Fatigue test simulation method for simulating microcrack degradation of continuous reinforced concrete pavement

Abstract

The invention provides a fatigue test simulation method for simulating microcrack degradation of a continuous reinforced concrete pavement, which comprises the following steps of: firstly, carrying out a four-point bending fatigue test on a standard cement concrete anti-bending test piece to determine fatigue parameters and failure load of a cement concrete material, and establishing a fatigue damage evolution equation; the fatigue damage evolution equation is written into a subprogram, and the microcrack fatigue degradation process of the continuous reinforced concrete pavement is simulated by connecting the subprogram into finite element software. According to the invention, the time cost and capital cost of the test can be greatly reduced, meanwhile, compared with the actual test, the stress and damage at the micro-crack in the test piece at any loading stage can be output, the micro-crack expansion condition in the test piece can be more conveniently observed, various mechanical and damage indexes in the test piece and at the micro-crack position can be monitored, and the test efficiency is improved. The method can be used for mechanism research on microcrack damage degradation of the continuous reinforced concrete pavement.

Description

Technical Field

[0001] The invention relates to a fatigue test method, in particular to a fatigue test simulation method for simulating micro-crack deterioration of a continuously reinforced concrete pavement. Background Art

[0002] Continuously reinforced concrete (CRC) refers to a reinforcement method in cement concrete structure that increases its bending and shearing capacity by arranging steel bars along the entire length of the beam or slab. In this reinforcement method, the steel bars are evenly distributed throughout the structure, forming a whole with the concrete, making full use of the advantages of concrete and steel bars. Compared with the local reinforcement method, it has better bearing capacity and greatly improves the stability and safety of the structure. Therefore, CRC is widely used in projects such as large-span beams, slabs, and bridges. For example, the continuously reinforced concrete pavement (CRCP) constructed with CRC is a cement concrete pavement with longitudinal continuous steel bars and transverse steel bars arranged in the concrete, and no shrinkage joints are required in the transverse direction. It has the advantages of high overall strength, long service life, and low maintenance costs. The structural design that no shrinkage joints are required in the transverse direction also greatly improves the comfort of CRCP.

[0003] As a long-life pavement structure suitable for heavy-load traffic environment, CRCP is built with cement concrete and steel bars as the main materials. This leads to the inevitable generation of tensile stress and microcracks in the mortar near the coarse aggregate during the early hardening process of CRCP due to environmental changes and differences in thermal properties between materials. These microcracks gradually expand and penetrate to form transverse cracks as the internal stress is released and the load is further applied. In the early stage of the cracks, there is generally no deterioration (crack expansion, widening, fragmentation, decreased load transfer capacity, etc.) to affect the overall performance of the pavement structure. However, due to the significant characteristics of heavy-load traffic in my country, the transverse microcracks of CRCP often begin to show progressive deterioration behavior in the early stage. The continuous deterioration of transverse microcracks will lead to the corrosion and expansion of the longitudinal steel bars in CRCP, the decrease of the longitudinal load transfer capacity of the plate, and the de-airing of the base after scouring, etc., and may eventually cause breakage and damage, which will significantly reduce the service performance and service life of CRCP.

[0004] The damage degradation process of CRCP transverse microcracks is the result of long-term effects of factors such as heavy loads. It is complex, time-consuming and costly to reproduce this process by repeated loading in physical projects or indoor tests. Most of the existing fatigue test simulations are 0-1 loading under a single force mode, and there are fewer fatigue test simulations under four-point bending loads. In addition, the loading waveform cannot be controlled in the existing fatigue test simulations. At the same time, the parameters of the existing fatigue damage evolution equations are often determined based on the damage characteristics of materials such as asphalt mixtures, cement mortars, and fibers. There are few damage evolution models and damage parameters determined based on the damage characteristics of cement concrete materials. In addition, most of the research on the fatigue performance of cement concrete materials at home and abroad relies on indoor tests. The fatigue test simulation of cement concrete materials is still in the preliminary research stage. In addition, the modeling of cement concrete standard flexural specimens (CRCP beams) with built-in continuous steel bars and four-point bending fatigue tests is quite different from the conventional fatigue test modeling methods. It is impossible to use the existing models and methods to study the damage degradation behavior of CRCP transverse microcracks. Summary of the invention

[0005] The technical problem to be solved by the present invention is as follows: In view of the above problems in the prior art, a fatigue test simulation method for simulating the degradation of microcracks in a continuously reinforced concrete pavement is provided. Considering the characteristics of the CRCP structure and transverse microcracks, a fatigue test for simulating the degradation process of transverse microcracks in CRCP is designed based on finite element analysis software, and the mechanical response and fatigue damage characteristics at the microcracks in CRCP can be analyzed. This simulation method helps researchers to analyze more complex working conditions, effectively saving time and cost.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A fatigue test simulation method for simulating microcrack degradation of a continuously reinforced concrete pavement comprises the following steps:

[0008] Conduct four-point bending fatigue tests on standard cement concrete flexural specimens under different stress ratios to obtain fatigue parameters of cement concrete materials;

[0009] Establishing a fatigue damage evolution equation, calculating parameters in the fatigue damage evolution equation according to the fatigue parameters, and writing the fatigue damage evolution equation into a subroutine;

[0010] The subroutine was connected to the finite element software, and the finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks was established using the finite element software to simulate the fatigue degradation process of microcracks in continuously reinforced concrete pavement.

[0011] Furthermore, when conducting four-point bending fatigue tests on standard cement concrete flexural specimens under different stress ratios, the following are included:

[0012] Prepare standard cement concrete flexural specimens, test and determine their flexural tensile strength f f ;

[0013] Four-point bending fatigue tests are carried out at different stress ratios, and multiple groups of four-point bending fatigue tests are carried out at each stress ratio. The magnitude of fatigue stress and fatigue load applied in the four-point bending fatigue test is calculated as the test result;

[0014] If the discreteness of the test results under the same stress ratio does not meet the requirements, the current standard cement concrete flexural specimens shall be abolished, and new standard cement concrete flexural specimens shall be prepared to test and determine their flexural tensile strength f f Afterwards, four-point bending fatigue tests were carried out at different stress ratios, and multiple groups of four-point bending fatigue tests were carried out at each stress ratio.

