏焊接件的使用性能,利用该技术可以对焊接缺陷进行评定检测,从而对焊接质量进行有效控制。近年来,无损检测技术日渐多样化,对较新型的超声波衍射时差法(TOFD)、发射检测(AE)、脉冲涡流检测、超声相控阵检测(PAUT)等焊接无损检测技术做了研究和探讨,可为实践应用提供依据。     

Considerations in the Validation and Application of Models for Eddy Current Inspection of Cracks Around Fastener Holes

To improve the detection and characterization of cracks around fastener holes in multilayer structures without removing the fastener, model-based approaches are proposed to support the design of advanced eddy current (EC) NDE systems. This work demonstrates the validation and application of models to simulate EC inspection as part of the design process. The volume integral method (VIM) and finite element method (FEM) are both used to simulate eddy current inspection of fastener sites for fatigue cracks. Convergence studies, validation with existing models, experimental validation studies and validation through inverse method demonstrations are presented, providing a continuum of methods to ensure the quality of measurement models. Consideration concerning convergence and validation is also given with features sensitive to the sample geometry and flaw characteristics. A novel calibration technique is also presented to practically evaluate the transformation between model-based impedance calculations and experimental voltage data. A series of studies are presented concerning the detection of cracks around fastener holes demonstrating the quality of the simulated data to represent experimental measurements.     

Analysis of Some Nondestructive Evaluation Experiments Using Eddy Currents

Nondestructive evaluation (NDE) is often conducted by inducing eddy currents in the materials to be examined, and characterizing those currents by the magnetic fields that they produce. Most papers describe experimental results produced by such methods; very few theoretical approaches have been proposed. In the present article, we develop some particularly very simple but ad-hoc analytical studies, notably in the case of various calibrated flaws such as cracks. The validity of the models has been checked with a classical High T_c radio frequency SQUID, which is described. Theory and measurement have been found to be in good agreement.     

Crack Characterization Method with Invariance to Noise Features for Eddy Current Inspection of Fasterner Sites

There is a need for research in eddy current (EC) nondestructive evaluation (NDE) to improve the reliability to detect, locate and size cracks around fastener sites in multi-layer structures while minimizing the overall cost of inspection. The objective of this work is to develop feature extraction and classification algorithms for crack characterization with invariance to noise features for eddy current inspection of fastener sites. Model-based parametric studies were first performed to explore potential features under a wide array of crack, noise and material conditions. Through these studies, several features were identified to have some invariance to the characteristic asymmetric response due to gaps between the fastener and hole, probe liftoff variation, and probe skew. In particular, a promising feature with noise invariance to all non-flaw conditions considered in this study was found through investigating changes in the eddy current response along a circumferential direction in an annulus region away from the hole center. To obtain a measure of this localized crack feature, an approach was developed using a fit of a characteristic function to the data through nonlinear least squares estimation. A model-based optimization approach was also implemented to evaluate the best signal processing algorithm design to distinguish between several classes of crack size. Using this approach, an optimized measure was found to be well correlated with subsurface crack size and insensitive to noise conditions included in this study.     

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Nickel-Base Superalloys

It is shown in this paper that, in contrast with most other materials, shot-peened nickel-base superalloys exhibit an apparent increase in eddy current conductivity at increasing inspection frequencies, which can be exploited for nondestructive residual stress assessment of subsurface residual stresses. It has been found that the primary reason why nickel-base superalloys, which are often used in the most critical gas-turbine engine components, lend themselves easily for eddy current residual stress assessment lies in their favorable electro-elastic behavior, namely that the parallel stress coefficient of the eddy current conductivity has a large negative value while the normal coefficient is smaller but also negative. As a result, the average stress coefficient is also large and negative, therefore the essentially isotropic compressive plane state of stress produced by most surface treatments causes a significant increase in conductivity parallel to the surface. The exact reason for this unusual behavior is presently unknown, but the role of paramagnetic contributions cannot be excluded, therefore the measured quantity will be referred to as apparent eddy current conductivity. Experimental results are presented to demonstrate that the magnitude of the increase in apparent eddy current conductivity correlates well with the initial peening intensity as well as with the remnant residual stress after thermal relaxation.     

