UHPC key tooth dry joint direct shear acoustic emission characteristic test method

Abstract

The invention discloses a UHPC key tooth dry joint direct shear acoustic emission characteristic test method, and belongs to the technical field of UHPC tests.The UHPC key tooth dry joint direct shear acoustic emission characteristic test method comprises the steps that S1, a UHPC test piece is designed and prepared; s2, building a test system which comprises a loading device, an acoustic emission monitoring device and a displacement and strain monitoring device; s3, the test piece is tested through a test system, and corresponding data are collected; s4, analyzing the data, and revealing a dry joint shear stress behavior and a damage failure mechanism under different parameter settings of the test piece by combining acoustic emission characteristic parameters; the UHPC key tooth dry joint direct shear acoustic emission characteristic test method provided by the invention breaks through the limitation of traditional load-displacement curve, crack observation and other methods, realizes real-time, continuous and multi-stage monitoring, and comprehensively masters the damage evolution dynamics.

Description

Technical Field

[0001] The present invention relates to the technical field of UHPC test, and in particular to a direct shear acoustic emission characteristic test method for UHPC key tooth dry joints. Background Art

[0002] In the field of bridge engineering, due to its excellent mechanical properties, UHPC (Ultra-High Performance Concrete) is increasingly widely used in segmental bridge structures. Key transportation facilities such as airport boarding bridges that connect terminals and airplanes often adopt the form of UHPC segmental bridges. Among them, the key tooth dry joint is a key part for structural force transmission. However, affected by discontinuous steel bars and discontinuous steel fiber distribution, the joint becomes a weak link in shear resistance. During the service of the bridge, it needs to bear complex actions such as vehicle loads and environmental vibrations. Micro-damages are likely to initiate and gradually evolve at the key tooth joints, threatening the structural safety.

[0003] Due to discontinuous steel bars and discontinuous steel fiber distribution, the key tooth dry joint of UHPC segmental bridges becomes a weak link in shear resistance. Traditional detection methods (such as load-displacement curves and crack observation) are difficult to capture the evolution of internal micro-damages in real time, while acoustic emission technology can monitor the cracking process of materials through elastic wave signals. Existing research shows that the shear resistance of UHPC key tooth joints is significantly affected by factors such as lateral stress, key tooth parameters (such as depth-to-height ratio, spacing-to-height ratio), and fiber content. However, there is a lack of a quantitative correlation model between acoustic emission characteristic parameters and mechanical responses, and there is an urgent need to establish a full-process damage monitoring method integrating acoustic emission technology. Summary of the Invention

[0004] The purpose of the present invention is to provide a direct shear acoustic emission characteristic test method for UHPC key tooth dry joints, which realizes the full-process monitoring of the initiation, propagation, and failure of damages during the shear process of dry joints by synchronously collecting acoustic emission characteristic parameters and mechanical data, and provides a theoretical basis for optimizing joint design and bearing capacity calculation.

[0005] To achieve the above purpose, the present invention provides a direct shear acoustic emission characteristic test method for UHPC key tooth dry joints, including the following steps: S1. Design and prepare UHPC specimens; S2. Build a test system, including a loading device, an acoustic emission monitoring device, and a displacement and strain monitoring device; S3. Conduct tests on the specimens through the test system and collect corresponding data; S4. Analyze the data and reveal the shear force behavior and damage failure mechanism of the dry joints under different parameter settings of the specimens in combination with acoustic emission characteristic parameters.

[0006] Preferably, the loading device in step S2 uses an electro-hydraulic servo press, and a prestressed screw is used to apply lateral stress; the acoustic emission monitoring device uses several sensors to collect acoustic emission characteristic parameters, including amplitude, energy, count, duration, rise time, and average frequency; the displacement and strain monitoring device uses a micrometer to measure the vertical slip, and strain gauges are pasted at the root of the key teeth to monitor stress concentration.

