A method for identifying mesostructure force chains of asphalt mixture based on finite element analysis

Abstract

The present invention discloses a method for identifying a mesoscopic structural force chain of an asphalt mixture based on finite element analysis, which belongs to the field of simulation of the mesoscopic mechanical behavior of particle-reinforced composite materials. The specific steps include: identifying a force chain based on the mechanical response of the asphalt mixture obtained by finite element analysis, and regarding the area with stress concentration in the asphalt mortar as the main force transmission area of ​​the aggregate; detecting whether stress concentration occurs in a small range for each asphalt mortar unit by a local detection method, and being able to accurately identify the main force transmission area of ​​the aggregate from the asphalt mortar with highly uneven stress distribution; defining the extensive contact area of ​​the aggregate, which is the asphalt mortar unit that shares nodes with the aggregate; if the main force transmission area is a subset of the intersection of the extensive contact areas of two or more aggregates, the aggregate that transmits force through the main force transmission area is determined; and connecting the centroid of the unit with the maximum stress in the main force transmission area with the centroid of the corresponding aggregate to obtain a force chain.

Description

Technical Field

[0001] The present invention belongs to the field of microscopic mechanical behavior simulation of particle reinforced composite materials, and relates to an asphalt mixture force chain identification technology, in particular to an asphalt mixture microscopic structural force chain identification method based on finite element analysis. Background Art

[0002] Asphalt mixture is a particle reinforcement material, usually composed of asphalt, coarse aggregate, fine aggregate, mineral powder and voids. The skeleton structure formed by aggregates is the main carrier of external loads inside the asphalt mixture. Therefore, the skeleton has an important influence on the performance of asphalt mixture (such as anti-rutting ability). According to the particle mechanics theory, the path formed by the transmission of force between multiple aggregates in contact with each other is called a force chain. Usually, the force chain is composed of aggregates that carry a force greater than the average force. The size and distribution of the internal force chain of the asphalt mixture can effectively reflect the interaction between aggregates at the mesoscale, and can be used as a basis for evaluating the performance of the asphalt mixture skeleton, thereby promoting the aggregate gradation design. Asphalt mixture at the mesoscale can be regarded as a two-phase composite material composed of coarse aggregate and asphalt mortar matrix. The core issue of identifying the mesostructure force chain of asphalt mixture is to identify the force transmission area and transmission force between coarse aggregates.

[0003] In existing studies, most methods based on discrete elements are used to identify the force chain of asphalt mixture. In these methods, the mesostructure model of asphalt mixture is usually simulated as a discrete body composed of round or spherical aggregates. The contact force between aggregates of asphalt mixture under external load is obtained by discrete element software. Aggregates carrying contact forces greater than the average are regarded as candidate aggregates for forming the force chain. If the angle between the direction of force transmitted by a certain aggregate and the centroid direction of the contacting aggregate is less than a preset angle (usually 45°), these aggregates are determined to be aggregates constituting the force chain. However, in road engineering, asphalt mixture is generally regarded as a continuum for modeling and analysis. Finite element method is widely used to simulate the mechanical behavior of asphalt mixture and asphalt pavement. However, methods for identifying force chains from the mechanical response of asphalt mixture obtained based on finite element method are rarely reported. The main reason is that the force chain identification criteria under the framework of continuum mechanics are not clear, and the number of units in the finite element model of asphalt mixture is huge, making it difficult to quickly obtain effective information from them. In addition, the shape of real aggregates is closer to a polyhedron with a random shape, and it is not reasonable to represent the force transmission area between them with simple contact points between circular or spherical aggregates, which is often used in the discrete element method.

[0004] In order to solve the above problems, there is an urgent need for a mesoscopic structural force chain identification method for asphalt mixture based on a continuous mechanics framework with more universal application scenarios. Summary of the invention

[0005] The present invention proposes a method for identifying the force chain of the asphalt mixture microstructure based on finite element analysis. The present invention defines the stress concentration area in the asphalt mortar matrix as the main force transmission area between coarse aggregates. According to the local detection method described in the present invention, stress concentration detection is performed on each asphalt mortar unit in a small range, which can more accurately identify the main force transmission area between aggregates, and realize the force chain identification of asphalt mixture based on finite element analysis. The present invention defines the extensive contact area of ​​aggregates. If the main force transmission area obtained by the local detection method is a subset of the intersection of the extensive contact areas of at least two aggregates, it is determined that the force is transmitted in these aggregates through the main force transmission area, that is, there is a force chain. The force chain is determined by connecting the centroid of the aggregate and the centroid of the unit with the maximum stress in the main force transmission area, which eliminates the cumbersome operation of detecting the angle between the force chain direction and the particle centroid direction in the existing force chain identification.

