CN114018705B - Concrete free fracture overall process control visualization tracking test system and method - Google Patents
Concrete free fracture overall process control visualization tracking test system and method Download PDFInfo
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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Abstract
The invention discloses a concrete free fracture overall process control visual tracking test system and a method, belonging to the technical field of concrete material performance test; the method comprises the steps of selecting a research carrier and calculating the crack initiation load of a concrete sample; arranging an ultrasonic thermal imaging detection system, a strain monitoring system, a negative feedback control system, a crack measuring system and a heat dissipation system; driving an ultrasonic vibration exciter group to excite a concrete test piece by using an ultrasonic generator, monitoring a surface temperature field of the concrete test piece by using a thermal infrared imager, and loading the concrete test piece; determining a microcrack temperature rise zone and a rapid increase condition of the reading of a strain gauge appearing in the field of vision of the thermal infrared imager, adjusting the applied pressure load, stopping exciting the test piece, and recording test data; recovering the servo loading system after the concrete test piece is damaged; the invention can realize the feedback control of the loading process on a microscopic scale, and presents the typical fracture occurrence development stage of the test piece and the transfer convergence rule of the stress field along with the growth of the microcrack group in the process.
Description
Technical Field
The invention relates to the technical field of concrete material performance testing, in particular to a concrete free fracture overall process control visual tracking test system and method.
Background
The concrete fracture mechanics is the basic science for researching the crack generation and development rule and relates to the prevention and control of concrete engineering accidents. Although the fracture mechanics test technology has been developed rapidly over the last half century, the servo loading is generally realized on the basis of a macroscopic parameter (such as deflection) of deformation of a loaded test piece in a servo press (including constant force loading and constant displacement loading) of key test equipment; in addition, mainstream monitoring means of fracture processes such as an acoustic emission technology and a DIC technology are mainly used for researching the single crack initiation and propagation rule of the prefabricated crack in the plain concrete test piece. Therefore, the servo scale of the current loading device, based on the feedback control of the deformation of the test piece and the characteristics of the study object applicable to the monitoring means, enables the microcracks of the concrete test piece in the free fracture state to appear in stages and cannot be visually tracked in the whole process from germination to growth to clustering to convergence to disaster.
The acoustic emission technology is widely applied to the positioning and identifying research of the cracking point in the material cracking process, and the acoustic emission technology is based on the acoustic energy released at the material cracking moment, inverts the cracking point of the material cracking by capturing the acoustic energy, and correlates the information such as stress, strain and the like in the cracking process by means of a strain acquisition system and the like. The acoustic emission technology is to calculate the position of the crack initiation point by calculation and analysis according to the received acoustic wave energy, so that the whole process of the crack initiation and expansion of the microcracks cannot be visually tracked in real time. In addition, the acoustic emission technology is usually used for monitoring the initiation and propagation of a crack, and for the initiation of multiple cracks in a reinforced concrete test piece, the acoustic energy signal collected by the receiver is easy to have inclusion interference and inaccurate in positioning. The digital speckle correlation method (DIC) is widely applied to fracture mechanics experiment research, and is a measurement method for performing correlation operation on speckle fields before and after deformation of a test piece (under the action of load) to obtain displacement full-field information. The technology needs to determine an observation area before monitoring, and the observation area is smaller; for free fracture of a crack initiation region unknown in advance, an observation region cannot be accurately arranged, namely, comprehensive monitoring of the fracture process of a test piece without a preset crack is difficult to realize. In addition, high-density speckles need to be uniformly marked in an observation area, and the use convenience is reduced.
The method for detecting the structural hidden cracks of the concrete test piece based on the ultrasonic excitation infrared thermal imaging method can position and track the crack development of the concrete test piece during free fracture in the whole process. However, the method is only used for tracking and detecting the growth process of the microcracks, cannot control the growth stage of the microcracks, and does not relate to the perception of stress field information of the test piece; therefore, the process of convergence of the microcrack group and the transfer convergence of the stress field during the growth of the microcrack group is difficult to effectively study.
Disclosure of Invention
The invention provides a system and a method for controlling and visualizing the tracking test of the whole process of free fracture of concrete to solve the problems.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention aims to overcome the defects of the prior art, provides a test system and a method for controllable free fracture stage and visual positioning and tracking of the whole fracture process of a concrete test piece without a preset crack, and can realize the following steps: based on the microscopic level perception, the full-coverage active positioning and tracking of the crack from the germination to the clustering to the convergence growth process is realized, the minimum opening displacement of the tip of the crack, which is effectively identified, can reach 0.005mm, and the comprehensive monitoring of stress field transfer convergence is realized.
Concrete free fracture overall process control visual tracking test system includes: the system comprises a concrete test piece, a servo loading system, a strain monitoring system, an ultrasonic thermal image detection system, a negative feedback control system, a crack measuring system and a heat dissipation system; the concrete test piece is a carrier which can be freely broken, the concrete test piece is arranged in a servo loading system, the servo loading system develops free fracture in the concrete test piece, the strain monitoring system monitors the dynamic transfer and convergence of stress in the stress field of the concrete test piece, the auxiliary ultrasonic thermal image detection system provides basis for the negative feedback control system to control the servo loading system, the ultrasonic thermal image detection system actively positions and tracks the growth of microcracks in the test piece, the distribution and the form of the microcracks are visualized to provide a basis for the negative feedback control system to drive the servo loading, the negative feedback control system controls the output load of the servo loading system according to the analysis results of the strain monitoring system and the ultrasonic thermography detection system, the crack measurement system measures the crack size after determining the crack distribution, and the heat dissipation system restores the concrete temperature of the area near the ultrasonic thermal excitation source to the ambient temperature in a short time.
Preferably, the concrete test piece comprises a reinforced concrete test piece and a plain concrete test piece.
Preferably, the servo loading system consists of a reaction frame, a universal material testing machine, a cast iron pressure head, a steel base plate and a support, the concrete test piece is arranged above the support, the universal material testing machine is arranged in the reaction frame, the displacement control rate of the universal material testing machine is 0.01-500 mm/min, and the maximum loading is 200 kN; the cast iron pressure head is a semi-cylinder, the diameter of the cast iron pressure head is 50-100 mm, and the length of the cast iron pressure head is 200-400 mm; the thickness of the steel backing plate is 20-40 mm, the length is 200-800 mm, and the width is 200-300 mm; the support is a cast iron hinged support, the number of the cast iron hinged supports is 2, the hinges are cylinders, the diameter of each cast iron hinged support is 30-50 mm, and the length of each cast iron hinged support is 200-400 mm; the concrete test piece breaking process is realized by a servo loading system.
