A method for realizing virtual fatigue simulation of cement stabilized macadam materials based on discrete element model

Abstract

The present invention proposes a method for realizing virtual fatigue simulation of water-stabilized macadam materials based on a discrete element model. The method includes: (1) preparing indoor fatigue test specimens of water-stabilized macadam and conducting indoor fatigue tests; (2) constructing a fatigue damage evolution model of water-stabilized macadam materials based on the results of indoor fatigue tests; (3) using PFC software to establish a particle system of water-stabilized materials, setting contact models and contact parameters to form numerical specimens of water-stabilized macadam materials; (4) establishing a virtual fatigue decay model based on the fatigue damage evolution model, and using the Fish command to embed the virtual fatigue decay model into each running step of the fatigue test to simulate the decay behavior of the material properties of the model over time; (5) establishing a virtual fatigue loading system, applying fatigue cyclic loads to the numerical specimens of water-stabilized macadam materials described in step 3 to realize the virtual fatigue simulation of water-stabilized macadam materials.

Description

Technical Field

[0001] The present invention relates to the technical field of numerical simulation based on the discrete element method, and particularly relates to a method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model. Background Art

[0002] The cement stabilized macadam base course has been widely used in highway construction due to its small elastic deformation and strong bearing capacity. As the load-bearing layer of the pavement structure, the quality of the cement stabilized base course is directly related to the bearing capacity and integrity of the entire pavement structure. During the service life of the pavement, fatigue cracking of the cement stabilized base course inevitably occurs under cyclic loading. Cracks in the base course will cause irreversible structural and functional defects. For example, it can cause reflection cracks in the overlying asphalt concrete layer. These cracks extend to the pavement surface, reducing the bearing capacity of the pavement structure, increasing the penetration of moisture into the pavement foundation, thereby increasing the maintenance cost and shortening the service life of the pavement. The fatigue fracture of the cement stabilized base course materials determines the long-term preservation of the pavement structure.

[0003] Currently, traditional indoor tests are still the basic method for studying the fatigue failure of the cement stabilized base course. Due to the influence of many factors, the repeatability of test results is poor. In particular, conventional tests cannot achieve a perfect reproduction of the specimen structure, which makes the research results of parameter analysis based on controlling variables generally have a large dispersion, and it is difficult to accurately analyze the influencing factors. In addition, the two key contents of fatigue cracking are crack propagation and material property degradation. The microscopic structure of materials significantly affects their macroscopic properties. Through the mesoscopic analysis of cracks, the propagation process of microcracks can be accurately obtained, and the fatigue damage degradation mechanism can be further clarified. Due to the limitation of testing means, the existing indoor tests are still difficult to analyze the crack morphology at the micro and mesoscopic scales. The numerical simulation method can realize the observation of microcracks and can conveniently obtain information such as stress and strain responses at each point inside the structure. Therefore, the research method based on numerical simulation has obvious advantages over traditional macroscopic tests. As a numerical method for solving discontinuous systems, the discrete element method allows large deformations and crack separation, and can effectively simulate discontinuous situations such as slip and cracking. Currently, there have been many numerical simulations of asphalt or cement concrete fracture based on the discrete element method. However, due to the complexity of the fatigue damage mechanism, the fatigue fracture model based on the discrete element has not been fully developed.

[0004] Aiming at the deficiencies of the current research, in order to study the fatigue cracking behavior of cement stabilized macadam materials from the mesoscopic scale and obtain its mesoscopic fatigue damage process, it is necessary to establish a virtual fatigue simulation model of cement stabilized macadam materials based on discrete element numerical means for the mesoscopic fatigue simulation analysis of cement stabilized macadam materials, so as to better serve the research on the fatigue damage degradation mechanism. Summary of the Invention

[0005] Technical problem: The technical problem to be solved by the present invention is to construct a fatigue damage evolution model of water-stabilized macadam materials based on macroscopic fatigue tests, and establish a virtual fatigue decay model based on the discrete element method to realize the decay behavior of material properties over time in the numerical specimens of water-stabilized macadam, so as to carry out fatigue simulation tests on water-stabilized macadam materials in the discrete element model, study the mesoscopic fatigue cracking process of water-stabilized macadam materials, and clarify its fatigue damage deterioration mechanism under cyclic loading.

