SU1714495A1 - Method of testing products for crack formation - Google Patents

Abstract

The invention relates to non-destructive quality control of materials and can be used in mechanical engineering, transportation, etc. The purpose of the invention is to improve the accuracy of control by bringing the test conditions to the conditions of operation. The product is loaded with vibration, and acoustic emission signals are received through the air gap. Additionally, the product is loaded with a gas stream with variable speed and temperature. The cracking in the product is judged by the time interval from the onset of vibration to the appearance of acoustic emission signals. 2 Il.

Description

The invention relates to non-destructive quality control of materials and products with the use of acoustic emission (AE) and can be used in various fields of engineering, energy and transport.

The purpose of the invention is to improve the accuracy of control due to the approach of the load of the product to the conditions of operation.

Figures 1 and 2 are diagrams of a device for implementing the method.

The diagrams show the vibrating table 1, product 2, air gap 3, piezotransducer 4 connected to the electric circuit of the amplifier 5, filter 6, comparator 7, intensity meter 8 and recorder 9. The delay element 10 is connected between the start block of the vibrospend 1 and the first input of element 11 the match is connected by the second input and output, respectively, between the output of the comparator 7 and the input of the intensity meter 8.

The method is carried out as follows.

The resonant frequency of the product and the frequency of the AE signals from the product when it is loaded are predetermined. Then, a resonant piezoelectric transducer 4 is disposed at a distance of 5-20 mm from the product 2 and the product 2 is loaded with vibration at a frequency different from the frequency of the AE. It is advisable to vibrate the product at the frequency of the mechanical resistivity of the product 2. The amplitude of the vibrations is established by a smaller distance between the piezoelectric transducer 4 and the product 2v. A time interval from the beginning of the vibration to the moment of occurrence of AE signals is detected by cracking in the product. This time can be measured, for example, by the recorder tape used as a recorder, on which the signal intensity is recorded, while the vibrator and the recorder are turned on simultaneously.

The choice of the distance between the piezoelectric transducer 4 and the product 2 is determined by the sensitivity of the piezoelectric transducer 4, the attenuation of the signal in the gap medium and the loading effect of a sufficient degree of vibration.

For long products modeled, in particular, by a one-sidedly fixed rectangular plate, the piezoelectric transducer 4 is placed over that part of the larger surface, where its displacement relative to other parts should take the minimum value, i.e. near the fastening of the plate or nodes of the antinodes of its oscillations. If there is a germinal defect in product 2, for which the motion parameters as the vibration leads to a stress-strain state of the local region of the material above the critical, then this defect (fatigue crack) begins to develop, simultaneously generating in the LE environment. In airspace, discrete AE pulses are distributed energetically according to the radiation pattern of the product 2 as an acoustically transmitting antenna and are filtered out from the shear component of the original elastic wave in the solid. As a result, piezotransducer 4 receives these pulses with minimal attenuation (approximately 15-17 dB at frequencies of 100-300 kHz for an air gap of mm length) and with the smallest amount of interference from product movement 2. The received AE and acoustic noise of the low frequency range are converted into electrical ones, amplified by amplifier 5, and the pulse part is passed by filter 6 in the form of radio pulses to comparator 7 with a trigger threshold. The cut-off frequency of the filter (CVF) is chosen at a slightly lower resonance frequency of the piezoelectric transducer (generator 4 (frequency of reception of AE signals), which ensures deep interference suppression due to the mismatch of their frequency properties with similar properties of AE signals from developing defects. Comparator 7 converts each radio pulse into a series of video signals , measured by the number per unit of time with an intensity meter 8, the length of which also depends on the threshold value of the comparator 7 and the AChAE path.

A higher quality product is considered to be one for which, with vibration, the measured time interval is either longer or has no end at all. In this case, the prediction of the destruction of the material of the product is assumed by the fatigue mechanism. In another case, if the specified interval is less than the statistical one for quality products and is 10-25% of it, the occurrence and development of the crack is due to a greater degree of technological reasons: a violation of the ratio of the material formulation, the mode of plastic deformation, heat treatment, etc. ,,:. .

Thus, the method very effectively and widely solves the problem of controlling cracking in products made of materials, mainly, high specific strength (alloyed steels, magnesium and titanium alloys, structural plastics, etc.) moving in air.

The limitation of the method to these materials results mainly from the fact that their crack formation is usually accompanied by large amplitudes of AE impulses than in the case of any other materials. This allows the method to be used without a fundamental increase in the sensitivity of the AE apparatus, which is approximately 1-2 µV for the band

200 kHz frequency.

However, for products to which no loads were applied before the vibration, the reception of AE signals begins with a slight delay relative to the starting moment of the vibrating stand 1, for example, for a time equal to 2-3 periods of the vibration frequency. By which, due to the Kaiser effect, information that is not directly related to the first discontinuous growth (formation) of a crack is excluded. In this regard, the circuit of FIG. 1 can be supplemented with an electrical delay line 10 and an electrical signal transmitter made, for example, in the form of

element 11 of the joint (figure 2).

The method also guarantees the long-term and reliable operation of the piezoelectric transducer, since it excludes the direct impact of the forces of friction and inertia from the movement of the product under test. At the same time, the design of the piezo-sensor can be simplified, since there is no need for a protector, it is possible to use a less durable case, etc.

To bring the test to the operating conditions for some products such as vehicles, it is necessary to use additional loading of the product by treating the gas flow with variable speed and temperature. If the product has an internal cavity, then it is advisable in this case to place the piezoelectric transducer from the internal cavity, thereby eliminating

thermal and acoustic noise masking AE signals from fatigue cracks.

Formulas of the method of controlling cracking in the product, namely, in the process of loading the product, acoustic emission signals are received, which are judged by declining, which, in order to improve accuracy, acoustic emission signals are carried out at a distance of 5–20 mm from the product, loading is carried out by vibration at a frequency different from the frequency of acoustic emission signals, and cracking is judged by the time interval from the start of vibration to the appearance of acoustic signals th emission.

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FIG, g

Claims (1)

Claim The method of controlling crack formation in the product, which consists in the fact that during the loading of the product, acoustic emission signals are received, which are used to judge crack formation, which is based on the fact that, in order to improve accuracy, acoustic emission signals are received at a distance of 5-20 mm from the product, loading is carried out by 5 vibrations with a frequency different from the frequency of acoustic emission signals, and crack formation is judged by the time interval from the start of vibration to the appearance of acoustic emission signals.