Preferably, the strain monitoring system consists of a strain gauge, a dynamic signal testing and analyzing system and dynamic signal acquisition and analysis software; the measuring range of the strain gauge is 1000 mu epsilon, the sensitivity coefficient is 2.06, and the resistance is 120.1 omega; the dynamic signal testing and analyzing system is embedded with an industrial computer, a high-speed hard disk and a Linux operating system, and is connected with the computer and then controlled by dynamic signal acquisition and analysis software to dynamically acquire data in real time.
Preferably, the ultrasonic thermography detection system comprises an ultrasonic generator with the output power of 200-3000W and the output frequency of 20-200 kHz, an ultrasonic vibration exciter group with the rated power of 50-200W and the output frequency of 20-200 kHz, a loading limiting device with glass fiber reinforced nylon as a supporting column and aluminum alloy as a main body frame, an infrared thermal imager with the minimum temperature difference resolution precision not exceeding 0.06 ℃ and a computer loaded with thermal image analysis processing software.
Preferably, the negative feedback control system loads control software for a servo loading system loaded in the computer, and can drive the servo loading system to adjust the output load according to the analysis results of the dynamic signal acquisition analysis software and the thermal image analysis processing software under the condition of presetting a test mode.
Preferably, the crack measuring system is composed of a crack observer with the resolution of not more than 0.0125 mm; the heat dissipation system is composed of a refrigeration air cooler, and the maximum air volume is not less than 18000m3And h, the swing can automatically swing at 90 degrees from side to side and adjust at 90 degrees from top to bottom.
The concrete free fracture overall process control visual tracking test method comprises the following steps:
s1, selecting a proper concrete sample as a research carrier of the fracture test according to the research purpose; configuring a loading device according to a load action form to be applied to the test piece, calculating the crack initiation load of the test piece, and determining the sensitive observation initial load of the occurrence of the crack;
s2, arranging a strain gauge group and an ultrasonic vibration exciter group along the opposite surface of the fracture observation surface of the concrete test piece and the bottom surface or the top surface of the test piece to form spatial three-dimensional strain acquisition and ultrasonic thermal excitation; the strain gauge groups uniformly cover the monitoring area of the surface of the concrete sample, and the ultrasonic vibration exciter groups cover the monitoring area of the strain gauge groups after the thermal excitation ranges of the respective distributed points are combined; each strain gauge is respectively connected to a dynamic signal testing and analyzing system through a lead, each vibration exciter is fixed through a loading limiting device adhered to the surface of the test piece, and the vibration exciter is connected with an ultrasonic generator to thermally excite the concrete test piece; the thermal infrared imagers are arranged at equal intervals along the test piece at a certain distance away from the concrete test piece, the thermal infrared imagers are 0.5 to 2.0 meters away from the concrete test piece, and monitoring surfaces of the thermal infrared imagers can completely cover a fracture observation surface of the concrete test piece after being sequentially spliced; the heat dissipation system is arranged around the excitation point of the test piece; the dynamic signal testing and analyzing system and the thermal infrared imager are connected to a computer integrating dynamic signal acquisition and analysis software, thermal image analysis and processing software and a negative feedback control system, and the computer is connected with the universal material testing machine;
s3, driving the ultrasonic vibration exciter group to excite the concrete test piece by using an ultrasonic generator, starting a video recording mode of the thermal infrared imager to monitor the surface temperature field of the concrete test piece, and recording the initial temperature field of the concrete test piece; starting a dynamic signal testing and analyzing system to monitor a stress field of a concrete sample, starting a servo loading system to load control software, setting a test mode according to the concrete sample, initially setting a loading displacement rate of the servo loading system, and then loading the concrete sample; when the pressure load is increased to a sensitive observation initial load, reducing the loading rate; monitoring and determining a first microcrack temperature rise zone appearing in the field of view of the thermal infrared imager through thermal image analysis processing software, and triggering a negative feedback control system to drive a servo loading system to adjust applied pressure load;
s4, stopping exciting the concrete test piece by the ultrasonic vibration exciter group, recording data of the strain gauge group, and radiating the area near the excitation point of the concrete test piece by using a radiating system; positioning the distribution of the microcracks in the concrete sample according to the thermograph, and measuring the length of each crack and the opening displacement of the tip of each crack;
s5, starting ultrasonic excitation, and continuing loading at the loading rate reduced in the step S3; when the number of microcrack temperature rise strips in a monitoring field of the thermal infrared imager is increased, or the microcrack temperature rise strip is elongated and the strain in the strain gauge group is increased steeply, the negative feedback control system drives the servo loading system to adjust the applied pressure load, and the step S4 is carried out; the step S5 is repeated until the concrete test piece is damaged;
and S6, after the concrete test piece is damaged, retrieving the vibration exciter group, the ultrasonic generating device, the loading limiting device, the thermal infrared imager, the dynamic signal testing and analyzing system, the computer loaded with dynamic signal acquisition and analysis software, thermal image analysis and processing software and a negative feedback control system, the crack measuring system and the heat radiating system, and recovering the servo loading system.
Preferably, the suitable concrete test piece in S1 is determined according to research needs, and includes: the concrete strength grade, the reinforcing steel bar reinforcement ratio, the shape and the size of the test piece; the action modes of the load comprise three-point bending and four-point bending damage, and wedging splitting and pulling damage; the observed sensitive initial load was less than 20% of the calculated crack initiation load.
Preferably, the principle of arranging the strain gauge groups in the test piece in S2 is as follows: when the strain gauge is arranged opposite to the observation surface, the strain gauge is arranged along the vertical direction of the main crack propagation direction; for the test piece damaged by the bending action, when the strain gauges are vertically arranged, the strain gauges are firstly arranged along a neutral layer of the test piece and then are symmetrically arranged up and down by taking the neutral layer as an axis, and the distance between the strain gauges is 30-100 mm; when the strain gauges are horizontally arranged, the first row of strain gauges are arranged along the vertical central axis of the test piece, the central point of each strain gauge is the intersection point of the horizontal line where the strain gauge is located and the vertical central axis, and the other rows of strain gauges are symmetrically arranged at intervals of 50-200 mm; for the test piece damaged by the splitting and pulling action, when the test piece is vertically arranged, the strain gauges are uniformly distributed along the vertical direction of the test piece at intervals of 50-100 mm; when the strain gauges are arranged in the horizontal direction, the strain gauges in the first row are arranged along the vertical central axis of the test piece, and the strain gauges in the other rows are symmetrically arranged at intervals of 50-200 mm. When the strain gauge is arranged on the bottom surface of the test piece, the strain gauge is arranged along the longitudinal center line of the bottom surface, is firstly arranged at the midpoint of the center line, is secondly arranged at intervals of 50-200 mm, is symmetrical to the midpoint of the center line, and is arranged at equal intervals along the center line.
