Neural network approach for determination of fatigue crack depth profile in a metal, using alternating current field measurement data

A neural-network-based technique is described to determine the depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the measurements along the crack opening. The network uses the multilayer perceptron structure for which the training database is established by systematically producing semi-elliptical multi-hump cracks with narrow openings and random lengths and depth profiles. A fast pseudo-analytic ACFM probe output simulator is also used to produce network input data around each crack for a specified inducer. To demonstrate the accuracy of the proposed inversion technique, the simulated results of cracks with both common and complex geometries are studied. The comparison of the actual and reconstructed depth profiles substantiates the technique introduced here. To further validate the technique, the experimental results associated with several fatigue cracks of complex geometries are presented.     

Scattering of an induced surface electromagnetic field by fatiguecracks in ferromagnetic metals

In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented     

THE MEASUREMENT OF SURFACE BREAKING CRACKS BY THE ELECTRICAL SYSTEMS ACPD/ACFM

Abstract— The successful implementation of a fracture mechanics approach to the in-service structural integrity assessment of welded and machined components requires an accurate and reliable NDT method for detecting and sizing surface breaking cracks. This paper discusses and compares the crack depth sizing abilities of two theoretically-based NDT techniques: alternating current potential difference (ACPD) and alternating current field measurement (ACFM). Surface fatigue cracks of high and low aspect ratio are used in the investigation. It is demonstrated that ACPD is capable of accurately sizing long shallow cracks and that ACFM provides accurate crack depth predictions for small deep cracks.     

交流磁场检测中激发频率与提离的影响分析

交流磁场检测是近年来电磁无损检测的重要进展之一,具有非接触、不需标定、检测速度快和适应性好等优点,在材料检测中具有广阔的应用前景。在检测过程中,激发频率的选取以及提离高度是影响信号分辨率的重要因素,以ANSYS软件系统为工具,建立了缺陷ACFM数值仿真模型,分析了信号对频率和提离高度的影响,结果显示ACFM方法更适合检测磁性材料。     

Techniques for depth-selective,low-frequency eddy current analysis for SQUID-based nondestructive testing

Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.     

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。     

Slots vs. cracks in eddy current NDE

The thin-skin theory of Lewis, Michael, Lugg, and Collins (LMLC) is applied to an analysis of the effects of slot closure on eddy current response. The LMLC model is shown to lead to a relatively simple expression for the ratio of the impedance change for an open slot to the impedance change for a closed slot, which represents a crack. Numerical calculations for a nonmagnetic material show that the impedance ratio is an almost linear function of the slot opening in skin depth units, and is relatively insensitive to slot length and aspect ratio. For ferromagnetic materials with large relative permeability, there is no significant slot closure effect. LMLC calculations are supplemented by volume integral calculations at low frequencies where the thin-skin theory fails.     

Removal of Probe Liftoff Effects on Crack Detection and Sizing in Metals by the AC Field Measurement Technique

In the alternative current field measurement (ACFM) technique, the nonzero value of liftoff distance for the magnetic sensor acts as a low-pass filter on surface crack signals, causing errors in crack detection and sizing. We present a blind deconvolution algorithm for liftoff evaluation and surface crack signal restoration. The algorithm employs the available closed-form expressions for the distribution of electromagnetic fields at the metal surface in the vicinity of a crack. To examine the accuracy of the algorithm, we use the original and the restored signals for crack sizing by a wavelet network inversion method. We present simulated and experimental results to demonstrate the role of the proposed algorithm in improving the inversion process.      

基于交变磁场测量技术的裂纹缺陷定量检测仿真分析与实验研究

裂纹缺陷的定量评估是无损检测的一项重要研究内容,本文采用交变磁场测量技术对平板裂纹缺陷进行了检测.在分析交变磁场测量技术原理的基础上,首先在大型电磁仿真软件AN SY S中建立了交变磁场测量模型,包括三组不同参数的裂纹缺陷和检测线圈相互垂直的两种传感器模型,然后用这两种传感器分别对不同缺陷进行了检测,研究了裂纹长度、深度和宽度变化对水平方向和垂直方向检测电压的影响规律,提取出了对长度和深度进行定量的特征量.最后,采用实验的方法对仿真结果进行了验证,实验结果的规律与仿真结论相一致,证明了仿真结果的正确性.从而为裂纹缺陷在实际中的检测和定量提供了借鉴.     