[0015] Furthermore, when calculating the magnitude of fatigue stress and fatigue load applied in the four-point bending fatigue test, the expression is as follows:

[0016] σ=S f ·f f

[0017]

[0018] Where: σ is the fatigue stress in fatigue parameters; S f is the stress ratio; F is the fatigue load in the fatigue parameters; L is the distance between supports; b and h are the width and height of the specimen, respectively.

[0019] Furthermore, the fatigue damage evolution equation is expressed as follows:

[0020]

[0021] Where: D is the damage degree, N is the number of times the current load acts, σ ​​is the fatigue stress in the fatigue parameter, a * , p and q are parameters related to fatigue damage.

[0022] Furthermore, when calculating the parameters in the fatigue damage evolution equation according to the fatigue parameters, it includes:

[0023] The fatigue life of the specimens under different stress ratios in the four-point bending fatigue test is calculated respectively, and the expressions are as follows:

[0024]

[0025] Where: N f is the fatigue life of the specimen, S f is the stress ratio, f fis the flexural tensile strength of the specimen, M, α, and γ are material parameters related to temperature, average stress, etc.;

[0026] The fatigue life N of cement concrete specimens under different stress ratios f Substitute into the formula lgN f =lgk-nlgS f Perform double logarithmic linear regression and fit the results, where k is the intercept of the fitted line and n is the slope of the fitted line;

[0027] The parameters related to fatigue damage in the fatigue damage evolution equation are calculated based on the intercept k and slope n of the fitting line. The expression is as follows:

[0028] p=n,q=0

[0029] Among them, f f is the flexural tensile strength of the specimen, k is the intercept of the fitting straight line, and n is the slope of the fitting straight line.

[0030] Furthermore, before writing the fatigue damage evolution equation into the subroutine, a fatigue damage calculation constitutive equation is also established. When writing the fatigue damage evolution equation into the subroutine, the fatigue damage evolution equation and the fatigue damage calculation constitutive equation are specifically written into the Umat subroutine. The expression of the fatigue damage calculation constitutive equation is as follows:

[0031]

[0032] Where: x , σ y , σ z are the principal stresses in the X, Y and Z directions respectively; τ xy , τ yz , τ xz are shear stress; x , ε y , ε z are the principal strains in the X, Y, and Z directions respectively; γ xy , γ yz , γ xz are shear strain respectively; the coefficient calculation formula in the equation is as follows:

[0033]

[0034]

[0035] d 44 =G xy , d 55 =G yz , d 66 =G xz

[0036] in:

[0037] E i =(1-D i )E,(i=x,y,z)

[0038] μ i =(1-D i )μ,(i=x,y,z)

[0039]

[0040] Where: E i is the Young's modulus after each damage in each principal stress direction, i = x, y, z; μ i is the Poisson's ratio after each damage in each principal stress direction, i = x, y, z; D i is the damage degree in each principal stress direction, i = x, y, z; G xy , G yz , G xz is the shear stiffness after each damage; D xy , D yz , D xz is the degree of shear damage; E, μ are the original Young’s modulus and Poisson’s ratio in each principal stress direction.

[0041] Furthermore, after the fatigue damage evolution equation is written into the subroutine, the method further includes: establishing a loading amplitude curve equation, and writing the loading amplitude curve equation into the Uamp subroutine. The expression of the loading amplitude curve equation is as follows:

[0042] p(t)=σ·sin(πt)

[0043] Where: p(t) is the magnitude of the concentrated load applied at time t, σ is the fatigue stress as the peak value of the applied concentrated load, 0≤t≤1.

[0044] Further, when using finite element software to establish a finite element model of fatigue test of continuously reinforced concrete pavement beams with preset microcracks to simulate the fatigue degradation process of microcracks of continuously reinforced concrete pavement, the steps of using finite element software to establish a finite element model of fatigue test of continuously reinforced concrete pavement beams with preset microcracks include:

[0045] Establish components: Establish the main part of cement concrete with transverse microcracks. The microcracks are set by the method of integral modeling and then partitioning. The three-dimensional line body without considering the shape of the steel bar transverse rib is used as the steel bar part. According to the specified size and layout position, establish the test fixture part on the main part of cement concrete with transverse microcracks.

[0046] Attribute assignment: setting the cement concrete material parameters in the main part of the cement concrete and the material parameters at the microcracks, and setting the material parameters of the steel bar part;

[0047] Assemble components: align the width direction of the cement concrete main part with the x direction of the model space, the length direction with the y direction of the model space, and the height direction with the z direction of the model space. Determine the positive direction along the z axis as the top of the model and the specimen, and select independent grid division for each component; embed the steel bar part into the center position of the cement concrete main part along the y direction; arrange the various parts of the test fixture at the corresponding positions above and below the cement concrete main body, where the fixture parts above the specimen body are named A1 and A2, and the parts below the specimen body are named B and A3;

[0048] Set the analysis step: Use the general static analysis step to set the boundary conditions and contact properties; use the general static analysis step to set the analysis step of the cyclic loading process, and select the required field output variables and process output variables;

[0049] Contact settings: define rigid body constraints in the test fixture; set "surface-to-surface contact" in the contact between the test fixture components and the main body of the cement concrete specimen, where the normal is set to "hard contact" and the tangential is set to "penalty friction"; the steel bars and the main body of the cement concrete specimen are coupled with "Embedded" in the constraint type;

[0050] Set the load and boundary conditions of the model: In the main part of the cement concrete specimen, define reference points at the center of the two end faces of the specimen in the y direction, and "couple" the two end faces with the reference points respectively, and then constrain the displacement of the two reference points in the y direction and the rotational freedom around the x-axis and the z-axis; in the boundary conditions of the test fixture, constrain the displacement of the reference points on the A1 and A2 components in the x and y-axis directions and the rotational freedom around the z-axis, and constrain all the degrees of freedom of the reference points on the A3 and B components; in the load part, define the concentrated force acting on the reference points on the A1 and A2 components, the direction is vertically downward, and the magnitude and amplitude change of the concentrated force are defined by the amplitude function;

[0051] Select the appropriate mesh type and assign sub-mesh properties: set linear mesh for steel bars and adopt global distribution; set hexahedral mesh for the test fixture and adopt global distribution; set hexahedral mesh for the cement concrete body and microcracks and adopt global distribution.

