CN109374452B - A method and testing device for characterization of fatigue damage status of prestressed concrete beams - Google Patents

Abstract

The invention discloses a fatigue damage state characterization method of a prestressed concrete beam, which comprises the following steps: measuring initial dynamic parameters of the prestressed concrete beam and performing a first static load test; static load test and dynamic test of prestressed concrete beams in fatigue loading process; analyzing the vibration mode parameters of the prestressed concrete beam; and (5) evaluating the fatigue damage state of the prestressed concrete beam. The invention also discloses a measuring device for realizing the scheme. According to the invention, the dynamic test is carried out in the fatigue test process of the prestressed concrete beam structure, and the position of the fatigue damage of the prestressed concrete beam and the damage degree thereof are analyzed based on the vibration mode parameters, so that the fatigue damage state of the prestressed concrete beam is represented based on the mode parameters, and a new thought is provided for the research and test of the fatigue performance of the prestressed concrete beam.

Description

A method and testing device for characterization of fatigue damage status of prestressed concrete beams

Technical field

The invention belongs to the technical field of fatigue damage testing of engineering structures, and in particular relates to a fatigue damage state characterization method and testing device for prestressed concrete beams.

Background technique

Prestressed concrete beams are widely used in long-span civil engineering structures such as highways and railway bridges. This type of prestressed concrete beam will withstand repeated vehicle fatigue loads for a long time during its design service life. Fatigue load will cause the occurrence and development of structural fatigue damage. As the damage accumulates, it will affect the normal performance of the structure and even endanger the safety of the structure. Therefore, the fatigue performance of prestressed concrete beams has attracted widespread attention from many scholars and engineering technicians at home and abroad.

In the traditional fatigue performance test of prestressed concrete beams, a pulsating fatigue testing machine is usually used to apply constant amplitude fatigue loads. At typical moments in the fatigue loading process, the machine is stopped to carry out static loading and unloading experiments, so as to determine the deflection, Stress and strain isostatic properties infer the fatigue damage development process of prestressed concrete beams. These methods have a single entry point and cannot reflect the stress characteristics of the structure under fatigue dynamic loads. It is difficult to reveal the development process and fatigue failure mechanism of fatigue damage of prestressed concrete beams. It is urgent to research and develop new dynamic testing methods to study prestressing. A new approach to fatigue problems in concrete beams.

Contents of the invention

The first technical problem to be solved by the present invention is to provide a method for characterizing the fatigue damage state of prestressed concrete beams based on modal parameters.

The second technical problem to be solved by the present invention is to provide a testing device that implements the above fatigue damage state characterization method.

In order to solve the above-mentioned first technical problem, the present invention adopts the following technical solutions:

A method for characterizing the fatigue damage state of prestressed concrete beams, which is characterized by including the following steps:

Step 1: Place the prestressed concrete beam to be tested on two simply supported supports. First, perform a dynamic test on the initial intact prestressed concrete beam to be tested, and obtain the initial modal frequency w ₀ of the prestressed concrete beam to be tested;

Step 2: Turn on the fatigue testing machine to perform a fatigue test on the prestressed concrete beam to be tested. After a certain number of fatigue cycles, stop the machine for dynamic testing. Obtain the modal frequency w _n of the prestressed concrete beam to be tested when the number of fatigue cycles is n million times;

Step 3: Obtain the damage variables of the prestressed concrete beam to be measured under different number of cycles according to the following formula:

Among them: w ₀ is the initial modal frequency, w _N is the modal frequency when the prestressed concrete beam is tested for fatigue failure, and N is the number of cycles when the prestressed concrete beam is tested for fatigue failure;

Step 4: Fit the damage variables of the prestressed concrete beam to be tested under different number of cycles obtained in step 3, and obtain the evolution law of damage variables in the entire fatigue process based on modal frequency, thereby characterizing the fatigue damage of the prestressed concrete beam to be tested. state.

