Nondestructive testing and crack evaluation of ferromagnetic material by using the linearly integrated hall sensor array

Magnetic flux leakage testing (MFLT), which measures the distribution of a magnetic field on a magnetized specimen by using a magnetic sensor such as a Hall sensor, is an effective nondestructive testing (NDT) method for detecting surface cracks on magnetized ferromagnetic materials. A scan-type magnetic camera, based on the principle of MFLT, uses an inclined Hall sensor array on a printed circuit board (PCB) to detect small cracks at high speed. However, the wave forms appear in a direction perpendicular to the scan because the sensors are bonded at different gradients and heights on the PCB despite careful soldering. In this paper, we propose linearly integrated Hall sensors (LIHaS) on a wafer to minimize these waves and to improve the probability of crack detection. A billet specimen is used to determine the effectiveness of the LIHaS in multiple crack detection. This paper was recommended for publication in revised form by Associate Editor Joo Ho Choi Prof. Jinyi Lee was born in Korea in 1968. He received the bachelor degree in mechanical design from Chonbuk University, Jeonju, Korea, in 1992. Also he received the master and Ph.D degree in mechanical and aeronautics & space engineering from Tohoku university, Sendai, Japan, in 1995 and 1998, respectively. He was a Researcher from 1998 to 2000 with the Tohoku university, Iwate university, Iwate Techno-Foundation and Saitama university, Japan. From 2000 to 2003, he worked for Lacomm Co., Ltd. and Gloria Techniques, Korea, as a researcher. In 2003, he was a lecturer with the Chosun university, Gwangju, Korea. Since 2005, he has been an Assistance Professor, Chosun university. His research interests are in application of magneto-optical film, laser and CCD line scan sensor, and development of magnetic camera. He is the author or coauthor of fifteen patents and over 50 scientific papers. Jiseong Hwang was born in Republic of Korea in 1979. He received the B.S and M.S degree in control and instrumentation engineering in 2005 and 2006, respectively, from the Chosun University, Gwangju, korea, where he is currently working toward the Ph.D. degree. His research interests are NDT and Evaluation, Magnetic camera. Jongwoo Jun was born in Korea in 1974. He received the bachelor degree in electronics engineering from Inje University, Kimhae, Korea, in 1999. He received the master degree in electronics engineering from Changwon University, Changwon, Korea, in 2005. Also he is currently working toward the Ph.D. degree in information & communication engineering from Chosun University, Gwangju, Korea. He worked for Lacomm Co., Ltd. and Gloria Techniques from 1999 to 2005, Korea, as a researcher. His research interests are development of magnetic camera, NDT and evaluation. Dr. Seho Choi was born in Korea in 1964. He received bachelor degree in the department of electrical and electronic engineering from Kyungpook National University, Daegu, Korea, in 1987. And he received master degree in the department of elec trical and electronic engineering from Korea Advanced Institute of Science and Technology in 1989. He received Ph.D. degree in the department of electrical and electronic engineering from the University of Sheffield in the U.K. in 2001. He had been worked for Agency for Defense Development as a Researcher from 1989 to 1992, Korea. Since 1993, he has been worked for POSCO Research Lab. as a principal researcher. His main research activities are developing Surface Defect Inspection System for hot and cold rolled steel strip, hot wire rod, and hot slab. He is also interested in developing Internal Defect Detection System for steel products by using Ultra-sonic and magnetic camera techniques. His major is image processing to detect tiny defect in high background noise image. He published many scientific papers as the author or coauthor.     

Application of a NDI method using magneto-optical film for micro-cracks

Leakage magnetic flux is occurred in the cracked area of magnetized specimens, and also it changes the magnetic domain area of the magneto-optical film positioned on the specimen. It causes the change of the optical permeability of the magnetic domain on the crack area. So crack images can be obtained easily using this principle. On the other hand, utilizing a laser in this method makes possible to perform a remote sensing by detecting the light intensity contrast between cracked area and normal area. This paper shows the application of non-destructive inspection system taking advantage of magneto-optical method for micro-cracks and presents examples applied to the several types of specimens having fatigue cracks and fabricated cracks using this method. Also the authors prove the possibility of this method as a remote sensing system under the oscillation load considering application to real fields.     

