Study on the Ezperiment of Weld Defect Testing by Using Two-dimensional Magnetic Memory Signals

Study on the Experiment of Weld Defect Testing by Using Two-dimensional Magnetic Memory SignalsZhang Weimin1, Liu Hongguang1,2, Gao Xuanyi3(1: School of Mechanical and Vehicular Engineering, Beijing Institute of Technology, Beijing, China, 100081;2: College of Material Science and Technology, Beijing Forestry University, Beijing, China, 100083;3: School of Information and Electronics, BeijingInstitute of Technology, Beijing, China, 100081)Abstract: The testing experiment and modeling analysis are applied on weld defects of the ferromagnetic component by using magnetic memory testing technology. The specific work include: a two-dimensional magnetic memory testing experiment instrument is designed and developed, a magnetic signal distributing model of plane weld seam is established by magnetic dipole theory, the distributing characters of normal and tangential components of magnetic memory signals on weld defects are theoretically analyzed, the weld parts with defects are tested by the two-dimensional magnetic memory testing instrument, it is indicated that two dimensional magnetic memory signals can preferably show the existence of actual defects.Keywords: metal magnetic memory； weld defect；magnetic dipole； magneto sensorI. IntroductionThe metal magnetic memory is a new method of nondestructive testing, applied in detecting and evaluating the early damage and stress concentration of ferromagnetic materials. The physical foundation of the work is based on the metal magnetic memory effect, which the magnetic domain of ferromagnetic material produces a directional and irreversible change because of phase change and local plastic deformation. The change information is picked up by magnetism sensor, and the state of damage and stress concentration is determined【1～4】.The method of metal magnetic memory is applicable to test weld defect, which is induced by organization phase change and uneven heat stress. Butthe method of metal magnetic memory is a weak magnetic testing method and easily disturbed in actual testing, it results in a high rate of misjudgment【5～8】. That multi-dimensional magnetic memory signals are used to judge the defect in testing is helpful to pickout “fake defect” and increase the accuracy of judgment.The weld defects are tested and studied by self-made two-dimensional magnetic memory testing instrument and the two-dimensional magnetic memory signals of weld defects are modeled and analyzed based on the magnetic dipole theory.II. Magnetic Dipole Model of Weld SeamThe three-dimensional model of weld seam is showed as Fig.1. The groove of seam is Y type. For convenience of computing, the cross section of defectis considered as a diamond with the length of side r. The defects are mainly some kinds of hole such as bubble, crack or faulty fusion. In the state of even magnetization, the defect can be considered as a magnetic dipole along the direction of outside field[9].The magnetic field H of point ),',(yxxP is avector sum of three components,22'2yxxHHHH++=.）（3])5.0()[(])5.0()[()'(23222322'⎪⎭⎪⎬⎫+++++−⎪⎩⎪⎨⎧+++−−=r h y f x fx r h y f x f x q x H d dxFig.1. Three-dimensional model of seamFig.2. Diagram of seam defect in sectionoy x 'The situation of section oy x '- along the direction of seam is discussed, as shown in Fig.2. When testingis along the axis 'x – the direction of seam, the normal component )'(x H y and the tangential component)'('x H x of P point in section oy x ' can beexpressed as [10]:)'()(21)'(''''0'x H H H H x H dx rx lx x x +++= (1))'()(21)'(0x H H H H x H dy ry ly y y +++= (2)Where, '0x H - the tangential component of earth field along the axis 'x ; 'lx H - the tangential component produced by seam and base metal without defect on the left of defect along the axis 'x ; 'rx H - the tangential component produced by seam and base metal without defect on the right of defect along the axis 'x ; ly H - the normal component produced by seam and base metal without defect on the left ofdefect along the axis y; ry H - the tangential component produced by seam and base metal without defect on the right of defect along the axis y.Previous parameters '0x H ,'lx H ,'rx H ,ly H andry H are obtained by experiments and statistics.)'