[0006] Technical solution: To achieve the above object, the present invention provides a method for realizing virtual fatigue simulation of water-stabilized macadam materials based on a discrete element model, and the method comprises the following steps:

[0007] Step 1, prepare indoor fatigue test specimens of water-stabilized macadam, formulate an indoor fatigue test plan, and conduct indoor fatigue tests;

[0008] Step 2, based on the indoor fatigue test results with different stress ratios, study the fatigue evolution curve and damage evolution parameters, and construct a fatigue damage evolution model of water-stabilized macadam materials to characterize the fatigue damage evolution law of water-stabilized macadam materials under cyclic loading;

[0009] Step 3, use the "wall" command in the PFC software to generate boundary walls to form a closed model boundary area, and fill the particle system within this model boundary area. Divide the aggregate-phase particles and mortar-phase particles according to the shape, size and distribution position of the aggregates, and assign the properties of each particle; respectively set the contact models of the aggregate phase, mortar phase and aggregate-mortar interface phase, and set the mesoscopic material parameters of various contact models; finally, delete the "wall" to form a numerical specimen of water-stabilized macadam materials composed of two phases of aggregate and mortar;

[0010] Step 4, according to the indoor fatigue test plan described in Step 1, use the "wall" command to generate virtual loading walls and supporting walls on the numerical specimen of water-stabilized macadam materials described in Step 3 to form a basic model for water-stabilized fatigue tests;

[0011] Step 5, perform static unidirectional loading on the basic model for water-stabilized fatigue tests described in Step 4 by referring to the indoor strength test method to obtain the peak load of the numerical specimen of water-stabilized macadam materials;

[0012] Step 6, establish a virtual fatigue decay model according to the fatigue damage evolution model described in Step 2;

[0013] Step 7: According to the indoor fatigue test scheme described in Step 1, establish a virtual fatigue loading regime. Use the Fish command to apply a fatigue cyclic load to the numerical specimen of the water-stabilized macadam material described in Step 3 through the loading wall described in Step 4. Embed the virtual fatigue attenuation model described in Step 6 during the operation of each time step using the Fish command. Calculate the parallel bond decay rate ω based on the contact force between adjacent particles, and update the contact bond radius coefficient within each operation time step. Simulate the damage deterioration behavior of the water-stabilized macadam material's performance over time, and achieve the virtual fatigue simulation of the water-stabilized macadam material.

[0014] Furthermore, the indoor fatigue test specimens of the water-stabilized macadam described in Step 1 can be taken from the core samples drilled on-site or the specimens formed indoors. The styles of the fatigue test specimens can be selected as beam-shaped specimens or semi-circular specimens.

[0015] Furthermore, the indoor fatigue test scheme described in Step 1 includes the loading frequency, load mode, and loading waveform. The loading frequency of the fatigue cyclic load is preferably 10 Hz, that is, the duration of a single action is about 0.1 s, to simulate the action time of a driving speed of 60 km / h to 80 km / h on the road surface. The fatigue test load mode is preferably the loading mode of controlling stress to improve the control accuracy of the test. The fatigue test loading waveform is preferably the loading waveform of the half-sine wave (Haversine).

[0016] Special note: Considering that the water-stabilized macadam material is similar to a brittle material and does not have the temperature sensitivity like asphalt mixture, the fatigue test of the water-stabilized macadam material is preferably carried out at room temperature without considering the influence of the test temperature on the fatigue behavior of the specimens.

[0017] Special note: Since the natural healing and repair process of cement-based materials is generally slow, its healing and repair effect on its own damage can be ignored during a short repeated load loading interval. Therefore, the fatigue test of the water-stabilized macadam material is preferably carried out in a non-intermittent loading method to improve the test efficiency of the test.

[0018] Furthermore, the fatigue test stress ratio range described in Step 2 is preferably 0.5 to 0.8, and the peak load value of the fatigue loading is obtained from the strength test of the parallel specimens.

[0019] Special note: To ensure the stability of the fatigue test loading process, the minimum load of the cyclic fatigue load is set to 0.1 kN.

[0020] Furthermore, during the fatigue test described in Step 2, the system automatically records the applied load value and time data. At the same time, a vertically installed linear variable differential sensor is used to measure the vertical deformation of the specimen. The data acquisition frequency is preferably 200 Hz, that is, 20 times of data are collected in each cycle.

