改进型低频涡流无损检测技术的应用

改进了低频涡流无损检测技术并设计了检测系统。检测系统的正交锁定放大部分的核心采用数字相敏检波技术,节约了硬件成本,并可以对检测信号进行正交分解。检测系统的传感器为双自感线圈构成的放置式探头,其检出电路为交流电桥。在绝对式检测方式下,通过峰值曲线对缺陷的长度实现了定量检测;利用差动检测方式获得了缺陷的“8”字型阻抗图;实验结果表明,改进的低频涡流检测技术可以对多层结构中的缺陷进行有效的检测。     

阻抗电压变换法无线涡流探头研究

研究的是一种无线涡流探头的设计。将探头线圈、激励源、信号调理电路集成于探头中,通过无线收发传输信号至仪器主机,可大大降低仪器的设计成本和体积,并提高涡流检测主机的探头兼容性,实现一套涡流仪接上各种不同探头就可对大多数工件探伤。解决传统涡流探伤仪在管材螺纹等环形工件检测的不便。检测电路采用阻抗电压变换法,检测涡流探头阻抗变化。使用该探头检测铁、铝两种材料缺陷,实验结果表明:经过调试之后发现,在提离效应和在对缺陷扫查过程中,本探头输出数据的变化很明显,并且这些变化可较容易将提离和缺陷区分出来,而且在对不同材料的工件进行检查时变化趋势及大小也有不同,这将为我们以后分析不同材料及缺陷提供帮助。     

非晶合金涡流探头性能的研究

用非晶合金作为磁芯制作涡流探头,运用正交试验设计方法,通过改变频率、磁芯直径、线圈匝数等参数对探头进行优化试验,对比各参数对探头检测灵敏度的影响。结果表明:设置合适的探头参数能够提高检测灵敏度;相同条件下非晶合金磁芯探头比铁氧体磁芯探头检测灵敏度要高。     

金属涡流探伤仪的原理与设计

金属在交变磁场中产生涡流,金属涡流探伤仪由信号振荡器、探头线圈、检波及测量比较电路、信号处理报警显示及电源等部分组成.当被测材料表面有缺陷或裂纹存在时,通过探头线圈的磁通量就发生变化,根据涡流大小及分布可检测出影响线圈电特性的参数,从而发现材料缺陷.     

阵列式脉冲远场涡流管道缺陷检测方法

针对采用单个枪测线圈在管道缺陷检测中存在的检测分辨力较低的问题,研究了管道缺陷定量检测中的阵列式脉冲远场涡流检测技术.利用有限元仿真的方法对阵列传感器结构进行了优化设计,并且分析了不同深度缺陷对感应电压信号的影响规律.最后通过实验对仿真结果进行了验证,实验结果表明本文设计的阵列传感器具有较高的检测精度和灵敏度,可以实现对管道缺陷的定量检测.     

铁磁性平板中不同走向裂纹脉冲远场涡流检测的仿真研究

远场涡流技术克服了传统涡流检测受集肤效应限制的不足，在铁磁性管道的检测中得到了广泛的应用。本文采用脉冲方波来激励探头，形成了铁磁性平板脉冲远场涡流检测新技术。首先分析了脉冲远场涡流的检测原理，然后采用ANSYS仿真软件建立了其仿真模型，仿真分析不同走向缺陷对空间磁场的扰动规律，对比分析了脉冲远场涡流技术对不同走向裂纹的检测灵敏度。研究结果对于深入明晰脉冲远场涡流的检测机理及传感器设计具有重要的理论价值。     

飞机多层金属板隐藏缺陷远场涡流探头设计及试验研究

由于飞机多层金属板结构厚度大、复杂等特性,现有检测方法无法发现原位内部缺陷,对于多层板金属缺陷的检测一直都是航空无损检测的难题。远场涡流检测技术因打破趋肤效应的限制,涡流能量可穿透较厚的被测试件,对金属板结构中隐藏缺陷的检测具有潜在优势。该文针对飞机多层金属板隐藏裂纹的原位检测,建立多层金属板构件隐藏裂纹平面远场涡流检测有限元仿真模型,研究不同角度、不同深度裂纹检测幅值、相位的变化规律。为验证该仿真模型的正确性,开展远场涡流检测多层金属板的试验。试验结果表明:设计开发的远场涡流探头可检测埋深13 mm的裂纹缺陷,当裂纹倾斜角度为0°时,检测灵敏度最高,当裂纹倾斜角度为90°时,检测灵敏度最低,与仿真结果保持一致,且能够对缺陷进行精准定位,可为飞机多层金属板构件隐藏裂纹的定量检测提供依据。     

M型电磁传感器检测裂纹的仿真分析及参数优化

传统的脉冲涡流传感器采用圆柱式结构,其磁场大部分在空气中传播,造成能量的损失,因此该文设计M型电磁传感器在单线圈传感器上增加一个U型磁轭来提高磁场的利用率。通过ANSYS仿真软件对M型传感器与单线圈传感器检测不同深度的裂纹进行仿真对比,结果证明M型传感器的灵敏度高,信号强,同时对于传感器激励源参数进行优化。     

