A Test System for the Frequency Characteristics of Acoustic Emission Signals in Concrete Structures and a Sensor Selection Method

Abstract

The present invention discloses a test system for the frequency characteristics of acoustic emission signals of a concrete structure and a method for selecting sensors, which relates to the technical field of concrete acoustic emission detection. The system includes an acoustic emission simulation source, N acoustic emission sensors disposed on the first surface of a concrete beam, and an acoustic emission signal acquisition unit. Based on the set system structure, the acoustic emission signal acquisition unit can analyze the simulated acoustic emission signals generated by the acoustic emission simulation source collected by the N acoustic emission sensors to determine the acoustic emission sensors with appropriate operating frequencies, so as to accurately collect the real damage signals in the concrete structure. The system structure is simple, and the sensor selection method implemented based on the system structure is highly efficient, and the selected sensors are more accurate.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete acoustic emission detection, and particularly to a test system for the frequency characteristics of acoustic emission signals of a concrete structure and a sensor selection method. Background Art

[0002] An acoustic emission source (defect and damage) will emit a stress pulse wave, i.e., an acoustic emission signal, under the induction of an external force, which is a mechanical vibration propagating in the material where the acoustic emission source is located. The so-called acoustic emission detection is to detect and receive the above acoustic emission signal and analyze it to obtain information about the acoustic emission source. Acoustic emission technology (AE) is a dynamic and overall non-destructive detection technology, which can evaluate and give early warning of the overall safety of a structure. Acoustic emission technology has been applied to the detection / monitoring of concrete structures of hydraulic buildings such as dams, sluices, pumping stations, dikes, bridges, culverts, etc.

[0003] Since the frequency spectrum of the acoustic emission signal can cover a frequency range from several Hz to dozens of MHz, therefore, when performing acoustic emission detection on a specific structure, it is first necessary to know the approximate frequency range of the acoustic emission signal generated by the defect to be detected under the action of an external force, and then select a most suitable frequency window from this large range to filter out the interference of noise and obtain the true original acoustic emission signal. For a concrete structure, which is an anisotropic and non-uniform composite material with a large number of natural microscopic pores and crack structures, the acoustic wave propagation characteristics are much more complex than those in an isotropic metal structure. The characteristics of the acoustic emission signal emitted by concrete damage are closely related to concrete types (strength, coarse aggregate particle size, mix ratio, sand content, etc.), damage types, and body structures, etc.

[0004] An important problem faced by AE signal processing technology is the diversity of AE sources, the suddenness and uncertainty of the signal itself. It can even be said that sometimes we hardly know what the true AE signal looks like. The acoustic signal received by the sensor is closely related to the transmission path, and this transmission path is related to many factors such as the position of the AE source, the nature of the object to be detected (material texture, material shape and geometric dimensions), the characteristics of the acoustic coupling agent, and the position of the receiving sensor. The sensor output signal is also closely related to the sensor type. The relationship between the sensor output signal and the acoustic emission source signal can be illustrated by Figure 1 as follows.

[0005] The signal obtained by the AE sensor is at least the comprehensive result of factors such as the sound source, transmission medium, coupling medium, and sensor response, and can be expressed by the mathematical formula as:

[0006] F(t) = S(t) * M(t) * C(t) * R(t) (1)

[0007] Where, S(t), M(t), C(t), and R(t) are the impulse response functions of the sound source, transmission medium, coupling medium, and sensor respectively, and the symbol * represents convolution. In the frequency domain, the above equation can be expressed as:

[0008] F(ω) = S(ω)·M(ω)*C(ω)*R(ω) (2)

[0009] Where, S(ω), M(ω), C(ω), and R(ω) are the frequency response functions corresponding to the above functions. The complexity of M(t), the uncertainty of C(t), and the differences in the sensor response R(t) result in a large difference between the sensor output signal F(t) and S(t). For an ideal and relatively certain same coupling state C(t), for the same sound source signal, the sensor response R(t) directly affects the output signal F(t).

