SU1698725A2 - Method of non-destructive testing of flat dielectric products - Google Patents

Abstract

The invention relates to methods for non-destructive testing of materials and products by examining the characteristics of a gas discharge in a high-voltage electric field and can be used to solve a wide class of flaw detection tasks in various areas of national economy. The purpose of the invention is to expand the functionality of the flaw detection method, where in addition to the presence and size of the defect, the depth of the defect is additionally determined. To do this, after determining the presence of a defect in the sheet material, acoustic and electromagnetic waves are jointly excited by a known method, the superposition of electromagnetic waves transmitted through the material and reflected from its surface is measured, and the depth of the defect is determined from measurements at several acoustic frequency frequencies. 2 Il. (Ls

Description

The invention relates to methods for non-destructive testing of materials and products by examining the characteristics of a gas discharge in an electric field of high strength and can be used to solve a wide class of flaw detection problems in various areas of the national economy.

A known method for defectometry of flat dielectric materials is based on the impact of a strong (ionizing) pulsed electric field and electromagnetic radiation on a material. The modulated electromagnetic signal is compared with the pulses of the electric field, and the results of joint measurements determine the presence of defects and their coordinates in the plane of the material.

However, this method does not allow determining the depths of the defects.

The purpose of the invention is to expand the capabilities of the method by determining the depth of the defects.

FIG. 1 shows a block diagram of a device implementing the proposed method; in fig. 2 - the main geometric relationships used in determining the coordinates of the defect.

The method of defectometry of flat dielectric materials is implemented as follows.

Electrodes connected to a pulsed high voltage source, i.e. place the test material in an electric field. The amplitude of the voltage is set such that

Ov O, 00 XI

to

SL

 ND

The working point in voltage was in the region of strong fields (a field of high tension), which caused the ionization effect in the space of local heterogeneities.

One electrode has a smaller area compared to the other and is made with a profiled surface, which allows the formation of a uniform electric field in the study area.

On the one hand, the studied flat dielectric sample is irradiated with electromagnetic radiation and the parameters of the signal passing through the sample are recorded. The electric field is excited by a pulse signal. The modulated signal transmitted through the sample is compared with a pulsed excitation signal of the electric field and, according to the results of joint measurements, the parameters of the defect are determined. The pulse frequency is varied from lower to upper values, which are related to the defect size being tested. The dependence of the gas discharge decay time (t) on the characteristic size of the defect (d), i.e. extended defect volume coordinate (d). When the frequency of the electric field F is greater than 1 / g for a particular defect, the effect of a steady decrease in conductivity is observed. However, at F 1 / g, parametric modulation occurs, which is an informative symptom of a defect. Information about a defect is highlighted by comparing two frequency signals.

After determining the presence of a defect and determining its coordinates on the sample plane, the depth of the value is determined. To do this, acoustic and electromagnetic waves are jointly excited, the superposition of the electromagnetic waves transmitted through the material and reflected from its surface is measured, and the depth of the defect is determined from measurements at several acoustic frequency frequencies. It uses the previously constructed coordinate nomogram.

A device for implementing the method comprises an electromagnetic wave generator 1, an emitter 2, electrodes 3 and 4 located on the surface of a dielectric material under investigation with a defect, a receiver 5, an electromagnetic radiation detector 6, a frequency detector 7, a high-impulse voltage source 8, a recorder 9, block 10 frequency, the exciting element 11, the generator 12

acoustic signals, frequency adjustment unit 13 and comparison unit 14.

Between the electrodes 3 and 4, connected to the source 8 of a high pulsed

voltage, put the investigated dielectric material with a defect. The emitter 2 connected to the generator 1, and the receiver 5 are located on the same level with one of the electrodes 3 of small size,

0 made profiled with the shape of the Rogowski electrode, at its opposite sides. The frequency of the high voltage pulses is set by the frequency tuning unit 10, the first output connected to the input of the high pulse voltage source 8. The output of the electromagnetic radiation detector 6 is connected to the measuring input of the frequency detector 7, to the reference input of which is fed

0 signal from the second output of the block 10 frequency adjustment. The inputs of the recorder 9 are connected to the output of the frequency detector 7 and the third output of the frequency tuning unit 10. The excitation element 1-1 is located on the sample surface and is connected to the generator output 12 of acoustic signals, to the control input of which the frequency tuning unit 13 is connected. A comparator unit 14 is connected to the second output of the electromagnetic radiation detector b.

The device works as follows.