Preferably, in S2, the principle of arranging the vibration exciter groups in the test piece is as follows: when the vibration exciters are arranged opposite to the observation surface, the vibration exciters are arranged among the rows of the strain gauge group, and for the adjacent vibration exciters, the distance between the projections of the two vibration exciters in the horizontal direction and the vertical direction is not less than 30mm, and the distance between the projections of the two vibration exciters in the vertical direction is less than 300 mm; the number of vibration exciters among each row of the strain gauge group is not less than 1, and the length of a test piece in the direction of each row of the strain gauge is divided by 300mm, and rounding is carried out to determine the length of a truncated digit; when the vibration exciters are arranged on the upper surface of the test piece, the vibration exciters are symmetrically arranged with the center of the loading area, and the distance between the vibration exciters and the end part of the test piece is not less than 300 mm; the frequency combination scheme of the vibration exciter is that the vibration exciter adjacent to the end part of the test piece in the horizontal direction preferably has the frequencies of 100kHz, 120kHz and 160kHz, the excitation frequency from the vibration exciter at the adjacent end part to the center shaft of the test piece is preferably selected from three frequency bands of 28-40 kHz, 68-100 kHz and 120-160 kHz, and the excitation frequency is increased in sequence; and applying 1500-2500N coupling pressure to each vibration exciter through a loading limiting device.
Preferably, the loading displacement control rate in the S3 is determined according to a test specimen, and for a reinforced concrete specimen, the loading displacement rate is 0.08-0.5 mm/min; for a plain concrete test piece, the loading displacement rate is 0.03-0.2 mm/min; the test mode set in the negative feedback control system includes: plain concrete test pieces and reinforced concrete test pieces.
Preferably, after the pressure load is added to the sensitive observation load in S4, the loading displacement rates in the reinforced concrete test piece mode and the plain concrete test piece mode are respectively adjusted to be 0.05-0.2 mm/min and 0.03-0.1 mm/min; the basis for identifying and determining the microcrack temperature rise zone by the thermal image analysis processing software is as follows: firstly, excluding a temperature rise region with the temperature difference of not less than 0.3 ℃ with the surrounding in the initial temperature field obtained in the step S3, secondly, the ratio of the length of a pixel point connecting line with the temperature difference of not less than 0.3 ℃ with the length of a test piece in the connecting line direction is not less than 5%, and the pixel points with the temperature difference of not less than 0.3 ℃ in the connecting line account for more than 60% of the total pixel points; the pressure testing machine under the drive of negative feedback control system, the adjustment of pressure load includes: for the reinforced concrete test piece mode, the pressure load is kept stable or unloaded, and for the plain concrete test piece mode, the pressure load is unloaded to zero.
Preferably, the steep increase of the strain in the strain gauge group in S5 is that the slope of the strain-versus-pressure curve is not less than 1; the elastic strain interval of the test piece collected by the strain gauge group is determined according to the yield strain of the test concrete test piece, and the test piece stress field transfer convergence rule is further reflected through the elastic strain change rule of the test piece.
Compared with the prior art, the invention provides a concrete free fracture overall process control visual tracking test system and method, which have the following beneficial effects:
1. the invention has the beneficial effects that: the method can be used for researching the fracture process of plain concrete and reinforced concrete test pieces without preset cracks, and the crack initiation and propagation of crack groups are more real.
2. The invention has the beneficial effects that: positioning the microcracks based on the analysis of the ultrasonic thermal excitation temperature field, monitoring the stress field of the test piece by combining a distributed strain gauge group, and realizing feedback control of the loading process on a microscopic scale, thereby presenting the typical generation and development stage of the test piece fracture and the transfer convergence rule of the stress field along with the growth of the microcrack group in the generation and development process as much as possible.
3. The invention has the beneficial effects that: the full-coverage type active positioning and tracking method can be used for actively positioning and tracking the occurrence and development process of the cracks under the component scale, and the form, distribution and size of the microcracks are visualized and quantifiable.
Drawings
Fig. 1 is a schematic diagram of a concrete free fracture overall process control visual tracking test system according to an embodiment of the concrete free fracture overall process control visual tracking test system and method provided by the invention;
fig. 2 is a spatial three-dimensional layout form diagram of a strain gauge group and a vibration exciter group in the concrete free fracture overall process control visual tracking test system and method according to a specific embodiment of the system and method for concrete free fracture overall process control visual tracking test provided by the invention;
fig. 3 is a reading chart of a number 10 strain gauge of a neutral layer at 36.0kN of an embodiment of the system and method for the concrete free fracture overall process control visual tracking test provided by the invention;
fig. 4 is a thermal image of the development state of microcracks in the beam span according to an embodiment of the system and method for the concrete free fracture overall process control visual tracking test provided by the invention;
fig. 5 is a DIC diagram of the development state of microcracks in the beam span according to an embodiment of the system and method for the concrete free fracture overall process control visualization tracking test provided by the present invention.
The reference numerals in the drawings denote: 1. a concrete sample; 2. a servo loading system; 3. a heat dissipation system; 4. a crack measuring system; 5. an ultrasonic generator; 6. an ultrasonic vibration exciter; 7. a thermal infrared imager; 8. a strain gauge; 9. a dynamic signal test analysis system; 10. the computer is provided with dynamic signal acquisition and analysis software, thermal image analysis and processing software and a servo loading system loading control software; 11. a reaction frame; 12. a steel backing plate; 13. a support; 14. and loading a limiting device.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
In the description of the present invention, it is to be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are merely for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention.
