Investigation into eddy current pulsed thermography for rolling contact fatigue detection and characterization

This paper reports on the use of eddy current pulsed thermography (ECPT) for detection and characterization of rolling contact fatigue (RCF). Detection mechanisms with eddy currents and heat propagation effects were discussed with RCF modeled as a simple angled defect. Two different angled defects were studied through numerical simulations and experimentally by using uniform magnetic field (UMF) excited by Helmholtz coils. Finally, a rail sample with RCF defects was inspected using UMF excitation. It is shown that ECPT with UMF excitation provides an efficient and robust method to detect angled defects, compared with nonuniform magnetic field (NUMF) excitation.     

High resolution approach for the localization of buried defects in the multi-frequency eddy current imaging of metallic structures

A high resolution approach is proposed to quantitatively estimate the depth of defects buried in planar metallic structures. This approach associates a multiple signal characterization (MUSIC) algorithm with an original eddy current imager. The interactions of the eddy currents and the defects are modelled by a set of virtual magnetic sources propagating in a spherical manner up to the surface of the structure. The defect localization is carried out using the MUSIC algorithm, applied to the multi-frequency observation of the magnetic field distribution at the surface of the structure. Accurate results obtained on simulated and experimental data relative to defects buried down to 5.4 mm in an aluminium layered structure validate the approach.     

小径管周向裂纹脉冲涡流检测仿真和试验研究

针对小径管周向裂纹缺陷,通过有限元仿真及试验,研究了利用磁导体环形激励脉冲涡流检测技术检测小径管周向裂纹缺陷的问题。仿真给出了管道在有缺陷和无缺陷状态下磁场分布、涡流分布以及接收线圈的电压值。从仿真结果可以观察出,周向裂纹端头处的磁场分布以及涡流分布会发生明显变化,产生沿管壁法向的磁场,检测线圈位于裂纹端头处正上方时检测灵敏度最高。实际检测结果与仿真结果一致,表明磁导体环形激励轴向涡流对小径管周向缺陷具有显著的检测效果。     

Evaluation of eddy current probes based on local field excitation

The paper deals with the eddy current testing by means of a technique based on local field excitation, with the objective to enhance sensitivity and spatial resolution of flaw detection. Two modifications of innovative eddy current probes are presented. Results of the application of the probes for an examination of thin Inconel plate specimens with small artificial defects on the opposite surface are compared. The criteria for comparison are signal-to-noise ratios and spatial resolution of the responses obtained. Performance of the probes is analyzed on the basis of the study of the distribution of their excitation fields with application of magneto-optic sensors and finite element modeling. The decrease of the local extent of the excitation field has been found to be the main cause of sensitivity decrease of the probes to detect opposite-surface defects.     

脉冲激励远场涡流管道检测技术的有限元仿真

研究了脉冲激励下的远场涡流管道检测技术,采用有限元仿真的方法对检测系统参数对检测结果的影响做了详细的分析。表明该技术将远场涡流和脉冲激励的优势有效结合,能提取较多检测信息,可同时测量管道内径和内、外壁缺陷信息,且具有信号幅值高,功耗低的优点。     

大口径管道远场涡流缺陷检测仿真建模

利用有限元分析软件ANSYS对大口径油气管道的远场涡流检测进行了仿真研究。根据远场涡流的特点,采取局部法进行三维仿真,即先对无缺陷的二维轴对称远场涡流模型做仿真,得到缺陷外围区域的磁场分布,再以此为边界条件对缺陷区域做三维仿真。该方案显著降低了模型的规模,提高了远场涡流缺陷检测仿真的效率和准确性,使得对大口径油气管道的远场涡流仿真得以在PC机上完成。     

3D Simulation of magnetic field distribution in electromagnetic forming systems with field-shaper

Impulse electromagnetic forming (IEMF) is an effective and powerful technique widely used for joining and shaping metals and field-shaper is a main part of the electromagnetic forming which has important effect on the distribution of magnetic field. In this technique, a metal work-piece is pushed to a die and formed by a pressure created using an intensive, transient magnetic field. This magnetic field is produced by passing a pulse of electric current through a forming coil in a pulsed power circuit. The produced transient magnetic field induces eddy currents in the surface of work-piece. Induced eddy currents in work-piece produce a magnetic field with reverse direction of initial magnetic field; this results in a mutual repulsion between coil and work-piece and in this way the work-piece is thrown toward the die. In this process created magnetic forces applied to work-piece are much like uniform, but in real applications, some regions of a work-piece have to be more deformed and therefore a much greater pressure has to be applied to these regions. The task of concentration of magnetic forces to some desired regions can be accomplished using field-shapers. Yu et al. [Yu, H., Li, C., Zhao, Z., Li, Z., 2005. Effects of field-shaper on magnetic pressure in electromagnetic forming. J. Mater. Process. Technol. 168, 245–249] have recently shown the effect of field-shaper on the distribution of the magnetic fields in electromagnetic forming, but because of the nature of 2D simulations some edge effects in real geometries could not be taken into consideration. In this paper, a 3D simulation using the FEA software MAXWELL has been applied to study the magnetic field distribution during an impulse electromagnetic forming process. Comparison of the 3D and 2D simulation results indicates that the maximum magnetic fields achieved in front of nodules of the field-shaper are about 15% stronger than those expected by 2D simulations.By changing the geometry of the field-shaper, the influence of the shape of the field-shaper on the distribution of the applied forces on the work-piece has been studied. Based on these simulations, some simple guidelines to design the field-shaper have been derived.     