[0021] It should be noted that since the development of fatigue damage of materials is a gradual evolution process and it is difficult to directly measure it, the irreversible effects brought by the damage can be characterized by the stress index of flexural modulus or the strain index of flexural strain. Further, the fatigue damage evolution law described in step 2 refers to the change relationship model of mechanical response indexes (modulus, strain, etc.) of the structure under cyclic loading. The macroscopic response changes of the flexural modulus and strain of the structure both reflect the degree and level of material damage development. The present invention preferably selects flexural deformation as the index reflecting the development of material fatigue damage.

[0022] Further, the damage evolution parameter described in step 2 is preferably the vertical cumulative deformation index of the specimen during the fatigue test.

[0023] Further, the fatigue evolution curve described in step 2 is preferably the evolution curve of the vertical cumulative deformation of the specimen with the fatigue loading process.

[0024] Further, the fatigue damage evolution model described in step 2 refers to the evolution relationship model of the vertical cumulative deformation of the specimen with the number of fatigue load cycles during the fatigue test, preferably a logarithmic regression model, and the expression is as follows:

[0025] D N = D0 + kln(N) (1)

[0026] In the formula, N is the number of cyclic loadings; D N is the vertical cumulative deformation at the Nth loading cycle; D0 is the vertical deformation at the initial moment; k is the fatigue damage rate, which is used to characterize the sensitivity of the load damage degree to the stress level, and k = f(S); S is the stress ratio level set for the fatigue test. Further, the fatigue damage rate k in the fatigue damage evolution model determines the change degree of the vertical cumulative displacement with the number of loadings. k is related to the stress level used in the fatigue test, and the change law of the fatigue damage rate k is obtained through load tests at different stress levels, preferably a two-parameter exponential model, and the expression is as follows:

[0027] k = Ae B*S (2)

[0028] In the formula, A and B are damage rate sensitive coefficients related to the material and are constant values.

[0029] It should be noted that to avoid the test errors brought by a single test sample and further improve the accuracy of the constructed fatigue damage evolution model, multiple comparison specimens can be used for parallel tests during the test at the same stress level.

[0030] Further, in step 3, the shape and size of the model boundary region refer to the shape and size of the numerical specimen of the cement stabilized macadam material.

[0031] Furthermore, the particle system described in step 3 is a collection of numerous "balls" with equal radii. The "ball" particles are preferably arranged in a regular pattern, and the regular arrangement pattern can be one of "cubic" and "hexagonal".

[0032] Furthermore, the particle properties described in step 3 are determined by the group name of the group to which the particles belong. Aggregate-phase particles are defined as "aggregate balls", and mortar-phase particles are defined as "mortar balls". That is, aggregate particles are composed of several "aggregate balls" bonded together, and the mortar matrix is composed of several "mortar balls" bonded together.

[0033] Specifically, the shape and size of the aggregates described in step 3 can be user-defined. The preferred range of aggregate particle size is 1.18 mm - 19 mm.

[0034] Specifically, for the purpose of mesoscopic simulation, the diameter of the "ball" is preferably 0.3 mm. At the same time, to improve the calculation efficiency, aggregates with a particle size less than 1.18 mm are regarded as the mortar phase.

[0035] Furthermore, in the numerical specimen of the water-stabilized macadam material described in step 3, the contact model within the "aggregate balls" is preferably a linear contact bonding model, and the contact models within the "mortar balls" and at the interface between the "aggregate balls" and the "mortar balls" are preferably parallel bonding models.

[0036] Furthermore, the mesoscopic parameters of the material of the contact model described in step 3 are related to the macroscopic properties of the material itself, and can be obtained through inverse analysis calculation by combining the macroscopic mechanical tests and simulation tests of the single-phase materials of aggregates or mortar.

[0037] Furthermore, the layout of the loading wall and the supporting wall described in step 4 is determined according to the style of the fatigue test specimen. For beam-shaped specimens, four-point contact is preferred to form a three-point loading method; for semi-circular specimens, three-point contact is preferred to form a two-point loading method, and the distance between the bottom supporting walls is preferably 0.8 times the diameter of the semi-circular specimen.