Example 1:
referring to fig. 1-2, the concrete free fracture overall process control visual tracking test system comprises: the system comprises a concrete test piece 1, a servo loading system 2, a strain monitoring system, an ultrasonic thermal image detection system, a negative feedback control system, a crack measuring system 4 and a heat dissipation system 3, wherein the concrete test piece 1 adopts one of a reinforced concrete test piece or a plain concrete test piece, the concrete test piece 1 is arranged in the servo loading system 2, the strain monitoring system is used for monitoring the transfer and convergence dynamic state of stress in a stress field of the concrete test piece and assisting the ultrasonic thermal image detection system to trigger the negative feedback control system, the ultrasonic thermal image detection system actively positions and tracks the growth of microcracks in the concrete test piece 1, visualizes the distribution and the form of the microcracks and triggers the negative feedback control system, the negative feedback control system controls the output load of the servo loading system 2 according to the analysis results of the strain monitoring system and the ultrasonic thermal image detection system, and the crack measuring system 4 measures the size of cracks after determining the crack distribution, and the heat dissipation system 3 restores the temperature of the concrete at the ultrasonic thermal excitation position on the concrete test piece 1 to room temperature.
Further, preferably, the servo loading system 2 is composed of a reaction frame 11, a universal material testing machine, a cast iron pressure head, a steel base plate 12 and a support 13; the concrete test piece 1 is arranged above the support 13, and the universal material testing machine is arranged in the reaction frame 11, wherein the displacement control rate of the universal material testing machine is 0.01-500 mm/min, and the maximum loading is 200 kN; the cast iron pressure head is a semi-cylinder, the diameter of the cast iron pressure head is 50-100 mm, and the length of the cast iron pressure head is 200-400 mm; the thickness of the steel base plate 12 is 20-40 mm, the length is 200-800 mm, and the width is 200-300 mm; the support 13 is a cast iron hinged support, the number of the cast iron hinged supports is 2, the hinges are cylindrical, the diameter is 30-50 mm, and the length is 200-400 mm; the breaking process of the concrete test piece 1 is realized by a servo loading system 2.
Further, preferably, the strain monitoring system is composed of a strain gauge 8, a dynamic signal testing and analyzing system 9 and dynamic signal acquisition and analysis software; the 8-range of the strain gauge is 1000 mu epsilon, the sensitivity coefficient is 2.06, and the resistance is 120.1 omega; the dynamic signal testing and analyzing system 9 is embedded with an industrial computer, a high-speed hard disk and a Linux operating system, the dynamic signal testing and analyzing system 9 is connected with the computer and then is controlled by dynamic signal acquisition and analysis software to dynamically acquire data in real time; the ultrasonic thermal image detection system comprises an ultrasonic vibration exciter group with the output power of 200-3000W, the output frequency of 20-200 kHz, an ultrasonic wave generator 5 with the rated power of 50-200W and the output frequency of 20-200 kHz, a loading limiting device 14 with glass fiber reinforced nylon as a supporting column and aluminum alloy as a main body frame, an infrared thermal imager 7 with the minimum temperature difference resolution precision of less than or equal to 0.06 ℃ and a computer loaded with thermal image analysis processing software; the negative feedback control system loads control software for a servo loading system loaded on a computer, and can drive the servo loading system 2 to adjust output load according to analysis results of the strain monitoring system and the ultrasonic thermography detection system under the condition of presetting a test mode.
Further, preferably, the crack measuring system 4 is composed of a crack observer with a resolution less than or equal to 0.0125 mm; the heat dissipation system 3 is composed of a refrigeration air cooler, and the maximum air volume is less than or equal to 18000m3And h, the swing can automatically swing at 90 degrees from side to side and adjust at 90 degrees from top to bottom.
A concrete free fracture overall process control visual tracking test method is applied to a concrete free fracture overall process control visual tracking test system and comprises the following steps:
s1, selecting a proper concrete sample 1 as a research carrier of the fracture test according to the research purpose; configuring a loading device according to a load action form to be applied to the test piece, calculating the crack initiation load of the test piece, and determining the sensitive observation initial load of the occurrence of the crack;
s2, arranging a strain gauge group consisting of strain gauges 8 and an ultrasonic vibration exciter group consisting of ultrasonic vibration exciters 6 along the opposite surface of the fracture observation surface of the concrete specimen 1 and the bottom surface or the top surface of the specimen to form spatial three-dimensional strain acquisition and ultrasonic thermal excitation; the strain gauge groups uniformly cover the surface of the test piece, and the ultrasonic vibration exciter groups cover the monitoring area of the strain gauge groups after being combined in the thermal excitation ranges of the respective distributed points; each strain gauge 8 is respectively connected to a dynamic signal testing and analyzing system 9 through a lead, each ultrasonic vibration exciter 6 is fixed through a loading limiting device 14 adhered to the surface of the concrete sample 1, and each ultrasonic vibration exciter 6 is connected with an ultrasonic generator 5 to thermally excite the concrete sample 1; the thermal infrared imagers 7 are arranged at equal intervals along the test piece at a certain distance away from the concrete test piece, the thermal infrared imagers are 0.5 to 2.0 meters away from the concrete test piece, and monitoring surfaces of the thermal infrared imagers 7 can completely cover a fracture observation surface of the concrete test piece 1 after being sequentially spliced; the heat dissipation system 3 is arranged around the vibration excitation point of the concrete test piece; the dynamic signal testing and analyzing system 9 and the thermal infrared imager 7 are connected to a computer 10 which is provided with dynamic signal collecting and analyzing software, thermal image analyzing and processing software and servo loading system loading control software, and the computer is connected with a universal material testing machine;
s3, driving the ultrasonic vibration exciter group to excite the concrete sample 1 by using the ultrasonic generator 5, starting a video recording mode of the thermal infrared imager 7 to monitor the surface temperature field of the concrete sample 1, and recording the initial temperature field of the concrete sample 1; starting a dynamic signal testing and analyzing system 9 to monitor a stress field of a concrete test piece, starting a servo loading system to load control software, setting a test mode according to the concrete test piece 1, setting two modes of a plain concrete test piece and a reinforced concrete test piece in a negative feedback control system, and initially setting a loading displacement rate of a servo loading system 2, wherein the loading displacement rate of the reinforced concrete test piece is 0.08-0.5 mm/min, and the loading displacement rate of the plain concrete test piece is 0.03-0.2 mm/min; then loading the concrete sample 1; when the pressure load is increased to a sensitive observation initial load, reducing the loading rate; monitoring and determining a first microcrack temperature rise zone appearing in the visual field of the thermal infrared imager 7 through thermograph analysis processing software, and triggering a negative feedback control system to drive a servo loading system 2 to adjust applied pressure load;
s4, stopping exciting the concrete test piece 1 by the ultrasonic vibration exciter group, recording data of the strain gauge group, and radiating the area near the excitation point of the concrete test piece by using the radiating system 3; positioning the distribution of the microcracks in the concrete sample 1 according to the thermograph, and measuring the length of each crack and the opening displacement of the tip of each crack;
s5, starting ultrasonic excitation, and continuing loading at the loading rate reduced in the step S3; when the number of microcrack temperature rise strips in a monitoring field of the thermal infrared imager 7 is increased, or the microcrack temperature rise zone is elongated and the strain in the strain gauge group is increased steeply, the negative feedback control system drives the servo loading system 2 to adjust the applied pressure load, and the step S4 is carried out; the process in the step S5 is circulated until the concrete test piece 1 is damaged;
s6, after the concrete test piece 1 is damaged, the ultrasonic vibration exciter group, the ultrasonic generator 5, the loading limiting device 14, the infrared thermal imager, the dynamic signal testing and analyzing system 9, the computer 10 loaded with dynamic signal acquisition and analysis software, thermal image analysis and processing software and servo loading system loading control software, the crack measuring system 4 and the heat dissipation system 3 are retrieved, and the servo loading system 2 is recovered.