The measurement and analysis of semi-elliptical surface fatigue crack growth

Fatigue tests in which the development of crack shape was studied with the a.c. field measurement technique have been conducted on semi-elliptical cracks growing in flat plate specimens under tension and bending stresses. The a.c. field measurement technique, which was used to measure the cracks, exploits the fact that high frequency alternating current tends to flow in a thin skin along the metal surface and the crack depths can be interpreted from the changes in the voltage distribution produced by the presence of a crack. In order to interpret the measured voltages so as to provide accurate estimates of the crack length and depth, a theoretical analysis of the a.c. field around a semi-elliptical crack is required. This analysis was used to interpret the measured voltages in terms of the changes in the crack shape. Finally the interpreted data on the changes in crack shape were then used to test the accuracy of various stress intensity factor solutions.     

Finite element modelling of closed cracks in eddy current testing

The eddy current inspection of small fatigue cracks in Ti–6AL–4V is evaluated in both a finite element model and experiments. The crack was created in a fatigue process and an eddy current measurement was carried out as the resulting crack was subjected to different levels of static load. The signal showed a strong dependency of the time between the creation of the fatigue crack and the eddy current measurement. This dependency is proposed to be related to oxides forming on the crack faces. The oxide is favourable for the detection of fatigue cracks. The narrow width of the fatigue crack is important to consider in eddy current inspection and as static loads are applied across the crack faces, electrical connections arise within the crack, which has a strong influence on the eddy current signal. Four different models of the contact behaviour were implemented within the finite element model. It is shown that the electrical connections that arise within small fatigue cracks, as well as the influence from the narrow opening as tensile loads are applied, can be predicted by a finite element model of the eddy current method.     

Finite element modelling and physics of remote field eddy currentresponses for axially aligned cracks

A simplified physical model for two-dimensional (2-D) finite element analysis of axially aligned crack responses for the remote field eddy current (RFEC) technique is proposed. Two formulations are applied to model the problems. Reasonable agreement between calculated results and some preliminary experimental results is achieved. A hypothesis, based on the physical model, is presented. It explains the RFEC signals from axially aligned cracks and makes some predictions of possible features for future verification     

Specific features of the space distribution of the signal of an eddy-current converter caused by cracks of different lengths

We calculate and analyze the space distribution of the signal of an eddy current converter for crack-like defects of different lengths. We have established the differences in eddy current signals between short and long cracks. We also present recommendations for the optimal selection of the parameters of an eddy current converter depending on the crack length. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 4, pp. 121–124, July–August, 2007.     

Eddy Current Signal Deconvolution Technique for the Improvement of Steam Generator Tubing Burst Pressure Predictions

Eddy current techniques are extremely sensitive to the presence of axial cracks in nuclear power plant steam generator tube walls, but they are equally sensitive to the presence of dents, fretting, support structures, corrosion products, and other artifacts. Eddy current signal interpretation is further complicated by cracking geometries more complex than a single axial crack. Although there has been limited success in classifying and sizing defects through artificial neural networks, the ability to predict tubing integrity has, so far, eluded modelers. In large part, this lack of success stems from an inability to distinguish crack signals from those arising from artifacts. We present here a new signal processing technique that deconvolves raw eddy current voltage signals into separate signal contributions from different sources, which allows signals associated with a dominant crack to be identified. The signal deconvolution technique, combined with artificial neural network modeling, significantly improves the prediction of tube burst pressure from bobbin-coil eddy current measurements of steam generator tubing.     

The detection and measurement of symmetric corner cracks by the a.c. field method

An unfolding method, developed earlier for cracks in plane surfaces, is extended in this work to provide an approximate mathematical model for the perturbations produced by a symmetrical corner crack when interrogated by a surface electric field. The model disregards the nonuniform nature of the current distribution upstream of the crack which is produced by the presence of the corner, and this simplification leads to unfolded field problems of plane Laplacian form. The Schwarz-Christoffel transformation is applied here in their solution. Experimental measurements on slots cut on steel blocks to simulate corner cracks are found to be in good agreement with the predictions of the model.     

