Research on High Resolution Stress Corrosion Crack Detection Based on Remote Field Eddy Current Non-Destructive Testing

Although remote field eddy current had achieved successive application in pipes and tubes non-destructive testing, the poor ability and sensitivity to axial crack such as stress corrosion crack prevented its further application. A novel high resolution remote field eddy current detection method which can be suitable for axial crack especially stress corrosion crack detection was proposed and verified by simulation and experiment. First of all, conventional detection ability of remote field eddy current for crack in different directions was researched with detail. Aside from this, applicability of orthogonal magnetic filed in remote field eddy current was analyzed and simulated by finite element method, and appropriate configuration using to generating orthogonal magnetic fields for tubular structure was discussed and verified. Thirdly, design of detection system including key parameters selection and detection coil mode were researched. At last, proposed high resolution detection system were verified experimentally using various type defects, such as parallel axial crack and real stress corrosion crack. From the above research, conclusions were drawn as followed: the detection ability and sensitivity of proposed remote field eddy current detection system could be improved significantly compared with conventional system, especially for axial crack which was not easily to detect, and real stress corrosion crack could be detected and evaluated successively.


INTRODUCTION
Stress corrosion crack (SCC) is typical defect of industry tubular structures such as oil and gas pipes [1].The main feature of SCC can be summarized as followed: tiny, cluster and often axial oriented.Non-Destructive Testing (NDT) is an effective method to detect it and avoid the occurrence of accident.Remote field eddy current (RFEC) is a "typical eddy current NDT method, which has advantages such as almost equal sensitivity to inner and outer defects, easy defect characterization and insensitivity to lift-off or wobble" [2,3].Principle of RFEC can be seen form Fig. (1).Remote field effect begins from where wave outside the pipe overruns it inside the pipe [4].Although RFEC can detect many type defects of pipes and tubes, poor ability to detect SCC prevents its further application.It is mainly because of that conventional RFEC sensor using two solenoid coils as exciting and detection electromagnetic disturbance, and is more sensitive to circumferential defect than axial defect.Therefore, the effective method to improve performance of RFEC SCC detection is designing a novel sensor and detection system, which is more sensitive to tiny axial crack.To verify detection ability and sensitivity of conventional sensor, two classes defects are manufactured in a section of ferromagnetic tube with 70 mm inner diameter, 82 mm outer diameter and 6 mm wall thickness.First class defects consist of circumferential and axial crack with 10 mm length and 2 mm width, circular defect with 6 mm diameter; second class defect consist of circumferential and axial crack with 10 mm length and 0.5 mm width, circular defect with 3 mm diameter.All defects are manufactured with depth of 3 mm, equal to 50% tube wall thickness.Fortunately, to solve the detection ability for crack in different directions, rotating excitation method has been proposed for eddy current NDT and its applicability has been verified [8][9][10].This type coil generates orthogonal magnetic field based on rectangular frame and two perpendicular windings on it.Unfortunately, it is not suitable for tubular structure, for its big air gap and un-uniform will cause serious problem for RFEC.The appropriate orthogonal pattern should be combined axial magnetic field and circumferential magnetic field for tubular structure.

Principle of Finite Element Method
Maxwell equation is basis of electromagnetic simulation.For remote filed eddy current simulation, because its low frequency, displacement current will be omitted [11][12][13], therefore the Maxwell equation can be simplified as followed: where,  A is called magnetic potential vector, the final governing equation can be rewritten as: Deeply, for harmonic condition, and suggests ∇ ⋅ Therefore, after mesh and discretion, magnetic potential vector  A can be solved from equation (5).And then, magnetic flux density  B and other quantity can be got.

Simulation of High Resolution RFEC
Finite element simulation parameters can be seen from Table 1, and Fig. (5) shows simulation model and mesh.

HIGH RESOLUTION RFEC DETECTION SYSTEM DESIGN
The proposed high resolution detection system is showed in Fig. (8).Signal generation unit uses AD7008 DDS chip to generate two orthogonal excitation waveforms.The power amplifier unit uses OPA548 to amplifier the orthogonal signal and applied to exciting coil.Fig. (9) shows the novel high resolution sensor.

Excitation
Conventional RFEC sensor always uses a solenoid coil to generate axial excitation.Therefore, only another coil which can get circumferential excitation is needed to generate orthogonal excitation.For the excitation configuration showed in Fig. (10), because of "skin effect" theory, when AC current was applied between the inside conductor's two ends, the current will focus on conductor's out side [14][15][16], then can obtain spatial circumferential excitation.Important phenomenon "phase knot" is used for test and verification.From the curve shown in Fig. (10), the remote field zone begin from about 3 times of inner diameter, and means it is an appropriate for RFEC detection.

Detection Coil
Differential detection coil is popular in RFEC tiny crack testing [17,18].Conventional differential mode coils has disadvantage such as susceptibility to jitter or off-center and difficulty to ensure windings symmetric.From the simulation results, to maximize the use of orthogonal excitation, the detection coil should be sensitive to radial perturbation field, and differential radial field detection coil shown in Fig. (11) is used to improve the detection ability and sensitivity [2,19].The detection coil mainly relies on the symmetry of the magnetic core and its basic principle can be summarized as followed: Firstly, in the case of defect free, according to magnetic path theory and its symmetry, the magnetic flux will be leaded from one of the shaft to another shaft, and no flux will enter the winding; Secondly, when crack locates at one side of shafts, the coil's symmetry will be broken, then some magnetic flux will change its path to the central winding, as shown in Fig. (12); Lastly, when crack locates at another side of shafts, some magnetic flux will also change its path to the winding, except for its direction is inverse, as shown in Fig. (13).