Further, preferably, the suitable concrete specimen 1 in S1 is determined according to research needs, and includes: the concrete strength grade, the reinforcing steel bar reinforcement ratio, the shape and the size of the test piece; the action modes of the load comprise three-point bending and four-point bending damage, and wedging splitting and pulling damage; the observed sensitive initial load was less than 20% of the calculated crack initiation load.
Further, preferably, in S2, the principle of arrangement of the strain gauge groups in the test piece is as follows: when the strain gauge 8 is arranged opposite to the observation surface, the strain gauge 8 is arranged along the vertical direction of the main crack propagation direction; for the test piece damaged by the bending action, when the strain gauges 8 are vertically arranged, the strain gauges are firstly arranged along a neutral layer of the concrete test piece 1 and then are vertically and symmetrically arranged by taking the neutral layer as an axis, and the distance between the strain gauges 8 is 30-100 mm; when the strain gauges 8 are horizontally arranged, the strain gauges 8 in the first row are arranged along the vertical central axis of the concrete sample 1, the central point of each strain gauge 8 is the intersection point of the horizontal line where the strain gauge is located and the vertical central axis, and the strain gauges 8 in the other rows are symmetrically arranged at intervals of 50-200 mm; for the concrete test piece 1 damaged by the splitting and pulling action, when the strain gauges 8 are vertically arranged, the strain gauges are uniformly distributed along the concrete test piece 1 in the vertical direction at intervals of 50-100 mm; when the strain gauges are arranged in the horizontal direction, the first strain gauge 8 is arranged along the vertical central axis of the concrete test piece 1, the other strain gauges 8 are symmetrically arranged at intervals of 50-200 mm, when the strain gauges 8 are arranged on the bottom surface of the concrete test piece 1, the strain gauges 8 are arranged along the longitudinal central line of the bottom surface, are firstly arranged at the central line midpoint, are arranged at intervals of 50-200 mm and are arranged at equal intervals along the central line, and the central line midpoint is taken as a symmetric point.
Further, preferably, the arrangement principle of the ultrasonic exciter group in the concrete sample 1 in S2 is as follows: when the ultrasonic vibration exciters 6 are arranged opposite to the observation surface, the ultrasonic vibration exciters 6 are arranged among the rows of the strain gauge group, the distance between every two adjacent ultrasonic vibration exciters 6 is less than 300mm, the vibration exciting frequencies are the same, and the distance between the projections of the two ultrasonic vibration exciters 6 in the horizontal direction and the vertical direction is more than or equal to 30 mm; the number of the ultrasonic vibration exciters 6 among each row of the strain gauge group is not less than 1, and the length of the concrete sample 1 in the direction of each row of the strain gauge 8 is divided by 300mm, and rounding is carried out to determine the length of the small digit; when the ultrasonic vibration exciters 6 are arranged on the upper surface of the concrete sample 1, the ultrasonic vibration exciters 6 are symmetrically arranged with the center of the loading area, and the distance between the ultrasonic vibration exciters 6 and the end part of the concrete sample 1 is more than or equal to 300 mm; the frequency combination scheme of the ultrasonic vibration exciter 6 is that the ultrasonic vibration exciter 6 adjacent to the end part of the concrete test piece 1 in the horizontal direction preferably has the frequencies of 100kHz, 120kHz and 160kHz, and the vibration exciting frequency from the ultrasonic vibration exciter 6 adjacent to the end part to the central axis of the concrete test piece 1 is preferably selected from three frequency bands of 28-40 kHz, 68-100 kHz and 120-160 kHz and is increased in sequence; 1500-2500N coupling pressure is applied to each ultrasonic vibration exciter 6 through a loading limiting device 14.
Further, preferably, after the pressure load is added to the sensitive observation load in the S4, the loading displacement rates in the reinforced concrete test piece mode and the plain concrete test piece mode are respectively adjusted to 0.05-0.2 mm/min and 0.03-0.1 mm/min; the basis for identifying and determining the microcrack temperature rise zone by the thermographic analysis software is as follows: firstly, excluding a temperature rise region with the temperature difference of not less than 0.3 ℃ with the surrounding in the initial temperature field obtained in the step S3, secondly, the ratio of the length of a pixel point connecting line with the temperature difference of not less than 0.3 ℃ with the length of a test piece in the connecting line direction is not less than 5%, and the pixel points with the temperature difference of not less than 0.3 ℃ in the connecting line account for more than 60% of the total pixel points; the universal material testing machine under the drive of negative feedback control system, the adjustment of pressure load includes: for the reinforced concrete test piece mode, the pressure load is kept stable or unloaded, and for the plain concrete test piece mode, the pressure load is unloaded to zero.
Further, it is preferable that the criterion of the steep increase of the strain in the strain gauge group in S5 is that the slope of the strain-versus-pressure curve is not less than 1; the elastic strain interval of the concrete sample 1 collected by the strain gauge group is determined according to the yield strain of the concrete sample 1 to be tested, and then the stress field transfer convergence rule of the concrete sample is reflected through the elastic strain change rule of the concrete sample 1.