[0038] Furthermore, the static unidirectional loading scheme described in step 5 involves a loading force control mode and a loading rate, and the loading of the numerical specimen of the water-stabilized macadam material is achieved by applying virtual loads to the loading wall. A static pressure splitting loading mode with displacement control is preferred, and the loading rate is preferably 0.86 mm / s.

[0039] It should be noted that for the water-stabilized macadam material, fatigue damage is mainly manifested as the attenuation of material properties at the mortar or aggregate-mortar interface. Further, the virtual fatigue decay model described in step 6 is preferably established through the attenuation evolution of the parallel bond contact model at the mortar and aggregate-mortar interface, so as to realize the natural attenuation of the bond parameters of the model under cyclic loading.

[0040] Further, in the virtual fatigue decay model described in step 6, the normal stress and tangential stress between particles are controlled by the particle contact force and the bond radius parameter, and the calculation methods of the normal stress and tangential stress between particles are as follows:

[0041]

[0042]

[0043] In the formula, is the parallel bond radius, R (1) , R (2) are the radii of the solid particles at both ends of the contact; is the bond radius coefficient, and the default value is 1.0; is the parallel bond normal stress component, is the tensile stress when; is the parallel bond tangential stress component; are the normal and tangential components and the bending moment of the parallel bond force respectively.

[0044] Further, in the virtual fatigue decay model described in step 6, the material damage is defined as the relative reduction of the parallel bond radius coefficient , and the expression is as follows:

[0045]

[0046] In the formula, ω is the parallel bond decay rate; t is the calculation time.

[0047] Further, in the virtual fatigue decay model described in step 6, it is assumed that the occurrence of damage has a critical stress value When the actual stress is less than the critical value , fatigue cracking will not occur, that is, the parallel bond does not decay; when the stress reaches , the parallel bond begins to decay, and the decay rate is related to the ratio of the stress between its units and the ultimate strength. The larger this ratio is, the faster the decay rate of the parallel bond radius is; when the stress value exceeds the strength limit , direct fracture failure will occur, and the decay rate is negative infinity at this time.

[0048] Therefore, combined with the fatigue damage evolution model described in Step 2, the virtual fatigue decay model described in Step 6 is expressed as follows:

[0049]

[0050] In the formula, is the tensile strength; is the damage threshold; is the damage limit strength; α1 and α2 are damage rate control constants varying with materials.

[0051] Furthermore, the virtual fatigue decay model described in Step 6 is embedded in the operation process of each time step by using the Fish command, and the bonded radius coefficient of the contact is updated according to the contact force between adjacent particles So far, by introducing the virtual fatigue decay model into the basic model of the water stable fatigue test, the damage deterioration behavior of the performance of the water stable macadam material changing with time is realized, and then it is used for the fatigue test simulation of the water stable macadam material. Within each time step, the bonded radius coefficient of the contact is updated in the following manner:

[0052]

[0053] In the formula, are the parallel bonded radius coefficients at the moments of t+Δt and t respectively; Δt is the calculation time step length.

[0054] It should be specially noted that considering that the fatigue test of the water stable macadam material is mainly tensile failure, the attenuation of the parallel bonded contact bond caused by shear stress is ignored in the discrete element fatigue simulation of the present invention.

[0055] Furthermore, the virtual fatigue loading regime described in Step 7 includes virtual loading frequency, virtual load mode and virtual loading waveform. The virtual loading frequency is preferably 100Hz, that is, the single action duration is about 0.01s to improve the calculation efficiency. The virtual load mode is preferably a stress-controlled cyclic loading mode without interruption. The virtual loading waveform is preferably a loading waveform of a semi-sine wave load.

[0056] Furthermore, the peak load value of the fatigue cyclic load described in Step 7 is the peak load of the numerical specimen of the water stable macadam material described in Step 5.

[0057] It should be specially noted that to ensure the stability of the virtual fatigue loading process, the minimum load of the virtual cyclic load is set to 0.1kN, and a preloading transition stage is set, and the preloading transition stage is preferably 0.005 - 0.01s.

[0058] Specific instructions: During the virtual fatigue test, the software automatically records the applied load value, the vertical displacement of the upper wall, and the mid-span deflection, and uses the number of broken parallel bond contacts to characterize the propagation of microcracks.

