EXPERIMENTAL STUDIES OF A DUAL-FREQUENCY SURFACE-MOUNTED SENSOR FOR MONITORING SURFACE CRACKS

V. I. Urusov, I. G. Leshchenko, B. B. Vinokurov

(Presented by the scientific seminar of the Department of Information and Measurement Technology)

When ferromagnetic products in the form of rods and tubes are acted upon by two alternating parallel magnetic fields of different frequencies and amplitudes, an emf appears in the measuring winding of the sensor, containing a broad spectrum of even and odd harmonics of the high-frequency and low-frequency fields. The second harmonic of the emf of the high-frequency component, isolated from this spectrum, is highly sensitive to transverse surface defects and only weakly sensitive to longitudinal defects [1]. For monitoring short defects in products of large diameter, feed-through sensors become practically unsuitable.

The authors considered it advisable to carry out studies of the indicated dual-frequency method for detecting defects by means of surface-mounted sensors, in which, unlike feed-through sensors, both the high-frequency and low-frequency fields are sharply nonuniform. In addition, cylindrical surface-mounted sensors must be equally sensitive to defects of arbitrary direction, if the latter are comparable with the dimensions of the sensor. Experimental studies were carried out with cylindrical sensors of two types, the designs and winding data of which are given in Fig. 1a, b. The surface-mounted sensor of the second type has a smaller gap between the low-frequency coil and the surface of the item being inspected, as well as a ferrite rod for strengthening the magnetic field. The dimensions of the second type of sensor are smaller than those of the first. A block diagram of the experimental setup is shown in Fig. 2. The low-frequency winding \(W_1\) is supplied through a large resistance \(R_1\) to create a prescribed-current mode (from a 50 Hz mains supply). The primary high-frequency winding \(W_2\) is supplied from generator 1, type G3-7A, in sinusoidal-current mode at a frequency of 100 kHz. To compensate the second harmonic, which is practically always present in the generator, compensator 2, analogous to that described in [1], is connected. During the experiment the amplitude of the high-frequency current remained unchanged owing to the large resistance \(R_2\) in the load circuit. The measuring winding of the sensor, wound together with \(W_2\), is connected to the input of selective voltmeter 3, type V6-1, tuned to twice the generator frequency (200 kHz). The signal from the output of the voltmeter amplifier is simultaneously fed to oscilloscope 4, type S1-19. With complete compensation of the second harmonic, the mag-

...of the magnetizing high-frequency current; with the source of the low-frequency field disconnected, only a horizontal straight line is observed on the oscilloscope screen. In the experiments carried out, as in [1], the amplitude of the high-frequency field (ampere-turns) at

Fig. 1

Fig. 2

several orders of magnitude smaller than that of the low-frequency magnetizing field of industrial frequency. On the experimental setup with surface-mounted sensors, as earlier with encircling sensors, it was established that the second harmonic of the high-frequency e.m.f. appears only when the ferromagnetic article is magnetized by an alternating current; the amplitude of the e.m.f. varies according to a periodic law with twice the frequency of the low-frequency field and is very sensitive to surface defects in the form of cracks, work hardening, mechanical stresses, scale, etc.

The results of experimental studies of the relative change in the period-average value of the high-frequency component of the emf of the second harmonic as a function of defect depth, the magnitude of the low-frequency magnetizing sinusoidal current, and the position of the sensors relative to the defect are shown in Figs. 3 and 4. Defects in the form of cracks 30 mm long, 0.5 mm wide, and 0.1 to 0.5 mm deep were made at the center of low-carbon steel plates measuring \(140 \times 140\) mm and 4 mm thick.

As follows from the graphs in Fig. 3, obtained for the sensor of the first type (Fig. 1, a), the relative changes of the second harmonic of the sensor over a defect 0.1 mm deep are 12%, and over a defect 0.5 mm deep, 47%. From the graphs

Fig. 3. Curves 1, 2, 3 for a defect depth of 0.1 mm and rms current values of 20, 50, and 100 mA, respectively.
Curves 4, 5, 6 for a defect depth of 0.5 mm and the same values of the magnetizing current.

in Fig. 4, obtained for sensors of the second design, the relative changes in emf are respectively 24% and 57%, which indicates a higher sensitivity to small defects. In the second sensor without a ferrite core, the sensitivity to defects decreases noticeably, and for a defect depth of 0.1 mm the maximum reaches 15%. For both coils, maximum sensitivity to defects is observed at a certain value of the low-frequency magnetizing current. Ne-

some decrease in sensitivity when the sensor is positioned directly over the defect is due to the fact that, in such a position, the defect is located practically along the magnetic lines of force; for an infinitely narrow slit without internal mechanical stresses this sensitivity, apparently, will tend to zero.

Figure 4. Curves 1, 2, 3 for a defect depth of 0.1 mm and effective current values of 20, 50, and 100 mA, respectively. Curves 4, 5, 6 for a defect depth of 0.5 mm and the same values of magnetizing current.

Conclusion

The method of superimposing two magnetizing fields of different amplitudes and frequencies, which is highly sensitive to defects in the form of cracks, etc., can be successfully applied with surface-mounted sensors as well, which significantly expands the possibilities of the higher-harmonics method in eddy-current flaw detection [2, 3].

LITERATURE

1. A. M. Gruznov. Development and investigation of new circuits for inspecting ferromagnetic products by the eddy-current method. Candidate dissertation, Tomsk, 1968.

2. I. G. Leshchenko. On the question of investigation and application in electromagnetic flaw detection of magnetization of ferromagnetic products simultaneously by two parallel fields of different frequencies and amplitudes. In: Collection of Reports “The Higher-Harmonics Method in Eddy-Current Flaw Detection.” Krasnoyarsk, 1969.

3. Abstracts of reports of the All-Union seminar on questions of the theory and application of higher harmonics of secondary emf in quality control of products made of ferromagnetic materials by the eddy-current method. Krasnoyarsk, 1968.