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.     

Using a wavelet network for reconstruction of fatigue crack depth profile from AC field measurement signals

This paper presents a wavelet-network-based technique for reconstructing the crack 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 ACFM probe output signals along the crack opening. The database for training the network is established by developing a random crack-depth generator, using a fast pseudo-analytic ACFM probe output simulator. The validity of the proposed technique is demonstrated by comparing the actual and reconstructed depth profiles of several simulated and machine-made cracks with no predetermined geometries.     

A fuzzy alignment approach to sizing surface cracks by the AC field measurement technique

In real world applications, one major issue with most of non-phenomenological methods is the need for a large and complete databanks. In most practical cases, it is impossible to obtain a complete and representative database that includes sufficient number of representative examples. This fact renders most of available defects and cracks databases useless. In this paper, an aligning method is formalized by a fuzzy recursive least square algorithm as a learning methodology for electromagnetic alternative current field measurement (ACFM) probe signals of a crack (data). This method along with a set of fuzzy linguistic rules, including adequate adaptation of different crack shapes (knowledge) for combining knowledge and data whenever the superposition theory can be utilized, provide a means to compensate for the lack of sufficient samples in available crack databases. We have shown that the combination of this fuzzy inference method and the method of the adaptation for different crack shapes provides sufficient means as a priori empirical knowledge for the training system. This approach significantly reduces the need for a large and complete crack databases. The validity of the proposed methodology is demonstrated by examining the sizing errors in the case of several surface cracks with elliptical depth profile when inverting their respective ACFM signals.     

飞机发动机叶片裂纹的交变磁场非接触原位探测

飞机发动机叶片基体出现裂纹时表面反映不明显,孔探仪检查表面裂纹实际判别的难度较大,因此用交变磁场非接触式探测法探测。介绍探伤原理、特点以及与其它无损检测方法的比较,最后给出了发动机叶片的检测实例。检测结果表明,该方法检出的大小裂纹的波形与实际裂纹相符甚好,因此较传统涡流法和孔探仪法更具优越性,具有实际应用价值。     

On the suitability of induction coils for crack detection and sizing in metals by the surface magnetic field measurement technique

The surface magnetic field measurement (SMFM) technique has proved to be an accurate means for crack detection and sizing cracks in ferrous metals. The technique involves the use of two U-shaped current-carrying wires of sufficiently high frequency while measuring the discontinuity in the resultant magnetic field at the crack edge with an appropriate magnetic field sensor. In this work, we describe a mathematical algorithm to obtain the crack signal from the output of an induction coil used in a SMFM probe. We also discuss the measurement errors due to the coil size and shape. To reduce the measurement errors, we present an algorithm in which the crack signal is recovered by appropriate deconvolution of the coil output signal and its spatial transfer function. The algorithm is then used to recover crack signals for various coil shapes and sizes. The study of the results demonstrates the effectiveness of the algorithm in the case of large coils.     

Modelling and experimental measurements of idealised and light-moderate RCF cracks in rails using an ACFM sensor

Alternating current field measurement (ACFM) sensors can be used to detect surface breaking defects in metal components. In rails rolling contact fatigue (RCF) cracks form due to the wheel–rail contact stresses. These cracks are surface breaking and can have complex shapes. A COMSOL model has been developed for a commercial ACFM system and RCF cracks in rails. In this paper model results are compared to experimental measurements using an ACFM pencil probe for calibration defects machined into a rail and real RCF defects (light and moderate categories; <20 mm surface length) in a rail removed from service. X-ray tomography has been used to determine the size and morphology of the real RCF cracks for input into the model. It has been shown that the model can be used to determine the change in normalised Bx signal due to the presence of calibration defects machined into a new rail. The model has also been used to compare the experimental data for the real RCF cracks to the reconstructed model cracks and a semi-elliptical approximation to their shape.     

基于ACFM技术的焊缝跟踪传感器设计与实现

根据交变磁场测量法（ACFM）的基本原理，设计了一种由激励线圈和感应线圈组成的交变磁场传感器，可对空间“点”磁场进行测量。通过大量实验，研究了传感器位置和不同焊缝宽度对传感信号的影响规律。根据传感信号的变化规律，提出了相应的信号处理的方法，为ACFM方法用于焊缝跟踪奠定了良好的基础。     

扰动磁场检测技术中裂纹磁场信号的反演

依据试验数据和有限元仿真结果,分析了裂纹缺陷特征与扰动磁场分布的关系,总结出交流场检测技术中磁信号裂纹尺寸反演的基本规律。借助于B_x扰动磁场信号形状与裂纹缺陷特征之间的相似性,建立了二维表面缺陷形状的反演算法。     

High sensitivity rotating alternating current field measurement for arbitrary-angle underwater cracks

Alternating current field measurement (ACFM) technology has been used for sizing underwater structure cracks. However, conventional ACFM is more sensitive to cracks perpendicular to the induced current than cracks with other angles. In this paper, a rotating alternating current field measurement (RACFM) method and underwater test system are present for the detection of arbitrary-angle cracks with high sensitivity. The RACFM is proved by simulations and experiments. Arbitrary-angle cracks detection results obtained from ACFM and RACFM have shown that the RACFM method overcomes the limitation of directional detection of ACFM and effectively achieves high detection sensitivity for arbitrary-angle cracks on underwater structures.     

