Study of the Effect of Defect Types and Sizes on the Harmonics of the Secondary EMF in Two-Frequency Testing of Ferromagnets
Abstract Generated abstract
This paper examines how defect type and size affect the fundamental and second harmonic components of secondary EMF during two-frequency nondestructive testing of ferromagnetic materials. Experiments used through-type and surface-mounted sensors, with a 50 Hz low-frequency magnetic field and high-frequency excitation, to compare two-frequency measurements with conventional eddy-current testing on steel tubes and plates containing artificial and introduced defects. The results indicate that the second harmonic of the high-frequency component is substantially more sensitive to surface, subsurface, and internal defects, with reported sensitivity on average about an order of magnitude greater than the eddy-current method. The study also observes that second-harmonic response depends on low-frequency field strength and reaches a maximum near conditions of greatest differential magnetic permeability.
Full Text
INVESTIGATION OF THE INFLUENCE OF DEFECT TYPES AND SIZES ON THE HARMONICS OF SECONDARY EMF IN TWO-FREQUENCY TESTING OF FERROMAGNETS
V. I. URUSOV, I. G. LESHCHENKO, B. B. VINOKUROV
(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)
In recent years, for nondestructive testing of ferromagnetic products, the method of higher harmonics has begun to be widely used; it is based on the nonlinear properties of a ferromagnet remagnetized by an alternating magnetic field. Nondestructive testing by the higher-harmonic method implies that the amount of information about the state of a product is increased through separate measurement of individual harmonic components of the signal.
In flaw detection of ferromagnetic products by the two-frequency method, when the product or a section of it is remagnetized simultaneously by two alternating fields of different frequencies and amplitudes, the emf of the second harmonic of the high-frequency component of the field is usually used as the measured quantity, since it is the most sensitive to surface and subsurface defects [1, 2].
In order to determine the advantages in sensitivity to defects of the two-frequency testing method over the ordinary eddy-current method, we conducted experiments that made it possible to compare the influence of the types and sizes of defects on the emf of the fundamental and second harmonics of the high-frequency
Table 1
| Single-frequency (eddy-current) method at the optimum frequency in the range 10–300 kHz | Two-frequency method at specified frequencies and amplitudes, at \(= 50\) Hz: \(= 100\) kHz | Two-frequency method at specified frequencies and amplitudes, at \(= 50\) Hz: \(= 200\) kHz | Type of defect |
|---|---|---|---|
| 7.4 | 39 | 52 | Annular burn, width 1.5–2.0 mm |
| 6.8 | 78 | 90 | Work hardening |
| 0.6 | 39 | 35 | Transverse annular score, depth 0.2 mm |
| 0.0 | 75 | 130 | Internal defect. Inside the tube, joint of rods |
frequency component of the field. The results of these experiments are presented in the present work.
The investigations were carried out with two-frequency sensors of the through-type and surface-mounted type. To create a strong low-frequency field, the industrial power network with a frequency of 50 Hz was used; the high-frequency field was created by a sinusoidal-oscillation generator. With the low-frequency supply switched off, the two-frequency sensor became an ordinary eddy-current sensor. To measure the fundamental and second harmonics of the high-frequency component of the field, a selective voltmeter V6-1 was used.
The test objects for the through-type sensor were tubes made of 30KhGSA steel with an outside diameter of 14 mm and a wall thickness of 0.8 mm, on the surface of which defects of various kinds had been introduced. The results of the experiment with the through-type sensor are summarized in Table 1.
The test objects for the surface-mounted sensor were two plates of carbon steel 10 mm thick. Artificial defects in the form of cracks, having the same opening but different depth (0.5; 1.0; 2.0; 4.0; 6.0; 8.0 mm), were introduced into one of them; defects having the same depth but different opening (0.5 mm and greater) were introduced into the other. The results of the experiment with the surface-mounted sensor are given in Fig. 1. This figure shows the relative change in the average value of the EMF of the fundamental and second harmonics as a function of defect depth under the same inspection conditions. It is interesting to note that the relative change in the EMF of the second harmonic, as the low-frequency field is increased, first grows, reaches a maximum, and begins to decrease. The maximum corresponds approximately to the fields at which the differential magnetic permeability of the ferromagnet has the greatest value.
Fig. 1
The relative changes in the EMF of the fundamental and second harmonics as a function of defect opening remain practically constant for any values of the low-frequency field.
The experiments performed make it possible to draw the following conclusions:
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The sensitivity to defects of the two-frequency inspection method is, on average, an order of magnitude higher than the sensitivity of the eddy-current method.
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The two-frequency method makes it possible to detect defects not only on the surface of the inspected article, but also throughout the entire depth to which the low-frequency field penetrates.
LITERATURE
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I. G. Leshchenko. Investigation and application in electromagnetic flaw detection of magnetization of ferromagnetic articles simultaneously by two parallel fields of different frequencies and amplitudes. In: The Method of Higher Harmonics in Eddy-Current Flaw Detection. Krasnoyarsk, 1969.
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V. I. Urusov, I. G. Leshchenko, B. B. Vinokurov. Studies on the detectability of defects in ferromagnetic articles by two-frequency surface-mounted sensors. Abstracts of reports of the Second All-Union Seminar on Higher-Harmonic Methods. Tomsk, 1970.