[0006] The technical solution of the present invention:

[0007] A method for identifying a mesoscopic structural force chain of asphalt mixture based on finite element analysis comprises the following steps:

[0008] Step 1. Use finite element software to obtain the mechanical response of the asphalt mixture microstructure under external load. Specifically: use finite element software to establish a finite element model of the asphalt mixture microstructure, and obtain the stress response of the asphalt mortar unit under the external load of the model.

[0009] Step 2. Use the local detection method to detect the units with stress concentration in the asphalt mortar. Specifically:

[0010] (2.1) For any asphalt mortar unit, the centroid of the unit is taken as the origin, and all the asphalt mortar units whose centroids of the surrounding units are less than the preset distance threshold D from the origin are set as the local detection area of ​​the unit. If the stress of the detected unit is greater than α (α ≥ 1) times the average stress of the local detection area where it is located, it is determined that the unit has stress concentration.

[0011] (2.2) Use the method in step (2.1) to test each asphalt mortar unit to obtain all stress concentration units in the asphalt mortar.

[0012] Step 3. Group the asphalt mortar stress concentration units obtained in step 2 to obtain the main force transmission area of ​​the aggregate. Specifically:

[0013] (3.1) In the asphalt mortar stress concentration unit, starting from any unit, find the unit that shares a node with the unit, and continue searching for the unit that shares a node with the newly found unit until no unit can be searched, and the searched units are automatically divided into a group.

[0014] (3.2) After obtaining a group of asphalt mortar stress concentration units, continue step (3.1) in the remaining ungrouped units until all asphalt mortar stress concentration units are grouped. Each group of asphalt mortar stress concentration units represents a main force transmission area.

[0015] Step 4. Define the extensive contact area of ​​aggregate as the asphalt mortar unit that shares the same node with the aggregate unit, and determine the extensive contact area corresponding to each aggregate accordingly.

[0016] Step 5. For each main force transmission area obtained in step 3, if it is a subset of the intersection of the extensive contact areas of at least two aggregates, these aggregates transfer force through the main force transmission area and these aggregates serve as part of the force chain; otherwise, the main force transmission area is deleted.

[0017] Step 6. Determine the force chain. Specifically, for each main force transmission area that is retained after being deleted in step 5, determine the unit with the largest absolute stress value, use the centroid of the unit as the representative force transmission point of the main force transmission area, and connect the representative force transmission point with the centroid of the aggregate that transmits force in the area obtained in step 5, thus establishing a force chain. Check the number of aggregates contained in each force chain, and remove force chains with less than three aggregates.

[0018] The present invention has remarkable effects and has the following advantages:

[0019] 1. The present invention proposes a method for identifying the force chain of asphalt mixture based on finite element analysis. Currently, most force chain identification methods are based on discrete element methods. Through the present invention, finite element analysis is expected to be widely used in the identification of force chains of different asphalt mixtures.

[0020] 2. The present invention uses a local detection method to detect whether there is stress concentration in a small range for each asphalt mortar unit. Because there are significant differences in the internal stress of asphalt mortar at a microscopic scale, this local detection method can reduce the error caused by using a traditional single stress threshold to identify aggregate contact, thereby improving the accuracy of force chain identification.

[0021] 3. The present invention defines the extensive contact area of ​​aggregates, and identifies the aggregates that transmit force in the main force transmission area by judging whether the main force transmission area is a subset of the intersection of the extensive contact area of ​​aggregates, thereby quickly determining the force chain. Therefore, the present method is suitable for aggregates with more realistic shapes, rather than aggregates represented by circles or spheres in the existing methods, and the contact between them is usually represented by points, which is inconsistent with the actual situation.

[0022] 4. The present invention can identify the force chain of a three-dimensional asphalt mixture model, while most existing methods are only applicable to a two-dimensional asphalt mixture model. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The present invention is a flowchart of a method for identifying mesoscopic structural force chains of asphalt mixtures based on finite element analysis.