Rotating field eddy current probe with bobbin pickup coil for steam generator tubes inspection

Eddy current probes are used extensively to evaluate the integrity of steam generator tubes in nuclear power plants and to detect cracks in tube walls. The probe design has evolved from simple bobbin probes to mechanically rotating multi-coil probes providing high resolution images of tube integrity. This paper presents a novel rotating field eddy current probe with bobbin pickup coil that generates a rotating magnetic field, thereby avoiding mechanical rotation of probe. Three balanced identical rectangular windings carrying excitation currents with 120° phase difference generate an electrically rotating magnetic field that is sensitive to defects of all orientations. A bobbin coil located in the center is used as a pick up sensor. The phase of induced voltage in the bobbin coil is interpreted in terms of the circumferential location of the defect. The properties of the probe are investigated using a three dimensional finite element model, and validated experimentally using measurements.     

曲面零件裂纹缺陷脉冲涡流热成像检测的仿真

脉冲涡流无损检测已成为零件表面和亚表面缺陷检测的重要手段之一。文章应用数值仿真方法,在COMSOL平台上模拟研究了复杂曲面零件的裂纹缺陷的脉冲涡流热特性,给出了在脉冲涡流作用下的裂纹缺陷周边温度场分布与变化规律。仿真结果表明,脉冲涡流热成像技术能有效检测复杂曲面零件的裂纹缺陷。研究成果可供检测系统设计作为参考。     

Composite magnetic flux leakage detection method for pipelines using alternating magnetic field excitation

Pipelines are an important transportation medium for petroleum and chemical products, but defects in the pipelines can present hidden dangers and affect the safe operation of the pipeline. The traditional pipeline magnetic flux leakage (MFL) scanning technique generally adopts the axial magnetization mode, which has increased the difficulty in detection and the possibility of missed detection of axial cracks. In this paper, a new composite MFL method using alternating magnetic field excitation is proposed for the detection of cracks in pipelines. The alternating magnetic field is first superimposed on the MFL magnetization field, which will form a parallel eddy current field perpendicular to the magnetization direction in the pipeline wall. The defects in the pipeline not only cause the flux leakage of the magnetization field, but also lead to the disturbance of the circumferential eddy current field. The disturbance signals can be picked up through a secondary induced magnetic field. Because the magnetic field and the eddy current field are orthogonal, the presented method can implement synchronous detection in two orthogonal directions to avoid missed detection caused by the crack orientation. A series of physical experiments are carried out in this paper. The results show that two orthogonal detection signals can be separated by a simple low pass filter. Therefore, with only one scan, the new detector can obtain the defect characteristics in the axial and circumferential directions to overcome the blind spot problem seen in traditional MFL detectors.     

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。     

涡流检测缺陷定量评估的研究

在涡流检测中实现缺陷定量评估具有重要意义。通常涡流检测是利用线圈阻抗变化来实现缺陷检测,分析受缺陷扰动的磁场量B的分布变化,比传统的涡流检测方法更有利于实现缺陷定量评估。利用有限元方法,对有裂纹的金属平板的空间磁场进行了数值仿真。仿真结果证实可通过分析磁场量B的变化来实现缺陷定量评估,并得到了缺陷的长度、深度与平板表面的磁场分量之间的量值关系。对平板裂纹的定量评估提供了理论上的依据。     

溢流阀阀体结构优化中均匀设计试验法的应用

针对溢流阀在线检测装置中,内置式涡流传感器的安装对阀腔流场产生影响,使溢流阀工作性能下降的问题,提出基于均匀设计试验法的阀体结构优化试验方案的研究方法。综合分析阀芯、阀座各结构参数的作用规律,确定优化的结构参数与优化指标,分析各结构参数间的交互效应,求解多优化目标的响应面函数。而后建立阀腔流场有限元分析模型,通过非线性二次规划算法,求解分析阀腔流场的优化结果。结果表明:均匀设计法适用于阀体流场结构优化的试验方案安排,优化后阀腔流场性能得到大幅提升。     

On the application of magneto-optic films in NDE

Some applications of magneto-optic (MO) films in the field of nondestructive evaluation (NDE) are presented. The method for imaging and measuring of magnetic fields on micrometer scale is based on the analysis of changes of the magnetic domain structure caused by the detected field. A process of MO film evaluation and calibration is described. The existence of the leakage field around a fatigue crack in nonmagnetic austenitic material is detected by a highly sensitive MO film. An experimentally determined distribution of the excitation field of an eddy current probe is compared with results obtained by the finite element method (FEM) numerical procedure.     