('x H dx is the tangential component produced bydefect along the axis 'x , approximately be expressed as [11,12]:Where,d q - the magnetic charge produced by two ends of defect along the length f direction, depending on tested sample material and obtained by experiments; h – the vertical length from top surface of defect to sample surface; r – the depth of defect; f – half of the length of defect.)'(x H dy is the normal component produced bydefect along the axis y, approximately be expressed as:)4(])5.0()[(1])5.0()[(1)'(23222322⎪⎭⎪⎬⎫++++−⎪⎩⎪⎨⎧+++−⋅=r h y f x r h y f x y q x H d dy The previous expression is simulated and analyzedby software Matlab. The distributing characters of the normal and tangential component of leakage field at defect are obtained – the normal component passes zero and the tangential component is maximum. The magnetic signal is influenced by the change of defect parameters such as location and dimension h ，rand f .For example, the magnetic signal curve with the change of f is shown as Fig.3. The peak value of curve)'('x H x and the wave crest and trough of curve)'(x H y are increased with the increment of defectdimension f. The distance from wave crest to trough of curve )'(x H y along the direction of length L equals the length of defect 2f. shownFig.3. Influence of change of f to curve )'('x H x .Fig.4. Influence of change of f to curve)'(x H yIII. Two-dimensional Magnetic Memory Signal Testing InstrumentTwo-dimensional magnetic memory signal testing instrument is composed of magneto sensor, pulse demagnetized circuit, post amplifier-filter circuit, signal scan device, signal sampling circuit and computer analysis system. System structure is shown in Fig.5.Probe is a magneto sensor of type HMC-1022[ 13，14]. The sensor is composed of four component Wheatstone bridges. The tested magnetic field is transferred to differential output voltage. That thesensor can test the lowest field of Gs μ30 with linear work range of Gs 6± and linear error less than 0.5% is satisfied with the requirement of testing weak magnetic signal on ferromagnetic sample surface. Pulse demagnetized circuit can eliminate the influence of outside interference field to keep the magneto sensor in a high sensitive state. The tested weak magnetic signal is amplified and noise is filtered out by amplifier-filter circuit to sample and analyze.Fig.5. Diagram of magnetic memory testing systemIV . Testing Experiment of Weld Samples8 plane welding samples with the same dimensionare prepared; the shape is shown as Fig.6. Length ofthe sample is 250mm(along the direction of seam), width is 200mm and thickness is 10mm. The groove is Y type and the weld seam is in the middle of sample, as shown in Fig.7. Where, angle of groove θ2＝60°，thickness d ＝10mm, seam root depth a ＝3mm and root width b ＝2mm. The material is pressure vessel steel 16MnR and weld mode is manual hiddenwelding. 4 samples are welded strictly according to the welding operation principles and obtained high welding quality. The others are prefabricated somedefects as bubbles and faulty fusion.Fig.6. Plate sampleFig.7. Dimension of seam in xoy sectionThe magnetic memory signals of welding seam – curves )'('x H x and )'(x H yare analyzed and abnormal distribution of signals are found. Curves)'('x H x and )'(x H y are the distribution ofsample 4, as shown in Fig.8. There is an obviousdefect character of magnetic memory signal at 168mm to the end along the seam. Obvious defect distributing character is also found at the location when it is tested vertically to the seam. The prefabricated defects of bubbles are found by x-ray testing. Fig.9 is the testing curve of sample 7. Obvious defect signal distributing characters occurred at 78mm and 105mm to the end. The prefabricated defects of faulty fusions are found by x-ray testing.Fig.8 curves)'('x H x and )'(x H y of sample 4Fig.9 curves )'('x H x and )'(x H y of sample 7There are 18 places with defect characters in)'('x H x and )'(x H y testing curves of 8 platesamples. The defects of bubbles and faulty fusions are found in 11 places by x-ray testing. But 5 places with defects are found by x-ray testing, there is no defect character in curves )'('x H x and )'(x H y . The results indicate that magnetic memory testing is easily disturbed because it is a weak magnetic testing method and sometimes abnormal signal is not corresponding to obvious defect.V . Conclusion(1) Two-dimensional magnetic memory signals are applied in welding defect testing and a new method that the welding defect is judged through normal component of magnetic field passes zero and tangential component is maximum is approved.