RU2403564C2 - Device for diagnosing limiting state and early warning on risk of breakage of materials and articles - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: device for diagnosing the limiting state and early warning on the risk of breakage of materials and articles has an acoustic-emission diagnosis system, wherein zones of the structure with the greatest load are fitted with sets of fragile strain indicators tuned to a given level of threshold deformation which is less or equal to the maximum allowable for safe use of the structure.

EFFECT: high degree of reliability of diagnosing the limiting state of materials and articles with subsequent warning on the risk of breakage of materials and articles.

2 cl, 8 dwg

Description

The invention relates to devices for technical diagnostics and non-destructive testing of materials and products and is intended for the diagnosis of their ultimate state and early warning of the danger of destruction.

 To solve the problem of diagnosing the limiting state of the structure, various means of technical diagnostics and non-destructive testing are used: local, integral and calculated. Local methods include visual, ultrasonic, magnetic, radiation, and eddy current monitoring [1]. These devices, as a rule, can provide point information about the stress-strain state of the structure at the place of their installation (strain gauges, ultrasonic transducers and radiation devices) or they can already record the fact of the beginning of the destruction of the structure (video surveillance system).

Acoustic-emission systems [2] are used as integral diagnostic systems for industrial facilities [2], which ensure the timely detection of developing damage, establish their nature and degree of danger, and determine the spatial coordinates of the destruction centers. To increase the probability of detecting defects and the reliability of determining their spatial coordinates, it is necessary to provide continuous measurements, i.e. to monitor the process of formation and development of cracks in highly loaded areas of the structure.

However, the use of traditional schemes of acoustic emission monitoring of the state of structures is not a sufficient measure that provides prevention and prevention of structural destruction. It is most advisable to combine various diagnostic methods in a monitoring system, since, firstly, it is necessary to take into account factors affecting structural damage and, secondly, the ability to detect defects by other control methods. The comprehensive use of diagnostic methods will allow, along with increasing the likelihood of detecting places of damage accumulation, measuring the stress state and movements of structural elements, controlling shrinkage, recording displacements and the growth of deformations in the supporting structural elements, assessing their current state and identifying areas most at risk of destruction.

The closest technical solution adopted for the prototype is an acoustic emission system [3], including acoustic emission transducers with built-in electric signal preamps, communication lines, analog-to-digital converter, digital block for recording acoustic emission signals (AE), system controller, cable interface communication, personal computer.

The registration of the parameters of the signals of the AE sources begins with their conversion into electrical ones by means of piezoelectric receivers. After conversion and amplification by preliminary amplifiers, the AE signals are fed to the corresponding inputs of the digital channels for recording acoustic emission. After passing the appropriate coordination and processing of parameters in the analog block (matching amplifier and low-pass filter) with the input parameters of the analog-to-digital converter (ADC), the AE signal is converted to digital form. The received data is already digitally received in a special event processing unit, where there is a continuous comparison of the instantaneous value of the incoming signal with the value of the discrimination threshold, which is set by the operator and stored in the same block. As soon as the current signal value exceeds the preset discrimination threshold, recording of the digitally generated acoustic emission signal into the buffer memory of the event processing unit begins. The received digital data is processed by a signal processor that calculates the parameters of the recorded hits (received and processed AE signals): maximum amplitude and pulse energy, number of pulses, signal energy value. After calculating the parameters of the registered pulse, the signal processor generates a block of information characterizing the event received in the channel, sets the data ready signal for the system controller and expects the controller to read these data. Upon completion of the reading of the information block by the system controller, the signal processor resumes the waiting and event processing cycle.

In this case, the acoustic emission diagnostic system operates as follows. AE receivers fixed at the monitoring object receive acoustic signals arising during the local dynamic restructuring of the structural material (transition to the area of plastic deformations, the formation of microcracks, their merging, the occurrence of macrocracks). Analyzing these processes, electronic equipment provides constant monitoring of the dynamics of their development. In cases of a critical increase in the rate of crack development and the occurrence of a structural failure risk, an early warning system about the danger of an accident automatically works.