[0052] Further, when a finite element model of a fatigue test of a continuously reinforced concrete pavement beam with preset microcracks is established using finite element software to simulate the fatigue degradation process of microcracks of the continuously reinforced concrete pavement, the steps of simulating the fatigue degradation process of microcracks of the continuously reinforced concrete pavement include:

[0053] Initialize the damage value D and the state variable STATEV, discretize the finite element model of the fatigue test of the continuously reinforced concrete pavement beam with preset microcracks using the finite element method, obtain the corresponding material parameters and boundary conditions, call the Uamp subroutine in the subroutine, use the loading amplitude curve equation to calculate the size of the concentrated load applied in the model under the current incremental step and perform the first loading;

[0054] Assign the initial stiffness matrix DDSDDE to the integration points of each unit of the discretized finite element model;

[0055] The stiffness matrix DDSDDE is used to calculate the stress at each Gaussian point inside each unit;

[0056] Call the Uamt subroutine in the subroutine and use the fatigue damage evolution equation to calculate the fatigue damage value D at each Gaussian point of each unit after this loading;

[0057] The fatigue damage value D at each Gaussian point of each unit after this loading is accumulated with the corresponding historical value to update the corresponding accumulated fatigue damage and record it in the state variable STATEV;

[0058] Substitute the stress and fatigue damage value D at each Gaussian point of each unit into the fatigue damage calculation constitutive equation, determine the influence of the fatigue damage value D on the unit stiffness, and update the stiffness matrix DDSDDE;

[0059] If the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is less than the target value, the step of calculating the stress of each Gaussian point inside each unit using the updated stiffness matrix DDSDDE is performed again until the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is greater than or equal to the target value;

[0060] Record the total number of load cycles and analyze the distribution of fatigue damage within the model elements and the fatigue damage evolution process.

[0061] The present invention also proposes a fatigue test simulation system for simulating microcrack degradation of a continuously reinforced concrete pavement, comprising a microprocessor and a computer-readable storage medium connected to each other, wherein the microprocessor is programmed or configured to execute any one of the fatigue test simulation methods for simulating microcrack degradation of a continuously reinforced concrete pavement.

[0062] Compared with the prior art, the advantages of the present invention are:

[0063] The present invention provides a fatigue test simulation method for simulating microcrack degradation of a continuously reinforced concrete pavement. This method requires firstly conducting a four-point bending fatigue test of a standard cement concrete flexural test piece to determine the fatigue parameters of the cement concrete material, establishing a fatigue damage evolution equation, calculating the parameters in the fatigue damage evolution equation according to the fatigue parameters, and then writing the fatigue damage evolution equation into a subroutine. The subroutine is connected to a finite element software and a finite element model of a fatigue test of a continuously reinforced concrete pavement beam with preset microcracks is established using the finite element software to simulate the fatigue degradation process of microcracks of the continuously reinforced concrete pavement. After the model is established, the stress ratio, steel bar diameter, steel bar layout position, microcrack width and depth can be arbitrarily changed to conduct fatigue tests of microcrack degradation of the continuously reinforced concrete pavement under different conditions, without having to go through the actual raw material preparation, test piece molding and crack prefabrication, cement concrete test piece standard maintenance, ultimate load determination, and finally the process of conducting fatigue tests of microcrack degradation of the continuously reinforced concrete pavement. This method can greatly reduce the time and financial costs of the test site. At the same time, compared with the actual test, it can output the stress and damage at the microcracks in the specimen at any loading stage, and can more conveniently observe the extension of microcracks in the specimen, monitor the mechanical and damage indicators inside the specimen and at the microcrack position, and can be used to study the mechanism of microcrack damage deterioration in continuously reinforced concrete pavements. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 Schematic diagram of a flow chart of an embodiment of the present invention.

[0065] Figure 2 This is a graph showing the result of performing a double logarithmic fatigue equation linear regression on the four-point bending fatigue test data of a cement concrete flexural specimen in an embodiment of the present invention.

[0066] Figure 3 The figure is a flow chart of finite element software calculation and subroutine calling in an embodiment of the present invention.

[0067] Figure 4 The figure is a detailed dimension drawing of the modeling and assembly of each component of the embodiment of the present invention.

[0068] Figure 5 This is a schematic diagram of preset micro cracks and steel bar assembly positions in the main body of the cement concrete specimen model according to an embodiment of the present invention.

[0069] Figure 6 Schematic diagram of model mesh division in an embodiment of the present invention.

[0070] Figure 7 It is a comparison chart of the load action times-maximum displacement curves of the test and simulation under a stress ratio of 0.7. DETAILED DESCRIPTION

[0071] The present invention is further described below in conjunction with the accompanying drawings and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

[0072] Embodiment 1

[0073] In order to solve the problem that the existing fatigue damage constitutive model and parameters cannot accurately reflect the microcrack damage degradation process in CRCP, and the simulation method for the seamed CRCP beam and the four-point bending fatigue test is incomplete, this embodiment proposes a fatigue test simulation method for simulating the microcrack degradation of a continuously reinforced concrete pavement. Considering the characteristics of the CRCP structure and transverse microcracks, an indoor four-point bending fatigue test is used to determine the constitutive model and parameters for fatigue damage analysis of CRCP microcracks. The finite element software is used to establish a CRCP beam with microcracks and a four-point bending test fixture model. The Umat and Uamp subroutines are used to define the damage characteristics of cement concrete and the frequency, size, and waveform of the fatigue load respectively. The damage degradation process of the transverse microcracks of CRCP under fatigue load is simulated, and the mechanical response and fatigue damage characteristics of the microcracks in CRCP can be analyzed.