[0010] The acoustic emission sensor should truly detect all information of the acoustic emission source. That is to say, when the sensor converts the detected mechanical vibration signal into an electrical signal, it should minimize distortion as much as possible. In order to truly, accurately, and objectively characterize the acoustic emission signal in concrete, it is necessary to test the frequency characteristics of the acoustic emission signal in the concrete structure and select an acoustic emission sensor with a suitable operating frequency. An unreasonable selection of the acoustic emission sensor will result in a large difference between the received signal and the expected acoustic emission signal, and the difference between the received acoustic emission signal and the actual acoustic emission signal directly affects the authenticity of the collected data and the data processing results. However, there is currently no clear method for quickly selecting a suitable acoustic emission sensor to detect concrete structures. Summary of the Invention

[0011] The purpose of the present invention is to provide a test system for the frequency characteristics of acoustic emission signals in concrete structures and a sensor selection method. The system structure is simple, and based on the system structure, the required sensor can be quickly selected to accurately collect the true damage signals in the concrete structure.

[0012] To achieve the above purpose, the present invention provides the following solutions:

[0013] The present application provides a test system for the frequency characteristics of acoustic emission signals in concrete structures, and the system includes: an acoustic emission simulation source, N acoustic emission sensors arranged on the first surface of the concrete beam, and an acoustic emission signal acquisition unit;

[0014] The acoustic emission simulation source is used to generate simulated acoustic emission signals at different positions on the first surface and the second surface of the concrete beam;

[0015] N of the acoustic emission sensors are all connected to the acoustic emission signal acquisition unit, and are used to collect the analog acoustic emission signals generated at different positions of the concrete, and transmit the analog acoustic emission signals to the acoustic emission signal acquisition unit;

[0016] The acoustic emission signal acquisition unit is used to determine the acoustic emission sensors with appropriate working frequencies according to the analog acoustic emission signals of the N acoustic emission sensors; the acoustic emission sensors with appropriate working frequencies are used to collect the real damage signals in the concrete.

[0017] Optionally, the N acoustic emission sensors are within a preset range on the first surface of the concrete beam; the area of the region within the preset range is less than a first preset value; the distance between adjacent two of the acoustic emission sensors is less than a second preset value.

[0018] Optionally, the distances from different signal generation position points on the first surface of the concrete beam to the preset range increase in sequence; the distances from different signal generation position points on the second surface of the concrete beam to the preset range increase in sequence.

[0019] Optionally, a preamplifier is also connected between each of the acoustic emission sensors and the acoustic emission signal acquisition unit.

[0020] Optionally, the N acoustic emission sensors include at least 1 broadband sensor and N - 1 resonant sensors with different working frequency ranges; the working frequency range of the broadband sensor is greater than the 20 - 250 kHz frequency band range.

[0021] Optionally, the working frequency ranges of the N - 1 resonant sensors are in sequence: f1~f1 + Δf, f1 + Δf~f1 + 2Δf, f1 + 2Δf~f1 + 3Δf,..., f1 + (N - 2)Δf~f2; where, f1~f2 constitutes the working frequency range of the broadband sensor; Δf = Δ / (N - 1) = (f2 - f1) / (N - 1); Δ = f2 - f1.

[0022] Optionally, the acoustic emission analog source is a broken lead analog source.

[0023] The present application provides a method for selecting sensors for a test system based on the frequency characteristics of acoustic emission signals of a concrete structure, and the method includes:

[0024] Obtain the analog acoustic emission signals generated at each position received by N acoustic emission sensors; the analog acoustic emission signals are generated by an acoustic emission analog source sequentially at signal generation positions according to a preset order; the N acoustic emission sensors include at least 1 broadband sensor and N - 1 resonant sensors with different operating frequency ranges; the operating frequency range of the broadband sensor is greater than the 20 - 250 kHz frequency band range;

[0025] Analyze the waveforms of the analog acoustic emission signals at each position received by each acoustic emission sensor;

[0026] Extract the ring count and duration of each analog acoustic emission signal waveform to obtain the average frequency value of each analog acoustic emission signal;

[0027] For each acoustic emission sensor, draw a two - dimensional scatter plot; the abscissa of the two - dimensional scatter plot is the distance between the signal generation position and the acoustic emission sensor, and the ordinate is the corresponding average frequency value;