Investigation of constantly and transiently propagating cracks in functionally graded materials by dynamic photoelasticity

In this study, the stress intensity factors (SIFs) for steady and transient propagation of cracks in transparent homogeneous functionally graded materials were analyzed by using the photoelasticity technique. The fracture analysis was carried out for the cracks propagating from a region with high elasticity towards low elasticity, as well as the cracks propagating from a region with low elasticity towards high elasticity. The analysis includes cracks propagating (1) at an almost steady speed, and (2) with the rapid increase, followed by a decrease in speed. For cracks with almost constant velocity, the SIFs were greater when a crack started from a high elasticity region, as compared to the cracks which initiated from a low elasticity region. For cracks propagating with rapid acceleration and deceleration, when the strain energy accumulated in the material due to an increase in load or stress was released at the moment of crack propagation, the SIF was momentarily lowered by approximately 45 %–50 % of the static SIF(before crack initiation), which subsequently increases by approximately 30 % eventually, the crack acceleration approaches nearly zero; the SIF decreases and increases respectively as the crack propagates in a material with decreasing and increasing modulus of elasticity.

     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

利用复合激励的无盲点管道裂纹漏磁检测新方法

管道是石油和天然气工业的重要组成部分,而轴向裂纹是管道安全运营的重要隐患。传统的漏磁检测技术(MFL)对管道中轴向裂纹的检测灵敏度不高,从而形成检测盲区。本文提出了一种利用复合激励的MFL检测新方法,可实现对轴向和周向缺陷的同步检测。首先,利用U型磁轭对管壁进行交直流复合磁化;直流磁化场直接作用于周向裂纹并形成有效的MFL检测信号,而交流磁化场则在管壁内形成垂直于磁化方向的均匀涡流场;当该涡流场受到轴向裂纹干扰时,将形成二次感生磁场的扰动,因此,新方法通过对管壁表面的漏磁场及二次感生磁场检测,同时获得周向和轴向两个方向的探测能力;最后开展仿真和实验,并分析了新方法中作用于轴向和周向裂纹的磁化场、涡流场和二次感生磁场的分布。结果表明,新方法只需通过一次扫描,即可以获得缺陷的轴向和周向特征,实现了对裂纹的无盲点检测。     

磨削力对HIPSN陶瓷磨削亚表面裂纹的影响

采用金刚石砂轮是磨削热等静压氮化硅(HIPSN)陶瓷最常用的加工方法,但是被磨零件亚表面常常伴随裂纹、崩碎等加工损伤,因此研究裂纹扩展一直是工程陶瓷的热点问题。对磨削加工后的HIPSN陶瓷亚表面裂纹进行探究,分析其在磨削加工过程中产生裂纹的原因以及去除机理,研究结果表明在磨削过程中对裂纹进行适当的控制,可以提高陶瓷零件的可靠性。设置单因素实验,对不同磨削参数下HIPSN陶瓷的磨削力进行测量,通过扫描电镜(SEM)对亚表面裂纹和表面形貌进行观察,分析磨削力对亚表面裂纹的影响。实验结果表明:磨削力随着砂轮线速度的增大而减小,随着工件进给速度和磨削深度的增大而增大;当磨削力变大时,陶瓷亚表面裂纹扩展程度增加,表面形貌变差。在粗磨加工HIPSN陶瓷时,可以通过减小工件进给速度和磨削深度,提高砂轮线速度的方法来降低裂纹的扩展程度,能够有效降低后续工艺的加工时间和难度,提高表面质量。     

Electromagnetic nondestructive testing at high lift-off using a magnetic image conduit

To protect sensors from the extreme environments, such as, heat, moisture, pollution and radiation, cracks must be inspected for; this can be done by measuring the distribution of magnetic fields at high lift-off through nondestructive electro-magnetic testing. However, as the intensity of an electro-magnetic field is inversely proportional to the square of the lift-off, it becomes increasingly difficult to effective inspect a crack as the lift-off increases. In this paper, a magnetic image conduit to minimize the intensity loss of an electro-magnetic field at high lift-off is proposed, and the effectiveness of a conduit for magnetic imaging is verified by means of both theoretical and experimental approaches.     

