Deep Defect Detection Using Eddy Current Testing with AMR ... - piers

Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 493

Deep Defect Detection Using Eddy Current Testing with AMR Sensor D. F. He and M. Shiwa National Institute for Material Science, Sengen 1-2-1, Tsukuba 305-0047, Japan

Abstract— We developed high sensitive anisotropic magneto resistive sensor and used it for eddy current testing (ECT). Due to the high magnetic field sensitivity of AMR sensor at low frequency, the ECT system can be used to detect deep defect. The excitation coil was 50 turns with the diameter of about 30 mm. The amplitude of the excitation current was about 20 mA. The AMR sensor was put at the center of the excitation coil, and the sensing direction of the AMR sensor was parallel with the plane of the excitation coil. By this arrangement, the background field produced the excitation coil can be compensated. Using this ECT system, we successfully detect the deep crack defect in an aluminum plate with the depth of 15 mm, and the signal-tonoise ratio was best for the excitation frequency of 32 Hz. 1. INTRODUCTION

Eddy current testing (ECT) is an efficient surface and near surface inspection method. Inductive coil, hall sensor, flux gate, giant magneto resistive (GMR) sensor, anisotropic magneto resistive (AMR) sensor, and Superconducting quantum interference device (SQUID) have been used to construct ECT systems [1–6]. To detect deep defect in conductive materials, like aluminum, copper and steel, high sensitive magnetic field sensors at low frequency are needed. SQUID had the best magnetic field sensitivity at low frequency and was used to detect deep defect in conductive material [7], but the need of low temperature cooling made the SQUID-based ECT system complex and limited its practical applications for ECT. We developed high sensitive AMR sensor and used it for ECT [8–10]. Compared with other sensors, AMR sensor had the advantages of good sensitivity at low frequency, easy operation, big dynamic range, and flat frequency response in a big bandwidth. In this paper, we will describe the experiments of deep defect detection using ECT with AMR sensor. 2. AMR-BASED ECT SYSTEM FOR DEEP DEFECT DETECTION

Figure 1 shows the schematic block diagram of ECT system with AMR sensor. AC magnetic field was produced by the excitation coil as AC current flew in it; and eddy current was induced in the specimen. The AMR sensor was used to measure the magnetic field produced by the eddy current. AMR sensor of HMC1001 was used. The lock-in amplifier was used to measure the amplitude and phase signal of the signal. In our experiment, only amplitude signal was used.

Figure 1: Schematic block diagram of ECT system with AMR sensor.

The excitation coil was a 50-turn circular coil with the diameter of about 30 mm. The AMR sensor was put in the center of the excitation coil, and the sensing direction was not perpendicular to, but parallel with the plane of the excitation. This configuration could compensate the background field produced by the excitation coil. Figure 2 shows the magnetic field noise spectrum measured in our laboratory using the AMR √ sensor. The magnetic field resolution was about 40 pT/ Hz at 32 Hz. The maximum magnetic

PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013

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Figure 2: Magnetic field noise spectrum measured in our laboratory using AMR sensor.

field detected by AMR was about 2 Gauss, so the dynamic range of the AMR sensor was over 120 dB at 32 Hz. 3. ECT EXPERIMENTS

The sample was an aluminum plate with the thickness of 16 mm and artificial defect was made on it. The depth of the defect was 15 mm. The AMR sensor was fixed with an X-Y stage and the scanning was done by moving the AMR sensor. The scanning √ speed was about 5 mm/s. The penetration depth of eddy current can be expressed by δ = 1/ πf µσ, where, f is the excitation frequency, µ is the permeability of the material and σ is the conductivity of the material. For aluminum, µ = µ0 ; σ = 3.5 × 107 S/m. If δ is 15 mm, the calculated excitation frequency is about 32 Hz. Figure 3 shows scanning results for the excitation frequency of 16 Hz, 32 Hz, 64 Hz, and 128 Hz. The amplitude of the excitation current was about 20 mA. The signal-to-noise ratio was best at the frequency of 32 Hz.

(a) 16 Hz

(b) 32 Hz

(c) 64 Hz

(d) 128 Hz

Figure 3: ECT signal of defect with the depth of 15 mm. (a) Excitation frequency was 16 Hz. (b) Excitation frequency was 32 Hz. (c) Excitation frequency was 64 Hz. (d) Excitation frequency was 128 Hz.

Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 495 4. SUMMARY

Using ECT with AMR sensor, the defect with the depth of 15 mm was detected and the signal-tonoise ratio was best for the excitation frequency of 32 Hz. REFERENCES

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