[0059] Advantages: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0060] A method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model, which is used for fatigue simulation analysis and research of cement stabilized macadam materials. The fatigue damage evolution model of cement stabilized macadam materials constructed based on macroscopic fatigue tests truly characterizes the decay characteristics of the material properties of cement stabilized macadam materials under cyclic loading, and can be used to guide the development of accurate virtual fatigue damage simulation test research. Based on the virtual fatigue decay model established by the present invention, the decay behavior of material properties over time is realized, and it can be used to carry out fatigue simulation tests on cement stabilized macadam materials in a discrete element model to study the mesoscopic fatigue cracking process of cement stabilized macadam materials and clarify its fatigue damage degradation mechanism under cyclic loading. The present invention provides practical verification and technical reference for in-depth fatigue simulation analysis and research of cement stabilized macadam materials. Brief Description of the Drawings

[0061] Figure 1 Schematic diagram of the preparation process of the semi-circular specimen of the present invention;

[0062] Figure 2 Schematic diagram of the loading method for semi-circular bending strength and fatigue test of the present invention;

[0063] Figure 3 Schematic diagram of the semi-versine load wave in the indoor fatigue test;

[0064] Figure 4 Schematic diagram of the vertical cumulative deformation evolution curve in the indoor fatigue test;

[0065] Figure 5 Typical curves of vertical cumulative deformation evolution at different stress levels in the indoor fatigue test;

[0066] Figure 6 Schematic diagram of the parallel bond attenuation of the virtual fatigue decay model;

[0067] Figure 7 Semi-circular numerical specimen of cement stabilized macadam materials of the present invention;

[0068] Figure 8 Basic model of the cement stabilized macadam fatigue test of the present invention;

[0069] Figure 9 Virtual load-displacement curve of the basic model of the cement stabilized macadam fatigue test;

[0070] Figure 10 Schematic diagram of the loading waveform of the virtual fatigue cyclic load

[0071] Figure 11 Semicircular fatigue test simulation results of the present invention Specific implementation manners

[0072] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The following specific embodiments will help to further understand the present invention, but do not limit the present invention in any form. It should be noted that without departing from the main concept of the present invention, several improvements and changes made all fall within the protection scope of the present invention

[0073] The present invention provides a method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model, including the following specific implementation steps

[0074] Step 1: Drill a cylindrical core sample of cement stabilized macadam on site. The core sample has a diameter of 150 mm, and prepare semicircular specimens according to the method of AASHTO TP 105-13; Two semicircular specimens obtained from the same cross-section can be considered to have the same mechanical properties, and are respectively used for the semicircular bending strength test and the fatigue test. The preparation process of the semicircular specimens is as Figure 1 .

[0075] Step 2: Use a UTM universal testing machine to conduct a semicircular strength test. Adopt a three-point loading mode. The distance between the two bottom supports is 0.8 times the diameter of the specimen, that is, 120 mm. The schematic diagram of the three-point loading mode is as Figure 2 shown. The indoor semicircular strength test adopts a static pressure splitting loading mode controlled by displacement, and the loading rate is 0.86 mm / s, which is used to obtain the peak load value of the cement stabilized macadam semicircular specimen

[0076] Step 3: Use a UTM universal testing machine to conduct a semicircular fatigue test according to the three-point loading mode described in Step 2. The semicircular fatigue test adopts a stress-controlled semi-sine wave cyclic loading mode without interruption, with a loading frequency of 10 Hz. The minimum load of the cyclic fatigue load is set to 0.1 kN. Determine the peak force of the fatigue test under the corresponding stress ratio according to the peak load value of the semicircular specimen. The semi-sine wave of the fatigue test is as Figure 3 shown. During the indoor semicircular fatigue test, the system automatically records the applied load value and time data. At the same time, a vertically installed linear variable differential sensor is used to measure the vertical deformation of the specimen. The data acquisition frequency is 200 Hz, that is, 20 times of data are collected in each cycle. The vertical cumulative deformation evolution curve of the semicircular specimen obtained from the semicircular fatigue test is as Figure 4 .

[0077] Step 4: According to the semi-circular fatigue test described in Step 3, conduct semi-circular fatigue tests with stress ratios of 0.5, 0.6, 0.7, and 0.8 respectively, and obtain the typical curves of the vertical cumulative deformation evolution of the semi-circular specimens under different stress ratios, as Figure 5 shown.