基于双频数据融合的电涡流焊接缺陷检测

焊缝表面纹理引起的干扰信号限制了电涡流检测技术对焊接中缺陷的检出,将矩形激励线圈和GMR器件相结合,设计了电涡流检测探头,仿真分析了在0.5,1和4 kHz激励频率条件下该探头检测的磁感应强度的分布规律,并选择0.5,4 kHz构成混频激励,针对这两个频率下的检测数据,提出了对于这两个激励频率数据的融合方法,以FPGA为核心,结合X-Y数控平台,构建了试验平台,针对实际铝合金焊接缺陷进行了试验. 结果表明,该方法在抑制焊接表面纹理造成的干扰信号的同时,也抑制了两个焊接铝合金板夹角引起的干扰,对于直径小于1 mm的亚表面缺陷,虽然两个激励频率下的数据很难清晰体现缺陷,但是采用该方法可被成功检测.     

有限元方法用于涡流电磁场计算的有效性分析

利用有限元方法，对涡流检测中的电磁场问题进行数值计算，有助于涡流检测线圈的优化设计、检测缺陷的识别与定量，从而提高涡流检测的效果与精度。为验证有限元数值计算结果的有效性，对TEAM（Testing Electromagnetic Analysis Methods）组织提出的Workshop问题（Problem 15）进行了求解与分析。利用解析方法计算了空芯线圈的阻抗，然后通过二维有限元模型进行了阻抗求解，对比结果有较好的一致性。针对厚板上裂纹的涡流检测，建立了三维有限元计算模型，对影响计算结果的因素进行了分析，得到了比较好的数值模拟结果。     

Dual frequency fusion for defect signal enhancement in EC-GMR inspecton of riveted multilayer structures

Reliable detection of defects under fasteners at rivet sites is one of the most critical problems in inspection of multilayer structures. Rotating eddy current (EC) excitation with giant magnetoresistive (GMR) sensor array field pick up (EC-GMR) has proved to be a promising approach in this application. In this paper, fusion of dual frequency is applied to EC-GMR scan data acquired with a novel differential array probe, to suppress the strong fastener response and hence improve the detectability of defects. Performance of the data fusion algorithm is evaluated on a two-layer aluminum sample with steel fasteners and notches introduced in the bottom layer using the electric discharge machine (EDM). Experimental results demonstrate that defects of different sizes can be successfully detected by evaluating the residual field after fusion. Defect orientation angle has minimal impact on the sensitivity of the probe owing to rotating eddy current excitation.     

管道腐蚀检测中新型脉冲涡流传感器的设计

提出了一种新型脉冲涡流传感器设计思路。它将脉冲涡流激励场从空间上转化为匀强涡流场,从而等效为一种自差分式的涡流检测技术,因此无需进行差分处理,可避免差分信号的不同步导致的检测精度和灵敏度不高。采用该新型脉冲涡流传感器对钢管腐蚀缺陷进行了检测,试验结果表明设计的传感器具有较高的灵敏度和检测精度。     

铁磁性平板构件腐蚀缺陷的脉冲涡流检测

为检测铁磁性平板构件背部的小型点腐蚀缺陷,使用脉冲涡流检测的方法实现缺陷的检测与识别。基于脉冲涡流检测技术,设计脉冲涡流传感器,在激励频率为32、16、8、4 Hz的低频下对厚度为3、6、9 mm的平板上的腐蚀缺陷点进行了试验。检测结果表明,使用脉冲涡流检测技术可检测到深度1 mm、直径2 mm的圆形点腐蚀缺陷,能有效检测出铁磁性平板表面及背部的小型点蚀缺陷。     

Analysis of eddy currents in a bar containing an embedded defect

The eddy current distribution is studied in a metal bar containing an embedded defect inserted within an encircling coil. Furthermore, the impedance change of the coil regarding the characteristics of the defect is determined. The defects investigated are nonmetallic inclusions and embedded cracks which may occur during the manufacturing processes of bars. To simulate these problems, a two-dimensional transverse-electric model is proposed, and then a set of coupled surface integral equations are formulated systematically. Since the magnetic field is unknown along the boundary of the defect, an additional boundary condition derived from Maxwell's equations is used. Using the boundary element method (BEM), these integral equations are solved in terms of nodal unknowns-current density. After the current distribution is obtained, the impedance changes of the coil caused by the defects are calculated vs. the configuration of defect for various frequencies. If the inclusion is circular and located at the center of the bar, good agreements are found by comparing the BEM solutions with the analytical ones. An auxiliary surface integral equation is also derived to further determine the currents on both sides of the crack.