阅读说明：本技术 宽频激励非线性声场调制的混凝土微裂缝检测系统及方法 (Concrete microcrack detection system and method based on broadband excitation nonlinear sound field modulation ) 是由 许颖 王青原 江雪雷 徐婷婷 于 2021-08-31 设计创作，主要内容包括：本发明涉及一种基于宽频激励非线性声场调制的混凝土微裂缝检测系统,包括：图像采集机构、压力试验机构、信号发射机构、信号采集机构和计算处理机构；所述图像采集机构能够采集混凝土材料试件在压力试验机构的压缩开裂实验中的全程图像数据和全程裂缝密度值数据；所述信号发射机构能够同时对处于压缩开裂实验中的混凝土材料试件发射高频信号和宽频信号；压缩开裂实验中的混凝土材料试件在高频信号和宽频信号的作用下能够反馈产生应力波信号数据；本发明提供的检测系统能够通过计算混凝土材料试件的对应裂缝密度值与损伤指标峰值回归得到对应回归方程,进而计算得到对应开裂状态下的混凝土材料试件的微裂缝密度。(The invention relates to a concrete microcrack detection system based on broadband excitation nonlinear sound field modulation, which comprises: the device comprises an image acquisition mechanism, a pressure test mechanism, a signal transmitting mechanism, a signal acquisition mechanism and a calculation processing mechanism; the image acquisition mechanism can acquire whole-course image data and whole-course crack density value data of the concrete material test piece in a compression cracking experiment of the pressure test mechanism; the signal transmitting mechanism can simultaneously transmit a high-frequency signal and a wide-frequency signal to a concrete material test piece in a compression cracking experiment; the concrete material test piece in the compression cracking experiment can feed back stress wave signal data under the action of a high-frequency signal and a broadband signal; the detection system provided by the invention can obtain the corresponding regression equation by calculating the regression of the corresponding crack density value and the damage index peak value of the concrete material test piece, and further calculate and obtain the micro-crack density of the concrete material test piece in the corresponding cracking state.)

1. A system and a method for detecting concrete microcracks based on broadband excitation nonlinear sound field modulation are characterized in that,

the method comprises the following steps: the device comprises an image acquisition mechanism, a pressure test mechanism, a signal transmitting mechanism, a signal acquisition mechanism and a calculation processing mechanism;

the image acquisition mechanism can acquire whole-course image data and whole-course crack density value data of the concrete material test piece in a compression cracking experiment of the pressure test mechanism;

the signal transmitting mechanism can simultaneously transmit a high-frequency signal and a wide-frequency signal to a concrete material test piece in a compression cracking experiment;

the concrete material test piece in the compression cracking experiment can feed back stress wave signal data under the action of a high-frequency signal and a broadband signal;

the signal acquisition mechanism can acquire the stress wave signal data;

the calculation processing mechanism can calculate the corresponding equation solution of the damage index value and the crack density value by adopting a sideband peak counting method according to the whole-course image data, the whole-course crack density value data, the measured and set pressure data of the pressure testing mechanism and the stress wave signal data.

2. The detection system of claim 1,

the signal transmitting mechanism includes: the device comprises a broadband exciter, a broadband signal transmitting probe, a function generator and a ceramic piezoelectric patch signal transmitter;

the function generator can generate a sine wave with single high frequency, is connected with the ceramic piezoelectric patch signal transmitter, and can transmit a high-frequency signal to the first side of the concrete material test piece by means of the ceramic piezoelectric patch signal transmitter;

the broadband exciter is connected with the broadband signal transmitting probe to transmit broadband signals to the first side of the concrete material test piece.

3. The detection system of claim 2,

the signal acquisition mechanism includes: an oscilloscope and a ceramic piezoelectric plate signal receiver;

the ceramic piezoelectric plate signal receiver is arranged on the other side of the concrete material test piece and receives stress wave signal data;

the ceramic piezoelectric sheet signal receiver is connected with the oscilloscope and can send received stress wave signal data to the oscilloscope;

the oscilloscope can send the received stress wave signal data to the calculation processing mechanism.

4. The detection system of claim 3,

the image acquisition mechanism includes: cameras, tripods, and illumination sources;

the camera is arranged on the tripod and used for acquiring whole-course image data and whole-course crack density value data and sending the acquired whole-course image data and the acquired whole-course crack density value data to the calculation processing mechanism;

the stability of the illumination environment is kept by adopting the illumination light source, and errors caused by light ray change are reduced.