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。     

亚表面缺陷的实时成像检测

提出一种适合于金属亚表面缺陷的可视化无损检测方法——磁光/脉冲涡流成像方法。该方法以脉冲信号激励产生涡流,以激光对被检测物体的照射取代传统涡流检测的线圈探头,通过磁光传感元件将缺陷引起的磁场变化转换成相应的光强度的变化,由传统的显微镜、照明系统、偏振器和CCD图像传感器组成的光学系统将光强变化转换为“明”或“暗”图像,实现了对缺陷的实时成像检测。本文论述了磁光/脉冲涡流实时成像检测机理,给出了一种实验装置。通过对金属表面/亚表面缺陷实验,表明该检测方法快速、准确,可实现微/纳米级缺陷的成像检测。     

Perturbation technique for the finite element modelling of differential probes in non-destructive eddy-current testing

The paper deals with a new finite element scheme for non-destructive eddy-current testing (ECT) problems involving multiply connected test pieces and differential probes. It concerns a perturbation technique applied to the magnetodynamic h-phi formulation. The unperturbed field (in the absence of the flaw) is conventionally computed in the complete domain. The source of the perturbation problem is then determined by the projection of the unperturbed field in a relatively small region around the defect, the optimum size of which depends on the working frequency. The discretisation of this reduced domain is well adapted to the size of the defect and chosen independently of the dimensions of the excitation probe and the specimen under study. At a discrete level, the voltage change is efficiently computed by integration only over the defect and a layer of elements in the reduced domain that touches the defect's boundary. The accuracy of the proposed perturbation model is illustrated by comparison of the results obtained for different dimensions of the reduced domain with those achieved in the conventional way. The considered test case involves a differential probe scanning the outer surface of a metal tube for the detection of through-wall cracks     

Volumetric inspection of moderately thick austenitic stainless steels by multifrequency eddy currents

This paper describes the initial phase of a project to develop eddy-current methods to inspect welds joining sections of austenitic stainless steel pipe having walls up to 13 mm (0.5 in.) thick. The objective of this phase was to demonstrate the feasibility of detecting and characterizing flaws in austenitic stainless steel base metals. These materials and welds present challenging eddy-current problems because of their relatively large thickness and ferromagnetism. Multiparameter analysis shows that a reflection coil probe operated with three discrete driver frequencies and phase detection can locate and size a cracklike defect in a single conductor in the presence of variations in conductor resistivity, permeability, and thickness and in the probe-conductor spacing (liftoff). Experiments were performed with a modular three-frequency instrument. Flat-plate specimens of types 304L and 347 stainless steel machined to 12.7 to 15.9 mm thickness simulated pipe walls; saw-cut slots 10 to 30% of nominal specimen thickness simulated cracklike defects. The same slots were used in duplicate experiments as near-side (directly under the test probe) or far-side (in the face opposite the probe) defects. Flaw detection and characterization capability was demonstrated by a series of experimental measurements fitted to specimen properties by least squares techniques. The quality of the fit determined the expected accuracy of measurement. Comparison of accuracy estimates determined the best choice of operating frequencies. From the 1,2,5 sequence of frequencies between 0.5 and 20 kHz, the optimum set of operating frequencies was selected to be 0.5, 2, and 10 kHz. Estimates of measurement accuracy for combined near- and far-side defect cases were: plate thickness, 0.74 mm; probe liftoff, 0.03 mm; defect location (depth of material above defect), 3.48 mm; and defect size (vertical slot depth), 1.09 mm. A few property values were back-calculated from instrument readings; the errors in these values were somewhat larger than the measurement accuracy estimates because of instrument drift and the absence of calibration circuits.     

Approximate model of eddy-current probe impedance for surface-breaking flaws

A theoretical model is derived for the prediction of eddy-current probe impedance changes caused by three-dimensional, surface-breaking flaws. Magnetic scalar potential theory and the surface impedance approximation are used to calculate fields on the flaw surface for arbitrary probe position and flaw geometry. Impedance changes are determined by a first-order perturbation calculation, with skin depth being the perturbation parameter. The end result is a relatively simple, three-dimensional model for simulating an eddy-current inspection. Numerical results for rectangular slots include maps of the impedance signals obtained in raster scan patterns and studies of skin-depth effects as a function of probe size, lift-off, and flaw dimensions.     