Example 2: based on example 1, but different;
referring to fig. 3-5, a concrete sample 1 is a 2000mm × 200mm × 300mm reinforced concrete test beam, the concrete strength grade is C30, and the beam is reinforced according to the structure: the longitudinal steel bar is HRB335, the diameter is phi 12mm, and the horizontal net spacing is 116 mm; the stirrup is HPB300, the diameter phi is 6mm, the stirrups are arranged at the end part of the beam in a dense mode, the distance between each stirrup and the beam end is 50mm, the distance between the 1 st stirrup and the 2 nd stirrup at the two ends is 150mm, and the distance between the other stirrups is 200 mm. The thickness of the steel bar protective layer is 30 mm. The four-point bending loading mode is adopted, two pressure heads on the upper part of the beam are symmetrically distributed along the span of the beam, the pressure heads are semi-cylinders with the diameter of 100mm and the length of 250mm and are made of cast iron, and the distance between the force application points of the two pressure heads is 460 mm; a cast iron plate with the thickness of 20mm is padded on the pressure head, and the length and the width of the plate are 560mm and 250 mm; the cast iron plate is directly contacted with a universal material testing machine. The distance between two stress points of the steel base plate at the lower part of the beam and the respective beam end is 150 mm. According to SL191-2008 'design specification of hydraulic concrete structure', the cracking load of the test beam is determined to be 27.8kN through calculation.
The strain acquisition system adopts a BX120-80AA resistance strain gauge, a DH5902 dynamic signal test analysis system and DHDAS dynamic signal acquisition analysis software. The length of the strain gauge 8 is 80mm, the width is 3mm, the measuring range is 1000 mu epsilon, the sensitivity coefficient is 2.06, and the resistance is 120.1 omega; the DH5902 system is embedded with an industrial computer, a high-speed hard disk and a Linux operating system, is controlled by DHDAS software after being connected with the computer, dynamically acquires data in real time and records the data. In view of the four-point bending loading form and the space between the force application points and the force bearing points, the strain gauges 8 are arranged in the dense growth area where cracks appear first in the beam, namely the span range of 800mm in the span of the beam. The strain gauge 8 is arranged opposite to the observation surface of the test beam, and in the horizontal direction, the strain gauge 8 is arranged on the neutral layer on the surface of the beam and on the horizontal line 75mm above and below the neutral layer; in the vertical direction, the strain gauges 8 are arranged along the vertical central axis of the beam, the central axis is taken as a symmetry axis, each strain gauge 8 is symmetrically arranged on two sides of the central axis, and the distance between each strain gauge 8 is 100 mm; 5 rows of strain gauges 8 are arranged on the opposite side of the observation surface of the test beam, and each row is provided with 3 strain gauges 8. In addition, the strain gauges 8 are arranged along the longitudinal center line of the bottom surface of the beam, 1 strain gauge is arranged at the center point, the other strain gauges 4 strain gauges are symmetrically arranged at the center point, the distance between every two strain gauges 8 is 100mm, and 5 strain gauges 8 are arranged in total. Each strain gauge 8 is connected to a dynamic signal testing and analyzing system 9 through a lead respectively. The strain gauges 8 are sequentially numbered from top to bottom in columns, the columns are sequentially numbered from right to left, namely from right to left, the first column to the fourth column are as follows: 1 to 4, 5 to 8, 9 to 12, 13 to 16, 17 to 20.
Selecting 51 ultrasonic generators with adjustable 28-40 kHz frequency and 3000W, and preparing 1 vibration exciter with 28kHz (power of 50W); 51 ultrasonic generators with the frequency of 40kHz and the power of 2500W, and 1 vibration exciter with the frequency of 40kHz (power of 50W) is arranged; the frequency of 80-120 kHz is adjustable, 2 ultrasonic generators of 2000W are provided with 2 100kHz vibration exciters (with power of 60W) and 2 120kHz vibration exciters (with power of 60W). The method comprises the following steps of selecting an excitation stroke self-adaptive loading device which takes glass fiber reinforced nylon as a support column and aluminum alloy as a main body frame, wherein 6 sets of loading devices are selected. The vibration exciter is arranged between the columns of the strain gauge group, and the frequency is arranged from the right end to the left end of the test beam in the sequence: 100kHz, 28kHz, 40kHz, 100 kHz; 1 piece of 120kHz vibration exciter is respectively arranged on the upper surface of the test beam 500mm away from the two ends. The vibration exciters arranged opposite to the observation surface of the test beam are wavy, and the distance between the projections of the adjacent vibration exciters in the horizontal direction is 30 mm. At the position where the vibration exciters are to be arranged, firstly, the loading device is bonded and fixed, then the vibration exciters are installed, and 2000N coupling pressure is applied to the vibration exciters by the loading device.
The excitation temperature field of the test beam is collected by a TH9100MV thermal imager, the infrared lens of the thermal imager is as high as the longitudinal center line of the test beam and is about 1m away from the test beam, and 4 thermal imagers are arranged from left to right along the longitudinal direction of the beam, so that the thermal image monitoring visual field completely covers the range of 1000mm across the test beam. DIC technology is also used for monitoring the growth process of hidden cracks in a rectangular area of 200mm multiplied by 300mm in the span of the observation surface of the beam, and the growth process is compared with the detection result of the ultrasonic thermal image in the area. The DIC technology synchronously acquires speckle images of a monitored area through two industrial cameras with 2448 x 2408 pixels, and calculates and analyzes the acquired images through DIC professional image analysis software (PMLAB DIC-3D) to obtain the form of microcracks.
The dynamic signal testing and analyzing system 9 and the thermal infrared imager 7 are connected to a computer 10 which is provided with dynamic signal collecting and analyzing software, thermal image analyzing and processing software and servo loading system loading control software, and the computer is connected with a pressure tester.
Setting a loading displacement rate of 0.3mm/min, and simultaneously driving a vibration exciter group to excite a test piece by using an ultrasonic generator 5; starting a video recording mode of a thermal infrared imager 7 to monitor the surface temperature field of the test piece, and collecting thermal image temperature field information of the test piece before loading to serve as a background temperature field of a subsequent identification temperature rise zone; and starting a dynamic signal testing and analyzing system 9 to monitor the stress field of the test piece, starting negative feedback control software, setting the control software to be in a reinforced concrete mode, and then loading the test piece. The whole process control visualization tracking test of the free fracture of the test beam is carried out from the initial loading until the beam is broken, and 27-level loading is carried out in total, wherein the typical loading process is shown in table 1. The loading column in table 1 is the maximum pressure value reached by loading, and unloading is the pressure value experienced in the beam after unloading is completed. The following is a detailed description of typical stages of fracture of a test beam.
