Detection of Cracks in Metallic Objects by Arbitrary Scanning Direction Using a Double U-Shaped Orthogonal ACFM Probe

Wei Li, Xin’an Yuan*, Guoming Chen, Xiaokang Yin, Jiuhao Ge, Qingxiao Kong, Yutian Zhang and Yanyun Wu

Center for Offshore Engineering and Safety Technology, China University of Petroleum, Qingdao, 266580, China

Alternating current �eld measurement (ACFM) technique is developed for sizing cracks on structures. According to the theory of ACFM, the induced current �eld should be perpendicular to the crack in metallic objects. So conventional ACFM probes should scan the crack in a particular direction. In this paper, a double u-shaped orthogonal ACFM probe is present for cracks detection at arbitrary scanning direction using induced rotating current �eld. The �nite element method (FEM) model is employed to analyze the rotating current �eld induced by the double u-shaped orthogonal ACFM probe. The experiments are carried out to test the crack on specimen at arbitrary scanning direction. Results show that the crack can be detected effectively at arbitrary scanning direction using the double u-shaped orthogonal ACFM probe. [doi:10.2320/matertrans.M2015384]

(Received October 13, 2015; Accepted February 5, 2016; Published April 8, 2016)

Keywords:　 ACFM, Arbitrary scanning direction, Double u-shaped orthogonal probe

1.　 Introduction

Alternating current �eld measurement (ACFM) technique is originally developed for sizing cracks on underwater struc-tures as an alternative to MPI1,2), which is developed from the alternating current potential drop (ACPD) technique3,4). ACFM probe induces a uniform alternating current �eld on the surface of conductive specimen. The uniform current �eld will be disturbed when the crack is present. The disturbed current �eld makes the magnetic �eld distorted above the crack. By measuring the distorted magnetic �eld, both the depth and length of crack can be deduced. As shown in Fig. 1, the magnetic �eld in X direction (BX) shows a deep trough, which contains depth information of crack. Meanwhile the magnetic �eld in Z direction (Bz) shows a negative and posi-tive peaks at both end of the crack, which gives an indication of crack length5,6). Because the theory of ACFM is based on the perturbation of induced surface current �eld in metallic objects, the thin defects can be detected effectively7).

However, the theory of ACFM is based on the assumption that the induced current is perpendicular to the crack. As

shown in Fig. 2a, when the u-shaped ACFM probe scans the specimen along the crack, the induced current �eld will be perpendicular to crack. Thus, the perturbation of induced cur-rent �eld is obvious and the characteristic signals of crack are in accord with the theory of ACFM. While the scanning direc-tion of u-shaped ACFM probe is perpendicular to the crack, as shown in Fig. 2b, the induced current �eld will be parallel to the crack. In this case, the perturbation of induced current �eld is inconspicuous. Due to leakage �ux effect, some mag-netic �eld leaks into the air. However, the leakage �ux effect does not match the theory of ACFM, which cannot make some quantitative detection of the crack (depth and length).

As mentioned above, the conventional ACFM probes should scan the metallic objects along the crack to get better characteristic signals8). In practice, it is not clear that the crack is present or not on the specimen, let alone the direction of the crack before the inspection. Hence, a double u-shaped

* Corresponding author, E-mail: xinancom@163.com

Fig. 1　Theory of ACFM.

Fig. 2　U-shaped ACFM probe for cracks detection. (a) The probe is parallel to the crack. (b) The probe is perpendicular to the crack.

Materials Transactions, Vol. 57, No. 5 (2016) pp. 608 to 612 ©2016 The Japan Institute of Metals and Materials

orthogonal ACFM probe is present in this paper to detect the cracks effectively at arbitrary scanning direction. The double u-shaped orthogonal ACFM probe can produce a rotating al-ternating current �eld on the specimen. The induced rotating current �eld will be perpendicular to the crack at any scan-ning direction. Thus the cracks can be detected effectively at arbitrary scanning direction using the double u-shaped or-thogonal ACFM probe.