Furthermore, the damage variables and cycle times are fitted using the following formula:

Among them: α, β are the parameters to be fitted, n is the number of cycles, and N is the number of cycles when the fatigue failure of the prestressed concrete beam is to be measured.

Further, the dynamic test adopts the vibration excitation method. By moving the vibration exciter to the maximum amplitude point of the theoretical vibration shape of each order of the prestressed concrete beam to be tested, frequency sweep is performed to excite the vibration. The acceleration signal is collected through the vibration pickup and then transmitted to The modal testing and analysis system performs modal analysis.

Further, the first-order modal frequency is selected as the damage variable defined by modal frequency.

Furthermore, during the dynamic test, the vibrator and vibration pickup were staggered.

In order to solve the above-mentioned second technical problem, the present invention adopts the following technical solutions:

A testing device that implements the above characterization method, including two simply supported supports set relatively on the ground, a prestressed concrete beam to be tested set on the two simply supported supports, an excitation system and a modal test analysis system;

The excitation system includes an exciter, a power amplifier and a signal amplifier connected in sequence. The exciter is freely movable and is located below the prestressed concrete beam to be tested and between two simply supported supports;

The modal testing and analysis system includes a vibration pickup, a signal acquisition system and a modal analysis system connected in sequence. The vibration pickup is arranged on the top surface of the prestressed concrete beam to be tested.

Further, the vibration pickups are arranged equally along the length direction of the prestressed concrete beam to be measured.

Further, the vibrator is installed on the walking trolley.

Further, a fatigue testing machine is set above the prestressed concrete beam to be tested. The fatigue testing machine is fixedly installed on the ground through a reaction frame. The top of the prestressed concrete beam to be tested is located below the actuator head of the fatigue testing machine and is provided with a load distribution beam. , the center point of the actuator head of the fatigue testing machine is directly facing the center point of the prestressed concrete beam to be tested and the center point of the load distribution beam.

Furthermore, displacement meters are installed at the upper parts of both ends of the prestressed concrete beam to be measured at the simply supported supports, at the mid-span position of the beam, and at the bottom of the load loading point.

Further, both ends of the load distribution beam are connected to the prestressed concrete beam to be measured through simply supported supports.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention utilizes the good mapping relationship between the fatigue damage state and modal parameters of the structure during the fatigue loading and failure process of prestressed concrete beams, and combines the fatigue test of the prestressed concrete beam structure with the dynamic test test, based on the vibration modal parameters. By analyzing the location and degree of fatigue damage of prestressed concrete beams, the fatigue damage state of prestressed concrete beams can be characterized based on modal parameters, which provides a new idea for the research and testing of fatigue performance of prestressed concrete beams.

2. The test device of the present invention improves the traditional fixed vibration equipment into a mobile vibration equipment, which can be quickly transported according to the location of the vibration point, making the test more convenient, and has the advantages of simple structure and convenient testing.

Description of the drawings

Figure 1 is a flow chart of the characterization method of the present invention;

Figure 2 is a schematic diagram of the power test of the present invention;

Figure 3 is a schematic diagram of the device for the fatigue test process of the present invention;

Figure 4 is a cross-sectional view of the present invention;

Figure 5 is a schematic diagram of excitation position selection;

Figure 6 shows the measured frequency degradation ratio curve during the fatigue process;

Figure 7 shows the fatigue damage evolution diagram of prestressed concrete beams characterized by the first-order modal frequency.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

Referring to Figure 1, a method to characterize the fatigue damage state of prestressed concrete beams includes the following steps:

Step 1: Place the prestressed concrete beam to be tested on two simply supported supports. First, perform a dynamic test on the initial intact prestressed concrete beam to be tested, and obtain the initial modal frequency w ₀ of the prestressed concrete beam to be tested;

Step 2: Turn on the fatigue testing machine to perform a fatigue test on the prestressed concrete beam to be tested. After a certain number of fatigue cycles, stop the machine for dynamic testing. Obtain the modal frequency w _n of the prestressed concrete beam to be tested when the number of fatigue cycles is n million times;