Acoustic emission system adopted as a prototype has the following significant disadvantages:

1) active acoustic emission is mainly recorded at the time of a significant restructuring of the material structure: upon transition to a plastic state, upon the formation of cracks and their development, which in some cases is already unacceptable (for example, when operating structures made from brittle and quasi-brittle materials);

2) it is impossible to quantify the level of the largest principal deformations (stresses) in the diagnosed zones of the structure;

3) the process of monitoring the state of the structure, as a rule, is accompanied by external and internal (hardware) noise effects, the amplitude and energy of which in many cases exceeds the specified parameters of the signals from plastic deformation, the formation and growth of cracks. This makes it difficult to identify the recorded signals, i.e. separation of “useful” AE signals from noise signals.

The essence of the invention consists in the creation of a device for diagnosing the limit state and early warning of the danger of destruction of materials and products, the technical result of which is achieved by the fact that in the most loaded areas of the structure sets of fragile strain gauges are installed that are set to a threshold strain level that is less than or equal to the maximum permissible safe operation of the structure, and for remote monitoring of their condition (registration and location of the crack process formation) uses an acoustic emission system. To separate the “useful” signals (cracking in the brittle layer of strain gauges) from the interference signals, the recorded hits are filtered by the pulse attenuation rate.

The specified technical result during the implementation of the invention is achieved by the fact that the known device (diagnostic acoustic emission system) is additionally equipped with a set of fragile strain gauges installed in the most loaded areas of the structure, and to separate the “useful” signals from the interference signals, they are specially filtered by the pulse attenuation rate .

An additional introduction to the device of a set of fragile strain gauges tuned to different threshold deformations (ε ₀ ) _i , less than or equal to the maximum permissible for safe operation of the structure (for example: (ε ₀ ) ₁ <(ε ₀ ) ₂ <... <(ε ₀ ) _i ≤ [ε ₁ ] ≤ε _B ) allows you to reliably diagnose the level of the highest tensile deformations (stresses) in the controlled area of the structure and signal the danger of failure.

Introduction to the filtering device by the attenuation rate of the recorded hits makes it possible to separate the crack formation signals in the strain gauge from the interference signals.

The proposed device through remote stationary monitoring of the level of greatest deformations in the most loaded areas of the structure allows with a high degree of reliability to diagnose their load and in case of reaching the maximum permissible state to signal the danger of destruction of materials and products.

The technical and economic efficiency of the invention follows from the technical result obtained during the implementation of the invention: preventing the destruction of materials and products by remote monitoring the level of greatest strains in the most loaded areas of the structure.

The analysis of the prior art, including a search by patents and scientific and technical sources of information and identification of sources containing information about analogues of the proposed invention, made it possible to find that no analogue was found, characterized by features identical to all the essential features of the proposed invention, and the definition from the list of identified analogues the prototype, as the closest in the totality of the features of the analogue, allowed to identify the set of essential in relation to the claimed device effective signs set forth in the claims.

To carry out the conformity of the present invention to the requirements of the inventive step, an additional search was carried out for known solutions in order to identify features that match the distinctive features of the proposed invention, the result of which shows that the proposed invention does not explicitly follow from the prior art.

As an example of a demonstration of the inventive device, let us consider the experiments conducted when testing flat plates (samples) of high-strength aluminum alloy B95 on an electro-hydraulic testing installation MTS.

To obtain brittle strain gauges on the surface of the samples, A7 aluminum foil 100 μm thick was used, which was electrochemically anodized in a 20% aqueous solution of sulfuric acid with a process duration of 30 to 60 minutes, an electrolyte temperature of 5–20 ° С and a current density of 4–6 A / dm ² . In order to increase the purity of the experiments, aluminum foil was first glued to the samples, and then it was anodized. This method of producing strain gauges made it possible to maximize their sensitivity to extremely low values ε ₀ = 400 ÷ 600 μm / m, at which spontaneous cracking is not observed in a brittle oxide film, and acoustic emission transducers can be installed and mounted on the surface of strain gauges.