[0028] Draw an average frequency histogram of the acoustic emission signals received by each acoustic emission sensor according to the two - dimensional scatter plot of each acoustic emission sensor; draw a kernel density curve of the average frequency of the acoustic emission signals received by each acoustic emission sensor according to the average frequency histogram; the abscissa of the average frequency histogram is the average frequency value, and the ordinate is the count; the abscissa of the kernel density curve is the average frequency value, and the ordinate is the density;

[0029] Determine two cut - off average frequency values corresponding to 1 / 2 of the curve peak in the kernel density curve of the broadband sensor;

[0030] Select acoustic emission sensors with appropriate operating frequencies from the N - 1 resonant sensors according to the frequency range formed by the two cut - off average frequency values and the distribution trend of the kernel density curve; the acoustic emission sensors with appropriate operating frequencies are used to collect real damage signals in concrete.

[0031] Optionally, selecting acoustic emission sensors with appropriate operating frequencies from the N - 1 resonant sensors according to the frequency range formed by the two cut - off average frequency values and the distribution trend of the kernel density curve specifically includes:

[0032] Select the resonant sensors from the N - 1 resonant sensors whose operating frequency range has a difference less than a preset difference value from the frequency range formed by the two cut - off average frequency values, and the distribution trend similarity of the kernel density curve to the kernel density curve of the broadband sensor is greater than a preset similarity value as the acoustic emission sensors with appropriate operating frequencies.

[0033] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0034] The present invention provides a test system for the frequency characteristics of acoustic emission signals of a concrete structure and a method for selecting sensors. The system includes an acoustic emission simulation source, N acoustic emission sensors disposed on the first surface of a concrete beam, and an acoustic emission signal acquisition unit. Based on the set system structure, the acoustic emission signal acquisition unit can analyze the analog acoustic emission signals generated by the acoustic emission simulation source collected by the N acoustic emission sensors to determine the acoustic emission sensors with appropriate operating frequencies, so as to accurately collect the real damage signals in the concrete structure. The system structure is simple, and the sensor selection method implemented based on the system structure is highly efficient, and the selected sensors are more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0036] Figure 1 Schematic diagram of various factors affecting acoustic emission signals provided by the present invention;

[0037] Figure 2 Schematic diagram of a test system for the frequency characteristics of acoustic emission signals of a concrete structure provided in Embodiment 1 of the present invention;

[0038] Figure 3 Flowchart of a sensor selection method based on a test system for the frequency characteristics of acoustic emission signals of a concrete structure provided in Embodiment 2 of the present invention;

[0039] Figure 4 Schematic diagram of acoustic emission characteristic parameters provided in Embodiment 2 of the present invention;

[0040] Figure 5 Scatter diagram of the AF value of the acoustic emission signal collected by a certain type of broadband sensor and the propagation distance Distance provided in Embodiment 2 of the present invention;

[0041] Figure 6 Histogram of the AF value distribution of the broken lead signal collected by a certain type of broadband sensor provided in Embodiment 2 of the present invention;

[0042] Figure 7 Schematic diagram of the -6dB cut-off frequency of the kernel density curves of the AF value distributions of the acoustic emission signals received by 4 types of acoustic emission sensors provided in Embodiment 2 of the present invention and the kernel density curve of the AF value distribution of the acoustic emission signal received by the broadband sensor;

[0043] Figure 8The kernel density curves of the AF values of the broken lead signals collected by three different types of sensors provided in Embodiment 2 of the present invention.

[0044] Symbol description:

[0045] 1 - Concrete beam; 2 - Broken lead simulation source; 3-1, 3-2, 3-3,..., 3-N - Preamplifiers; 4-1, 4-2, 4-3,..., 4-N - Acoustic emission sensors; 5 - Acoustic emission signal acquisition unit, 6 - Signal cable. Specific implementation manners

[0046] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] The purpose of the present invention is to provide a test system for the frequency characteristics of acoustic emission signals of concrete structures and a sensor selection method. The system structure is simple, and the required sensors can be quickly selected based on the system structure.

[0048] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0049] Embodiment 1

[0050] As Figure 2 shown, this embodiment provides a test system for the frequency characteristics of acoustic emission signals of a concrete structure. The system includes: a concrete beam 1, an acoustic emission simulation source, N preamplifiers 3-1 to 3-N, N acoustic emission sensors 4-1 to 4-N provided on the first surface of the concrete beam 1, and an acoustic emission signal acquisition unit 5;

[0051] Among them, the number of sensors is N, and 3 ≤ N ≤ 10. The value of N should not be too large, otherwise the workload required for the test will be too large.