[0007] Preferably, in step S3, a loading control method is adopted, which is divided into two stages: preloading and formal loading. The same load level is used in the two stages, and the acoustic emission signal, load-displacement data, and strain data are recorded respectively. Before the test, the shear bearing capacity of the specimen is estimated, where is the flat joint area, is the lateral compressive stress on the joint, is the friction coefficient of the joint; at the same time, the following formula is used to estimate the shear bearing capacity of the key tooth joint specimen: ; where is the joint surface area; is the ultimate tensile strength of the specimen.

[0008] Preferably, step S4 specifically includes the following steps: S41. Analyze the acoustic emission characteristic parameters to clarify the development trend from microcracks to failure cracks, and provide data information for the final failure mode and failure mechanism; S42. Construct a mechanical-acoustic emission coupling model that correlates mechanical responses with acoustic emission characteristics to understand the damage and failure process of the specimen; S43. Consider factors such as lateral compressive stress, key tooth geometric parameters, and fiber content, and establish a bearing capacity prediction model in combination with acoustic emission characteristic parameters, so as to establish a quantitative relationship between the above parameters and the bearing capacity of the UHPC key tooth dry joint, and realize the prediction of the bearing capacity.

[0009] Preferably, step S41 specifically includes: Microcrack identification: amplitude < 40 dB, corresponding to early damage inside the key teeth, and the calculation formula is: ; where is the amplitude of the th acoustic emission event, is the indicator function; Macroscopic crack propagation: 40 dB ≤ amplitude ≤ 60 dB, corresponding to crack propagation at the root of the key teeth, and the energy release rate formula is: ; where is the energy of a single event, is the time interval; Failure crack determination: amplitude > 60 dB, corresponding to the shear failure of key teeth. Calculation of the sound source coordinates based on the time difference positioning method: ; Among them, is the sensor coordinate, is the signal arrival time, is the elastic wave velocity.

[0010] Preferably, the mechanical-acoustic emission coupling model in step S42 is based on the following correlation methods: Cracking threshold determination correlation: a sudden increase in the acoustic emission time rate corresponds to the cracking load , combined with the Mohr stress circle criterion formula , jointly determine the critical state of the specimen starting to damage and crack from the mechanical and acoustic perspectives; Ultimate failure point correlation: the peak value of acoustic emission energy corresponds to the ultimate load , based on the shear-compression strength criterion formula , comprehensively judge the moment when the specimen reaches the bearing limit and fails based on the acoustic emission characteristics and mechanical theory. Among them, is the key tooth area; , are coefficients; Failure mode correlation: judge the failure mode according to the frequency of the acoustic emission signal. During ductile failure, low-frequency signals dominate, corresponding to the steel fiber bridging effect, and there is a residual load formula ; during brittle failure, high-frequency signals dominate, corresponding to the shear of key teeth, and there is a damage factor formula , among them, is the residual energy, and the failure mode and post-failure performance state of the specimen are clarified through the correlation between the acoustic emission signal frequency and mechanical properties.

[0011] Preferably, step S43 studies the influence laws of lateral pressure stress, key tooth geometric parameters, and fiber content on the specimen performance, and then integrates and establishes a bearing capacity prediction model. The influence laws of single factors are as follows: Lateral pressure stress: Increasing it delays the peak value of the acoustic emission event rate, and the energy corresponding to the ultimate load increases. The fitting formula is: ; Key tooth depth-to-height ratio : When = 0.5, the acoustic emission events are concentrated at the root, and the failure mode is ductile; when < 0.3, the failure mode is brittle, and the formula is: , among them, is the damage uniformity coefficient; The steel fiber content formula is: , where is the fiber volume fraction; is the reference energy density.

[0012] Preferably, the expression of the bearing capacity prediction model is: ; where is the constant term; , , , , , are the regression coefficients of the corresponding variables respectively; is the acoustic emission energy; is the acoustic emission event count; is the average frequency.

[0013] Therefore, the present invention adopts the above-mentioned test method for the direct shear acoustic emission characteristics of UHPC key tooth dry joints, and has the following beneficial effects: (1) Multi-stage monitoring from microcrack initiation to macroscopic failure is realized, and the dynamic correlation of stress-damage-acoustic signal is achieved; (2) By integrating parameters such as lateral compressive stress, key tooth geometry, and fiber content, a bearing capacity prediction model based on acoustic emission characteristics is established, so as to establish a quantitative relationship between the above parameters and the bearing capacity of UHPC key tooth dry joints, and realize the prediction of the bearing capacity.