2.　 FEM Model of Double U-shaped Orthogonal ACFM Probe

As Ferraris effect, a rotating magnetic �eld can be con-structed using two orthogonal coils driven by alternating cur-rents with 90° phase difference9). In this way, two same exci-tation coils are wound on the U-shaped Mn-Zn ferrite material magnetic yokes. The magnetic yokes are placed or-thogonally along X direction (excitation X) and Y direction (excitation Y). Excitation X coils and excitation Y coils are driven by one pair alternating current (ix(t) and iy(t)) with 90° phase difference, which are de�ned as follows

ix(t) = I0 sin(ωt + α0) (1)

iy(t) = I0 sin(ωt + α0 + 90◦) (2)

Where, I0 is the amplitude of the alternating current, ω is the angular frequency of the alternating current, and α0 is the initial phase of the ix(t).

Because the two excitation coils are very close to the con-ductor surface, the conductor will be assumed to be a half-in-�nite plate10,11). Because the angular frequencies of ix(t) and iy(t) are the same, the excitation X coils and excitation Y coils will produce a complex alternating magnetic �eld. Mean-

while, the phase of iy(t) delays 90° compared with ix(t) and the two excitation coils are placed orthogonally, so the amplitude of the complex alternating magnetic �eld is constant and the phase is changed periodically. According to the principle of electromagnetic �eld propagation, the complex alternating magnetic �eld can induce a rotating alternating current �eld in the conductor with the same amplitude and angular fre-quency12).

The FEM model of the double U-shaped orthogonal ACFM probe is set up in ANSYS13). The alternating current �eld is analyzed by the transient analysis method. The model con-sists of two U-shaped orthogonal yokes wound with exci-tation coils and a mild steel sheet sample, as shown in Fig. 3. The excitation coils X and Y are driven by the alternating current (1 V amplitude, 6000 Hz frequency) with 0° and 90° initial phase respectively.

Fig. 3　The FEM model of the double u-shaped orthogonal ACFM probe.

Fig. 4　The rotating current �eld on the surface of specimen at different transient time, (a) t =  0, (b) t =  0.25 T, (c) t =  0.5 T, and (d) t =  0.75 T.

609Detection of Cracks in Metallic Objects by Arbitrary Scanning Direction Using a Double U-Shaped Orthogonal ACFM Probe

A complete period is divided into 4 transient steps equally. The transient induced current density on the surface are sim-ulated and analyzed. The simulation results show that the di-rection of induced current �eld revolves periodically. The di-rection is negative Y at t =  0, and negative X at t =  0.25 T, and positive Y at t =  0.5 T, and positive X at t =  0.75 T, as shown in Fig. 4. Thus, the double U-shaped orthogonal ACFM probe produces a rotating current �eld in metallic objects. Due to the skin effect, the induced rotating current �eld will �ow on the surface of metallic objects. If the surface cracks (such as initial stress corrosion cracking) are present, the induced ro-tating current �eld will be perpendicular to the cracks at arbi-trary scanning direction, which produces a larger distorted magnetic �eld. The simulation results provide a strong evi-dence for surface cracks detection at arbitrary scanning direc-tion using the double u-shaped orthogonal ACFM probe.

3.　 Crack Detection Experiments

The double u-shaped orthogonal ACFM probe is set up, as shown in Fig. 5a. The two orthogonal U-shaped Mn-Zn fer-rite yokes are wound with 500 turns excitation coils (0.15 mm enameled copper wire), according to the theory model and FEM model. The experimental system is shown in Fig. 5b. The signal generator provides a sine signal with 6 kHz fre-quency and 1 V voltage as the initial driving signal. The phase of initial driving signal is changed 90° by a phase shifter. The initial driving signal and the changed 90° phase driving signal are used to drive the two excitation coils of the double u-shaped orthogonal ACFM probe. The detecting sensor picks up the distorted magnetic �eld (BX and BZ) caused by the disturbed rotating alternating current �led. After signal ampli�cation, phase sensitive recti�cation and low pass �lter-ing, the analog signals are transformed to digital signals and sent into the PC using the DAQ module. The intelligent iden-

ti�cation software in PC will display the signals and identify cracks with one pass scanning.

The specimen is a Q235 mild steel sheet with a 45 mm length and 7 mm depth arti�cial rectangular crack. As shown in Fig. 5c, the double u-shaped orthogonal ACFM probe scans the crack from 0° to 90° direction with 10° increase sim-ulating the detection of cracks at arbitrary scanning direction. The 0° direction indicates that the scanning direction of probe is parallel to the crack, while the 90° direction indicates the scanning direction of probe is perpendicular to the crack.