5. The detection system of claim 3,

the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver both adopt ceramic piezoelectric patches with the diameters of 15mm and the thicknesses of 4 mm;

the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver are both covered and protected by BOPP adhesive tapes, and then are adhered to the opposite sides of the concrete material test piece by 502, so that the integrity of the piezoelectric patches in the detection process is ensured.

6. The detection system of claim 3,

the couplant between the broadband emission signal probe and the concrete material test piece is HC-98 type medical ultrasonic couplant, and the main component is aqueous polymer gel;

the acoustic coupling of the whole experimental process can be realized between the broadband transmitting signal probe, the ceramic piezoelectric patch signal transmitter, the ceramic piezoelectric patch signal receiver and the concrete material test piece to be tested.

7. The detection system of claim 3,

the distance between the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver is 100mm-250 mm.

8. A concrete material microcrack density testing method based on the detection system of any one of claims 1 to 7, characterized by comprising the following steps:

s1, acquiring and detecting the whole compression cracking image data of the concrete material test piece by means of an image acquisition mechanism, and further obtaining the whole crack density value data of the micro crack generation and expansion stage of the concrete material test piece;

s2, acquiring stress wave signal data of the concrete material test piece in a loading state by a signal acquisition mechanism;

the calculation processing mechanism calculates the damage index peak value of the nonlinear ultrasonic detection result of the concrete material test piece in a corresponding loading state by adopting a side-band peak counting method according to the collected stress wave signal data;

s3, the calculation processing mechanism obtains a corresponding regression equation through regression of the corresponding crack density value and the damage index peak value of the concrete material test piece, and equation parameters are obtained;

and S4, substituting the damage index values corresponding to the same mixing ratio into a regression equation, and calculating to obtain the micro-crack density of the concrete material test piece in the corresponding cracking state.

Technical Field

The invention belongs to the technical field of nondestructive testing of microcracks of concrete members, and particularly relates to a system and a method for detecting microcracks of concrete by modulating a broadband excitation nonlinear sound field.

Background

Concrete is dominant in building structural materials, a concrete structure can usually work with seams, but the generation and development of micro-cracks (cracks with the width of less than 150 micrometers) have great influence on concrete structures with higher requirements on durability and fatigue failure resistance in inland harbor and harbor engineering, nuclear power engineering, subway and tunnel engineering, so that the detection and evaluation of the micro-cracks in the concrete have important significance for controlling the development to form macro-cracks. The traditional concrete damage detection and evaluation methods such as an ultrasonic pulse echo method and a rebound method for detecting the concrete strength cannot be used for early-stage micro-crack detection of concrete materials.

Because the concrete material is formed by mixing sand, stone and cement paste, the internal structure of the concrete material is complex, and the micro-scale anisotropy appears. When no external action exists, certain nonlinear characteristics exist in the mechanical property of the ultrasonic probe, which brings great limitation to the conventional ultrasonic detection.

The conventional ultrasonic technology can detect macro cracks in a structure based on reflection, transmission, attenuation and other phenomena of ultrasonic waves at defects, but the detection of early damage and closed microcracks of a concrete material is difficult to realize. The nonlinear ultrasonic characteristic shown after the material is damaged is utilized, and the nonlinear theory is applied to realize the early detection of the material damage.