Step 3: Obtain the damage variables of the prestressed concrete beam to be measured under different number of cycles according to the following formula:

Among them: w ₀ is the initial modal frequency, w _N is the modal frequency when the prestressed concrete beam is tested for fatigue failure, and N is the number of cycles when the prestressed concrete beam is tested for fatigue failure;

Step 4: Fit the damage variables of the prestressed concrete beam to be tested under different number of cycles obtained in step 3, and obtain the evolution law of damage variables in the entire fatigue process based on modal frequency, thereby characterizing the fatigue damage of the prestressed concrete beam to be tested. state.

Furthermore, the damage variables and cycle times are fitted using the following formula:

Among them: α, β are the parameters to be fitted, n is the number of cycles, and N is the number of cycles when the fatigue failure of the prestressed concrete beam is to be measured.

Further, the dynamic test adopts the vibration excitation method. By moving the vibration exciter to the maximum amplitude point of the theoretical vibration shape of each order of the prestressed concrete beam to be tested, frequency sweep is performed to excite the vibration. The acceleration signal is collected through the vibration pickup and then transmitted to The modal testing and analysis system performs modal analysis.

Further, the first-order modal frequency is selected as the damage variable defined by modal frequency.

Furthermore, during the dynamic test, the vibrator and vibration pickup were staggered.

During the modal test, the selection of the excitation position follows the following principles: 1) Move the exciter to the maximum amplitude point of the theoretical vibration shape of each order of the beam to be tested for frequency sweep excitation; 2) The exciter The placement location should avoid being directly below the vibration pickup location to avoid distortion of the vibration signal measured at this point due to overload.

The modal analysis method selected for the modal test analysis system is the random subspace SSI method. This method does not require the input of external excitations or the external excitations are unmeasurable, and can effectively extract structural modal parameters from the structural response to environmental excitations.

Referring to Figures 2 to 5, a testing device for implementing the above characterization method includes two simply supported supports 1 arranged relatively on the ground 10, and a prestressed concrete beam to be tested arranged on the two simply supported supports 1. 2. Excitation system 3 and modal test analysis system 4.

Specifically, the excitation system 3 includes an exciter 301, a power amplifier 302 connected to the exciter 301, and a signal amplifier 303 connected to the power amplifier 302. The exciter 301 is freely movable and arranged on the prestressed concrete beam to be tested. The lower part of 2 is located between two simply supported supports 1. The modal test analysis system 4 includes a vibration pickup 401, a signal acquisition system 402 connected to the vibration pickup 401, a modal analysis system 403 connected to the signal acquisition system 402, a modal analysis system 403 connected to the signal acquisition system 402, and other components. They are all existing structures and will not be described in detail here.

The vibration pickup 401 is arranged on the top surface of the prestressed concrete beam 2 to be tested. The vibration pickup 401 is adsorbed on the magnetic support, and the magnetic support is fixedly installed on the prestressed concrete beam 2 to be tested. There is also a fatigue testing machine 5 with an actuator facing the prestressed concrete beam 2 to be tested above the prestressed concrete beam 2 to be tested. The fatigue testing machine 5 is fixedly installed on the ground through a reaction frame 6 .

Preferably, the vibration pickups 401 are equally arranged along the length direction of the prestressed concrete beam 2 to be measured, the output end of the vibration pickup 401 is connected to the input end of the signal acquisition system 402, and the output end of the signal acquisition system 402 is connected to the modal analysis system. The input end of 403 is connected. The signal acquisition system 402 collects the vibration signal measured by the vibration pickup 401 and transmits it to the modal analysis system 403. The modal analysis system 403 obtains the modal parameters of the beam to be measured through analysis and processing.

It can be imagined that in the actual design, the exciter 301 is set on the walking trolley 11, and the walking trolley 11 is set on the ground. For each modal test, the vibration exciter is quickly transported to the designated excitation site through the walking trolley. points to make modal testing more convenient and effective.