[0074] like Figure 1 As shown, the method of this embodiment includes the following steps:

[0075] S101) performing a four-point bending fatigue test on a standard cement concrete flexural test piece under different stress ratios to obtain fatigue parameters of the cement concrete material;

[0076] S102) establishing a fatigue damage evolution equation, calculating parameters in the fatigue damage evolution equation according to the fatigue parameters, and establishing a fatigue damage calculation constitutive equation and a loading amplitude curve equation of fatigue load, writing the fatigue damage evolution equation and the fatigue damage calculation constitutive equation into the Umat subroutine, and writing the loading amplitude curve equation into the Uamp subroutine;

[0077] S103) The Umat subroutine and the Uamp subroutine are respectively connected to the finite element software, and the finite element model of the fatigue test of the continuously reinforced concrete pavement beam with preset microcracks is established using the finite element software to simulate the fatigue degradation process of the microcracks of the continuously reinforced concrete pavement.

[0078] The following is a detailed description of each step.

[0079] In step S101 of this embodiment, when performing a four-point bending fatigue test on a standard cement concrete flexural test piece under different stress ratios, the steps include:

[0080] S201) Prepare standard cement concrete flexural test specimens according to the requirements of JTG 3420-2020, and test and determine their flexural tensile strength f according to the method therein. f ;

[0081] S202) Four-point bending fatigue tests are carried out at different stress ratios, and multiple groups of four-point bending fatigue tests are carried out at each stress ratio, and the magnitude of fatigue stress and fatigue load applied in the four-point bending fatigue test are calculated as test results, and the calculation formula is as follows:

[0082] σ=S f ·f f (1)

[0083]

[0084] Where: σ is the fatigue stress in fatigue parameters; S f is the stress ratio; F is the fatigue load in the fatigue parameter; L is the distance between supports; b and h are the width and height of the specimen respectively;

[0085] In this embodiment, when conducting a four-point bending fatigue test, at least three different stress ratio values ​​are determined, and at least five sets of parallel tests are conducted under each stress ratio;

[0086] S203) If the discreteness of the test results under the same stress ratio does not meet the requirements, the current standard cement concrete flexural test specimen is discarded, and a new standard cement concrete flexural test specimen is prepared to test and determine its flexural tensile strength f f Afterwards, four-point bending fatigue tests were carried out at different stress ratios, and multiple groups of four-point bending fatigue tests were carried out at each stress ratio.

[0087] In this embodiment, the discreteness does not meet the requirements specifically refers to that the variance or standard deviation is greater than a preset threshold value, resulting in a large discreteness. In addition, during the four-point bending fatigue test, when the fatigue load is applied on the servo testing machine, a half-sine wave is selected as the loading waveform, the loading frequency is 10 Hz, and the ratio of unloading stress to peak stress is 0.2.

[0088] In step S102 of this embodiment, when establishing the fatigue damage evolution equation, a model for analyzing the fatigue damage evolution of CRCP microcracks is determined based on the fatigue damage accumulation characteristics of CRCP beams in indoor tests. The expression is as follows:

[0089]

[0090] Where: D is the damage degree; N is the number of times the current load acts; M, α, and γ are material parameters related to temperature, average stress, etc.

[0091] By transforming and integrating the above formula, we can get:

[0092]

[0093] Where: N f is the fatigue life of the specimen.

[0094] Substituting formula (1) into formula (4), we get:

[0095]

[0096] make n=α, then:

[0097] lgN f =lgk-nlgS f (6)

[0098] The fatigue life N of the cement concrete specimen in the above four-point bending fatigue test is f Substitute the stress level σ into the calculation formula (6) for double logarithmic regression, and the regression coefficients k and n are the intercept and slope of the straight line.

[0099] Let a * =(2M) -α , p = α, q = γ, then another form of the above nonlinear fatigue damage evolution equation is as follows:

[0100]

[0101] Where: a * , p and are parameters related to fatigue damage; since q is generally 0, we have:

[0102]

[0103] According to the above formula, in step S102 of this embodiment, when calculating the parameters in the fatigue damage evolution equation according to the fatigue parameters, it includes:

[0104] According to formula (5), the fatigue life of the specimens under different stress ratios in the four-point bending fatigue test is calculated respectively;

[0105] Substitute the fatigue life of cement concrete specimens under different stress ratios into formula (6) for double logarithmic linear regression and fit the results, as shown in the attached figure. Figure 2 As shown;

[0106] According to formula (8), the parameters related to fatigue damage in the fatigue damage evolution equation are calculated based on the intercept k and slope n of the fitting line, thereby calculating the damage parameters of cement concrete.

[0107] In step S102 of this embodiment, the fatigue damage calculation constitutive equation specifically refers to the fatigue damage calculation constitutive equation of each unit of the cement concrete main part of the continuously reinforced concrete pavement, and the expression is as follows:

[0108]

[0109] Where: x , σ y , σ z are the principal stresses in the X, Y and Z directions respectively; τ xy , τ yz , τ xz are shear stress; x , ε y , ε z are the principal strains in the X, Y, and Z directions respectively; γ xy , γ yz , γ xz are shear strain respectively; the coefficient calculation formula in the equation is as follows:

[0110]

[0111]

[0112] d 44 =G xy , d 55 =G yz , d 66 =G xz (16)

[0113] in:

[0114] E i =(1-D i )E, (i=x,y,z) (17)

[0115] μ i =(1-D i )μ, (i=x,y,z) (18)

[0116]

[0117] Where: E i is the Young's modulus after each damage in each principal stress direction, i = x, y, z; μ i is the Poisson's ratio after each damage in each principal stress direction, i = x, y, z; D i is the damage degree in each principal stress direction, i = x, y, z; G xy , G yz , G xz is the shear stiffness after each damage; D xy , D yz , D xz is the degree of shear damage; E, μ are the original Young’s modulus and Poisson’s ratio in each principal stress direction.