When the pressure load is loaded to 22.2kN, the loading rate is reduced to 0.1 mm/min; when the pressure load is loaded to 30.0kN, after a temperature rise region with the temperature difference not less than 0.3 ℃ with the surrounding temperature in a background temperature field is eliminated by the thermal image analysis processing software, the software automatically determines the first microcrack appearing in the beam (in a pixel point connecting line with the surrounding temperature difference not less than 0.3 ℃, pixel points with the temperature difference not less than 0.3 ℃ account for 80% of the total pixel points), and then triggers a negative feedback control system to drive the pressure tester to unload the pressure load to 20 kN; stopping exciting the test piece by the vibration exciter group, recording data of the strain gauge group, and radiating the area near the excitation point of the test piece by using a radiating system 3; the position of the micro-crack in the test piece is positioned according to the thermography and measured by using a intelligent Bo-Di F130 crack width meter: the length of the crack along the height direction of the beam is about 30mm, and the width of the opening of the crack tip is 0.02 mm.
Driving the ultrasonic excitation test piece to continuously load at a loading rate of 0.1mm/min, and when the pressure load is increased to 36.0kN, rapidly increasing the reading of a No. 10 strain gage 8 of the neutral layer, wherein the slope of a strain change curve along with the pressure under the action of the pressure load is 5.6; at this point, the microcracks A, B, E in the test beam spread further in the direction of the beam height, with typical results shown in FIG. 3. The negative feedback control system drives the pressure tester to unload the pressure load to 31.2 kN; stopping exciting the test piece by the vibration exciter group, recording data of the strain gauge group, and radiating the area near the excitation point of the test piece by using the radiating system 3. The position of the micro-crack A, B, E in the test piece is positioned according to the thermograph, and the three cracks are respectively measured to be 147mm, 62mm and 183mm along the height direction of the beam, and the width of the opening of the crack tip is 0.01 mm. When the pressure load is increased to 40.8kN, the reading of the No. 6 strain gauge 8 of the neutral layer is increased sharply, and the slope of the strain-pressure change curve under the action of the pressure load is 6.2; at this point, the microcracks C, D in the test beam propagate in the beam height direction and the microcracks F are also newly generated. The negative feedback control system drives the pressure tester to unload the pressure load to 31.2 kN; and stopping exciting the test piece by the vibration exciter group, recording the data of the strain gauge group, and radiating the area near the excitation point of the test piece by using a radiating system 3. The position of the micro-crack C, D, F in the test piece was located according to the thermography and it was found that the three cracks were 152mm, 114mm and 40mm respectively in the beam height direction and the split tip opening width distribution was about 0.02mm, 0.01mm and 0.01 mm.
And obtaining the yield strain of the test beam according to the material parameters of the test beam and the load loading form, determining the elastic strain interval of the test beam acquired by the strain gauge group, and further reflecting the stress field transfer convergence rule of the test piece through the elastic strain change rule of the test piece.
The following table is a typical change condition table of the servo pressure tester step-by-step loading part process, the development condition of the microcrack at the corresponding loading stage and the data of the strain gauge 8 under negative feedback control;
the above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (7)
1. Concrete free fracture overall process control visual tracking test system includes: the system comprises a concrete test piece, a servo loading system, a strain monitoring system, a negative feedback control system, a crack measuring system and a heat dissipation system; the concrete test piece is the carrier of free fracture, the concrete test piece sets up in servo loading system, its characterized in that:
the system also comprises an ultrasonic thermal image detection system, wherein the ultrasonic thermal image detection system actively positions and tracks the growth of microcracks in the concrete test piece, visualizes the distribution and the form of the microcracks and provides a basis for the negative feedback control system to drive the servo loading system; the ultrasonic thermal image detection system comprises an ultrasonic generator with the output power of 200-3000W and the output frequency of 20-200 kHz, an ultrasonic vibration exciter group with the rated power of 50-200W and the output frequency of 20-200 kHz, a loading limiting device, an infrared thermal imager with the minimum temperature difference resolution precision of less than or equal to 0.06 ℃ and a computer loaded with thermal image analysis processing software;
the strain monitoring system monitors the transfer and convergence dynamics of stress in the stress field of the concrete test piece, and the auxiliary ultrasonic thermal image detection system provides a basis for the negative feedback control system to control the servo loading system; the strain monitoring system consists of a strain gauge, a dynamic signal testing and analyzing system and dynamic signal acquisition and analysis software; the measuring range of the strain gauge is 1000 mu epsilon, the sensitivity coefficient is 2.06, and the resistance is 120.1 omega; the dynamic signal testing and analyzing system is embedded with an industrial computer, a high-speed hard disk and a Linux operating system, and is controlled by dynamic signal acquisition and analysis software to dynamically acquire data in real time;
the negative feedback control system controls the output load of the servo loading system according to the analysis results of the strain monitoring system and the ultrasonic thermography detection system;
the servo loading system develops free fracture in the concrete test piece;
the crack measuring system measures the crack size after determining the crack distribution;
the heat dissipation system restores the temperature of the concrete test piece in the area near the ultrasonic thermal excitation source to the ambient temperature in a short time;
strain gauge groups are arranged along the opposite side of the fracture observation surface of the concrete sample and the bottom surface of the concrete sample, and ultrasonic vibration exciter groups are arranged along the opposite side of the fracture observation surface of the concrete sample and the upper surface of the concrete sample to form spatial three-dimensional strain acquisition and ultrasonic thermal excitation; the strain gauge group uniformly covers the test piece surface monitoring area, and the ultrasonic vibration exciter group covers the monitoring area of the strain gauge group after the ultrasonic vibration exciter group is combined in the thermal excitation range of each distributed point; the thermal infrared imagers are arranged at equal intervals along the concrete sample at a certain distance away from the concrete sample, and monitoring surfaces of the thermal infrared imagers are sequentially spliced to completely cover a fracture observation surface of the concrete sample; the heat dissipation system is arranged around the vibration excitation point of the concrete test piece;
the arrangement principle of the strain gauge group in the concrete sample is as follows:
when the strain gauge is arranged opposite to the observation surface, the strain gauge is arranged along the vertical direction of the main crack propagation direction;
for a test piece damaged by the bending action, when the strain gauges are vertically arranged, the strain gauges are firstly arranged along a neutral layer of a concrete test piece and then are vertically and symmetrically arranged by taking the neutral layer as an axis, and the distance between the strain gauges is 30-100 mm; when the strain gauges are horizontally arranged, the first row of strain gauges are arranged along the vertical central axis of the concrete test piece, the central point of each strain gauge is the intersection point of the horizontal line where the strain gauge is located and the vertical central axis, and the other rows of strain gauges are symmetrically arranged at intervals of 50-200 mm;
for the concrete test piece damaged by the splitting and pulling action, when the strain gauges are vertically arranged, the strain gauges are uniformly distributed along the vertical direction of the concrete test piece at intervals of 50-100 mm; when the strain gauges are arranged in the horizontal direction, the first row of strain gauges are arranged along the vertical central axis of the concrete test piece, and the other rows of strain gauges are symmetrically arranged at intervals of 50-200 mm;
when the strain gauge is arranged on the bottom surface of the concrete test piece, the strain gauge is arranged along the longitudinal center line of the bottom surface, is firstly arranged at the midpoint of the center line, is secondly arranged at equal intervals along the center line by taking the midpoint of the center line as a symmetrical point at intervals of 50-200 mm;
the arrangement principle of the ultrasonic vibration exciter group in the concrete sample is as follows:
when the ultrasonic vibration exciters are arranged opposite to the observation surface, the ultrasonic vibration exciters are arranged among the rows of the strain gauge group, and for the adjacent ultrasonic vibration exciters, the distance between the projections of the two ultrasonic vibration exciters in the horizontal direction and the vertical direction is greater than or equal to 30mm, and the distance between the projections of the two ultrasonic vibration exciters in the horizontal direction and the vertical direction is less than 300 mm; the number of the ultrasonic vibration exciters among each row of the strain gauge group is not less than 1, and the length of the concrete sample in the direction of each row of the strain gauge is divided by 300mm, and the length is rounded and determined by rounding off decimal places;
when the ultrasonic vibration exciters are arranged on the upper surface of the concrete test piece, the ultrasonic vibration exciters are symmetrically arranged by taking the center of the loading area as a center, and the distance between the ultrasonic vibration exciters and the end part of the concrete test piece is more than or equal to 300 mm.