Research has shown that nonlinear ultrasonic modulation technology has high sensitivity to microcracks or composite material delamination in structures, and the sensitivity can be maintained by opening and closing the crack surface, and is suitable for detecting microcracks [ Jhang K Y. nonlinear ultrasonic technology for non-destructive evaluation of micro-large in material: a review. int J Precis Eng Manual, 2009(10): 123-) 135 ]. By conducting experiments on CFRP plates, the harmonic and modulation phenomena caused by crack excitation and wave vibration [ Solodov I, Krohn N, Busse G. nonlinear ultrasonic NDT for Early detection and imaging, Proceedings of the 10th European Conference on Non-Destructive Testing, Moscow,2010 ]. Observing the influence of low-frequency vibration excitation on a modulation index by carrying out a Nonlinear modulation test on a cracked aluminum plate [ Klepka A, Staszewski W J, Jenal R B, et al. 11:197-211]. In a laser ultrasonic modulation test of a complicated aluminum fitting sample, internal defects were found by observing spectral sidebands [ Sohn H, Lim H J, Yang S.A Fatigue Crack Detection method [ M ]. Springer Netherlands,2015 ]. The nonlinearity is derived from the integration of the stress-strain relationship that varies with the crack width and amplitude [ Kawashima K, Ryuji O, Toshihiro I, et al.nonlinear acid stress through surface cracks: FEM simulation and evaluation [ J ]. Ultrasonics 2002; 40:611-5]. Nonlinear ultrasound techniques have also been used in qualitative studies to detect closed cracks in Concrete weak layers [ Antonaci P, Bruno C, Bocca PG, et al. nonlinear ultrasonic evaluation of load effects on semiconductors in semiconductors [ J ]. center & Concrete Research, 2010,40(2): 340-. However, the cracking research on concrete materials is limited to the research on the critical width and length of a single crack, and the research does not research the detection of the microcrack density which has a great influence on the durability, strength and service life of the material, wherein the microcrack density is defined as the cumulative length of the microcracks in an observation area.

The subject group, Zhang He Yong et al, in the experimental research on the detection of microcracks in thermally damaged concrete based on nonlinear ultrasonic technology, detects the feasibility of microcracks in thermally damaged concrete based on a nonlinear sound field modulation method, and introduces the detection results of a nonlinear sound field resonance method for comparison. Experimental results show that the obtained damage index characteristic is increased along with the increase of the nonlinearity of the solid material, and the operation requirement of the sound field modulation method on external excitation is obviously lower than that of the sound field resonance method. However, the existing research results only consider the introduction of the damage by adopting different high temperature degrees, and do not give a more definite damage characterization physical quantity.

Disclosure of Invention

Technical problem to be solved

In order to solve the above problems in the prior art, the invention provides a system and a method for detecting concrete microcracks based on broadband excitation nonlinear sound field modulation, which overcome the problem that the quantitative values of concrete damage and cracks cannot be given in the application of the existing ultrasonic technology to concrete detection.

(II) technical scheme

In order to achieve the purpose, the invention adopts the main technical scheme that:

a concrete microcrack detection system based on broadband excitation nonlinear sound field modulation comprises: the device comprises an image acquisition mechanism, a pressure test mechanism, a signal transmitting mechanism, a signal acquisition mechanism and a calculation processing mechanism;

the image acquisition mechanism can acquire whole-course image data and whole-course crack density value data of the concrete material test piece in a compression cracking experiment of the pressure test mechanism;

the signal transmitting mechanism can simultaneously transmit a high-frequency signal and a wide-frequency signal to a concrete material test piece in a compression cracking experiment;

the concrete material test piece in the compression cracking experiment can feed back stress wave signal data under the action of a high-frequency signal and a broadband signal;

the signal acquisition mechanism can acquire the stress wave signal data;

the calculation processing mechanism can calculate the corresponding equation solution of the damage index value and the crack density value by adopting a sideband peak counting method according to the whole-course image data, the whole-course crack density value data, the measured and set pressure data of the pressure testing mechanism and the stress wave signal data.

Preferably, the signal emitting mechanism includes: the device comprises a broadband exciter, a broadband signal transmitting probe, a function generator and a ceramic piezoelectric patch signal transmitter;

the function generator can generate a sine wave with single high frequency, is connected with the ceramic piezoelectric patch signal transmitter, and can transmit a high-frequency signal to the first side of the concrete material test piece by means of the ceramic piezoelectric patch signal transmitter;

the broadband exciter is connected with the broadband signal transmitting probe to transmit broadband signals to the first side of the concrete material test piece.

Preferably, the signal acquisition mechanism comprises: an oscilloscope and a ceramic piezoelectric plate signal receiver;

the ceramic piezoelectric plate signal receiver is arranged on the other side of the concrete material test piece and receives stress wave signal data;

the ceramic piezoelectric sheet signal receiver is connected with the oscilloscope and can send received stress wave signal data to the oscilloscope;

the oscilloscope can send the received stress wave signal data to the calculation processing mechanism.

Preferably, the image acquisition mechanism comprises: cameras, tripods, and illumination sources;

the camera is arranged on the tripod and used for acquiring whole-course image data and whole-course crack density value data and sending the acquired whole-course image data and the acquired whole-course crack density value data to the calculation processing mechanism;

the stability of the illumination environment is kept by adopting the illumination light source, and errors caused by light ray change are reduced.