[0118] In step S102 of this example, when the fatigue damage evolution equation and the fatigue damage calculation constitutive equation are written into the Umat subroutine, the above calculation formula is compiled in the Fortran programming language according to the Umat subroutine format, and is accessed using the User Material subroutine interface in the finite element software, and the number of state variables and variable controls related to the solution are set to 12.

[0119] In step S102 of this embodiment, the expression of the loading amplitude curve equation is as follows:

[0120] p(t)=σ·sin(πt), 0≤t≤1 (21)

[0121] Where: p(t) is the magnitude of the concentrated load applied in the model at time t; σ is the fatigue stress as the peak value of the applied concentrated load; t is the model time.

[0122] When writing the loading amplitude curve equation into the Uamp subroutine, specifically, the above formula (21) is compiled in the Fortran programming language according to the Uamp subroutine format, accessed using the Uamp subroutine interface, and merged with the previous Umat subroutine into a for file.

[0123] In step S301 of this embodiment, when a finite element model of a fatigue test of a continuously reinforced concrete pavement beam with preset microcracks is established using finite element software to simulate the fatigue degradation process of microcracks of the continuously reinforced concrete pavement, the following steps are included:

[0124] First, the finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks was established using finite element software. The specific steps include:

[0125] Components are established to establish the finite element model of the fatigue test of the CRCP beam with preset microcracks, including:

[0126] Establish a cement concrete main body with transverse microcracks, and the microcracks are set by using the method of integral modeling and then partitioning; take the three-dimensional line body without considering the shape of the steel bar transverse rib as the steel bar part; establish the test fixture part on the cement concrete main body with transverse microcracks according to the specified size and layout position;

[0127] In this embodiment, in the main part of the cement concrete, it is characterized by being provided with transverse microcracks with a width of 0.5 mm and a depth of 3 cm. The position of the transverse microcracks is the central cross section of the cement concrete main body perpendicular to the y direction, and the preset microcrack starting point is located at the lower surface of the specimen; in the steel bar part, the length of the steel bar is equal to the length of the main body of the cement concrete specimen; in the test fixture part, the test fixture includes two types, a total of 4 components, and reference points are defined on the side surfaces of one end of the length direction of the 4 fixture components. The reference point position in the test fixture is the center point of any one of the two end surfaces of the fixture component along the length direction. Each component should be in close contact with the cement concrete main body, and the size and layout position of each component shall be determined in accordance with GB / T 50081-2019.

[0128] Attribute assignment is used to assign material properties to cement concrete, steel bars, and micro-cracks, including:

[0129] Set cement concrete material parameters, including cement concrete density, elastic modulus, Poisson's ratio, and damage parameters; set steel bar material parameters, including steel bar density, elastic modulus, Poisson's ratio, and cross-sectional area; set microcrack material parameters, including elastic modulus and Poisson's ratio.

[0130] In this embodiment, the cement concrete material parameters are the same as those of the standard cement concrete flexural specimens. The damage parameters are obtained by performing a four-point bending fatigue test on the standard cement concrete flexural specimens at different stress ratios, substituting the fatigue life of the specimens at different stress ratios into formula (6) for double logarithmic linear regression, and further substituting the parameters of the fitting line into formula (8) for calculation. In the steel bar material parameters, the steel bar cross-sectional properties are selected as trusses, and the steel bar cross-sectional area is calculated and input according to the unit dimension determined when the model is established; the material modulus of the microcrack part is 1 / 50 of the cement concrete material.

[0131] Assembly components, used to assemble and combine steel bars, specimen fixture components and cement concrete components, including:

[0132] When assembling components, align the width direction of the cement concrete specimen body with the x direction of the model space, the length direction with the y direction of the model space, and the height direction with the z direction of the model space. The positive direction along the z axis is determined as the top of the model and the specimen, and each component is divided into independent grids; embed the steel bars into the center of the cement concrete body along the y direction; arrange the various parts of the test fixture at the corresponding positions above and below the cement concrete body in accordance with the requirements of the above-mentioned specifications, among which the fixture parts above the specimen body are named A1 and A2, and those below the specimen body are named B and A3.

[0133] Set the analysis step, select the program type of the analysis step, and check the required factory output variables and process output variables, including:

[0134] Set analysis step 1 (boundary conditions and contact property settings), use the general static analysis step, and set the step size to 1; set analysis step 2 (cyclic loading process), use the general static analysis step, set the step size to 1500, and the fixed incremental step size to 0.1; check the required field output variables and process output variables, specifically check the output variables stress (S), total strain (E), nominal strain (NE), user-defined output variables (SDV), rotation and displacement (U) and reaction force (RF), and keep the default options for the process output variables.

[0135] Contact settings are used to set the rigid body constraints of the fixture components and the contact properties between the component models, including:

[0136] Define rigid body constraints in the test fixture, and rigidly constrain the four components to their reference points established in step 1; set "surface-to-surface contact" in the contact part between each component of the test fixture and the main body of the cement concrete specimen, where the normal is set to "hard contact", the tangential is set to "penalty friction", and the friction coefficient is 0.25; the "Embedded" in the constraint type is used for coupling constraints between the steel bars and the main body of the cement concrete specimen.

[0137] Loads and boundary conditions are used to set the loads and boundary conditions of the model, including:

[0138] In the main body of the cement concrete specimen, reference points are defined at the center of the two end faces of the specimen in the y direction, and the two end faces are "coupled" with the reference points respectively, and then the displacement in the y direction and the rotational freedom around the x-axis and the z-axis of the two reference points are constrained; in the boundary condition part of the test fixture, the displacement in the x-axis and y-axis directions and the rotational freedom around the z-axis of the reference points on the two components A1 and A2 are constrained, and all the degrees of freedom of the reference points on the two components A3 and B are constrained; in the load part, the concentrated force acting on the reference points on the two components A1 and A2 is defined, and the direction is vertically downward. The magnitude and amplitude change of the concentrated force are defined by the amplitude function. Specifically, the peak value of the concentrated force is calculated according to formula (1), and the half-sine wave amplitude curve is defined according to formula (21).