2. The concrete free fracture overall process control visualization tracking test system according to claim 1, characterized in that:
the negative feedback control system comprises a computer loaded with a servo loading system loading control software; the negative feedback control system drives the servo loading system to adjust the output load according to the analysis result of the dynamic signal acquisition analysis software and the thermal image analysis processing software under the condition of presetting a test mode; the crack measuring system is composed of a crack observation instrument with the resolution of less than or equal to 0.0125 mm.
3. The concrete free fracture overall process control visual tracking test method adopts the concrete free fracture overall process control visual tracking test system in claim 1, and is characterized by comprising the following steps of:
s1, selecting a proper concrete sample as a research carrier of the fracture test according to the research purpose; configuring a servo loading system according to a load action form to be applied to the concrete sample, calculating the crack initiation load of the concrete sample, and determining the sensitive observation initial load of the occurrence of the crack;
s2, arranging an ultrasonic thermography detection system, a strain monitoring system, a negative feedback control system, a crack measuring system and a heat dissipation system;
s3, driving the ultrasonic vibration exciter group to excite the concrete test piece by using an ultrasonic generator, starting a video recording mode of the thermal infrared imager to monitor the surface temperature field of the concrete test piece, and recording the initial temperature field of the concrete test piece; starting a dynamic signal testing and analyzing system to monitor a stress field of a concrete sample, starting a servo loading system to load control software, setting a test mode according to the concrete sample, initially setting a loading displacement rate of the servo loading system, and then loading the concrete sample;
when the pressure load is increased to a sensitive observation initial load, reducing the loading rate; monitoring and determining a first microcrack temperature rise zone appearing in the field of view of the thermal infrared imager through thermal image analysis processing software, and triggering a negative feedback control system to drive a servo loading system to adjust applied pressure load;
s4, stopping exciting the concrete test piece by the ultrasonic vibration exciter group, recording data of the strain gauge group, and radiating the area near the vibration excitation point of the concrete test piece by using a radiating system; positioning the distribution of the microcracks in the concrete sample according to the thermograph, and measuring the length of each crack and the opening displacement of the tip of each crack;
s5, starting ultrasonic excitation, and continuing loading at the loading rate reduced in the step S3; when the number of microcrack temperature rise strips in a monitoring field of the thermal infrared imager is increased or the microcrack temperature rise strips stretch and the strain in the strain gauge group increases steeply, the negative feedback control system drives the servo loading system to adjust the applied pressure load, and the step S4 is carried out; the step S5 is repeated until the concrete test piece is damaged;
and S6, after the concrete test piece is damaged, retrieving the ultrasonic thermal image detection system, the strain monitoring system, the negative feedback control system, the crack measuring system and the heat dissipation system, and recovering the servo loading system.
4. The concrete free fracture overall process control visualization tracking test method according to claim 3, characterized in that: the concrete test pieces in the S1 comprise reinforced concrete test pieces and plain concrete test pieces; the load action forms comprise three-point bending and four-point bending damage, and wedging splitting and pulling damage; the sensitive observation initial load is less than 20% of the calculated crack initiation load.
5. The concrete free fracture overall process control visualization tracking test method according to claim 3, characterized in that: the frequency combination scheme of the ultrasonic vibration exciters is that the ultrasonic vibration exciters adjacent to the end part of the concrete specimen in the horizontal direction select 100kHz, 120kHz or 160kHz frequencies, and the vibration exciting frequencies from the ultrasonic vibration exciters adjacent to the end part to the center shaft of the concrete specimen are selected from three frequency bands of 28-40 kHz, 68-100 kHz and 120-160 kHz and are sequentially increased; applying 1500-2500N coupling pressure to each ultrasonic vibration exciter through a loading limiting device.
6. The concrete free fracture overall process control visualization tracking test method according to claim 4, characterized in that: the basis for determining the microcrack temperature rise zone in the S4 by monitoring the thermal image analysis processing software is as follows: firstly, excluding a temperature rise region with the temperature difference of not less than 0.3 ℃ with the surrounding in the initial temperature field obtained in the step S3, secondly, the ratio of the length of a pixel point connecting line with the temperature difference of not less than 0.3 ℃ with the length of a test piece in the connecting line direction is not less than 5%, and the pixel points with the temperature difference of not less than 0.3 ℃ in the connecting line account for more than 60% of the total pixel points; the servo loading system under the drive of negative feedback control system, the adjustment of pressure load includes: for the reinforced concrete test piece mode, the pressure load is kept stable or unloaded, and for the plain concrete test piece mode, the pressure load is unloaded to zero.
7. The concrete free fracture overall process control visualization tracking test method according to claim 3, characterized in that: the standard of the steep increase of the strain in the strain gauge group in the S5 is that the slope of the strain-pressure change curve is not less than 1; and determining a test piece elastic strain interval collected by the strain gage group according to the yield strain of the test concrete test piece, and further reflecting the test piece stress field transfer convergence rule through the test piece elastic strain change rule.
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