Preferably, the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver both adopt ceramic piezoelectric patches with the diameters of 15mm and the thicknesses of 4 mm;

the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver are both covered and protected by BOPP adhesive tapes, and then are adhered to the opposite sides of the concrete material test piece by 502, so that the integrity of the piezoelectric patches in the detection process is ensured.

Preferably, the couplant between the broadband emission signal probe and the concrete material test piece is HC-98 type medical ultrasonic couplant, and the main component is aqueous polymer gel;

the acoustic coupling of the whole experimental process can be realized between the broadband transmitting signal probe, the ceramic piezoelectric patch signal transmitter, the ceramic piezoelectric patch signal receiver and the concrete material test piece to be tested.

Preferably, the distance between the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver is 100mm-250 mm.

The technical scheme also provides a concrete material micro-crack density testing method based on the detection system, which comprises the following steps:

s1, acquiring and detecting the whole compression cracking image data of the concrete material test piece by means of an image acquisition mechanism, and further obtaining the whole crack density value data of the micro crack generation and expansion stage of the concrete material test piece;

s2, acquiring stress wave signal data of the concrete material test piece in a loading state by a signal acquisition mechanism;

the calculation processing mechanism calculates the damage index peak value of the nonlinear ultrasonic detection result of the concrete material test piece in a corresponding loading state by adopting a side-band peak counting method according to the collected stress wave signal data;

s3, the calculation processing mechanism obtains a corresponding regression equation through regression of the corresponding crack density value and the damage index peak value of the concrete material test piece, and equation parameters are obtained;

and S4, substituting the damage index values corresponding to the same mixing ratio into a regression equation, and calculating to obtain the micro-crack density of the concrete material test piece in the corresponding cracking state.

(III) advantageous effects

The invention has the beneficial effects that: the invention provides a system and a method for detecting concrete microcracks based on broadband excitation nonlinear sound field modulation, which have the following beneficial effects:

1. compared with linear ultrasonic detection which can only qualitatively judge damage, the nonlinear ultrasonic detection has higher sensitivity, and the nonlinear sound field modulation technology is adopted to evaluate the concrete compression damage, so that the change of the microcrack density can be represented by the change of the damage index peak value along with the increase of loading.

2. In combination with the observation area, the method links the internal force output of the interaction of the nonlinear modulation ultrasonic wave and the micro-crack contact surface generated by compression with the crack density of the micro-crack, and can realize the response evaluation of the nonlinear sideband peak counting result under the conditions of different micro-crack densities.

3. Aiming at the crack density of the concrete under the action of a compressive load in the multi-crack cracking state, the solution of a wave equation under the multi-crack condition when broadband is coupled with single frequency under broadband excitation is deduced based on the theory of nonlinear sound field modulation, and accordingly, a sideband peak counting method is provided as a damage index to evaluate the change condition of the concrete micro-crack density. The method has important theoretical significance and engineering application value for timely measuring the concrete microcrack generation condition and the expansion trend thereof, effectively preventing the microcracks from developing into harmful deep cracks due to stress concentration, ensuring the safety and reliability of the concrete structure and the like.

Drawings

FIG. 1 is a diagram of a theoretical model of the present invention;

FIG. 2 is a load-displacement curve of a concrete cubic test piece under uniaxial compression in the invention;

FIG. 3 is a schematic view of the measuring apparatus according to the present invention;

FIG. 4 is a schematic diagram of a non-destructive testing method for axial stress of a concrete member based on nonlinear sound field modulation according to the present invention;

FIG. 5 is a schematic diagram of the fundamental principle of the time domain and frequency domain signals modulated by the nonlinear sound field of the present invention, wherein FIG. 5a) is a graph of normalized time domain signals in a lossless state; wherein FIG. 5b) normalized time domain signal plot under damage state; FIG. 5c) a comparison of normalized frequency domain in lossless and in damaged state;

FIG. 6 is a schematic diagram of DIC corresponding to different damage states of a concrete member axially stressed and loaded test piece in the invention, and the schematic diagram of DIC cracks under the conditions of 1.41% and the peak compressive stress respectively in the states of FIG. 6a), FIG. 6b), FIG. 6c), FIG. 6d), FIG. 6e), FIG. 6f) and FIG. 6 g);

FIG. 7 is a graph showing the peak value of the damage index and the variation trend of the crack density of the test piece according to the present invention.