[0139] Mesh division and cell type, select the appropriate mesh type, and assign sub-mesh properties, including:

[0140] A linear grid is set for the steel bars, with global steel bar layout and T3D2 truss unit as the grid type. A hexahedral grid is set for the test fixture, with global layout and C3D8 unit. A hexahedral grid is set for the cement concrete body and microcracks, with global layout and C3D8 unit.

[0141] After using finite element software to establish a finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks, the finite element model is used to simulate the fatigue degradation process of microcracks of continuously reinforced concrete pavement. The specific steps include:

[0142] Create a work task, select a subroutine file, submit the finite element model to the software for operation, and use the finite element software to call the subroutine to calculate and obtain the calculation results;

[0143] Post-processing, obtain and analyze software calculation results.

[0144] In this embodiment, when the finite element software is used to call the subroutine to calculate and obtain the result, such as Figure 3 As shown, the following steps are included:

[0145] Initialize the damage value D and the state variable STATEV, discretize the finite element model of the four-point bending fatigue test of the continuously reinforced concrete pavement beam with preset microcracks using the finite element method, obtain the material parameters and boundary conditions set in the model, call the Uamp subroutine part in the subroutine, use the loading amplitude curve equation to calculate the size of the concentrated load applied in the model under the current incremental step and perform the first loading;

[0146] Assign the initial stiffness matrix DDSDDE to the integration points of each unit of the discretized finite element model;

[0147] The stiffness matrix DDSDDE is used to calculate the stress at each Gaussian point inside each unit;

[0148] The Uamt subroutine in the subroutine is called, and the fatigue damage evolution equation is used to calculate the fatigue damage value D at each Gaussian point of each unit after this loading;

[0149] The fatigue damage value D at each Gaussian point of each unit after this loading is accumulated with the corresponding historical value to update the corresponding accumulated fatigue damage and record it in the state variable STATEV;

[0150] Substitute the stress and fatigue damage value D at each Gaussian point of each unit into the fatigue damage calculation constitutive equation, determine the influence of the fatigue damage value D on the unit stiffness, and update the stiffness matrix DDSDDE;

[0151] If the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is less than the target value, the step of calculating the stress of each Gaussian point inside each unit using the updated stiffness matrix DDSDDE is performed again until the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is greater than or equal to the target value;

[0152] Record the total number of load cycles and analyze the distribution of fatigue damage within the model elements and the fatigue damage evolution process.

[0153] In the following, this model will be established by combining the continuously reinforced concrete beam specimens with preset microcracks in the previous experiments, and the model calculation results will be compared with the test results to verify the correctness of the model.

[0154] Step 1: Establish components for establishing the finite element model of the fatigue test of the CRCP beam with preset microcracks: In the main part of the cement concrete, it is characterized by transverse microcracks with a width of 0.5 mm and a depth of 3 cm. The transverse microcracks are set by the method of integral modeling and then partitioning; in the steel bar part, it is a three-dimensional line body without considering the shape of the steel bar transverse ribs; in the test fixture part, the test fixture includes two types, a total of 4 parts, and the reference points are defined on the side of one end of the length direction of the 4 fixture parts. Each part should be in close contact with the main body of the cement concrete. The size and layout of each part are as shown in the attached figure. Figure 4 As shown in the figure, the preset micro cracks and steel bar assembly positions in the main body of the cement concrete specimen model are shown in the attached figure. Figure 5 shown.

[0155] Step 2: Attribute assignment, used to assign material attributes to cement concrete, steel bars, and microcracks: Set cement concrete material parameters, including: cement concrete density 2450kg / m 3 , elastic modulus 31GPa, Poisson's ratio 0.167, four-point bending fatigue test of standard cement concrete flexural specimens with the same cement concrete material parameters under different stress ratios was carried out in advance, the fatigue life of the specimens under different stress ratios was substituted into formula (6) for double logarithmic linear regression, and the parameters of the fitting line were further substituted into formula (8) to calculate the parameters of the fatigue damage evolution equation: a * =1.284×10 -19 , p = 22.741, q = 0, compile equation (8) and equations (10) to (20) and the determined damage parameters into the Umat subroutine and connect it to the finite element main program; set the steel bar material parameters, the parameters include: steel bar density 7850kg / m 3 , elastic modulus 206GPa, Poisson's ratio 0.28, cross-sectional area of ​​steel bar with diameter of 16mm is 201.1mm 2 ; Set the material parameters at the microcracks, the parameters include: elastic modulus 620MPa, Poisson's ratio 0.167.

[0156] Step 3: Assemble components, which are used to assemble and combine the steel bars, specimen fixture components and cement concrete components: When assembling components, align the width direction of the cement concrete specimen body with the x direction of the model space, the length direction with the y direction of the model space, and the height direction with the z direction of the model space. The positive direction along the z axis is determined as the top of the model and the specimen, and each component is divided into independent grids; embed the steel bars into the center of the cement concrete body along the y direction; arrange the various parts of the test fixture at the corresponding positions above and below the cement concrete body according to the requirements of the above specifications, among which the fixture components above the specimen body are named A1 and A2, and those below the specimen body are named B and A3.

[0157] Step 4: Set the analysis step, select the program type of the analysis step, and check the required factory output variables and process output variables: Set analysis step 1 (boundary conditions and contact property settings), use the general static analysis step, and set the step size to 1; set analysis step 2 (cyclic loading process), use the general static analysis step, set the step size to 1500, and the fixed incremental step size to 0.1; check the output variables stress (S), total strain (E), nominal strain (NE), user-defined output variables (SDV), rotation and displacement (U) and reaction force (RF), and keep the default options for the process output variables.

[0158] Step 5: Contact settings, used to set the rigid body constraints of the fixture components and the contact properties between the component models: define rigid body constraints in the test fixture part, and rigidly constrain the four components to their reference points established in step 1; set "surface-to-surface contact" in the contact part between the test fixture components and the main body of the cement concrete specimen, where the normal is set to "hard contact", the tangent is set to "penalty friction", and the friction coefficient is 0.25; the "Embedded" in the constraint type is used for coupling constraints between the steel bars and the main body of the cement concrete specimen.