[0052] The acoustic emission simulation source is used to generate simulated acoustic emission signals at different positions on the first surface and the second surface of the concrete beam 1. Among them, the acoustic emission simulation source is a broken lead simulation source 2. The broken lead simulation source 2 uses a 0.5mm HB lead core to break (PLB).

[0053] The N acoustic emission sensors are all connected to the acoustic emission signal acquisition unit 5, and are used to collect the simulated acoustic emission signals generated at different positions of the concrete and transmit the simulated acoustic emission signals to the acoustic emission signal acquisition unit 5.

[0054] Among them, the operating frequency range of the broadband sensor is greater than the 20 - 250 kHz frequency band range. The operating frequency ranges of N - 1 of the resonant sensors are successively: f1 to f1 + Δf, f1 + Δf to f1 + 2Δf, f1 + 2Δf to f1 + 3Δf,..., f1 + (N - 2)Δf to f2; where f1 to f2 constitutes the operating frequency range of the broadband sensor; Δf = Δ / (N - 1) = (f2 - f1) / (N - 1); Δ = f2 - f1.

[0055] According to the standard ASTM E 3100 - 2017 "Standard Guide for Acoustic Emission Testing of Concrete Structures", sensors sensitive within the 20 - 250 kHz frequency band are usually used for the testing of concrete structures. The N acoustic emission sensors include at least 1 broadband sensor and N - 1 resonant sensors with different operating frequency ranges.

[0056] Among them, there is a broadband acoustic emission sensor 4 - 1 containing low frequencies, whose operating range is f1 to f2, f1 ≤ 20 kHz, f2 ≥ 300 kHz, and its sensitivity response curve should be relatively flat within the range of f1 to f2, and within other ranges, the flatness fluctuation range can increase relatively.

[0057] The operating frequency bandwidth of the broadband sensor 4 - 1 is Δ, and the operating frequency bandwidth of the selected resonant sensor is Δf.

[0058] Δ = f2 - f1 (3)

[0059] Δf = Δ / (N - 1) = (f2 - f1) / (N - 1) (4)

[0060] It is recommended to select N - 1 resonant sensors with the operating frequency ranges as shown in Table 1 below as the resonant sensors 4 - 2, 4 - 3... 4 - N respectively.

[0061] Table 1 Recommended Operating Frequency Ranges of the Selected Resonant Sensors

[0062] Resonant sensor number Operating frequency range of the selected resonant sensor / kHz 4-2 f1 to f1+Δf 4-3 f1+Δf to f1+2Δf … … 4-N f1+(N-2)Δf to f2

[0063] As an example, the acoustic emission sensor 4-1 adopted in this embodiment is of the model VS45-H@Vallen Systeme GmbH, which is a broadband sensor with a working frequency range of 20 to 450 kHz, that is, f1 = 20 kHz and f2 = 450 kHz; the preamplifier adopted is AEP5@Vallen Systeme GmbH with a gain of 34 dB; the acoustic emission sensor 4-2 is of the model VS30-SIC@Vallen Systeme GmbH, which is a low-frequency resonant sensor with a working frequency range of 25 to 80 kHz and an integrated preamplifier; the acoustic emission sensor 4-3 is of the model VS150-RIC@Vallen Systeme GmbH, which is a resonant sensor with a working frequency range of 100 to 450 kHz and an integrated preamplifier.

[0064] The acoustic emission signal acquisition unit 5 is configured to determine an acoustic emission sensor with a suitable working frequency based on the analog acoustic emission signals of the N acoustic emission sensors; the acoustic emission sensor with a suitable working frequency refers to a sensor that can collect real damage signals in the concrete.

[0065] The model of the acoustic emission signal acquisition unit 5 is: AMSY-6@Vallen Systeme GmbH, and the sampling rate is set to 5 MHz.

[0066] The filter of the acoustic emission signal acquisition unit 5 is set to 20 to 300 kHz, the gate threshold is set to 40 dB, and the retrigger time and the continuous discrimination time during data acquisition are set to 200 and 400 μs respectively.