[0078] Step 5: Taking the vertical cumulative deformation of the semi-circular specimen as the damage evolution parameter index and the vertical cumulative deformation evolution curve as the fatigue evolution curve, establish a fatigue damage evolution model in the form of a logarithmic regression model. The expression is as follows:

[0079] D N = D0 + kln(N) (8) where N is the number of cycles of the cyclic load; D N is the vertical cumulative deformation at the Nth loading cycle; D0 is the vertical deformation at the initial moment; k is the fatigue damage rate; S is the stress ratio level set for the fatigue test.

[0080] Step 6: Based on the typical curves of the vertical cumulative deformation evolution under different stress ratios, establish the fatigue damage evolution models corresponding to different stress ratios, and statistically obtain the results of the fatigue damage rate k values under different stress levels S, as shown in Table 1. Further, establish a relationship model between the fatigue damage rate k and the stress level S used in the fatigue test in the form of a two-parameter exponential model. The expression is as follows:

[0081] k = 0.00944e 1.68481*S (9) Table 1 Results of fatigue damage rate k values under different stress levels S

[0082]

[0083] Step 7: Taking the center of the model as the coordinate origin, use the "wall" command in the PFC2D software to generate a circular boundary wall with a diameter of 150 mm, and regularly arrange "balls" with a diameter of 0.3 mm in a "cubic" manner within the circular boundary wall area. According to the custom virtual aggregate shape, size, and distribution position, assign the "balls" within the aggregate phase range to "aggregate balls" and give them the group name "aggregate", and assign the remaining "balls" without the group name "aggregate" to "mortar balls" and give them the group name "mortar". Use the "contact model" command to apply linear contact bonding between adjacent "balls" within the "aggregate" group, apply parallel bonding between adjacent "balls" within the "mortar" group, and apply parallel bonding between the "balls" of the "aggregate" group and the "mortar" group.

[0084] Step 8: Traverse each contact described in Step 7, and use the command word "contact property" to assign the mesoscopic parameters of the material to each contact. The mesoscopic parameters of the cement stabilized macadam material model are set as shown in Table 2.

[0085] Table 2 Mesoscopic parameters of the cement stabilized macadam material model

[0086]

[0087] Step 9: Delete the wall "wall", and evenly divide the circular specimen to form a semi-circular numerical specimen of the cement stabilized macadam material composed of aggregate and mortar phases. The result is as Figure 7 .

[0088] Step 10: According to the three-point loading mode described in Step 2, use the "wall" command to generate virtual loading walls and support walls on the semi-circular numerical specimen of the cement stabilized macadam material described in Step 9 to form a basic model for the water stability fatigue test. The result is as Figure 8 .

[0089] Step 11: Refer to the semi-circular strength test method described in Step 2, adopt a displacement-controlled unidirectional loading mode, apply a vertical displacement load through the top wall, and perform static unidirectional loading on the basic model of the water stability fatigue test described in Step 10. Automatically record the load value and vertical displacement of the top loading wall through the Fish command for plotting the load-displacement curve, as Figure 9 . Finally, obtain the peak load of the numerical specimen of the cement stabilized macadam material. It should be noted that to improve the simulation test efficiency, the virtual loading speed is set to 10 times that of the indoor test, i.e., 8.6 mm / s.

[0090] Step 12: According to the relationship model between the fatigue damage rate k and the stress level S described in Step 2, establish a virtual fatigue decay model through the parallel bond contact model at the mortar and aggregate-mortar interface. The expression is as follows:

[0091]

[0092] In the formula, is the normal stress component of the parallel bond contact in the basic model of the water stability fatigue test; is the tensile strength parameter value in the basic model of the water stability fatigue test. Step 13: Use the Fish command, according to the virtual fatigue decay model described in Step 12, and based on the contact force between adjacent particles at both ends of the parallel bond contact at the mortar and aggregate-mortar interface, update the parallel bond radius coefficient of the parallel bond contact at the mortar and aggregate-mortar interface. The update method of the bond radius coefficient of the contact is as follows:

[0093] In the formula, are the parallel bond radius coefficients at the start and end of the calculation time step respectively; Δt is the calculation time step length, preferably 10 -6 s.