A novel signal processing technique for eddy-current testing ofsteam generator tubes

In eddy-current testing of steam generator tubes of nuclear power plants, the signals of defects may be corrupted by noise and other nondefect signals arising from the probe lift-off and the structures attached to the tubes, resulting in unreliable detection and inaccurate characterization of defects. In this paper, a novel signal processing technique is presented to reduce the noise and nondefect signals by the use of a wavelet transform. The noise and nondefect signals are reduced by first decomposing testing signals into wavelet components and then modifying the wavelet coefficients. The defect signals embedded in noise and nondefect signals are reconstructed through the inverse wavelet transform of the modified wavelet coefficients. The results of processing the one-dimensional and two-dimensional signals from eddy-current testing of tube test pieces show that this signal processing technique is effective for extracting defect signals embedded in noise and nondefect signals     

The reduced impedance function for cup-core eddy-current probes

Eddy-current nondestructive evaluation makes use of ferrite-core probes for exciting alternating currents in the test piece. The effectiveness of the probe for detecting flaws in the material through changes in its impedance may be assessed from its interaction with an unflawed conductor. For an important class of eddy-current probes, those with ferrite cup-cores, the measured impedance characteristics are governed by a rule-based behavior found from experiments. These findings are in conflict with the elementary circuit theory model of probe-workpiece interaction but are confirmed by a general field-theory probe model     

Multiple sensors on pulsed eddy-current detection for 3-D subsurface crack assessment

This paper proposes the use of multiple sensors in pulsed eddy-current detection for three-dimensional (3-D) subsurface flaw imaging. A normalization technique has been proposed to eliminate the characteristic variation among the Hall devices used in the probe and lift off effects. A principal component analysis-based feature extraction that provides orthogonal information for multiple sensor fusion has been introduced and investigated. Using the features of multiple projection coefficients, 3-D surface flaws can be measured and reconstructed. The experimental tests have illustrated that the proposed method has delivered more defect information than the conventional peak value and time for pulsed eddy-current sensors.     

A theoretical and computational model of eddy-current probesincorporating volume integral and conjugate gradient methods

A general three-dimensional computational model of ferrite-core eddy-current probes has been developed for research and design studies in nondestructive evaluation. The model is based on a volume integral approach for finding the magnetization of the ferrite core excited by an AC-current-carrying coil in the presence of a conducting workpiece. Using the moment method, the integral equation is approximated by a matrix equation and solved using conjugate gradient techniques. Illustrative results are presented showing the impedance characteristics and field distributions for practical eddy-current probe configurations     

Correlation between He-Ne scatter and 2.7-microm pulsed laser damage at coating defects

A reported correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings has been analyzed in detail. Examination of a much larger data base confirms the previous result on dielectric-enhanced reflectors with polished substrates over a wide range of energy densities above the damage onset. Scatter signals from individual undamaged defects were detected using a He-Ne scatter probe with a focal spot that nearly coincides with the 150-microm-diam (D1/e(2)) focal spot of the damage-probe beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45 degrees incidence with 100-ns pulses at 2.7-microm wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal and by equivalence of the defect densities indicated by the two probes. Characteristics of the correlation are compared with a simple model based on focal spot intensity profiles. Conditions that limit correlation are discussed, including variable scatter from defects and background scatter from diamond-turned substrates. Results have implication for nondestructive defect detection and coating quality control.     

Millimeter-Wave Differential Probe for Nondestructive Detection of Corrosion Precursor Pitting

Critical aircraft structural components, such as wings and fuselages, are exposed to harsh environments that vary considerably in temperature and moisture content. In most cases, the corrosion is hidden under paint and primer and cannot be visually detected. The initiation of corrosion is preceded by the presence of corrosion precursor pitting. Near-field millimeter-wave nondestructive testing (NDT) methods have been successfully used for detecting corrosion precursor pitting in exposed as well as painted aluminum substrates. However, near-field millimeter-wave measurements are susceptible to clutter that may mask indications of small defects such as pitting. Standoff distance variation produces an unwanted intensity gradient on an image and may be considered the most undesired clutter-producing effect. This paper presents a differential millimeter-wave probe consisting of a pair of radiating apertures. It is shown that the differential nature of this probe tends to significantly reduce the undesired effect of standoff distance variation, thereby enhancing probe detection sensitivity. Furthermore, when this probe is used for the purpose of millimeter-wave imaging, it produces defect indications with unique features that help in distinguishing the defect from noise. This dual differential probe was used for detecting corrosion precursor pitting. The design of the probe and the results of detecting various pittings are presented in this paper.     

Eddy-current reflection probes: Theory and experiment

A general Z theory of reflection-type eddy-current probes was recently developed for new types of robotic proximity sensors. In this paper, the theory is applied to the characterization of surface-connected flaws. Flaw signals were calculated using a finite-difference implementation of the Z theory developed at Stanford University. A special air-core eddy-current reflection probe was fabricated at the National Institute of Standards and Technology (NIST) and used to obtain flaw signals for a number of rectangular-shaped electrical-discharge-machined (EDM) notches in aluminum. An automatic impedance analyzer was used to measure flaw signals as the probe was scanned over the length of the flaw. Experimental results were then compared to predictions of the theory.This paper is a contribution of the National Institute of Standards and Technology, not subject to copyright in the United States.