It should be noted that in practical applications, the simply supported support 1 includes a base 101 fixedly installed on the ground and a hinge support 102 provided on the base 101. The two hinge supports 102 are used to support the beam to be measured. (prestressed concrete beam), the base 101 is fixedly installed on the laboratory floor 10 through fastening bolts. There is also a load distribution beam 7 on the top of the prestressed concrete beam 2 to be tested for transmitting the load applied by the fatigue testing machine 5 to the beam to be tested. Both ends of the load distribution beam 7 are connected to the prestressed concrete beam to be tested through hinge supports 8 The concrete beam 2 is connected. Displacement meters 9 are provided at the upper end of the beam, the mid-span position of the beam and the bottom of the load loading point corresponding to the simply supported supports at both ends of the prestressed concrete beam to be measured. During the test, according to the loading procedure of the static monotonic loading test, graded loading is carried out to the fatigue upper limit load, and the strains, cracks, deflections, etc. and their development under each level of load are measured.

The present invention will be further described below in conjunction with specific embodiments.

Example

A 32m ordinary height standard railway bridge prestressed concrete simply supported T-beam was selected as the prototype beam. Based on the similarity theory, a 1:6 scale model of the prototype beam was made as the prestressed concrete beam to be tested 3. The design parameters are shown in Table 1 below. A total of three model beams were prepared in this example, one of which (No. 1) was used for static load testing to determine the static limit load P _u required for the fatigue test. The actual measured P _u =265kN; the other two ( No. 2, No. 3) are used for fatigue testing.

Table 1 Model beam design parameters

The concrete mix ratio is cement: water: stone: sand: water-reducing agent = 460: 118: 1092: 735: 4.2. Concrete test blocks are reserved when pouring each test beam, and its mechanical properties are tested at the same time as the model beam test. The measured The mechanical properties are shown in Table 2 below:

Table 2 Measured concrete mechanical performance parameters

The longitudinal bars use HRB335 grade steel bars with a diameter of 10mm; according to the design and construction requirements of railway bridges, stirrups with a diameter of 8mm (HPB300) and a spacing of 100mm are arranged in the pure bend section of the beam, and 50mm in other sections. The measured mechanical property parameters of steel bars are shown in Table 3 below:

Table 3 Measured mechanical properties parameters of steel bars

The prestressed steel bars use two bundles of 7φ5 steel strands, with a nominal diameter d = 15.2mm, a standard ultimate strength value f _ptk = 1860MPa, and a parabolic layout. The prestressed tendons are tensioned at both ends (single-hole jacks and single steel strands are stretched in two steps), the tensioning control stress σ _con = 1116MPa, over-tensioning 5%, and the age of the concrete during tensioning exceeds 28 sky.

A method for characterizing the fatigue damage state of prestressed concrete beams, which mainly includes the following steps:

1) Measurement of the initial dynamic parameters of the prestressed concrete beam and the first static load test: Place the prestressed concrete beam 2 to be tested on the simply supported support 1, and first perform a dynamic test on the initial intact prestressed concrete beam 2 to be tested. The test adopts the excitation method, by moving the exciter 301 to the vicinity of the maximum amplitude point of the theoretical vibration shape of each order to perform frequency sweep excitation, and collects the acceleration signal through the vibration pickup 401 and then transmits it to the modal test analysis system for modal analysis. Analyze and obtain the modal parameters of each order; then arrange the load distribution beam 7 according to the four-point bending loading method, and place the load distribution beam 7 on the prestressed concrete beam 2 to be tested through the hinge support 8, and ensure the operation of the fatigue testing machine 5 The center point of the moving head is directly opposite to the center point of the prestressed concrete beam 2 to be tested and the center point of the load distribution beam 7; according to the loading procedure of the static monotonic loading test, graded loading is carried out to the fatigue upper limit load, and the load under each level of load is measured. Strain, crack, deflection, etc. and their development;

2) Static load test and dynamic test of prestressed concrete beams during the fatigue loading process: After completing step 1), start the fatigue testing machine 5 to perform fatigue loading, and the number of fatigue load cycles reaches 10,000, 50,000, 100,000, and 250,000. After the second stop (and by analogy, until the prestressed concrete beam 2 to be tested is close to fatigue failure), a dynamic test and a static load test loaded to the fatigue upper limit load are performed as described in step 1); the prestressed concrete beam to be tested is After the fatigue failure of beam 2, a dynamic test and a static load test were conducted.