[0094] Step 14: Referring to the indoor fatigue test plan described in Step 3, according to the stress-controlled semi-sine cyclic loading mode without interruption, use the Fish command to control the movement of the upper loading wall to achieve fatigue loading on the water-stable fatigue test basic model described in Step 10. The fatigue loading frequency is 100 Hz, the minimum load is set to 0.1 kN, and a preloading transition stage of 0.005 s is set. The virtual fatigue cyclic load loading waveform is as Figure 10 .

[0095] Step 15: During the fatigue loading process described in Step 14, use the "set fish callback" command word to call the parallel bond radius coefficient update function within each time step to achieve automatic update of the parallel bond radius coefficient of the parallel bond contact at the mortar and aggregate-mortar interface, simulate the decay behavior of the performance of the water-stable macadam material with the change of fatigue loading time, and then achieve the virtual fatigue simulation of the water-stable macadam material. During the simulation test process, the system automatically records the load value, vertical displacement, and mid-span deflection applied by the upper loading wall, the decay curve of the parallel bond contact key coefficient, and uses the number of fractures of the parallel bond contact key to characterize the propagation of microcracks. The semi-circular fatigue test simulation results are shown in Figure 11 .

[0096] Specially noted that both the indoor semi-circular strength and fatigue tests are carried out under room temperature conditions.

[0097] Specially noted that the support shaft described in Step 2 uses a smooth round rod to reduce the friction between the specimen and the device.

[0098] Specially noted that to avoid the test error caused by a single test sample, the typical curve of the vertical cumulative deformation evolution of the semi-circular specimen described in Step 4 can be determined by the test results of multiple parallel specimens.

[0099] Specially noted that in the vertical cumulative deformation evolution curve of the semi-circular specimen described in Step 3, with the increase of the number of load applications, the vertical cumulative deformation has successively experienced three stages: rapid growth in the initial stage, stable growth in the middle stage, and rapid growth in the later stage. There are two inflection points in the evolution process of the vertical cumulative deformation amount from the rapid growth stage to the stable growth stage and then to the rapid growth stage. The number of load cycles corresponding to the second inflection point is used as the fatigue life N of the fatigue test. The third stage is the failure stage, and the maintenance time is short. When fitting the fatigue damage evolution model described in Step 5, the vertical deformation in the third stage of fatigue is not considered to improve the accuracy of the fitting curve.

[0100] Specifically, the particle size range of the virtual aggregate described in step 7 is 1.18 mm - 19 mm, and the aggregate with a particle size less than 1.18 mm is regarded as the mortar phase.

[0101] Furthermore, the mesoscopic parameters of the material of the contact model described in step 8 are related to the macroscopic properties of the material itself, and can be obtained through inverse analysis and calculation by combining the macroscopic mechanical tests and simulation tests of the aggregate or mortar single-phase material.

[0102] Specifically, since it is difficult to directly determine the decay coefficient of the virtual fatigue decay model described in step 12, the present invention intends to control the fatigue life corresponding to a stress ratio of 0.8 to be hundreds of times, and taking this as the optimization goal, the parameters in the virtual fatigue decay model are determined by the trial algorithm.

[0103] Specifically, considering that the semi-circular fatigue test of the water-stabilized macadam material is mainly tensile failure, the present invention ignores the attenuation of the parallel bond contact bonds caused by shear stress in the discrete element fatigue simulation.