The brittle oxide strain gauge [4, 5] is a thin aluminum foil subjected to electrochemical anodization to obtain an oxide film (15–40 μm thick) and glued to the structural element under study (see FIG. 1). When deformations ε ₁ exceeding the threshold strain ε ₀ occur in the aluminum foil substrate, crack patterns form in the oxide film of the strain gauges (see Fig. 2), which reflect the force field of the largest principal deformations on the surface of the structure. Using the characteristics of the strain sensitivity of the strain gauges (ε ₀ , σ ₀ ) and the graph of the change in the number of cracks in the oxide film (Ψ) from the level of deformations in the substrate (see Fig. 3), it is possible to estimate the values of the largest principal deformations (stresses) on the surface of the structure under study in the area of crack propagation in a brittle strain gauge.

Oxide strain gauges can be used to conduct research in the field of elastic and elastoplastic deformations (from 400 to 4000 μm / m) at temperatures from -200 ° C to + 200 ° C in various environments (water, oil, liquid nitrogen, etc.), They have a fairly simple manufacturing technology and for a long time (at least 10 years) keep their characteristics stable.

In the experiments under consideration, R15I integrated piezoelectric resonance detectors were used to detect AE pulses. The operating range and resonant frequency of the receivers were 70–200 kHz and 150 kHz, respectively. The electrical signals received at the output of the AE converter were amplified, recorded and further processed and interpreted using the DiSP-2 acoustic emission system manufactured by RAS (Physical Acoustic Corporation). The system and processing of recorded signals were controlled using the AEwin software package [6].

The test sample was installed in the grips of the upper and lower yoke of the MTS electro-hydraulic test setup, after which a piezo-electric receiver R15I was attached to the surface of the strain gauge. The test mode of the sample during its step loading is shown in the table.

Table Registration time load 136 220 240 320 350 450 490 550 600 from Level load 6 10 12 fourteen fifteen 16 17 21 24 kN Oxide Strain Indicator Status No cracks were observed First cracks Active fissure imaging Cracking peaked

After the sample was completely unloaded, it was re-loaded, during which the load gradually increased over 420 seconds to the maximum permissible level F = 25 kN. In this case, a high intensity of AE signals was observed only when the load increased to 23-24 kN, after which the emission sharply decreased and actually stopped at F = 25 kN. The crack pattern recorded in the strain gauge after reloading is shown in Fig.4.

Figure 5 shows the AE signals recorded by the R15I receiver during the first loading of the sample with a stepwise increase in load to 24 kN. As follows from the histogram of signal activity, shown in Fig.6, a, in stage III, when the load increased to 23.5 kN, the intensity of the cracking process reached its maximum. A further increase in the load to F _max = 24 kN caused a noticeable decrease in the emission of recorded signals. Figure 6, b shows a histogram of AE activity recorded during re-loading of the sample to F _max = 25 kN. From the drawing it can be seen that active crack formation in the strain gauge begins only when the load increases to 22 kN and reaches its maximum at F = 23-24 kN. A further increase in the load practically did not cause the formation of new cracks: the AE signal registration process slowed down at first, and stopped at F = 25 kN, which actually testifies to the complete destruction of the oxide film of the strain gauge (see Fig. 6, b).

In Fig. 7, for the hits highlighted in Fig. 5, typical forms of recorded signals are shown: the interference signal is hit No. 1 and the crack signals in the oxide film of the strain gauge are hit No. 2. Analysis of AE signals recorded during testing of samples at the MTS stand allows us to note the following

1. In the course of the research, both “useful” acoustic signals associated with the formation of cracks in the oxide film of strain gauges and signals from mechanical noise, noise of the electric motor and pump of the test bench were recorded.

2. The AE transducer made it possible to register crack formation signals in a strain gauge, starting from the moment of crack initiation in the oxide film until its complete destruction. Moreover, the number of recorded signals, as a rule, was a large number of cracks formed. This is due to the fact that the crack propagation did not always occur in a single slip.