[0159] Step 6: Load and boundary conditions, used to set the load and boundary conditions of the model: in the main part of the cement concrete specimen, define reference points at the center of the two end faces of the specimen in the y direction, and "couple" the two end faces with the reference points respectively, and then constrain the displacement of the two reference points in the y direction and the rotational freedom around the x-axis and the z-axis; in the boundary condition part of the test fixture, constrain the displacement of the reference points on the A1 and A2 components in the x and y axis directions and the rotational freedom around the z-axis, and constrain all the degrees of freedom of the reference points on the A3 and B components; in the load part, define the concentrated force acting on the reference points on the A1 and A2 components, the direction is vertically downward, according to the standard cement concrete flexural tensile strength of 6.7MPa, the ultimate failure load of 22.21kN, according to formula (1), the peak value of the concentrated force under the stress ratio of 0.7 is calculated to be 15.55kN, and the half-sine wave amplitude curve is defined by the Uamp subroutine according to formula (21).

[0160] Step 7: Mesh division and unit type, select the appropriate mesh type, and assign sub-mesh properties: set linear mesh for steel bars, use global steel bar type, and select T3D2 truss element for mesh type; set hexahedral mesh for test fixture, use 5mm size global type, and select C3D8 element; set hexahedral mesh for cement concrete body and microcracks, use 0.25mm size global type, and select C3D8 element. The model mesh division is as shown in the attached figure. Figure 6 As shown;

[0161] Step 8: Create a work task, select the subroutine file, and submit the model to the software for execution.

[0162] Step 9: Post-processing, obtain and analyze the software calculation results. The comparison of the load action times-maximum displacement curves of the test and simulation under the stress ratio of 0.7 is shown in the attached figure. Figure 7 As shown in the figure, the model calculation results are basically consistent with the experimental results.

[0163] It can be seen that the method of this embodiment effectively supplements the fatigue test simulation of cement concrete materials, helps researchers to analyze the microcrack degradation behavior of CRCP under complex working conditions, and can greatly save time and economic costs.

[0164] Embodiment 2

[0165] This embodiment proposes a fatigue test simulation system for simulating microcrack degradation of a continuously reinforced concrete pavement, comprising a microprocessor and a computer-readable storage medium connected to each other, wherein the microprocessor is programmed or configured to execute the fatigue test simulation method for simulating microcrack degradation of a continuously reinforced concrete pavement described in Embodiment 1.

[0166] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A fatigue test simulation method for simulating microcrack degradation of a continuously reinforced concrete pavement, characterized in that: The following steps are involved: Conduct four-point bending fatigue tests on standard cement concrete flexural specimens under different stress ratios to obtain fatigue parameters of cement concrete materials; Establishing a fatigue damage evolution equation, calculating parameters in the fatigue damage evolution equation according to the fatigue parameters, and writing the fatigue damage evolution equation into a subroutine; The subroutine was connected to the finite element software, and the finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks was established using the finite element software to simulate the fatigue degradation process of microcracks in continuously reinforced concrete pavement.

2. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: When conducting four-point bending fatigue tests on standard cement concrete flexural specimens under different stress ratios, the following are included: Prepare standard cement concrete flexural specimens, test and determine their flexural tensile strength f f ; Four-point bending fatigue tests are carried out at different stress ratios, and multiple groups of four-point bending fatigue tests are carried out at each stress ratio. The magnitude of fatigue stress and fatigue load applied in the four-point bending fatigue test is calculated as the test result; If the discreteness of the test results under the same stress ratio does not meet the requirements, the current standard cement concrete flexural specimens shall be abolished, and new standard cement concrete flexural specimens shall be prepared to test and determine their flexural tensile strength f f Afterwards, four-point bending fatigue tests were carried out at different stress ratios, and multiple groups of four-point bending fatigue tests were carried out at each stress ratio.

3. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 2, characterized in that: When calculating the magnitude of fatigue stress and fatigue load applied in a four-point bending fatigue test, the expression is as follows: σ=S f ·f f Where: σ is the fatigue stress in fatigue parameters; S f is the stress ratio; F is the fatigue load in the fatigue parameters; L is the distance between supports; b and h are the width and height of the specimen, respectively.

4. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: The fatigue damage evolution equation is expressed as follows: Where: D is the damage degree, N is the number of times the current load acts, σ ​​is the fatigue stress in the fatigue parameter, a * , p and q are parameters related to fatigue damage.

5. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 4, characterized in that: When calculating the parameters in the fatigue damage evolution equation according to the fatigue parameters, it includes: The fatigue life of the specimens under different stress ratios in the four-point bending fatigue test is calculated respectively, and the expressions are as follows: Where: N f is the fatigue life of the specimen, S f is the stress ratio, f f is the flexural tensile strength of the specimen, M, α, and γ are material parameters related to temperature, average stress, etc.; The fatigue life N of cement concrete specimens under different stress ratios f Substitute into the formula lgN f =lgk-nlgS f Perform double logarithmic linear regression and fit the results, where k is the intercept of the fitted line and n is the slope of the fitted line; The parameters related to fatigue damage in the fatigue damage evolution equation are calculated based on the intercept k and slope n of the fitting line. The expression is as follows: Among them, f f is the flexural tensile strength of the specimen, k is the intercept of the fitting straight line, and n is the slope of the fitting straight line.

6. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: Before writing the fatigue damage evolution equation into the subroutine, a fatigue damage calculation constitutive equation is also established. When writing the fatigue damage evolution equation into the subroutine, the fatigue damage evolution equation and the fatigue damage calculation constitutive equation are specifically written into the Umat subroutine. The expression of the fatigue damage calculation constitutive equation is as follows: Where: x , σ y , σ z are the principal stresses in the X, Y and Z directions respectively; τ xy , τ yz , τ xz are shear stress; x , ε y , ε z are the principal strains in the X, Y, and Z directions respectively; γ xy , γ yz , γ xz are shear strain respectively; the coefficient calculation formula in the equation is as follows: d 44 =G xy ,d 55 =G yz ,d 66 =G xz in: E i =(1-D i )E,(i=x,y,z) μ i =(1-D i )μ,(i=x,y,z) Where: E i is the Young's modulus after each damage in each principal stress direction, i = x, y, z; μ i is the Poisson's ratio after each damage in each principal stress direction, i = x, y, z; D i is the damage degree in each principal stress direction, i = x, y, z; G xy , G yz , G xz is the shear stiffness after each damage; D xy , D yz , D xz is the degree of shear damage; E, μ are the original Young’s modulus and Poisson’s ratio in each principal stress direction.

7. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: After the fatigue damage evolution equation is written into the subroutine, the method further includes: establishing a loading amplitude curve equation, and writing the loading amplitude curve equation into the Uamp subroutine. The expression of the loading amplitude curve equation is as follows: p(t)=σ·sin(πt),0≤t≤1 Where: p(t) is the magnitude of the concentrated load applied in the model at time t; σ is the fatigue stress as the peak value of the applied concentrated load; t is the model time.

8. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: When using finite element software to establish a finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks to simulate the fatigue degradation process of microcracks of continuously reinforced concrete pavement, the steps of using finite element software to establish a finite element model of fatigue test of continuously reinforced concrete pavement beam with preset microcracks include: Establish components: Establish the main part of cement concrete with transverse microcracks. The microcracks are set by the method of integral modeling and then partitioning. The three-dimensional line body without considering the shape of the steel bar transverse rib is used as the steel bar part. According to the specified size and layout position, establish the test fixture part on the main part of cement concrete with transverse microcracks. Attribute assignment: setting the cement concrete material parameters in the main part of the cement concrete and the material parameters at the microcracks, and setting the material parameters of the steel bar part; Assemble components: align the width direction of the cement concrete main part with the x direction of the model space, the length direction with the y direction of the model space, and the height direction with the z direction of the model space. Determine the positive direction along the z axis as the top of the model and the specimen, and select independent grid division for each component; embed the steel bar part into the center position of the cement concrete main part along the y direction; arrange the various parts of the test fixture at the corresponding positions above and below the cement concrete main body, where the fixture parts above the specimen body are named A1 and A2, and the parts below the specimen body are named B and A3; Set the analysis step: Use the general static analysis step to set the boundary conditions and contact properties; use the general static analysis step to set the analysis step of the cyclic loading process, and select the required field output variables and process output variables; Contact settings: define rigid body constraints in the test fixture; set "surface-to-surface contact" in the contact between the test fixture components and the cement concrete specimen, with "hard contact" set for the normal direction and "penalty friction" set for the tangent direction; use "Embedded" in the constraint type to couple the steel bars and the cement concrete specimen; Set the load and boundary conditions of the model: in the main part of the cement concrete specimen, define reference points at the center of the two end faces of the specimen in the y direction, and "couple" the two end faces with the reference points respectively, and then constrain the displacement of the two reference points in the y direction and the rotational freedom around the x-axis and the z-axis; in the boundary conditions of the test fixture, constrain the displacement of the reference points on the A1 and A2 components in the x and y-axis directions and the rotational freedom around the z-axis, and constrain all the degrees of freedom of the reference points on the A3 and B components; in the load part, define the concentrated force acting on the reference points on the A1 and A2 components, the direction is vertically downward, and the magnitude and amplitude change of the concentrated force are defined by the amplitude function; Select the appropriate mesh type and assign sub-mesh properties: set linear mesh for steel bars and adopt global distribution; set hexahedral mesh for the test fixture and adopt global distribution; set hexahedral mesh for the cement concrete body and microcracks and adopt global distribution.

9. The fatigue test simulation method for simulating microcrack degradation of continuously reinforced concrete pavement according to claim 1, characterized in that: When a finite element model of fatigue test of a continuously reinforced concrete pavement beam with preset microcracks is established using finite element software to simulate the fatigue degradation process of microcracks of the continuously reinforced concrete pavement, the steps of simulating the fatigue degradation process of microcracks of the continuously reinforced concrete pavement include: Initialize the damage value D and the state variable STATEV, discretize the finite element model of the fatigue test of the continuously reinforced concrete pavement beam with preset microcracks using the finite element method, obtain the corresponding material parameters and boundary conditions, call the Uamp subroutine in the subroutine, use the loading amplitude curve equation to calculate the size of the concentrated load applied in the model under the current incremental step and perform the first loading; Assign the initial stiffness matrix DDSDDE to the integration points of each unit of the discretized finite element model; The stiffness matrix DDSDDE is used to calculate the stress at each Gaussian point inside each unit; Call the Uamt subroutine in the subroutine and use the fatigue damage evolution equation to calculate the fatigue damage value D at each Gaussian point of each unit after this loading; The fatigue damage value D at each Gaussian point of each unit after this loading is accumulated with the corresponding historical value to update the corresponding accumulated fatigue damage and record it in the state variable STATEV; Substitute the stress and fatigue damage value D at each Gaussian point of each unit into the fatigue damage calculation constitutive equation, determine the influence of the fatigue damage value D on the unit stiffness, and update the stiffness matrix DDSDDE; If the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is less than the target value, the step of calculating the stress of each Gaussian point inside each unit using the updated stiffness matrix DDSDDE is performed again until the maximum value of the accumulated fatigue damage at each Gaussian point of each unit is greater than or equal to the target value; Record the total number of load cycles and analyze the distribution of fatigue damage within the model elements and the fatigue damage evolution process.

10. A fatigue test simulation system for simulating microcrack degradation of continuously reinforced concrete pavement, characterized in that: The invention comprises a microprocessor and a computer-readable storage medium connected to each other, wherein the microprocessor is programmed or configured to execute the fatigue test simulation method for simulating microcrack deterioration of a continuously reinforced concrete pavement as claimed in any one of claims 1 to 9.

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