The Fig. 6 (a) to (d) show the 0°, 30°, 60°, 90° direction crack detection experimental results using the double u-shaped orthogonal ACFM probe. Comparing Fig. 6 (a) with Fig. 1, it is clear that the BX and BZ signals are in accordance with the principle of ACFM, which veri�es the feasible of the experimental system. As shown in Fig. 6 (b), the perturbation of characteristic signals above the crack is still obvious at 30° direction. At 60° direction, as shown in Fig. 6 (c), the BX and BZ characteristic signals are also in accord with the principle of ACFM. Finally, when the scanning direction is perpendic-ular to the crack (90° direction), as shown in Fig. 6 (d), the crack still can be recognized by the double u-shaped orthog-onal ACFM probe perfectly.

The parameters for characterizing the inspection sensitivi-ty, SX and SZ, are given as follows to reduce detection error and improve the SNR (Signal to Noise Ratio)14).

S X =MXmax

MX0× 100%

S Z =MZmax

MX0× 100%

(3)

Where, MX0 is the amplitude of BX signal without crack, and MXmax and MZmax are the maximum perturbation of BX and BZ caused by crack respectively.

As shown in Fig. 7, there is a little decrease of sensitivity

Fig. 5　The double u-shaped orthogonal ACFM probe. (a) The double u-shaped orthogonal ACFM probe and detecting sensor. (b) The experimental system. (c) Detection of cracks at arbitrary scanning direction.

610 W. Li, et al.

by the double u-shaped orthogonal ACFM probe at different scanning detection. The maximum sensitivity of SX is 32.6%, while the minimum sensitivity of SX is 27.2%. The maximum decrease of sensitivity in SX is 16.8%, which meets require-ment of sensitive detection of cracks at arbitrary scanning direction. Meanwhile, the maximum sensitivity of SZ is 69.1% and the minimum sensitivity of SZ is 59.8%. The maximum decrease of sensitivity in SZ is 13.5%. It suggests that the de-tection sensitivity of double u-shaped orthogonal ACFM probe changes a little for inspecting cracks at arbitrary scan-ning direction. The crack length can be calculated from the distance between the two peaks of BZ, while the crack depth can be estimated from the sensitivity of BX.

4.　 Conclusion

In this paper, a double u-shaped orthogonal ACFM probe is present for cracks detection at arbitrary scanning direction on

structures. The induced rotating current �eld is analyzed by simulations. The crack detection experiments are carried out at arbitrary scanning direction by the double u-shaped orthog-onal ACFM probe test system. From these results, we con-clude that the feasibility of the double u-shaped orthogonal ACFM probe is veri�ed by both EFM model and experiments. It is apparent that the double u-shaped orthogonal ACFM probe can detect cracks effectively in metallic objects at arbi-trary scanning direction.

During the practice, many challenges must be addressed to implement this technology. Due to the skin effect, the pene-tration depth of the induced rotating current �eld is limited. Thus, detection of subsurface defects is still a big challenge for the double U-shaped orthogonal ACFM probe. The quan-titative evaluation of cracks also needs to be studied in future work.

Fig. 6　BX and BZ signals from experiments using the double u-shaped orthogonal ACFM probe at different scanning direction; (a) 0° direction, (b) 30° direc-tion. (c) 60° direction. (d) 90° direction.

Fig. 7　Detection sensitivity for cracks at different scanning direction; (a) BX, (b)BZ.

611Detection of Cracks in Metallic Objects by Arbitrary Scanning Direction Using a Double U-Shaped Orthogonal ACFM Probe

Acknowledgements

Wei Li and Xin’an Yuan contributed equally to this work.This work is supported by the National Natural Science

Foundation of China (No. 51574276), the Shandong Provin-cial Natural Science Foundation, China (No. ZR2015EM009), the Fundamental Research Funds for the Central Universities (No.15CX05024A), the Postgraduate Innovation Project of China University of Petroleum (No. YCX2015039 and YCX2014040), the Applied Basic Research Program of Qin-gdao City, China (14-2-4-49-jch).

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