3. The signal amplitudes caused by the formation of cracks in the oxide film were in the range from 30 dB (the lower threshold value for registering cracks) to 90-100 dB. The signals were generated either by the nucleation of the crack, or by its propagation along the width of the strain gauge, and the level of their amplitude was determined by the amount of energy released in this case.

4. The waveforms caused by the formation of cracks in the oxide film of the strain gauge and the noise arising from the testing of samples differ markedly from each other. This is clearly seen when comparing the graphs in Fig. 8, which shows the waveforms allocated for the strongest (right) and weak (left) pulses of the considered sources.

5. As can be seen from the graphs, for the compared amplitudes, the duration of the attenuation of the interference signal is 3-4 times longer than the length of the signal attenuation from the formation of a crack in the strain gauge. Therefore, to identify the signal source, a parameter can be used that characterizes the pulse attenuation rate, in particular, a coefficient (K) equal to the ratio of the signal amplitude (A) to the number of oscillations (C) that exceed the threshold level.

6. The calculation of the values of the coefficient of attenuation of the pulse for the signals recorded during the tests showed that the value of this coefficient for the interference signals was in the range 0.08–0.4, and for the signals of the crack of the oxide film of the strain gauge, the level of the coefficient K was significantly higher and varied from unity for low values of amplitudes (A = 30-50 dB) up to 8 ÷ 10 units for high values of A≥90 dB.

According to the test results it is possible to draw the following conclusions

1. The use of AE transducers with a system for monitoring and processing acoustic signals makes it possible to register pulses of cracking in a brittle oxide film, from the moment of their nucleation to the complete destruction of the strain gauge.

2. Using a set of fragile strain gauges adjusted to a predetermined level of threshold deformation (in the range of values ε ₀ = 400-4000 μm / m), together with a system for recording and processing acoustic signals, an effective non-destructive testing device can be obtained for remote monitoring of the level of major deformations (stresses) in the studied structural elements and notify in the current time mode of reaching the limit state (for example, according to the strength conditions).

3. Signals related to the various observed classes: cracks in the strain gauge and interference arising during loading are quite well separated by the pulse decay rate. This makes it possible to use the coefficient K = A / C≥1 as a filter to store only “useful” information — impulses of cracking in the oxide film of the strain gauge.

Literature

1. Non-destructive testing and diagnostics: Handbook / V.V. Klyuyev, F.R.Sosnin, A.V. Kovalev and others; Ed. V.V. Klyueva. 2nd ed., Rev. and add. - M.: Mechanical Engineering, 2003, 656 p.

2. V.I. Ivanov, I.E. Vlasov. Acoustic emission method. Non-Destructive Testing: Directory; In 8 t. Under the total. ed. V.V. Klyueva. T.7. Book 1. M .: Engineering. 2005, 340 p.

3. A. N. Seryoznov, V. V. Muraviev, L. N. Stepanova and others. High-speed diagnostic acoustic-emission system. Defectoscopy, 1998, No. 7, 9-14 p.

4. Prigorovsky N.I. Methods and means of determining the fields of strains and stresses. Directory. M .: Engineering, 1983, 248 p.

5. Prigorovsky N.I., Panskih V.K. The method of fragile strain-sensitive coatings. M .: Nauka, 1978, 183 p.

6. Physical Acoustics Corporation. Aewin User Guide, Version 1.90, Princeton, New Jersey, USA, 2002, 150 pp.

Claims (2)

1. A device for diagnosing the limit state and early warning of the danger of destruction of materials and products, including an acoustic emission diagnostics system, characterized in that in the most stressed areas of the structure sets of fragile strain gauges are installed that are set to a predetermined level of threshold deformation less than or equal to the maximum permissible for safe operation of the structure. 2. The device according to claim 1, characterized in that to separate the signals of cracking in the brittle layer of the strain gauges from the interference signals, the registered, received and processed acoustic emission signals are filtered by the pulse attenuation rate.

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