A flat dielectric material is placed between the electrodes 3 and 4 with the radiator 2 and the electromagnetic radiation receiver 5. The frequency of generation of electromagnetic radiation is chosen such that the wavelength in the material does not exceed its military thickness. The latter condition follows from the conditions for the propagation of electromagnetic radiation in a waveguide filled with a dielectric. In the absence of a defect in the volume of the dielectric material 5, a constant signal level is established at the output of the electromagnetic radiation detector 6. This signal from the output of the electromagnetic radiation detector 6 is fed to the measuring input of the frequency

0 of the detector 7, the reference input of which receives a variable reference signal from the second output of the frequency tuning unit 10. The frequency of the reference signal is equal to the frequency of tuning the output voltage at the output of a source 8 of a high pulse voltage. On the frequency detector 7 output signal, there is no signal in this case, since the information signal is constant.

The device in the presence of a defect works in the following mode.

From the output of the high pulse voltage source to the electrodes 3 and 4, a sequence of pulses is applied, the amplitude of which is such that the intensity of the electric field in the gas cavity of the defect exceeds the breakdown value. The pulse repetition frequency is set by the frequency tuning unit 10. In a defect in the form of a closed cavity, the gas is ionized and its conductivity increases sharply. If the repetition time of high voltage pulses is shorter than the recombination time of charged particles inside the cavity of the defect and on its walls (the gas discharge decay time), then a region of high conductivity always exists in the dielectric material. Otherwise, a region of high conductivity (at a defect) arises in accordance with the repetition rate of high voltage pulses. The high conductivity region causes additional losses of electromagnetic energy and, naturally, the level of the electromagnetic radiation signal received by the receiver 5 changes. In the first case (the high conductivity region constantly exists) at the output of the detector b, the signal level changes (compared to the defect-free area), but will remain constant, and the signal at the output of frequency detector 7 will still be absent. In the second case, the periodic appearance of a high conductivity region causes a periodic change in the level of the electromagnetic radiation signal at the receiver 5.

Thus, at the output of the electromagnetic radiation detector 6, an amplitude-modulated signal appears with a frequency equal to the frequency of the following high-voltage pulses ionizing the cavity of the defect. In this case, variable frequency signals of the same frequency are output to the output of frequency detector 7, and a maximum amplitude signal appears at its output. This signal arrives at the first input of the recorder 9 and permits recording the value of the pulse repetition frequency of high voltage supplied to the second input of the recorder 9 from the third output of the frequency tuning unit 10. If there are several defects of different sizes, forming a high-voltage pulse train with different repetition frequency (reducing frequency), it is possible to select defects by size using the previously obtained calibration curve for the relationship between the size of the defect and the time (frequency) of gas discharge attenuation.

At the end of the measurement of the dimensions of the defect, the acoustic signal generator 12 is turned on and acoustic waves are excited in the sample through the excitation element 11. With

A constant frequency of the acoustic wave generator 12 through time t i on the electromagnetic radiation detector 6 is set to zero beats, which are fixed by the comparator unit 14. When the oscillation frequency of the generator 12 of acoustic signals is changed by the control signal from the frequency tuning unit 10 due to the final speed of the acoustic waves on the detector 6 of electromagnetic oscillations,

a bundle of pulses.

By measuring the duration of the burst of pulses in unit 14 comparing the frequency, knowing the coordinates on the plane of the defect and the exciting element, and the speed of propagation of acoustic waves, calculate the depth of the defect.

The main geometrical relations used in the construction of the coordinate nomogram are shown in FIG. 2, The nomogram is calculated by the formula

(h + | 3) 2

l2 V2li / Vi + Vt A tp,

where Vi is the velocity of the surface acoustic wave;

Ј is the velocity of the acoustic wave in the material;

D tp - zone of zero beats in the time interval;

And - the distance between the exciting element 11 and the emitter 2;

2 - distance to the defect from the excitation element 11;

1h is the distance from the radiator 2 to the electrode 3;

h is the depth of the defect. The measurements are repeated at different acoustic exposure frequencies, which allows one to determine the depths of several defects at the same time in the usual cumulative measurement procedure.

Claims (1)

Invention Formula Method of defectometry of flat dielectric materials according to ed. No. 1550407, characterized in that, in order to expand the possibilities of the method by determining the depth of defects, the material is additionally affected by electromagnetic and acoustic waves at several frequencies, the superposition of electromagnetic waves is measured, those that have passed through the material and reflected dome measurements judge the depth of the defect from its surface, and according to the results of the combinations.