Analysis of the inducing frequency of a U-shaped ACFM system

According to the principle of the alternating current field measurement (ACFM), the inducing frequency has a significant influence on the signal acquisition and the measurement accuracy of an ACFM system. To design an ACFM prototype system with a U-shaped probe, the inducing frequency of the ACFM system is determined through simulation analysis and an experimental study in this paper. A large number of simulations are designed and run to analyze the influences of the inducing frequency on characteristic vectors of the induction electromagnetic field. By analyzing the simulation results, 6 kHz is selected to be the optimal inducing frequency for the U-shaped probe of an ACFM prototype. This frequency is tested by real crack inspection experiments using the U-shaped probe of the ACFM prototype in laboratory. The results show that 6 kHz is appropriate to realize the crack inspection and sizing with reasonable accuracy. The result in this paper will benefit the design and manufacturing of the prototype for the U-shaped probe ACFM system.     

High-speed inspection of rails using ACFM techniques

High-speed ACFM tests were carried out using a rotary test piece that contained spark-eroded notches. The ACFM sensor detected the induced notches during inspection at 121.5 km/h. The recorded signal remained unaffected by the increases in inspection speed under constant lift-off. To simulate actual rail inspection conditions at high speed, further tests were carried out using a spinning rail rig and a set of rails that contained spark-eroded notches of various shapes and sizes up to a speed of 32 km/h. Although, the ACFM sensor successfully detected the majority of the notches, the signal obtained was affected by lift-off variations.     

Elimination of liftoff effect using a model-based method for eddy current characterization of a plate

Model-based inversion method has been studied extensively for characterization of a metal plate in eddy current testing. However, few reports cover liftoff elimination. In this work, model-based inversion method is evaluated in terms of liftoff reduction. For better inversion accuracy, a complex yet accurate procedure is presented to do calibrations of coil parameters before use. The results from simulations and experiments demonstrate that model-based inversion method has an exceptional ability to compensate influence of large liftoff variations by considering it as an unknown variable to be determined.     

Sizing vertical cracks using burst vibrothermography

We apply burst vibrothermography to characterize (area and depth) vertical cracks in a fast way. We present the calculation of the evolution of the surface temperature distribution produced by a vertical crack excited by an ultrasound burst. We develop a stabilized inversion algorithm that is able to retrieve the area and depth of the crack from one thermogram and one timing-graph. We check its performance by inverting both, synthetic data and experimental data taken on samples containing calibrated heat sources. Depth limits are established for accurate reconstructions of the defects as a function of the noise in the data.     

直流电位法对Q235钢材表面裂纹的检测仿真

目的针对Q235钢材的表面裂纹,在电位法检测原理的基础上,进行有限元仿真模拟。方法首先,设计不同宽度、不同深度及不同的裂纹与激励夹角等多组方案仿真计算。其次,基于直流电位法原理的解析解与有限元模拟对比验证,证明其有限元模拟的准确性。结果不同宽度的表面裂纹,两侧的电位值不同,有明显的极值拐点,且随着裂纹宽度的增加,两侧的电压降趋势逐渐平缓。不同深度的表面裂纹,两侧的电位幅值随着裂纹深度增加而增大,最小可检测0.02 mm的裂纹深度。不同方向的裂纹,其周围的电场线疏密程度及走向趋势不一样,当夹角30o≤α≤90o时,直流电位法可以较好地检测表面裂纹。通过解析解验证有限元仿真结果,在裂纹处的电位信号趋势一致,幅值略有不同,表明结果可靠。结论直流电位法对Q235钢材表面不同宽度、不同深度及不同夹角的裂纹检测分辨率较好。该研究为电位法检测Q235钢材表面裂纹提供了一定的可行性依据,证明此检测方法具有重要的工程实用价值。     

Nondestructive testing of train wheels using differential-type integrated Hall sensor matrixes embedded in train rails

A nondestructive testing method for inspecting cracks in train wheels is proposed in this study. The train wheels can be inspected without disassembly, as soon as the train enters its shed, using this method. The proposed method uses differential-type integrated Hall sensor matrixes (D-IHaSMs) embedded in the rails. This D-IHaSM can inspect cracks over a large area at high speed with a high spatial resolution. In addition, a custom signal processing circuit is designed to match the requirements of the D-IHaSM. The artificial cracks formed in the train wheels are inspected and the crack volume is estimated using the D-IHaSM system.     