考虑磁导率非线性的铁磁材料脉冲涡流检测仿真研究

长期以来,脉冲涡流检测中铁磁材料的磁导率多被视为常数,但这种简化的合理性及其影响尚缺乏充分的证明.本文基于有限元法,建立了探头置于Q235钢板上方的脉冲涡流检测模型,研究了钢板磁导率的空间分布和时间变化规律及其对探头信号的影响,并进行了实验验证.结果表明:钢板中瞬态磁场的工作点与激励电流幅值有关,增大激励电流,磁场可能超出磁化曲线的起始磁化区而进入到瑞利区甚至陡峭区;由于磁导率非线性的影响,激励方波高、低电平段的感应电压信号不成奇谐对称,高电平段的晚期感应电压比低电平段的大;钢板浅层磁导率的变化显著,不能简化为常数,而深层的磁导率变化很小,可视为常数.     

基于脉冲涡流检测技术的缺陷定量检测研究

对飞机多层结构中出现的缺陷进行定量检测具有重要意义.文章将脉冲涡流检测技术应用其中,设计并实现了一套完整的脉冲涡流检测系统.在对检测信号进行分析处理的基础上,提取峰值、峰值时间、过零时间等时域特征量,实现对缺陷的定量检测,并通过进一步的实验加以验证.     

涡流检测中轴对称场的快速计算

本对涡流检测中解析求解轴对称场时遇到的无穷积分给出一种简单有效的数值积分方法,提出一种改进积分收敛性的方法,加速了无穷积分的截断,显提高了轴对称场的计算效率。几组典型数据的计算结果与ＦＥＭ－ＢＥＭ组合法计算结果的比较表明,本的方法是准确、快速的,这一计算方法的高效率使其可应用于涡流无损检测中缺陷的重构。     

Use of Eddy Current Technology to Assist in the Evaluation of the Fatigue Damage of Electrical Conductors

The article deals with the evaluation of fatigue damage on overhead conductors which have been in service for a period exceeding 50 years. Under wind excitation, electrical transmission lines are subjected to aeolian vibrations that may induce wire fatigue of conductor at the exit of supporting equipment. This fatigue phenomenon is a fretting fatigue problem which is located at contact points between wires belonging to adjacent wire layers or between external layer wire and the supporting equipment. This article reports the methodology followed: 1) in selecting wires with the more severe damaged contact point using eddy current technology and 2) in comparing, through a metallographic examination, the contact points of those selected wires coming from in situ worn conductors and from laboratory fatigue tested virgin conductors used as reference.     

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Inhomogeneous Nickel-Base Superalloys

Recently, it has been shown that shot-peened nickel-base superalloys exhibit an approximately 1% increase in apparent eddy current conductivity at high inspection frequencies, which can be exploited for nondestructive subsurface residual stress assessment. Unfortunately, microstructural inhomogeneity in certain as-forged and precipitation hardened nickel-base superalloys, like Waspaloy, can lead to significantly larger electrical conductivity variations of as much as 4–6%. This intrinsic conductivity variation adversely affects the accuracy of residual stress evaluation in shot-peened and subsequently thermal-relaxed specimens, but does not completely prevent it. Experimental results are presented to demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence. This characteristic independence of frequency can be exploited to distinguish these inhomogeneities from near-surface residual stress and cold work effects caused by surface treatment, which, in contrast, are strongly frequency-dependent.     

A New Approach for Restoration of Eddy Current Images

Eddy current images of defects are blurred due to convolution of point spread function of eddy current probe with defects. Disturbing variables such as lift-off, surface roughness, and material property variations influence the eddy current images. In order to restore the length, width, depth, and orientation of surface-breaking defects in the presence of disturbing variables, a new and comprehensive approach has been developed. This approach uses artificial neural network and image processing methods. Studies on austenitic steel plates confirm that through this approach it is possible to restore the spatial information of surface-breaking defects of uniform or slowly varying depth and also to form their accurate three-dimensional pictures. This approach is fast as well as amenable for automation.