[ description of reference ]

1: a concrete material test piece; 2: PZT;

3: a broadband signal emission probe;

4: a broadband exciter (ZBL-520 nonmetal ultrasonic detector);

5: a function generator (DG1022U type digital signal generator);

6: a pressure testing mechanism (SANS60T hydraulic universal tester);

7: oscilloscopes (TREK MDO3024 type mixed domain digital oscilloscopes);

8: a support; 9: a calculation processing mechanism; 10: a camera (canon 1200D); 11: a tripod; 12: an illumination source.

Detailed Description

For the purpose of better explaining the present invention and to facilitate understanding, the present invention will be described in detail by way of specific embodiments with reference to the accompanying drawings.

Embodiment 1 is a device composition testing method for nondestructive testing of micro cracks of a concrete member by nonlinear sound field modulation effect and a relevant principle explanation. The calculation principle of the concrete microcrack damage index under the action of the pressurized load based on the broadband excitation nonlinear sound field modulation is as follows.

The internal force of the micro-cracks can be evaluated by the mechanical property of the tested piece as follows:

Δσ＝K(ξ)ξ (1a)

ξ＝U₊-U_- (1b)

xi represents the amount of change in crack interface spacing, U₊And U_-Respectively representing the deformation quantity of the interfaces on two sides of the microcrack, and K (xi) represents the elastic coefficient. The first order Taylor expansion of K (ξ) is:

K(ξ)＝K₀+K₁ξ (2)

supposing microcrackingThe amount of deformation on both sides of the slot is very small, then there is K₁＜＜K₀. When formula (2) is substituted for formula (1a), the following are provided:

Δσ＝K₀ξ+K₁ξ² (3)

wherein, K₁The nonlinearity of the material after microcracking can be used for characterizing the damage degree of the material.

And deducing related internal stress caused by microcracks under the broadband vibration modulation of the multi-crack condition. Two columns of sound waves with different frequencies are incident simultaneously (shown in figure 1), and the displacement excited at the crack infinitesimal is as follows:

u_h(x,t)＝U_h(x)cos2πf_ht (5)

an infinite small segment of length (unit infinitesimal) is taken on a certain crack in the solid medium for analysis,

d represents the crack width at the crack infinitesimal position, and the interval change is as follows under the influence of the stress field of the modulated ultrasonic signal:

when the formula (6) is substituted into the formula (3), the internal stress generated by the sound wave at the crack infinitesimal position is shown as the formula (7).

From the internal stress result corresponding to the final unit infinitesimal derived in the equation (7), the internal force output of the system includes the fundamental frequency signal (coefficient K)₀The first two terms corresponding to D are the fundamental frequency signal of wide frequency and high frequency, and the second harmonic signal (coefficient) of fundamental frequencyTo what is providedSecond order harmonic signals of wide and high frequencies, respectively, modulated side-frequency signals and other signals (coefficient 4K) in response to the first two terms₁D²Term, broadband and high frequency and modulated signals generated between broadband and broadband, respectively).

In FIG. 1, a crack is selected from the cracks to study the total length of the crack (the kth crack is s in length)_kAnd k is 0,1,2,.. multidot.m) integral, calculating the internal force generated on the microcracks according to a formula (8), superposing the integral of the cracks in the region to obtain the internal force output F of the system in the region, and calculating the internal force output F according to a formula (9).

The concrete is in the initial stage of compression cracking, the micro cracks are stably generated and expanded, and two sides of the micro cracks can be contacted under the excitation of broadband signals. The induced crack density at this time represents the total length of cracks per unit area of the concrete material in the case of multiple cracks. And (3) cracking the concrete sample under the compressive load to change the accumulated length of the microcracks, so as to cause concrete damage. The crack density is defined as the crack length in the observed area and is given by equation (10).

Wherein A is the observation area (unit: mm)²)，L_iThe crack length (unit: mm). The internal force output value F for the cracked region can be expressed by the formula (11).

According to the formula, at the initial stage of compression cracking, along with the increase of the crack density, the internal force output of the system comprises a fundamental frequency signal, a fundamental frequency second harmonic signal, a modulation side frequency signal and other signals, the stress waves are generated under the combined action of the internal forces, and the amplitude of the frequency domain signal is inevitably increased in a corresponding calculated frequency domain diagram.