[0067] N preamplifiers are respectively arranged between the N acoustic emission sensors and the connection with the acoustic emission signal acquisition unit 5.

[0068] The size of the concrete beam 1 is 150×150×1500 (mm). The acoustic emission sensors 4 are placed on the concrete beam 1 and are well coupled and fixed. The preamplifiers (3-1, 3-2, 3-3,... 3-N) and the acoustic emission sensors (4-1, 4-2, 4-3,... 4-N) are connected through the signal cable 6. The preamplifiers (3-1, 3-2, 3-3,... 3-N) are respectively connected to the acoustic emission signal acquisition unit 5 through the signal cable 6. The signal cable 6 is a coaxial cable with electromagnetic shielding function.

[0069] The acoustic emission sensors are arranged as close as possible. If the sensor diameter is ignored, the sensor arrangement positions are understood as the same point. Therefore, the N acoustic emission sensors are within a preset range on the first surface of the concrete beam 1; the area of the region within the preset range is less than a first preset value; the spacing distance between two adjacent acoustic emission sensors is less than a second preset value.

[0070] Among them, the distances between different signal generation position points on the first surface of the concrete beam 1 and the preset range increase sequentially; the distances between different signal generation position points on the second surface of the concrete beam 1 and the preset range increase sequentially.

[0071] In this embodiment, in view of the fact that the frequency characteristics of acoustic emission signals depend on the concrete materials and structures to be measured, the types of concrete damage, and the types of acoustic emission sensors, the lead core break (PLB) simulation signal is used as the acoustic emission signal in the concrete to replace the acoustic emission signals generated by concrete damage during mechanical tests such as uniaxial compression, three-point bending, and shear tests of concrete specimens on a testing machine. This can greatly simplify the test process, improve the test efficiency, and reduce the test time cost and economic cost. Based on the fact that the working frequency range of broadband sensors is relatively wide and the sensitivity responses to various different frequency signals are relatively consistent within the working frequency range (although its sensitivity is lower than that of narrowband resonant sensors), N different types of acoustic emission sensors are used in the present invention. One of them is a broadband sensor including low frequencies, and the other N - 1 are resonant sensors with different working frequencies and resonant frequencies. Without interfering with each other, the N sensors are arranged as close to each other as possible. On the premise of ignoring the sensor diameter, they are approximately understood to be arranged at the same position. The N sensors are used to synchronously receive acoustic emission signals at different positions and with different propagation distances in the concrete structure, ensuring the consistency of the sound source signal S(t) at the same position and for the same lead break for the N different sensors.

[0072] Embodiment 2

[0073] As Figure 3 shown, this embodiment provides a method for selecting sensors of the concrete structure acoustic emission signal frequency characteristic test system described in Embodiment 1. The execution subject is the acoustic emission signal acquisition unit 5, and the method includes:

[0074] S1: Obtain the simulated acoustic emission signals generated at each signal generation position received by N acoustic emission sensors; the simulated acoustic emission signals are generated by an acoustic emission simulation source at the signal generation positions in a preset order; the N acoustic emission sensors include at least 1 broadband sensor and N - 1 resonant sensors with different working frequency ranges; the working frequency range of the broadband sensor is greater than the 20 - 250 kHz frequency band range.

[0075] First, set up a test system for the frequency characteristics of acoustic emission signals of a concrete structure according to the solution of the embodiment, and set up a broken lead simulation source 2. Set the signal generation position (the position where the lead core breaks) on the concrete beam 1. As an example, at different distances from the acoustic emission sensor 4 on the concrete beam 1 (on a straight line parallel to the beam length direction in the plane where the acoustic emission sensor is located, such as at distances of 25 mm, 50 mm, 75 mm... from the acoustic emission sensor 4, on the opposite side of the plane where the acoustic emission sensor is located, along a straight line parallel to the beam length direction, at distances of 0, 25, 50 mm, 75 mm... from the projection position of the acoustic emission sensor 4 on this plane) set the signal generation position. As Figure 2 shown, the acoustic emission sensors are arranged in a line along the beam width direction (for example, 150 mm), and as close as possible. If the sensor diameter is ignored, that is, assuming the sensor has no size, the sensor arrangement position is understood as a point. In theory, it is most appropriate to arrange multiple sensors at the same point; in this way, in theory, the distance for the broken lead signal at a certain position to be transmitted to each sensor is equal, which can reduce the difference in the signal caused by the unequal transmission distance of the same broken lead signal. The broken lead positions are collinear (such as at 25 mm, 50 mm, 75 mm...), and the connection line of the broken lead positions (in the direction corresponding to the beam length of 1500 mm) passes through the center of the sensor. Although the centers of each sensor are not at one point, they are approximately understood to be at the same position and the same point.