Rolling friction and adhesion between smooth solids

Rolling friction has been explained in terms of crack propagation through an adhesive joint. The contact between a smooth cylinder and flat has been regarded as an adhesive junction bounded by two cracks moving in the same direction at the same speed, one crack continually opening and one closing. Propagation of these cracks requires a force which is calculated from crack theory and shown to be equal to the friction.The theory has been verified experimentally using glass cylinders rolling on smooth rubber. Results show that rolling friction is closely connected with peel adhesion. Moreover, this adhesion interpretation of rolling friction between smooth surfaces explains several observations: (a) the existence of a static rolling friction, (b) the unusually high value of friction and its independence from load and roller radius, (c) the marked effect of lubricant or dust.     

Deformations of a simplified flip chip structure under thermal testing inspected using a real-time Moiré technique

Deformations of a Si-epoxy-FR4 (simplified flip chip) structure under thermal testing were inspected with a real-time Moiré technique. Specimens without cracks and specimens with a crack at the silicon-epoxy interface were prepared. The measurement results showed that the maximum deformation appeared at the edge. When the specimen was cooled to 20 °C, there was residual plastic deformation in the specimen. The creep effect was more dominant in the FR4-epoxy interface. Upon cooling to 20 °C, the specimen experienced partial strain recovery. To characterize the behavior of the interfacial crack, stress intensity factors K_I and K_II, and the strain energy release rate G in the vicinity of the crack tip were calculated using the measured deformations to conduct a quantitative study. It was observed that a sharp strain gradient occurred at the crack tip. K_I and K_II were dependent on temperature, and G was dominated by K_I for the interfacial crack in the specimen.     

多条滚动接触疲劳裂纹共存时的瞬态扩展行为分析

利用有限元法建立考虑轮轨瞬态滚动接触载荷的三维滚动接触疲劳斜裂纹扩展分析模型,分析现场中成群出现的钢轨滚动接触疲劳裂纹之间的相互影响。该模型可以准确反映高频轮轨瞬态滚滑行为对裂纹载荷边界和裂纹面接触的影响,突破了以往研究中基于赫兹接触的稳态接触载荷边界假设带来的种种局限。基于实测,模型中裂纹扩展角度取典型值30°,长度和深度分别取10～20 mm和2～4 mm (长深比固定为5),考虑间距在5～20 mm范围内变化,分析多至5条裂纹共存情况下的裂尖应力场强度因子。300 km/h运行速度下的计算结果表明:相同尺寸的多裂纹共存时某一裂纹尖端的节点力相较于单裂纹时低,但多裂纹时的接触刚度更低,使得裂纹尖端附近两裂纹面间的相对位移较单裂纹更大,最终使得多裂纹工况的裂尖应力场强度因子随裂纹数量的增多而增大;对于特征尺寸(长度)为15 mm的等间距多裂纹,当裂纹间距大于5 mm时,3条裂纹共存模型即足以将裂纹间相互影响精确考虑在内,较5条裂纹共存模型的误差仅为1.7%。对于文中所研究的多裂纹,当裂纹间距大于裂纹特征尺寸时,裂纹间的相互影响可以忽略,即可采用单裂纹模型进行计算。     

Investigation of Behavior of Stress-Corrosion Cracks in a Pipes under Internal Pressure

Corrosion induced by ambient conditions causes generation of longitudinal cracks on outside surfaces of gas pipelines, and this can lead to breaks in main pipelines. The paper presents experimental data on behavior of cracks in sections of pipelines with aws under constant and cyclically varied (45 cycles) hydrostatic pressure, the pressure magnitude corresponding to operational parameters. Crack parameters (increasing depths and openings of cracks) were monitored by sensors of magnetic induction placed on pipes immediately over cracks. The data indicated that cracks with depths of up to 30–35% of the wall thickness did not show residual changes (in their depths and openings) after tests. Residual changes were detected in deeper cracks, which manifested as critical ows of metal near the crack edge and growth of cracks to their critical dimensions. Photographs of fractured surfaces showed that crack depths started to increase long before the ultimate fracture of a pipeline. Thus, tests conducted by applying high pressure to operating gas pipelines can lead to increases in crack dimensions without detecting them. This effect is actually detrimental to the tested pipeline's safety. The absence of residual changes in cracks with depths of up to 30–35% of the wall thickness indicates that, possibly, new limits on aw dimensions should be established for decommissioning unsafe sections of pipelines, and these limits should take account of the hazard due to stress-corrosion cracks, depending on their dimensions and inside gas pressure.     