Experimental Condition
(1) Frequency: Remote field eddy current always operate at low frequency, for example, 10 Hz~120 Hz.Main criteria to select frequency is signal strength and place where remote field zone begin.From experimental test, 30 Hz~60 Hz should be appropriate frequency, for under these frequency, a suitable detection signal can be obtained and the remote field zone is about 2～3 times of pipe diameter.At last, 60 Hz is chosen to get a better signal to noise ratio.
(2) Exciting power: Because of the "pass through twice" character of remote field eddy current, it is not an appropriate method to increase detection signal strength by increasing exciting power; exciting power should be set at an appropriate level.According to skin effect equation: Threshold of exciting power can be determined by change of phase.Finally exciting power of axial excitation is chosen as 1.1 A, and circumferential excitation 1.7 A.
(3) Sensor length reducing: In conventional RFEC, detection coil always fixed at 4~5 times tube diameter to avoid the jitter or off-center of sensor, and forbidden the ability to pass through the bend and elbow.Using shielding is a popular method to solve the problem.By experimental test, we use the mode: steel first, copper second, aluminum last, which can be seen in Fig. (14).After using shielding, the remote field zone begins from 1.8 inner diameters.And 2.3 inner diameter should be appropriate, because of irregular of pipe and problem caused by inaccuracy of transmission gear.manufactured axial cracks with 10 mm length, 0.5 mm width, and different depth of 15%, 20%, 40%, 60% tube wall thickness; Second type is two parallel axial crack with 3 mm separation distance; Third type defect is a real stress corrosion crack.

Results of Axial Cracks with Different Depth
Figs. (16,17) show experiment results of axial cracks with different depth.From the amplitude and phase of detection coil, except for defects with depth of 15% wall thickness, all of other defects can be detected; especially, the proposed high resolution detection system can detect and identify axial crack with depth of 20% wall thickness.Compared with the section 2, the detection ability is improved significantly.

Results of Two Parallel Axial Cracks
Figs. (18,19) show experiment results of two parallel axial cracks.The amplitude and phase can detect the crack clearly, and from the contour plot of amplitude, the shape of crack can be evaluation, although the phase failed to discrimination of two cracks.but it is not easily to determine the shape of the crack even in the contour plot.Fortunately, we can use the contour plot of phase to identify shape of the crack, for phase is usually chosen to determine defect in RFEC testing.

CONCLUSION
Stress corrosion crack is not easily to detect using conventional remote field eddy current method.It is mainly because of that conventional RFEC sensor only using axial magnetic filed excitation, and detection coil usually is designed to detect axial disturbed magnetic field.A novel high resolution remote field eddy current detection system is proposed and verified.Firstly, the poor detection ability to axial crack of conventional RFEC detection system is discussed with detailed.Secondly, appropriate orthogonal excitation and detection coil mode is design and tested.Experiment results show that proposed novel detection system has high resolution to various type of axial cracks, especially to stress corrosion crack which is difficult to detect by conventional system, and the shape and size of stress corrosion crack can be evaluated by combination utilize the amplitude and phase of detection coil.

Fig. ( 2 )
Fig. (2) shows conventional remote field eddy current sensor, which usually adopts a coaxial solenoid coil excited by sinusoid to generate exciting field, and adopts another solenoid coil placed at about 2~3 times pipes inner diameter to pick up disturbed electromagnetic field [5-7].

Fig. ( 3 )
Fig. (3) shows experiment results of first class defects.All of defects can be detected and identified clearly, although sensitivity to axial crack is relative lower than circumferential crack and circular defect.Fig. (4) shows experiment results of second class defects.With reduction of size of defect, detection ability to all of the defects decreases, especially for axial crack.Both amplitude and phase of detection coil for axial cracks reduce too low to identify axial crack.

Fig. ( 6
Fig.(6) shows simulation results of time-varying magnetic flux density in remote field zone.In remote field zone, the magnetic flux density is orthogonal also like the excitation, and can be divided to in-phase part and quarter part, which can be sensitive to defect in different directions.

Fig. ( 7
Fig.(7) shows radial direction magnetic flux density around the axial crack.Significant change around crack confirms that the remote field zone can be use for tiny axial crack detection.

Fig. ( 15
Fig. (15) shows reference materials for experiment test.The specimen is a section of ferromagnetic tube with 70 mm inner diameter, 82 mm outer diameter and 6 mm wall thickness.Three type defects are used for test.First class defect consists of

Filter
Fig. (8).Principle and photography of detection system.

Figs
Figs.(20, 21)  show experiment results of the real stress corrosion crack.From the 3D plot of detection coil, the proposed method can detect stress corrosion crack obviously,

Table 1 . Parameters of finite element simulation.
Author Xu Xiaojie, Institute of Information and Navigation of Air Force Engineering University, Ph.D. The author's major is Eddy Current Non-Destructive Testing and Signal Processing. 5 papers received by EI or SCI and 1 patents had been published.Second Author Jia Yueling, Institute of Equipment management and Engineering of Air Force Engineering University.The author's major is Equipment Engineering and Computer Simulation.Third Author Zang Zhanbin, Institute of Information and Navigation of Air Force Engineering University.The author's major is Techtronic Engineering and Signal Processing.