[0076] When breaking the lead on the first surface, the path for the broken lead signal to be transmitted to each sensor is mainly along the surface of the concrete beam 1. When breaking the lead on the second surface (the opposite surface of the first surface), if the broken lead signal is received by the sensor on the opposite side, the transmission path needs to cross the entire thickness (150 mm) direction of the concrete beam, so that the influence of different transmission paths of the broken lead signal (surface propagation, propagation through the concrete structure) on the signal characteristics can be comprehensively considered.

[0077] S2: Analyze the simulated acoustic emission signal waveforms at each signal generation position received by each acoustic emission sensor; extract the characteristic parameters (ring count and duration) of each simulated acoustic emission signal waveform to obtain the average frequency value of each simulated acoustic emission signal.

[0078] For the frequency feature analysis of the waveform signals collected by acoustic emission sensors, a common method is to obtain the frequency corresponding to the peak point in the energy spectrum through the fast Fourier transform (FFT) of the time-domain waveform - the main frequency (or peak frequency), and often the weighted average with the amplitude of the energy spectrum after FFT transformation as the weight - the centroid frequency. However, the above two frequency calculations require Fourier transformation of the time-domain waveform, with a large amount of calculation, and the waveform is vulnerable to noise interference, and the noise has an obvious impact on the calculation results of the signal peak frequency and centroid frequency. The present invention constructs a new physical parameter - the average frequency AF (average frequency) to measure the frequency characteristics of acoustic emission signals, which has the characteristics of being insensitive to noise and simple and fast calculation - it can be calculated by dividing the ring count of acoustic emission characteristic parameters by the duration.

[0079] Figure 4 As an acoustic emission characteristic parameter, the average frequency AF of an acoustic emission signal is defined as the ratio of the ring count (Count) to the duration (Duration), as shown in formula (5), and the unit of measurement of AF is kHz.

[0080]

[0081] S3: For each of the acoustic emission sensors, draw a two-dimensional scatter plot; the abscissa of the two-dimensional scatter plot is the distance between the signal generation position and the acoustic emission sensor, and the ordinate is the corresponding average frequency value.

[0082] According to the sensor model, draw two-dimensional scatter plots with the abscissa being the lead break distance Distance (the distance between the signal generation position and the acoustic emission sensor) and the ordinate being the AF value of the corresponding lead break signal, as Figure 5 shown. The two-dimensional scatter plot drawn by the acoustic emission signal acquisition unit 5 reflects the relationship between the AF value of the acoustic emission signal and the acoustic wave propagation distance Distance in the current concrete beam 1 (the attenuation of AF with the propagation distance).

[0083] S4: Draw a histogram of the average frequency of the acoustic emission signals received by each acoustic emission sensor according to the two-dimensional scatter plot of each acoustic emission sensor; draw a kernel density curve of the average frequency of the acoustic emission signals received by each acoustic emission sensor according to the average frequency histogram; as Figure 6 and 7 shown, the abscissa of the average frequency histogram is the average frequency value, and the ordinate is the count; the abscissa of the kernel density curve is the average frequency value, and the ordinate is the density.

[0084] According to the sensor type, draw the vectors α1, α2, α2...α of the AF values of the signals received by the acoustic emission sensors 4-1, 4-2, 4-3,...4-N respectively NThe histogram and kernel density curve reflect the distribution range and concentration degree of the AF value.

[0085] S5: Determine two cut-off average frequency values corresponding to the 1 / 2 of the curve peak value in the kernel density curve of the broadband sensor.