The fatigue test adopts constant amplitude sine wave loading with a loading frequency of 3.5Hz. The main test parameters are shown in Table 4 below. The lower limit of fatigue load is P _min = 0.2P _u and the upper limit of fatigue load P _max is 0.45P _u and 0.5 respectively. _Pu .

Table 4 Model beam test parameters and fatigue life

3) Analysis of vibration modal parameters of prestressed concrete beams: As shown in steps 1) and 2), perform dynamic tests on the initially intact prestressed concrete beam 2 to be tested before the initial static load and during the fatigue process. Through the modal The test analysis system conducts vibration modal parameter analysis and obtains modal parameters of each order in the fatigue process.

When measuring the initial dynamic parameters, the load distribution beam 7 and the simply supported support 8 are removed, and the magnetic support matched with the vibration pickup 401 is pasted on the prestressed concrete beam 2 to be measured. The vibration pickup 401 is adsorbed on the magnetic On the support, it is fixed on the prestressed concrete beam 2 to be tested; during the fatigue test cycle loading, the vibration pickup 401 is removed and kept. When the fatigue test is stopped for a certain number of times for dynamic testing, the vibration pickup 401 is taken out and adsorbed on the magnetic support. Measure again.

The modal analysis method selected for the modal test analysis system is the random subspace SSI method. This method does not require the input of external excitations or the external excitations are unmeasurable, and can effectively extract structural modal parameters from the structural response to environmental excitations.

4) Fatigue damage status assessment of prestressed concrete beams: The modal parameters of each order in the fatigue history obtained through the analysis in step 3) are analyzed to evaluate the fatigue damage status of the prestressed concrete beam 2 to be measured.

The instrument models and manufacturers used in this example are shown in Table 5 below:

Table 5 Test Instruments

Based on the fatigue damage state characterization method and testing device of prestressed concrete beams based on modal parameters described in the present invention, the frequencies of the two fatigue test beams in the fatigue history are obtained as shown in Table 6.

Table 6 Summary table of measured fatigue history and degradation ratio

Define the frequency degradation ratio under fatigue:

γ(n)＝w _n /w ₀

In the formula, ω ₀ is the initial frequency of the intact beam; ω _n is the frequency of the beam after fatigue n thousand times. Then the frequency degradation ratio of the first three measured frequencies in the fatigue history is obtained in Table 6, and the frequency degradation ratio curve is drawn as shown in Figure 6.

It can be seen from Table 6 above and Figure 6 that as the number of fatigue times increases, the first three frequencies of prestressed concrete beams all decrease. At the beginning of loading, the modal frequency shows a significant decrease; after entering the middle stage of fatigue, the frequency decrease rate slows down and gradually decreases to a smaller value, fluctuating but basically remaining stable; when the fatigue life is reached, an amplitude appears. Smaller reduction, the final frequency reduction amplitudes of the first three orders of beam No. 2 are 19.5%, 15.8%, and 9.0% respectively, and the reduction amplitudes of the first three frequencies of beam No. 3 are 19.4%, 13.6%, and 7.4% respectively. It can be seen that under the effect of fatigue, the first-order frequency has the greatest frequency reduction; the second-order frequency follows; and the third-order frequency has the smallest frequency reduction.