[0014] The technical solution of the present invention will be further described in detail below with reference to the drawings and embodiments. Brief Description of the Drawings

[0015] Figure 1 is a flowchart of a test method for the direct shear acoustic emission characteristics of UHPC key tooth dry joints according to the present invention; Figure 2 is a specific flowchart of data analysis in an embodiment of the present invention; Figure 3 is a superimposed diagram of the load-slip curve and the acoustic emission energy rate in an embodiment of the present invention. Detailed Embodiments

[0016] The following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0017] In the construction and operation and maintenance of bridge engineering, UHPC segment bridges are widely used due to their excellent mechanical properties. Especially in facilities such as airport boarding bridges where strict requirements are imposed on structural safety and durability, the key-tooth dry joint, as a key load-transfer part, its shear resistance and damage evolution process are directly related to the overall service safety of the bridge. To accurately explore the acoustic emission characteristics of UHPC key-tooth dry joints under direct shear force, clarify the damage development law, and support the construction of a health monitoring and safety assessment system for bridge structures throughout their life cycle, this test is specifically carried out. The following details the specific implementation process of the test method for the direct shear acoustic emission characteristics of UHPC key-tooth dry joints in combination with the actual service environment and loading characteristics of the bridge.

[0018] Please refer to Figures 1 - 3 , a test method for the direct shear acoustic emission characteristics of UHPC key-tooth dry joints, comprising the following steps: S1. Design and prepare UHPC specimens; S2. Set up a test system, including a loading device, an acoustic emission monitoring device, and a displacement and strain monitoring device; The loading device uses an electro-hydraulic servo press, and a prestressed screw rod is used to apply lateral stress. Loading rate: 0.5 kN / s in the elastic stage and 0.1 mm / min after cracking; The acoustic emission monitoring device uses several sensors (frequency 100 kHz - 1 MHz, threshold 40 dB) to collect acoustic emission characteristic parameters, including amplitude, energy, count, duration, rise time, and average frequency; The displacement and strain monitoring device uses a dial gauge to measure vertical slip (accuracy 0.001 mm), and strain gauges (gage length 5 mm) are pasted at the root of the key teeth to monitor stress concentration.

[0019] S3. Conduct tests on the specimens through the test system and collect corresponding data; adopt a loading control method, which is divided into two stages: preloading and formal loading. The same load level is used in both stages, and acoustic emission signals, load-displacement data, and strain data are recorded respectively. Preloading: Preload at 10% of the estimated ultimate load for 5 minutes to calibrate the equipment. Formal loading: Force control stage: 20 kN / level, hold each level for 2 minutes, and record the initial acoustic emission signal; Displacement control stage: After cracking, switch to 0.1 mm / min until the load drops to 50% of the ultimate load or the slip reaches 5.5 mm and stop. Before the test, use to estimate the shear bearing capacity of the specimens, where is the area of the flat joint surface, is the lateral compressive stress on the joint, is the friction coefficient of the joint; at the same time, the following formula is used to estimate the shear bearing capacity of the key-tooth joint specimens: ; Where is the joint surface area; is the ultimate tensile strength of the specimen.

[0020] S4. Analyze the data and reveal the dry joint shear stress behavior and damage failure mechanism of the specimen under different parameter settings in combination with the acoustic emission characteristic parameters, which specifically include the following steps: S41. Analyze the acoustic emission characteristic parameters, clarify the development trend from microcracks to failure cracks, and provide data information for the final failure mode and failure mechanism, which specifically include: Microcrack identification: amplitude < 40 dB, corresponding to early damage inside the key teeth, and the calculation formula is: ; where, is the amplitude of the th acoustic emission event, is the indicator function; Macroscopic crack propagation: 40 dB ≤ amplitude ≤ 60 dB, corresponding to the crack propagation at the root of the key teeth, and the energy release rate formula is: ; where, is the energy of a single event, is the time interval; Failure crack determination: amplitude > 60 dB, corresponding to the shear failure of the key teeth, and the calculation of the sound source coordinates based on the time difference positioning method: ; where, is the sensor coordinate, is the signal arrival time, is the elastic wave velocity.