Simulations and experimental investigation on motion stability of a flexible rotor-bearing system with a transverse crack

In the classical process for stability studies on the rotor-bearing system with crack faults, the simple discrete model is adopted for research on such problems, which neglect some needful dynamical influence factor, such as the material damping, shearing effect and gyroscopic effects, etc. Therefore, it is necessary to find a precise calculation model for simulation of the rotor-bearing system with cracks faults. In this paper, instead of the traditional simple discrete model, finite element （FE） model is adopted to investigate the motion stability of a nonlinear rotor system with crack fault. According to finite element theory, the FE model of the cracked rotor system is established firstly. It should be pointed out that the element where the crack occurs is modeled by a particular crack element and the supports at both ends are simulated by two nonlinear loads. Then, based on dimensionless and dimensionality reduction, the Newmark-[3 method and the shooting method are employed to study the effect of eccentricity and the depth of crack on instability speed and bifurcation feature. Furthermore, the simulation results are verified by some corresponding experiments. The simulation and experimental results show that instability speed does not change monotonically, but decreases firstly and then increases when the amount of eccentricity increases. Moreover, as the type of instability changes, the instability speed jumps concomitantly. Additionally, the presence of crack fault can disturb the oil whirl, as a result, instability speed tends to increase slightly, but it does not affect the type of instability and jumping phenomenon. This research presents an effective and convenient method which uses the finite element method （FEM） to research the motion stability of the nonlinear rotor-bearing system with cracked faults and other nonlinear force, and the proposed method can provide a theoretical reference for stability analysis and vibration control in more complex relevant rotor-bearing system.     

新型霍尔传感器在电流检测中的应用

相对于传统的集成霍尔传感器，磁集极霍尔传感器具有高线性度、低磁滞、高灵敏度、体积小、价格低等优点。随着该技术的推广，磁集极霍尔传感器必将广泛应用于很多领域。介绍了磁集极霍尔传感器检测电流的基本原理及应用，并且提出了对于不同范围电流检测的解决办法。     

Effects of cracks on rotor system instability

The stability threshold of a rotor-bearing system having a transverse crack has been studied by using FEM and considering the various crack parameters and shaft internal damping (viscous and hysteretic damping) and geometric parameters. It has been observed that the instability speed has reduced considerably with increase in crack depth and influenced more with hysteretic damping compared to viscous damping. The stability of the rotor system including shaft with two open cracks has also been carried out and observed the influence of one crack over the other for threshold speed limits.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

Driving and braking control of PM synchronous motor based on low-resolution hall sensor for battery electric vehicle

Resolvers are normally employed for rotor positioning in motors for electric vehicles,but resolvers are expensive and vulnerable to vibrations.Hall sensors have the advantages of low cost and high reliability,but the positioning accuracy is low.Motors with Hall sensors are typically controlled by six-step commutation algorithm,which brings high torque ripple.This paper studies the high-performance driving and braking control of the in-wheel permanent magnetic synchronous motor(PMSM) based on low-resolution Hall sensors.Field oriented control(FOC) based on Hall-effect sensors is developed to reduce the torque ripple.The positioning accuracy of the Hall sensors is improved by interpolation between two consecutive Hall signals using the estimated motor speed.The position error from the misalignment of the Hall sensors is compensated by the precise calibration of Hall transition timing.The braking control algorithms based on six-step commutation and FOC are studied.Two variants of the six-step commutation braking control,namely,half-bridge commutation and full-bridge commutation,are discussed and compared,which shows that the full-bridge commutation could better explore the potential of the back electro-motive forces(EMF),thus can deliver higher efficiency and smaller current ripple.The FOC braking is analyzed with the phasor diagrams.At a given motor speed,the motor turns from the regenerative braking mode into the plug braking mode if the braking torque exceeds a certain limit,which is proportional to the motor speed.Tests in the dynamometer show that a smooth control could be realized by FOC driving control and the highest efficiency and the smallest current ripple could be achieved by FOC braking control,compared to six-step commutation braking control.Therefore,FOC braking is selected as the braking control algorithm for electric vehicles.The proposed research ensures a good motor control performance while maintaining low cost and high reliability.     

