RU2712773C1 - Optical fiber acoustic emission method with multilayer coating of optical fiber with substances with different brittleness - Google Patents

Abstract

FIELD: defectoscopy.SUBSTANCE: invention relates to prediction at all stages of occurrence and development of defects in large engineering structures. Optical fiber acoustic-emission method comprises applying brittle strain-sensitive material on multimode optical fiber, material hardening and determination of dangerous plastic deformations zone as per formation of crack material. Multilayer adhesive coating with different brittleness of each layer after hardening is applied onto optical fiber at its fixation on engineering structure. When observing and classifying defects using acoustic emission signals, their impact spectra are used, which are extracted from the obtained signal by frequency filters.EFFECT: high sensitivity when recording plastic deformation of parts of engineering structures at all stages of formation and development of defects.1 cl, 3 dwg

Description

The invention relates to predicting at all stages of the occurrence and development of defects using fiber-optic methods for recording an acoustic emission signal (AE), and can be used to identify the most likely zones of destruction of metal or concrete structures of engineering structures, for example gas pipelines, bridges, hydroelectric dams and other large and extended objects.

Known acoustic emission method of zone control, including the installation of local acoustic emission transducers (PAE), usually piezoelectric, on previously cleaned contact surfaces. (Guiding document RD 03 131-97. Acoustic emission control method. - S. 8-11, http://snipov.net/c_4653_snip_99823.html). The accumulation, recording and operational processing of AE control data is carried out using special software included in the acoustic emission systems. This method is complex and expensive, has a low sensitivity, requires the use of a large number of sensors, connecting wires, and other various equipment.

Also known is a fiber-optic acoustic emission method in which an optical fiber (S) is used as a distributed sensing element (RF patent No. 2650799). This method is much simpler and cheaper than the zone method, since the OB performs two functions at once: a vibration sensor, and an information signal transmission line. But this method can only control the initial stage of the destruction of the object. The sensor has high sensitivity, but it is disposable, because after complete cracking of a brittle coating during deformation of the object, or from external vibrations, the AE disappears, and the sensitivity of the RFE decreases sharply.

The technical result of the proposed method is to increase the sensitivity when registering plastic deformation of parts of engineering structures at all stages of the formation and development of defects, starting from a small plastic deformation, and ending with a breakdown of organic matter, and the destruction of the controlled structure. This result is achieved by applying a multilayer coating with different brittleness deposited on the OM. At the same time, the frequency spectrum of the recorded AE signals is expanding significantly. All coating layers have different fragility, and therefore generate signals of different frequencies. The method allows both early prediction of the formation of defects and the further process of their development. The use of a multilayer coating with various brittleness helps to increase the service life of the fiber-optic distributed sensor, and more reliable prevention of accidents and technological disasters.

SUMMARY OF THE INVENTION: FIG. 1 shows a fiber optic sensor with a brittle multilayer coating, in section. Using the first extruder, a first layer with a hardness of HSxl (for example, according to Shore) and a resonant frequency of the AE signal upon cracking f _{1 is} applied to the OM. Next, using a second extruder, a second layer is applied with a lower hardness (greater ductility) HSx2 and a lower resonant frequency f ₂ . Then applied the 3rd layer (HSx3, f ₃ ), the fourth (HSx4, f ₄ ), etc. The farther from the OM, the lower the hardness (brittleness) of the coating:

HSxl> HSx2> HSx3> HSx4,

f ₁ > f ₂ > f ₃ > f ₄

The hardness of the coating (for example, epoxy) depends on the percentage of plasticizer and hardener in it. The frequency of the attenuating AE wave f (Fig. 2) is proportional to the fragility of the coating (Shore - HSx). With increasing hardness (brittleness) of the coating substance (HSx), the frequency of damped oscillations of a mechanical wave (sound) after the occurrence of an impact, when another crack is formed, increases

f = 1 / T,

(where T is the period of oscillation). Each value of hardness (brittleness), with the same dimensions of the mechanical system, has its own resonant frequency of damped oscillations.

https://zetlab.com/podderzhka/vibratsionnyie-ispyitaniya/teoriya-vibroispyitannniy/udarnyiy-spektr-i-dobrotnost-kolebatelnoy-sistemyi/

Thus, with an increase in the plastic deformation of the controlled object, first cracks appear in the first layer with great brittleness (HSxl), then in the second layer (HSx2), then in the third (HSx3), etc. Each of the layers of brittle coating generates a sound wave of natural frequency: f _1, > f _2, > f _3, > f ₄ . With an increase in the plastic deformation of an object (engineering structure), the probability of its destruction (P) increases, which is proportional to the number of AE pulses (n) during their accumulation, and inversely proportional to the carrier frequency of the AE signal (f).

P = n / f

Where: P is the probability of destruction of the engineering structure, n is the number of AE pulses (cracks) during the accumulation time, f is the carrier frequency of the AE of the damped signal

f = kHSx

Where: k is a constant coefficient, HSx is the brittleness (hardness) of the coating.