It can also be seen from Figure 6 that the degradation process of modal frequency of prestressed concrete beams also has a three-stage law similar to fatigue stiffness degradation. The modal frequency decreases significantly in the early stage of fatigue. This is because at the beginning of loading, the development of concrete cracks and the large loss of effective prestress cause the stiffness of the test beam to decrease significantly, resulting in the modal frequency of the beam showing Characteristics of rapid reduction. After entering the middle stage of fatigue, the cracks are slowly extending and expanding, the effective prestress loss rate decreases and becomes stable, and local bond slip failure occurs between the steel bars and concrete, causing the beam stiffness to develop in an approximately linear state, and the modal frequency also decreases. It decreases approximately linearly, and the relative development is relatively stable. At the end of fatigue, concrete cracks expand sharply again and dendritic cracks appear. At this time, the stiffness of the beam body decreases again, so the frequency once again has a decreasing stage.

Define damage variables based on modal frequencies:

Among them: w ₀ is the initial modal frequency, w _N is the modal frequency when the prestressed concrete beam is tested for fatigue failure, and N is the number of cycles when the prestressed concrete beam is tested for fatigue failure. The damage variable D defined by this formula ranges from 0 to 1; D=0 corresponds to the damage-free state of the test beam; D=1 corresponds to complete fatigue failure of the beam. The damage variable D is a monotonically increasing function, that is, the fatigue damage degree of the test beam increases with the increase in the number of load cycles, and the damage is irreversible.

Considering that in actual engineering bridge dynamic testing, since the energy of the first-order modal frequency occupies a larger proportion, it has higher accuracy. At the same time, it was found from previous research that the frequency degradation amplitude of the first-order modal frequency is the largest, so The first-order modal frequency is used here to define the damage variables. Based on the research of many scholars on the damage cumulative fitting curve, after comparison and selection, the following formula was selected for fitting:

Among them: α, β are the parameters to be fitted, n is the number of cycles, and N is the number of cycles when the fatigue failure of the prestressed concrete beam is to be measured. The test results were analyzed using the least squares method for nonlinear regression, and the parameters obtained are shown in Table 7 below. The fitting degree ^R2 of the two beams is close to 100%, indicating that the model fits well.

Table 7 Fatigue damage nonlinear model fitting parameters

According to the above fitting curve and fitting parameters, the fatigue cumulative damage evolution law of the two beams based on the first-order modal frequency can be obtained, as shown in Figure 7.

It can be seen from Figure 7 that the fatigue damage evolution law of each test beam has obvious nonlinearity. The entire fatigue damage evolution can be divided into three stages. In the first stage of the initial development of damage, the cumulative fatigue damage increases sharply and reaches a stable level; in the second stage, the cumulative fatigue damage increases steadily and slowly; as the number of cycles increases, the cumulative fatigue damage enters the third stage, and in the third stage, the cumulative fatigue damage On the basis of the accumulated damage in the second stage, it began to increase sharply until the test beam was completely destroyed and lost its bearing capacity.

Comparing the fatigue damage evolution curves of the two beams, it can also be seen that the greater the fatigue stress amplitude, the more severe the damage development. In the early stage of fatigue, the damage of beam No. 2 with larger stress amplitude develops more rapidly than that of beam No. 3. When the two beams reach the damage thresholds of approximately 0.68 and 0.56 respectively, they enter the middle fatigue stage in which damage develops stably; at the end of the damage period The threshold value of beam No.2 is about 0.85, which is larger than the threshold value of beam No.3, 0.82. Throughout the entire development process, the damage development of beams with larger stress amplitude is always ahead of that of beams with smaller stress amplitude. In comparison, the low life characteristics of beams with large stress amplitude also show the rationality of this evolution law.

This embodiment uses the first-order natural frequency as the damage variable to effectively simulate the three-stage nonlinear fatigue damage evolution law of prestressed concrete beams. It can be seen that through the study of the fatigue damage accumulation curve, combined with the identification of the three-stage fatigue damage threshold, it can provide a research basis for the determination of the degree of structural performance degradation and the prediction of the remaining life. It has certain application prospects and provides a basis for research on the fatigue performance of prestressed concrete beams. Provides new ideas.