HIGH TEMPERATURE ELASTIC-PLASTIC SMALL CRACK GROWTH BEHAVIOR IN A NICKEL-BASE SUPERALLOY

Abstract— An experimental study of the elastic-plastic fatigue behavior of small surface cracks in Alloy 718 at 650°C was conducted under conditions of total strain control. During cycling, the crack growth was continuously monitored using a direct current potential drop technique while the influence of crack closure was monitored using a laser interferometry technique measuring the crack mouth opening displacement. The crack tip plastic zone size was also measured using a post-test delta phase decoration technique. Results show that the growth rates of the small cracks correlate well with long crack LEFM data when using an appropriate elastic-plastic driving force parameter. The anomalous crack growth rates observed in some experiments were found to be experimental transients dominated by the crack initiation fracture and do not represent an intrinsic behavior of Alloy 718. No contribution to the anomalous small crack growth could be attributed to either crack closure or enhanced crack tip plasticity.     

金属表面裂纹涡流检测系统

针对现实生活中由于金属裂纹引发的众多灾难性事故,研究设计了一种结构简单、灵敏度高而且便于携带的金属表面裂纹涡流检测装置.该装置应用涡流探伤的基本原理,采用磁性材料,制作了一种精细的涡流传感器,用于前端提取微弱的裂纹信号;后台应用单片机控制电路,处理提取的滤波、放大、A/D转换等信号,最后由单片机驱动声、光报警,显示裂纹缺陷的相对大小,实现了防患于未然的目的.     

岩石类材料裂隙形成和扩展的相场方法模拟

相场方法通过定义一个连续分布函数来近似表示自由不连续裂纹,在此基础上建立最小能量变分框架,从而得到描述裂纹发展的控制方程。不需要提前设定裂纹的形状、尺寸和方向,相场方法就能很好地描述裂纹的形成和扩展,为利用数值方法模拟裂纹扩展提供了一个严谨准确的理论框架。该文首次将相场方法用于模拟岩石裂隙扩展问题,预测包含不同岩桥倾角的预制双裂隙岩石类材料在单轴压缩作用下的损伤和破坏过程,并与室内试验结果进行对比。结果表明相场方法非常适合模拟岩石类材料内部复杂裂隙的萌生、扩展和连接过程,在岩体工程稳定性分析领域具有广阔的应用前景。     

Methodology of short fatigue crack detection by the eddy current method in a multi-layered metal aircraft structure

The field of non-destructive inspection (NDI) is an integral part of aircraft maintenance and service. It is optimal to apply the eddy current (EC) method on the complex multi-layered metal aircraft structure to detect cracks and other damage. A range of standard eddy current probes giving satisfactory results with respect to crack detection are available. However, these EC probes are not always suitable for the detection of short fatigue cracks hidden under a rivet head. This article presents a new methodology for eddy current inspection of a critical area on the wing. The development of the new methodology was inspired by a catastrophic glider accident in 2010. Because of this accident, all glider service was prohibited. The critical area is located in the structure of the wing spar, which consists of six layers. The position of the critical area is hidden under the countersunk rivet head in the third layer of the spar flange and under two layers of metal sheets. Thus, the actual location of the area is in the fourth layer, created by the countersunk rivet head. In addition, another rivet of a smaller diameter is inserted into the rivet in the flange. This article presents the work related to the development of a special EC probe that is able to measure in an operating frequency range between 200 Hz and 100 kHz with a single-value interpretation of the individual EC signals.     

A novel microwave method for detection of long surface cracks inmetals

A novel microwave technique for detecting long surface cracks in metals is described. This technique utilizes an open-ended waveguide to probe the surface of a metal. In the absence of a crack the metal surface is seen as a relatively good short-circuit load. However, in the presence of a crack higher order modes are generated which in turn change the reflection properties at the waveguide aperture. This change brings about a perturbation in the standing wave characteristics which is then probed by a diode detector. The experimental and theoretical foundations of this technique are given, along with several examples. It is shown that cracks a fraction of a millimeter in width are easily detected at around 20 GHz or lower. Smaller cracks can be detected at higher microwave frequencies