The invention mainly adopts broadband signal excitation, the total number of frequency domain signals is more obvious, so that the edge frequency counting method is adopted to calculate the corresponding edge frequency number ratio under different compressive loads when the nonlinear response numerical value is researched, and the calculation formula is shown as the formula (12).

In the formula, N_peak(th)Number of frequency amplitudes above threshold, N_totalThe number of amplitude values of all frequencies.

Damage Index (DI) is defined as the value of the peak-to-peak count (SPC) of a material after Damage_damage(th)) Subtracting the Sideband Peak Count (SPC) at which the material is not damaged (referred to herein as the first measured reference data)_intact(th)) As shown in formula (13). The invention adopts the sideband peak number change value to determine the material damage degree (namely the change condition of the microcrack density).

DI＝SPC_damage(th)-SPC_intact(th) (13)

Compression damage cracks were made in concrete test pieces by a cubic compression test in which the test pieces were loaded down at a constant loading rate of 0.5mm/min (e.g., the force-displacement curve of the test piece is shown in FIG. 2).

Example 2

As shown in fig. 3: the embodiment of the invention discloses a concrete microcrack detection system based on broadband excitation nonlinear sound field modulation, which comprises: the device comprises an image acquisition mechanism, a pressure test mechanism 6, a signal emission mechanism, a signal acquisition mechanism and a calculation processing mechanism 9; the image acquisition mechanism can acquire whole-course image data and whole-course crack density value data of the concrete material test piece in a compression cracking experiment of the pressure test mechanism 6; the signal transmitting mechanism can simultaneously transmit a high-frequency signal and a wide-frequency signal to a concrete material test piece in a compression cracking experiment.

The concrete material test piece in the compression cracking experiment can feed back stress wave signal data under the action of a high-frequency signal and a broadband signal; the signal acquisition mechanism can acquire the stress wave signal data; the calculation processing mechanism 9 can calculate the corresponding equation solution of the damage index value and the fracture density value by adopting a sideband peak counting method according to the whole-course image data, the whole-course fracture density value data, the measured pressure data and the stress wave signal data of the pressure testing mechanism 6.

In this embodiment, the signal transmission mechanism includes: the broadband exciter 4, the broadband signal transmitting probe 3, the function generator 5 and the ceramic piezoelectric patch signal transmitter; the function generator 5 can generate a sine wave with a single high frequency, is connected with the ceramic piezoelectric patch signal transmitter, and can transmit a high-frequency signal to the first side of the concrete material test piece 1 by means of the ceramic piezoelectric patch signal transmitter; the broadband exciter 4 is connected with the broadband signal emission probe 3 to emit broadband signals to the first side of the concrete material test piece 1.

It should be noted that: the broadband signal emission probe is arranged on the bracket and used for emitting a high-frequency signal to the first side of the concrete material test piece 1.

In this embodiment the signal acquisition mechanism includes: an oscilloscope 7 and a ceramic piezoelectric patch signal receiver; the ceramic piezoelectric plate signal receiver is arranged on the other side of the concrete material test piece 1 and used for receiving stress wave signal data; the ceramic piezoelectric sheet signal receiver is connected with the oscilloscope 7 and can send received stress wave signal data to the oscilloscope 7; the oscilloscope 7 can send the received stress wave signal data to the calculation processing mechanism 9.

In this embodiment the image acquisition mechanism includes: a camera 10 and a tripod 11, and an illumination light source 12; the camera 10 is arranged on the tripod 11 and is used for acquiring the whole-course image data and the whole-course crack density value data and sending the acquired whole-course image data and the whole-course crack density value data to the calculation processing mechanism 9.

In the embodiment, the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver both adopt ceramic piezoelectric patches with the diameters of 15mm and the thicknesses of 4 mm; the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver are both covered and protected by BOPP adhesive tapes, and then are adhered to the opposite sides of the concrete material test piece by 502, so that the integrity of the piezoelectric patches in the detection process is ensured.

In the embodiment, the coupling agent between the broadband emission signal probe 3 and the concrete material test piece 1 is HC-98 type medical ultrasonic coupling agent, and the main component is aqueous polymer gel; the acoustic coupling of the whole experimental process can be realized among the broadband transmitting signal probe 3, the ceramic piezoelectric patch signal transmitter, the ceramic piezoelectric patch signal receiver and the concrete material test piece to be tested.