[0021] S42. Construct a mechanical-acoustic emission coupling model that correlates the mechanical response with the acoustic emission characteristics to understand the damage failure process of the specimen; the mechanical-acoustic emission coupling model is based on the following correlation methods: Cracking threshold determination correlation: the sudden increase in the acoustic emission time rate corresponds to the cracking load , combined with the Mohr stress circle criterion formula , jointly determine the critical state of the specimen starting to damage and crack from both mechanical and acoustic perspectives; Ultimate failure point correlation: the peak value of the acoustic emission energy corresponds to the ultimate load , based on the shear compression strength criterion formula , comprehensively judge the moment when the specimen reaches the bearing limit and fails by combining the acoustic emission characteristics and mechanical theory, where, is the key tooth area; , are coefficients; Failure mode correlation: The failure mode is discriminated according to the acoustic emission signal frequency. During ductile failure, low-frequency signals dominate, corresponding to the steel fiber bridging effect, and there is a residual load formula ; During brittle failure, high-frequency signals dominate, corresponding to the shear of key teeth, and there is a damage factor formula , where is the residual energy. By correlating the acoustic emission signal frequency with the mechanical properties, the failure mode of the specimen and the performance state after failure are clarified.

[0022] S43. Considering the factors of lateral compressive stress, key tooth geometric parameters and fiber content, a bearing capacity prediction model is established by combining the acoustic emission characteristic parameters, so as to establish a quantitative relationship between the above parameters and the bearing capacity of the UHPC key tooth dry joint, and realize the prediction of the bearing capacity. Through the influence laws of the single factors of lateral compressive stress, key tooth geometric parameters and fiber content on the specimen performance, and then integrating to establish a bearing capacity prediction model. Among them, the influence laws of the single factors are as follows: Lateral compressive stress: Increasing it delays the peak value of the acoustic emission event rate, and the energy corresponding to the ultimate load increases. The fitting formula is: ; Key tooth depth-to-height ratio : When = 0.5, the acoustic emission events are concentrated at the root, and the failure mode is ductile; when < 0.3, the failure mode is brittle, and the formula is: , where is the damage uniformity coefficient; The steel fiber content formula is: , where is the fiber volume fraction; is the reference energy density.

[0023] The expression of the bearing capacity prediction model is: ; Among them, is the constant term; , , , , , are the regression coefficients of the corresponding variables respectively; is the acoustic emission energy; is the acoustic emission event count; is the average frequency.

[0024] Therefore, the present invention adopts the above-mentioned test method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint, breaks through the limitations of traditional methods such as load-displacement curves and crack observations, realizes real-time, continuous and multi-stage monitoring, and comprehensively grasps the dynamic damage evolution.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the present invention or make equivalent replacements, and these modifications or equivalent replacements do not enable the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An experimental method for the direct shear acoustic emission characteristics of UHPC key tooth dry joints, characterized in that, It includes the following steps: S1. Design and prepare UHPC specimens; S2. Build a test system, including a loading device, an acoustic emission monitoring device, and a displacement and strain monitoring device; S3. Conduct tests on the specimens through the test system and collect corresponding data; S4. Analyze the data and reveal the shear force behavior and damage failure mechanism of the dry joint under different parameter settings of the specimens by combining acoustic emission characteristic parameters. Specifically, it includes the following steps: S41. Analyze the acoustic emission characteristic parameters, clarify the development trend from microcracks to failure cracks, and provide data information for the final failure mode and failure mechanism; S42. Construct a mechanical-acoustic emission coupling model that correlates mechanical responses with acoustic emission characteristics to understand the damage failure process of the specimens; S43. Consider the factors of lateral compressive stress, key tooth geometric parameters, and fiber content, and establish a bearing capacity prediction model by combining acoustic emission characteristic parameters, so as to establish a quantitative relationship between the above parameters and the bearing capacity of the UHPC key tooth dry joint and realize the prediction of the bearing capacity.