The above-described embodiments are merely examples to clearly illustrate the present invention, rather than limiting the implementation. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. An exhaustive list of all embodiments is neither necessary nor possible. However, obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims (9)

1. The method for representing the fatigue damage state of the prestressed concrete beam is characterized by comprising the following steps of:

step 1: placing the prestressed concrete beam to be tested on two simply supported supports, and firstly, performing power test on the initial intact prestressed concrete beam to be tested to obtain the initial modal frequency w of the prestressed concrete beam to be tested ₀ ；

Step 2: starting a fatigue testing machine to perform fatigue test on the prestressed concrete beam to be tested, stopping the machine after a certain number of fatigue cycles to perform power test, and obtaining the modal frequency w of the prestressed concrete beam to be tested when the number of fatigue cycles is n ten thousand times _n ；

Step 3: obtaining damage variables of the prestressed concrete beam to be tested under different circulation times according to the following formula:

wherein: w (w) ₀ For the initial modal frequency, w _N The mode frequency of the prestressed concrete beam to be tested in fatigue failure is N, which is the cycle number of the prestressed concrete beam to be tested in fatigue failure;

step 4: fitting the damage variables of the prestressed concrete beam to be tested under different cycle times obtained in the step 3 to obtain a fatigue whole-process damage variable evolution rule based on modal frequency, thereby representing the fatigue damage state of the prestressed concrete beam to be tested;

the damage variable and the cycle number are fitted by adopting the following formula:

wherein: alpha and beta are parameters to be fitted, N is the cycle number, and N is the cycle number when the prestressed concrete beam to be tested is subjected to fatigue failure.

2. The method for characterizing the fatigue damage state of the prestressed concrete beam according to claim 1, wherein: the dynamic test adopts an excitation method, and the vibration exciter is moved to the vicinity of the maximum amplitude point of the theoretical amplitude of each order of the prestressed concrete beam to be tested to carry out sweep frequency excitation, and the vibration exciter is used for collecting acceleration signals and then transmitting the acceleration signals to a modal test analysis system to carry out modal analysis.

3. The method for characterizing the fatigue damage state of the prestressed concrete beam according to claim 1, wherein: and the damage variable selects a first-order modal frequency.

4. The method for characterizing the fatigue damage state of the prestressed concrete beam according to claim 1, wherein: during power test, the vibration exciter and the vibration pickup are staggered.

5. A testing device for realizing the characterization method according to any one of claims 1-4 is characterized by comprising two simply supported supports oppositely arranged on the ground, a prestressed concrete beam to be tested arranged on the two simply supported supports, an excitation system and a modal test analysis system;

the vibration excitation system comprises a vibration exciter, a power amplifier and a signal amplifier which are sequentially connected, wherein the vibration exciter is arranged between two simple support seats below the prestressed concrete beam to be tested in a free moving way;

the modal test analysis system comprises a vibration pickup, a signal acquisition system and a modal analysis system which are connected in sequence, wherein the vibration pickup is arranged on the top surface of the prestressed concrete beam to be tested.

6. The test device of claim 5, wherein: the vibration pickers are equally arranged along the length direction of the prestressed concrete beam to be tested, and the vibration exciter is arranged on the travelling trolley.

7. The test device of claim 5, wherein: a fatigue testing machine is arranged above the prestressed concrete beam to be tested, the fatigue testing machine is fixedly arranged on the ground through a reaction frame, a load distribution beam is arranged below an actuating head of the prestressed concrete beam to be tested, and the central point of the actuating head of the fatigue testing machine is opposite to the prestressed concrete to be tested

A beam center point and a load distribution beam center point.

8. The test device of claim 5, wherein: and displacement meters are arranged at the upper parts of two ends of the prestressed concrete beam to be tested, at the positions of the simple support and Liang Kuazhong and at the bottom of the load loading point.

9. The test device of claim 5, wherein: and two ends of the load distribution beam are connected with the prestressed concrete beam to be tested through simple supports.