In the embodiment, the distance between the ceramic piezoelectric patch signal transmitter and the ceramic piezoelectric patch signal receiver is 100mm-250 mm.

The embodiment also provides a concrete material microcrack density testing method based on the detection system in the embodiment, which comprises the following steps:

s1, acquiring and detecting the whole compression cracking image data of the concrete material test piece by means of an image acquisition mechanism, and further obtaining the whole crack density value data of the micro crack generation and expansion stage of the concrete material test piece;

s2, acquiring stress wave signal data of the concrete material test piece in a loading state by a signal acquisition mechanism;

the calculation processing mechanism calculates the damage index peak value of the nonlinear ultrasonic detection result of the concrete material test piece in a corresponding loading state by adopting a side-band peak counting method according to the collected stress wave signal data;

s3, the calculation processing mechanism obtains a corresponding regression equation through regression of the corresponding crack density value and the damage index peak value of the concrete material test piece, and equation parameters are obtained;

and S4, substituting the damage index values corresponding to the same mixing ratio into a regression equation, and calculating to obtain the micro-crack density of the concrete material test piece in the corresponding cracking state.

Example 3

And in the quasi-static loading process, transmitting and receiving signals subjected to ultrasonic excitation broadband modulation, and taking pictures through DIC (sampling frequency is 1Hz) to obtain a cracking image of the test piece. The test system mainly comprises a universal loading tester (SANS60T hydraulic universal tester), a generator (DG1022U type digital signal generator), an oscilloscope (TREK MDO3024 type mixed domain digital oscilloscope), a broadband exciter (ZBL-520 nonmetal ultrasonic detector), a camera (Canon 1200D), an illumination, a notebook computer and the like, and the whole test device is shown in figure 3.

The invention selects piezoelectric ceramic Pieces (PZT) as a sensor and an exciter, respectively adopts epoxy resin to be pasted at the center of the left and right surfaces of a test piece, a ZBLU520 nonmetal ultrasonic detector is adopted by a signal source 1 to emit 500V, a radio frequency pulse signal with the pulse width of 0.32ms is adopted as a broadband signal, and a DG1022U type digital signal generator is connected by a piezoelectric ceramic transducer by a signal source 2 to emit a signal with the amplitude of 15V and the period of 50 kHz. The couplant between the transducer close to the signal source 1 and the concrete is HC-98 type medical ultrasonic couplant, and the main component is aqueous polymer gel. As shown in fig. 4, the test piece was always in a pressed state during the measurement, and in order to ensure that the vibration amplitudes in the vicinity of the cracks were substantially uniform during the measurement, the measurement in a non-destructive state was performed in a state before the press was started and the entry force 2KN was entered.

The normalized frequency domain results were examined using the signals obtained from the receiving sensor of the present invention shown in fig. 4, with the test piece in a non-destructive state at 0.14% peak stress (shown in fig. 5 (a)), and the test piece in a 28.97% peak stress state prior to peak stress (shown in fig. 5 (b)) as an example of a destructive state. A normalized frequency domain plot comparison is given, as shown in fig. 5 (c).

And inputting the signals acquired by the oscilloscope into a storage device, and storing the signals in a csv format. And (3) opening Matlab software, running codes, filtering data in the file, referring to corresponding file data, inputting a filtering bandwidth interval, and running calculation.

And performing Fourier transform on the ultrasonic signals output after filtering, converting time domain signals into frequency domain signals, and normalizing and subtracting the results of the lossless state respectively by capturing side frequency count values corresponding to different thresholds of a research interval near the fundamental wave on the spectrogram and obtaining damage index values corresponding to different thresholds. The results of the simultaneous DIC images (shown in fig. 6) were compared to obtain a numerical relationship between the peak value of the corresponding damage index and the crack density (shown in fig. 7). It is found that when the damage index peak value calculation result is in the (0.02, 0.2) interval in the compressed state, it is judged that the concrete material starts to crack.

The technical principles of the present invention have been described above in connection with specific embodiments, which are intended to explain the principles of the present invention and should not be construed as limiting the scope of the present invention in any way. Based on the explanations herein, those skilled in the art will be able to conceive of other embodiments of the present invention without inventive efforts, which shall fall within the scope of the present invention.