2. The test method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint according to claim 1, wherein: In step S2, the loading device uses an electro-hydraulic servo press, and a prestressed screw is used to apply lateral stress; the acoustic emission monitoring device uses several sensors to collect acoustic emission characteristic parameters respectively, including amplitude, energy, count, duration, rise time, and average frequency; the displacement and strain monitoring device uses a micrometer to measure the vertical slip, and strain gauges are pasted at the root of the key teeth to monitor stress concentration.

3. The experimental method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint according to claim 1, wherein: In step S3, a loading control method is adopted, which is divided into two stages: preloading and formal loading. The same load level is used in both stages, and acoustic emission signals, load-displacement data, and strain data are recorded respectively. Before the test, is used to estimate the shear bearing capacity of the specimen. Among them, is the flat joint area, is the lateral compressive stress on the joint, is the friction coefficient of the joint; at the same time, the following formula is used to estimate the shear bearing capacity of the keyed joint specimen: ； wherein, is the joint surface area; is the ultimate tensile strength of the test piece.

4. The test method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint according to claim 3, wherein Step S41 specifically includes: Microcrack identification: amplitude < 40 dB, corresponding to early damage inside the key teeth, and the calculation formula is: ； Among them, is the amplitude of the nth acoustic emission event, and Macroscopic crack propagation: 40 dB ≤ amplitude ≤ 60 dB, corresponding to the crack propagation at the root of the key tooth, and the energy release rate formula is: ; where is the energy of a single event, is the time interval; Failure crack determination: amplitude > 60 dB, corresponding to the shear failure of the key teeth, and the sound source coordinate calculation based on the time difference positioning method: ； Among them, is the sensor coordinate, is the time of arrival of the signal, is the elastic wave velocity.

5. The test method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint according to claim 4, wherein, The mechanical-acoustic emission coupling model in step S42 is based on the following correlation method: Cracking threshold determination correlation: The sudden increase in the acoustic emission time rate corresponds to the cracking load , combined with the Mohr stress circle criterion formula , jointly determine the critical state of the specimen's initial damage and cracking from the mechanical and acoustic perspectives; Ultimate failure point correlation: peak acoustic emission energy Corresponding to the ultimate load , based on the shear-compression strength criterion formula , comprehensively judge the moment when the specimen reaches the bearing limit and fails based on acoustic emission characteristics and mechanical theory. Among them, is the key tooth area; 、 are coefficients; Failure mode correlation: Determine the failure mode based on the frequency of acoustic emission signals. During ductile failure, low-frequency signals dominate, corresponding to the bridging effect of steel fibers, and there is a residual load formula ; During brittle failure, high-frequency signals dominate, corresponding to the shearing of key teeth, and there is a damage factor formula , where is the residual energy. By correlating the acoustic emission signal frequency with the mechanical properties, the failure mode of the specimen and the performance state after failure are clarified.

6. The test method for the direct shear acoustic emission characteristics of the UHPC key tooth dry joint according to claim 5, characterized in that In step S43, the influence laws of single factors such as lateral compressive stress, key tooth geometric parameters, and fiber content on the specimen performance are considered, and then a bearing capacity prediction model is established by integration. Among them, the influence laws of single factors are as follows: Lateral compressive stress: Increasing it delays the peak value of the acoustic emission event rate and increases the energy corresponding to the ultimate load. The fitting formula is: ; Key tooth depth-to-height ratio : When = 0.5, acoustic emission events are concentrated at the root and the failure mode is ductile; when < 0.3, the failure mode is brittle, and the formula is: , where is the damage uniformity coefficient; The formula for the steel fiber content is as follows: , where is the fiber volume fraction; is the reference energy density.

7. The test method for direct shear acoustic emission characteristics of UHPC key tooth dry joints according to claim 6, characterized in that The expression of the bearing capacity prediction model is: ； Among them, is the constant term; , , , , , are the regression coefficients of the corresponding variables respectively; is the acoustic emission energy; is the acoustic emission event count; is the average frequency.

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