[0104] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for realizing virtual fatigue simulation of water-stabilized macadam materials based on a discrete element model, characterized in that, The method comprises the following steps: Step 1: Prepare indoor fatigue test specimens of water-stabilized macadam, formulate an indoor fatigue test plan, and conduct an indoor fatigue test; Step 2: Based on the indoor fatigue test results under different stress ratios, study the fatigue evolution curve and damage evolution parameters, and construct a fatigue damage evolution model of water-stabilized macadam materials to characterize the fatigue damage evolution law of water-stabilized macadam materials under cyclic loading; Step 3: Use the "wall" command in the PFC software to generate boundary walls, form a closed model boundary area, and fill the granular system within this model boundary area. Divide the aggregate-phase particles and mortar-phase particles according to the shape, size, and distribution position of the aggregates, and assign the properties of each particle; Set the contact models for the aggregate phase, mortar phase, and aggregate-mortar interface phase respectively, and set the mesoscopic material parameters of various contact models; finally, delete the "wall" to form a numerical specimen of water-stabilized macadam material composed of two phases of aggregates and mortar; The granular system is a collection of numerous "ball" particles with equal radius. The "ball" particles are arranged in a regular pattern, and the regular arrangement pattern is selected from "cubic" and "hexagonal"; the diameter of the "ball" is selected as 0.3 mm, and the aggregates with a particle size less than 1.18 mm are regarded as the mortar phase; Step 4: According to the indoor fatigue test plan described in Step 1, use the "wall" command to generate virtual loading walls and support walls on the numerical specimen of water-stabilized macadam material described in Step 3 to form a basic model for the water-stabilized fatigue test; Step 5: Refer to the indoor strength test method to conduct static unidirectional loading on the basic model for the water-stabilized fatigue test described in Step 4 to obtain the peak load of the numerical specimen of water-stabilized macadam material; Step 6: Establish a virtual fatigue decay model according to the fatigue damage evolution model described in Step 2; Step 7: According to the indoor fatigue test scheme described in Step 1, establish a virtual fatigue loading regime. Using the Fish command, apply fatigue cyclic loads to the numerical specimens of the water-stabilized macadam material described in Step 3 through the loading wall described in Step 4. Embed the virtual fatigue decay model described in Step 6 in the operation process of each time step using the Fish command. Calculate the parallel bond decay rate ω based on the contact force between adjacent particles, and update the contact bond radius coefficient within each operation time step. Simulate the damage deterioration behavior of the water-stabilized macadam material's performance over time, and achieve the virtual fatigue simulation of the water-stabilized macadam material. The fatigue evolution curve described in Step 2 is the evolution curve of the vertical cumulative deformation of the specimen with the fatigue loading process; the fatigue damage evolution model described in Step 2 refers to the evolution relationship model of the vertical cumulative deformation of the specimen with the number of fatigue loads during the fatigue test, which is selected as a logarithmic regression model, and the expression is as follows: D N = D0 + kln(N) (1) where N is the number of cyclic load applications; D N is the cumulative vertical deformation at the Nth loading cycle; D0 is the vertical deformation at the initial moment; k is the fatigue damage rate, which is used to characterize the sensitivity of the load damage degree to the stress level, S is the stress ratio level set in the fatigue test, and the expression of k is as follows: k = Ae B*s (2) In the formula, A and B are damage rate sensitive coefficients related to the material; In the virtual fatigue decay model described in Step 6, the normal stress and shear stress between particles are controlled by the particle contact force and bond radius parameters. The calculation methods for the normal stress and shear stress between particles are as follows: In the formula, is the parallel bonding radius, R (1) , R (2) are the radii of the solid particles at both ends of the contact; is the bonding radius coefficient; is the normal stress component of the parallel bond, is the tensile stress when is the tangential stress component of the parallel bond; are the normal and tangential components of the parallel bonding force and the bending moment, respectively; Combined with the fatigue damage evolution model described in Step 2, the virtual fatigue decay model described in Step 6 is expressed as follows: In the formula, is the tensile strength; is the damage threshold; is the damage limit strength; α1 and α2 are damage rate control constants varying with the material; Within each time step, the update method of the bonding radius coefficient of contact is as follows: ​ In the formula, are the parallel bond radius coefficients at the times of t+Δt and t respectively; Δt is the calculation time step.

2. A method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model according to claim 1, characterized in that The indoor fatigue test specimens of water-stabilized macadam described in Step 1 are cored samples taken from the site or specimens formed indoors. The styles of the fatigue test specimens are beam-shaped specimens or semi-circular specimens. Moreover, the indoor fatigue test plan includes the loading frequency, load mode, and loading waveform.

3. A method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model according to claim 1, characterized in that The layout of the loading walls and support walls described in Step 4 is determined according to the style of the fatigue test specimen. For beam-shaped specimens, four-point contact is selected to form a three-point loading method; for semi-circular specimens, three-point contact is selected to form a two-point loading method.

4. A method for realizing virtual fatigue simulation of cement stabilized macadam materials based on a discrete element model according to claim 1, characterized in that The static unidirectional loading scheme described in step 5 involves a loading force control mode and a loading rate, and the loading of the numerical specimen of the water-stabilized macadam material is achieved by applying virtual loads to the loaded wall.

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