金属回转体表面涡流探伤装置设计

为更准确地对昂贵金属回转体进行裂纹定位工作,根据涡流检测原理设计一种用于金属回转体表面的裂纹检测装置。该检测装置采用涡流传感器直线移动、回转体旋转的检测方式对回转体的外表面进行全面检测。检测装置主要由变速器、传动结构、涡流传感器、提离调整机构和坐标记录结构组成,通过涡流传感器-数据采集卡-计算机之间的数据传递来对输出数据进行降噪、显示及储存。该装置可用于较大直径回转体表面裂纹的检测与储存,给裂纹车削工作提供方便,也为其他设计人员提供了参考。     

Large-scale parallel computation for the reconstruction of natural stress corrosion cracks from eddy current testing signals

An inversion algorithm for the reconstruction of cracks from eddy current signals is developed in this study and applied to the profile evaluation of natural stress corrosion cracks that were found in steam generator tubes of a nuclear power plant. A crack is modeled as an assembly of small regions having conductivities inside so that eddy currents that flow across the cracks are considered. The conductivity of each region, which is assumed to be a discrete value, is reconstructed by means of the algorithm. Since the algorithm is based upon a tabu search that usually requires a large number of evaluating solution candidates, simulations are carried out on a supercomputer with the use of parallel computation using up to 128 CPUs so as to reconstruct the crack profiles within a reasonable computational time. It is demonstrated that the algorithm can estimate the profiles of the natural cracks with sufficient accuracy. The simulations also show that the algorithm is highly compatible with parallel computation. Additional simulations using other models of natural cracks are performed. Reconstructed profiles of the natural cracks, as a notch with zero conductivity, are very different from the true profiles, even though the reconstructed signals agree well with the measured values. This reveals that it is necessary to take the internal conductivity into consideration when dealing with natural cracks.     

30CrMnSiNi2A高强钢的疲劳小裂纹扩展特性及寿命预测

研究了３０ＣｒＭｎＳｉＮｉ２Ａ高强钢在恒幅载荷下的小裂纹起始特征和裂纹扩展特性。试验结果表明，小裂纹起始于缺口表面初始缺陷（如夹杂或孔洞）处。在应力比Ｒ＝０的恒幅载荷下，未显示小裂纹效应：在Ｒ＝－１的恒幅载荷下，则显示出小裂纹效应的存在。利用裂纹闭合模型和微观结构缺陷作为初始裂纹尺寸的方法预测了小裂纹扩展速率和疲劳全寿命，预测值与实验值符合良好。     

Structural optimization of 2-D array probe for alternating current field measurement

A double U-shaped orthogonal inducer for the alternating current field measurement (ACFM) is presented, which could extend the limitation of the direction of tested cracks and decrease the loss of magnetic flux compared to the single rectangular inducer. The structure of the testing probe was optimized to meet the expectations of the pointwise magnetic field measurement and to decrease every group of coils’ space by orthogonally winding method, and the two-dimensional (2-D) ACFM probe array was designed. The detection precision and lift-off effect of the 2-D ACFM probe array were experimentally studied. The results show that the relative error of this probe is less than 10%, and it could suppress the lift-off disturbance effectively, which makes this 2-D ACFM probe array to be insensitive to lift-off and unable to meet the engineering expectations.     

Analysis technique for interaction of rectangular open-ended waveguides with surface cracks of arbitrary shape in metals

The use of microwave technique has proved to be a viable means for crack detection and sizing surface cracks in metals. In this technique, the surface of the specimen is interrogated with a radiating open-ended waveguide probe and a crack is assumed to be a simple short-circuited rectangular waveguide, causing peturbations in the probe reflection coefficient. Since the growth of fatigue in metals is a stochastic process, the cracks do not have a constant predetermined shape. We describe a new formulation to model the problem of an open-ended waveguide probe radiating into a conducting metal with a surface-breaking crack of arbitrary shape. In this formulation, the crack is first modeled by an appropriate number of short rectangular waveguides. The generalized scattering matrix technique is then used to calculate the scattering matrix of the new segmented waveguide structure. The probe reflection coefficient of the dominant mode, TE₁₀, is finally calculated for various positions of the crack in order to predict the probe output signal. To demonstrate the accuracy of the model, we consider two special cases of a long and an elliptical-shape cracks. The comparison of our results with those available in the literature substantiates the model introduced in this paper. To further validate the model, we present results associated with a fatigue crack of complex geometry which are compared with those obtained using a finite element code.