(2) The weld seam model of two-dimensional magnetic memory signals deduced by magnetic dipole theory is exactly reflected the distributing trend of two-dimensional components of magnetic field at weld seam, and quantitatively analyzed the influenceof the changes of defect dimensional parameters to two-dimensional magnetic memory signal results.(3) Two-dimensional magnetic memory signal testing instrument is built by high sensitive magneto sensors. The welding samples are tested by the instrument and the situation that normal component of field )'('x H x passes zero and tangential component)'(x H y is maximum is found. It is indicated that itis feasible to test welding defect by usingtwo-dimensional magnetic memory signals.(4) The method of magnetic memory testing is a weak magnetic testing method and the signals areeasily disturbed to decrease the accuracy of judgment.The method eliminating the disturbance of noise from magnetic memory signals should be further studied.REFERENCES[1] Doubov A A. Diagnostics of metal items and equipmentby means of metal magnetic memory[C] Proc of ∥CHSNDT 7th Conference on NDT and International Research Symposium. Shantou , China : Non-Destructive Testing Institution , 1999 :181-187.[2] Zhang Weimin , Dong Shaoping , Zhang Zhijing.State-of-the-art of metal magnetic memory testing technology[J ] .China Mechanical Engineering , 2003 ,14 (10) :892 - 896.(in Chinese)[3] DUBOV A A. A study of metal properties using themethod of magnetic memory [J]. Metal Science and Heat Treatment, 1997, 39(9-10): 401-405.[4] WILSON J W, TIAN G Y , BARRANS S. Residualmagnetic field sensing for stress measurement [J]. Sensors and Actuators A, 2007, 135(2): 381-387.[5] Di Xinjie, Li Wushen, Yan Chunyan, et al. Featureextraction of metal magnetic memory signal and its application for weld crack[J ] . Transactions of the China Welding Institution, 2006,27(2) :19 - 22. (in Chinese) [6] Liang Zhifang, Wang Yingna, Li Wushen, et al. Progressin the study of metal magnetic memory signal features of welding cracks [ J ] . Mechanical Science and Technology, 2007 ,26 (1) :81 - 83. (in Chinese)[7] Dong Lihong, Xu Binshi, Dong Shiyun, Chen Qunzhi,Wang Dan. Variation of stress-induced magnetic signals during tensile testing of ferromagnetic steels [J]. NDT&E International, 2008, 41(3): 184-189.[8] Dong Lihong, Xu Binshi, Dong Shiyun, Ye Minghui,Chen Qunzhi, Wang Dan, Yin Dawei. Metal magnetic memory signals from surface of low-carbon steel and low-carbon alloyed steel [J]. Journal of Central South University of Technology, 2007, 14(1): 24-27.[9] Zhong Weichang. Why the model of magnetic dipole, but not the model of magnetic domain is used for the research on magnetic testing principle[J ] . Nondestructive Testing, 2006, 28 (2): 94- 95. (in Chinese)[10] Liu Hongguang, Zhang Weimin, Wang Zhaoxia, et al.Study onWeld Defect Testing Technology Based on the Method of Magnetic Memory [J] .Transactions of Beijing Institute of Technology, 2007,27(9):811~814.(in Chinese)[11] Ren Laiping, Zhao Junsheng, Hou Shixi. The DistributionModel of Magnetic Dipole in Magnetic Field[J]. Hydrographic Surveying and Charting, 2002, 22(2): 18～21. (in Chinese)[12] Wang Zhaoxia, Zhang Weimin, Song Jingang, et al.Model of Magnetic Dipole Established in Weak Magnetic Field[J]. Transactions of Beijing Institute of Technology, 2007, 27(5): 395～398(in Chinese)[13] Pei Yi, Yu Nanfang, Liu Qi, et al. Theory and Applicationof Anisotropic Magnetoresistive Sensor [J]. Instrument Technique and Sensor, 2004,8:26~32(in Chinese) [14] Zhang Qingxin, Zhang Zhaohui, Lan Jinhui, et al.Analysis of SET/RESET Circuits in AMR Sensors[J]. Chinese Journal of Scientific Instrument, 2005,8:118~119 (in Chinese)Study on the Ezperiment of Weld Defect Testing by Using Two-dimensional Magnetic Memory Signals作者：Zhang Weimin， Liu Hongguang， Gao Xuanyi作者单位：Zhang Weimin(School of Mechanical and Vehicular Engineering, Beijing Institute of Technology, Beijing, China, 100081)， Liu Hongguang(School of Mechanical and Vehicular Engineering, BeijingInstitute of Technology, Beijing, China, 100081 College of Material Science and Technology,Beijing Forestry University, Beijing, China, 100083)， Gao Xuanyi(School of Information andElectronics, Beijing Institute of Technology, Beijing, China, 100081)本文链接：/Conference_WFHYXW348067.aspx。