[0086] As Figure 7 shown, in the kernel density curve of the AF value of the signal received by the broadband acoustic emission sensor 4-1 including low frequencies, the AF value corresponding to the peak point Dm is Fm, and FL and FH are the left and right two frequencies corresponding to the density value of 1 / 2 of the peak Dm in the kernel density curve of the AF value, that is, the -6dB cut-off frequencies.

[0087]

[0088] The AF value of the acoustic emission signal collected by the broadband sensor 4-1 is mainly distributed in 30 - 40 kHz, the -6dB frequency band range is 31.0 - 37.1 kHz, and the AF value has a certain attenuation with the propagation distance but is not serious, as Figure 5 and Figure 6 shown.

[0089] S6: Select an acoustic emission sensor with a suitable operating frequency from the N-1 resonant sensors according to the frequency range composed of the two cut-off average frequency values and the distribution trend of the kernel density curve; the acoustic emission sensor with a suitable operating frequency refers to the sensor that can collect the real damage signal in the concrete.

[0090] Step S6 specifically includes:

[0091] Select a narrowband resonant sensor from the resonant sensors 4-2, 4-3,... 4-N whose operating frequency range is relatively consistent with FL~FH and the distribution trend of the kernel density curve of its AF value is closest to that of the broadband acoustic emission sensor 4-1, which is a narrowband resonant acoustic emission sensor with good frequency response and high sensitivity to the damage signal in the concrete structure to be measured.

[0092] The main consideration factor when selecting a sensor is the frequency characteristic of the acoustic emission wave generated by the expansion of potential defects in the structure to be detected. The operating frequency range of the above broadband sensor 4-1 is relatively wide, but its sensitivity is lower than that of the resonant sensors 4-2, 4-3,... 4-N. Therefore, on the basis of testing the frequency characteristic of the acoustic emission signal in the concrete structure using the broadband sensor 4-1, a high-sensitivity, narrowband resonant sensor with a corresponding frequency range is selected.

[0093] In fact, it may not be possible to find a resonant sensor that exactly meets the operating frequency range in Table 1 above. However, it should be as close as possible to the recommended frequency range in Table 1 above. It is recommended that the value of N be in the range of 3 ≤ N ≤ 10. The value of N should not be too large. On the one hand, if it is too large, the workload required for the experiment will be too large. Moreover, it is more difficult to find a resonant sensor that exactly meets the recommended operating frequency range above (because the larger N is, the smaller the operating frequency bandwidth Δf of the resonant sensor, and there are fewer commercial sensors that can exactly meet such a narrow bandwidth).

[0094] As Figure 8 shown, taking three acoustic emission sensors as an example, the operating frequencies of acoustic emission sensors 4-1, 4-2, and 4-3 are respectively in the ranges of 20 - 450 kHz, 25 - 80 kHz, and 100 - 450 kHz. The AF values of the received lead break signals are mainly distributed in the ranges of 30 - 40 kHz, 25 - 35 kHz, and 40 - 80 kHz respectively. The AF values corresponding to the peak points of the AF value kernel density curves are 33.2 kHz, 28.8 kHz, and 55.4 kHz respectively. A high-sensitivity, low-frequency resonant sensor 4-2 with an operating frequency in the range of 25 - 80 kHz can be selected to monitor the damage of this concrete structure.

[0095] In this embodiment, the higher the sampling rate of the acoustic emission signal acquisition unit 5, the more real the digitized waveform signal obtained, and the higher the accuracy of the duration. However, the frequency of the acoustic emission signal in concrete is relatively low. It is preferred that the acoustic emission signal acquisition unit 5 is an acquisition device with a sampling rate of more than 5 MHz.

[0096] In this embodiment, a new physical parameter - average frequency AF (average frequency) that is insensitive to noise and has the characteristics of simple and fast calculation is constructed through the conventional characteristic parameters of acoustic emission to measure the frequency characteristics of the acoustic emission signal. Then, according to the histogram distribution and kernel density curve of the AF value of the lead break signal received by each sensor, a high-sensitivity, narrow-band resonant sensor with a corresponding operating frequency range is selected to perform acoustic emission monitoring on this type of concrete. The method based on this embodiment not only has high efficiency in sensor selection, but also the selected sensors can perform acoustic emission monitoring more accurately.