使用恒Kmax方法测试物理小裂纹门槛值研究

大量研究表明长裂纹和物理小裂纹扩展行为差异的主要原因是由于裂纹闭合程度不同造成的,消除裂纹闭合效应影响后的长裂纹与小裂纹的裂纹扩展行为应趋于一致.文中使用恒Kmax/增Kmin值降低△K的试验方法来测试裂纹扩展门槛值.此方法在测出△Kth时应力比R已经很高,实际上已不存在裂纹闭合效应的影响.通过对比试验结果表明,恒Kmax法测试长裂纹扩展门槛值能有效去除裂纹闭合效应的影响,用测得的门槛值近似表述小裂纹门槛值,能给出偏于保守的结果.     

绝对式多极磁电轴角编码器的设计

为了实现多对极磁电式轴角编码器的高分辨率绝对式检测并降低其成本,基于改进格雷码构建了一种新型多极磁电轴角编码器模型,提出一种基于校准查表的信号处理方式,以消除磁场非线性和装配误差对测量精度的影响。设计了两个由永磁体磁环构成的码道:粗码道,根据改进格雷码生成N和S极充磁顺序,采用圆周均匀分布的线性霍尔元件得到转子所处磁极区域的绝对偏移量;细分码道,N和S极等距间隔排列,定子上3个线性霍尔元件将磁信号转换为电信号,查表得到转子于所处信号周期内的相对偏移量。在离线状态下,用高分辨率的增量式光电轴角编码器进行校准,对其A/B相脉冲输出和磁电式轴角编码器的霍尔信号同时采样并上传到计算机进行高精度信号重构,得到标准角位移和霍尔信号映射关系,通过单片机的自编程技术将数据存储于主控芯片中固定地址以供查表;角位移检测状态下,根据霍尔信号查表得到绝对角位移。根据上述原理研制出12极磁电式轴角编码器样机,实现了分辨率为±0.72′,精度达±1.2′以下的单圈绝对位置检测。该编码器通过增加磁极数还可进一步提高测量精度。     

高精度霍尔翼片传感器的磁路仿真与设计

采用ANSYS有限元分析软件对霍尔翼片传感器进行磁路仿真和设计。在对2AV霍尔翼片传感器进行仿真的基础上，对新型高精度Sm2Co17霍尔翼片传感器进行静态和动态仿真，并根据仿真结果进行结构调整和优化。通过磁路仿真设计，适当调整霍尔器件与永磁体之间的空气间隙，使霍尔器件、永磁体和叶片相互匹配，得到符合要求的工作点检测参数，研制出的新器件达到国外同类产品水平，满足了霍尔翼片传感器的高精度要求，成本更低，也可大批量生产。     

Fatigue growth simulation for cracks in notched and unnotched round bars

Fatigue crack growth for various cracks in both unnotched and notched round bars is directly modelled by using an automated numerical technique, which calculates the stress intensity factors at a set of points on the crack front through the three-dimensional finite element method and then applies an appropriate fatigue crack growth law to this set of points to obtain a new crack front. This technique also has the capability of automatic remeshing so that the crack propagation can conveniently be followed. The crack geometries modelled in the present paper include a small internal crack near the free boundary and several initially part-elliptical or irregular surface cracks in a smooth round bar under tension, a surface crack in different semi-circularly notched bars under both tension and bending, a surface crack initiated from the root of a V-notched bar and an initially twin crack configuration within a smooth tension bar. Some fatigue growth characteristics relevant to each type of cracks are also revealed. It is demonstrated that the fatigue growth analyses of various cracks commonly occurring in bars can reliably be made by using the automated finite element technique proposed.