In FIG. 3 is a simplified block diagram of a device explaining a method of fiber-optic acoustic emission control of plastic deformation of an object using a fiber optic sensor with a multilayer brittle coating. The circuit contains power supplies (17БП), a unit for generating an optical sounding signal — an optical emitter (18ОИ), and a device for continuous directional input of an optical signal (19УВ) into a distributed sensitive element (20РЭЭ). An LED is used as an optical emitter (for small RFE lengths), and for RFE lengths of more than 30 meters, a semiconductor laser is used. RFE is fixed on the object (engineering structure), for which, during installation, a multilayer brittle coating is polymerised material is used successively using extruders. This multilayer coating ensures its fixation and mechanical contact with the controlled object. After hardening, the multilayer coating becomes brittle, and generates an acoustic emission signal, cracking from mechanical stress, upon deformation of the controlled object. Moreover, with an increase in plastic deformation, cracking occurs first in the first layer with maximum fragility, and ends in the last layer (in Fig. 1, this is the 4th layer), with minimal fragility. Thus, the controlled object can remain under observation from the beginning of the formation of the defect, until the complete destruction of the object. An acoustic emission signal originates in the immediate vicinity of the optical fiber, and acts on it, changing the mode field propagated through the fiber. AE signal almost without attenuation reaches the RFE, because brittle material is in close proximity to the surface of an optical fiber. Changes in the mode field are recorded at the output end of the fiber (by a change in the static speckle structure). After the spatial filter (22PF), the light signals are converted by the optical radiation receiving unit (23BPr) into electrical signals, which are fed to the low-pass filter unit (24BFCH), and are separated by the frequency spectrum (in Fig. 3, four output signals). After the processing unit (25BO), the signals are sent to a display (26D), which gives information about the state of the controlled object (21O), and recommendations for repairing the object or protecting it from further destruction. A multilayer brittle coating on the OM creates a shock spectrum of the AE signal, which is an excellent indicator of the reliability of an engineering structure.

Conclusion: The more cracks (n), and the lower the resonant frequency (f) of damped mechanical vibrations (AE signals), the higher the probability of destruction of the engineering structure in this zone. With an increase in the AE amplitude and a decrease in the duration of signals (damped oscillations), the probability of destruction also increases.

The inventive method has passed numerous tests using a fiber optic system "SOVA".

(http://www.altsvet.ru/content/files/tso_SOVA.pdf).

This system was supplemented by low-pass filters (24BNCHF), separating the signals according to the frequency of the AE wave from different layers. The SOVA system allows you to control the plastic deformation of extended objects (up to 1 km) along the entire length of the optical fiber. The experimental results showed not only the high sensitivity of the proposed method, but also the ability to work throughout the entire period of occurrence and development of the defect, when determining the plastic deformation of large objects. The experiments showed the prostate of the implementation of the method, manufacturability, and economic efficiency, compared with the zone methods of acoustic emission.

The following are some results of one of the experiments.

1. The “accumulation time” of pulses exceeding the set “sensitivity threshold” did not change: 5 s.

2. The mechanical load on the object (N, kg) increased according to a linear law.

3. "Number of pulses" of different carrier sound frequency: N (f1),

N (f2), N (f3), N (f4), for the "accumulation time": pcs.

4. After recording the response, a pause of 200 ms was formed (read ban) to combat the oscillatory process in the optical cable.

Figure captions with explanations

FIG. 1 Sectional optical fiber with a multilayer coating. Graphs of the hardness of the coating material (HSx) versus the distance to the surface of the RFE (d). Frequency response graph of the amplitude of the signal (A) versus frequency (f)

1, 2, 3, 4 - Layers of brittle coating having different hardness

5 - Sectional optical fiber with a multilayer coating

6 - Fragility of the layers (hardness - HSx)

7 - The distance from the surface of the RFE to the next layer (d)

8 - Amplitude of AE waves (AFC) generated by the 1st, 2nd, 3rd, 4th layers

9 - frequency

10, 11, 12, 13 — Shock spectra and resonance frequencies (fl, f2, f3, f4) of AE waves generated by the 1st, 2nd, 3rd, 4th layers.

FIG. 2 Damped oscillations in a solid after impact

14 - The amplitude of the damped oscillations of the AE wave (A)

15 - Time (t)

16 - Oscillation period (T)

FIG. 3 Simplified block diagram of the device for fiber-optic acoustic emission control of plastic deformation of an object using a fiber optic sensor with a multilayer brittle coating.

17 - power supply; 18 - optical emitter; 19 - input device; 20 - distributed sensing element; 21 - controlled object; 22 - spatial filter; 23 - reception unit; 24 - block low-pass filters; 25 - processing unit; 26 - display; 27 - the most probable zones of occurrence of AE; 28 - mechanical load.

Claims (1)

Acoustic-emission fiber-optic method for controlling plastic deformations at all stages of the formation and development of defects in large engineering structures, including applying a brittle strain-sensitive material to a multimode optical fiber, hardening the material, determining the zone of dangerous plastic deformations formed in it, characterized in that on the optical fiber when it is fixed, a multilayer adhesive coating is applied to the engineering structure with different fragility of each layer after hardening Evan, while detecting and classifying defects on the acoustic emission signals using them shock spectra allocated from the received signal frequency filters.

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