[0097] In this article, specific examples are used to elaborate on the principles and implementation manners of the present invention. The descriptions of the above embodiments are only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, based on the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for selecting sensors of a test system based on the frequency characteristics of acoustic emission signals of concrete structures, characterized in that, The system includes: an acoustic emission simulation source, N acoustic emission sensors disposed on the first surface of the concrete beam, and an acoustic emission signal acquisition unit; The acoustic emission simulation source is used to generate simulated acoustic emission signals at different positions on the first surface and the second surface of the concrete beam; The N acoustic emission sensors are all connected to the acoustic emission signal acquisition unit, and are used to collect the simulated acoustic emission signals generated at different positions of the concrete, and transmit the simulated acoustic emission signals to the acoustic emission signal acquisition unit; The acoustic emission signal acquisition unit is used to determine the acoustic emission sensors with appropriate operating frequencies according to the simulated acoustic emission signals of the N acoustic emission sensors; the acoustic emission sensors with appropriate operating frequencies are used to collect real damage signals in the concrete; The sensor selection method specifically includes: Obtain the analog acoustic emission signals generated at each position received by N acoustic emission sensors; the analog acoustic emission signals are generated by an acoustic emission analog source sequentially at signal generation positions in a preset order; the N acoustic emission sensors include at least 1 broadband sensor and N - 1 resonant sensors with different working frequency ranges; wherein, the working frequency range of the broadband sensor is ; the working frequency ranges of the N - 1 resonant sensors are sequentially: , , ,..., ; represents the working frequency bandwidth of the resonant sensor; Analyze the waveforms of the simulated acoustic emission signals at each position received by each acoustic emission sensor; Extract the ring count and duration of each simulated acoustic emission signal waveform to obtain the average frequency value of each simulated acoustic emission signal; For each acoustic emission sensor, draw a two-dimensional scatter plot; the abscissa of the two-dimensional scatter plot is the distance between the signal generation position and the acoustic emission sensor, and the ordinate is the corresponding average frequency value; Draw an average frequency histogram of the acoustic emission signals received by each acoustic emission sensor according to the two-dimensional scatter plot of each acoustic emission sensor; draw a kernel density curve of the average frequency of the acoustic emission signals received by each acoustic emission sensor according to the average frequency histogram; the abscissa of the average frequency histogram is the average frequency value, and the ordinate is the count; the abscissa of the kernel density curve is the average frequency value, and the ordinate is the density; Determine the two cut-off average frequency values corresponding to half of the curve peak in the kernel density curve of the broadband sensor; Select the acoustic emission sensors with appropriate operating frequencies from the N - 1 resonant sensors according to the frequency range formed by the two cut-off average frequency values and the distribution trend of the kernel density curve; the acoustic emission sensors with appropriate operating frequencies are used to collect real damage signals in the concrete.

2. The sensor selection method according to claim 1, wherein, The N acoustic emission sensors are within a preset range on the first surface of the concrete beam; the area of the region within the preset range is less than a first preset value; the distance between adjacent two acoustic emission sensors is less than a second preset value.

3. The sensor selection method according to claim 2, wherein The distances between the different signal generation position points on the first surface of the concrete beam and the preset range increase in sequence; the distances between the different signal generation position points on the second surface of the concrete beam and the preset range increase in sequence.

4. The sensor selection method according to claim 1, wherein A preamplifier is also connected between each acoustic emission sensor and the acoustic emission signal acquisition unit.

5. The sensor selection method according to claim 1, characterized in that ， 。 6. The sensor selection method according to claim 1, wherein The acoustic emission simulation source is a broken lead simulation source.

7. The sensor selection method according to claim 1, characterized in that Select the acoustic emission sensors with appropriate operating frequencies from the N - 1 resonant sensors according to the frequency range formed by the two cut-off average frequency values and the distribution trend of the kernel density curve, specifically including: Select the resonance sensor from the N-1 resonance sensors whose difference between the working frequency range and the frequency range formed by the two cut-off average frequency values is less than the preset difference value, and the distribution trend similarity of the kernel density curve and the kernel density curve of the broadband sensor is greater than the preset similarity value as the acoustic emission sensor with the appropriate working frequency.

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