[0024] Figure 2 It is the asphalt mixture microstructure finite element model used in the embodiment of the present invention.

[0025] Figure 3 Schematic diagram of the local detection area of ​​the asphalt mortar unit in an embodiment of the present invention.

[0026] Figure 4 Schematic diagram of the extensive contact area of ​​aggregates in an embodiment of the present invention.

[0027] Figure 5 Schematic diagram of the main force transmission area between aggregates in an embodiment of the present invention.

[0028] Figure 6 It is the force chain of the mesostructure of the asphalt mixture in the embodiment of the present invention. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0030] The present invention is a method for identifying the mesoscopic structural force chain of asphalt mixture based on finite element analysis. The whole method process is as follows: Figure 1 As shown, the specific steps include:

[0031] Step 1. Use finite element software to obtain the mechanical response of the asphalt mixture microstructure under external load. Specifically: use finite element software to establish a finite element model of the asphalt mixture microstructure, and obtain the stress response of the asphalt mortar unit under the external load of the model.

[0032] For the finite element model under two dimensions, the finite element mesh is divided by three-node units and four-node units, and for the finite element model under three dimensions, the finite element mesh is divided by tetrahedron units and hexahedron units, but not limited to. The density of the mesh has an important influence on the recognition accuracy of the force chain. In the present invention, it is recommended to take 0.1 to 0.5 times of the minimum aggregate particle size.

[0033] In this embodiment, a three-dimensional asphalt mixture microstructure composed of coarse aggregate and asphalt mortar is used, and the coarse aggregate is the aggregate with a particle size greater than 2.36 mm. The finite element model of the asphalt mixture microstructure composed of tetrahedral units is established using ABAQUS software. Figure 2 The grid density is set to 1 within the range of 0.1 to 0.5 times of 2.36, i.e. (0.236, 1.18).

[0034] In this embodiment, the third principal stress is extracted, and the force chain obtained based on this can reflect the pressure transmission characteristics between aggregates. This method can also be used to identify force chains based on other types of stress, such as equivalent stress and first principal stress. Accordingly, the force chain obtained will reflect the mechanical significance of the selected stress.

[0035] Step 2. Use the local detection method to detect the units with stress concentration in the asphalt mortar. Specifically:

[0036] (2.1) For any asphalt mortar unit, the centroid of the unit is taken as the origin, and all the asphalt mortar units whose centroids of the surrounding units are less than the preset distance threshold D from the origin are set as the local detection area of ​​the unit. If the stress of the detected unit is greater than α (α ≥ 1) times the average stress of the local detection area where it is located, it is determined that the unit has stress concentration.

[0037] (2.2) Use step (2.1) to test each asphalt mortar unit and obtain all stress concentration units in the asphalt mortar.

[0038] Figure 3 It is the local detection area of ​​the asphalt mortar unit in this embodiment.

[0039] The stress referred to in this embodiment is specifically the third principal stress. The recommended value of the preset distance threshold D is twice or more of the unit density, which is 2 mm, and α is 1.5.

[0040] Step 3. Group the asphalt mortar stress concentration units obtained in step 2 to obtain the main force transmission area of ​​the aggregate. Specifically:

[0041] (3.1) In the asphalt mortar stress concentration unit, starting from any unit, find the unit that shares a node with the unit, and continue searching for the unit that shares a node with the newly found unit until no unit can be searched, and the searched units are automatically divided into a group.

[0042] (3.2) After obtaining a group of asphalt mortar stress concentration units, continue step (3.1) in the remaining ungrouped units until all asphalt mortar stress concentration units are grouped. Each group of asphalt mortar stress concentration units represents a main force transmission area.

[0043] After grouping, 5961 main force transmission areas were initially obtained in this embodiment.

[0044] Step 4. Define the extensive contact area of ​​aggregate as the asphalt mortar unit that shares the same node with the aggregate unit, and determine the extensive contact area corresponding to each aggregate accordingly.

[0045] Figure 4It is the extensive contact area of ​​the aggregate in the microstructure of the asphalt mixture in this embodiment, that is, it refers to the asphalt mortar unit that has a common node with the aggregate and tightly wraps the aggregate.

[0046] Step 5. For each main force transmission area obtained in step 3, if it is a subset of the intersection of the extensive contact areas of at least two aggregates, these aggregates transfer force through the main force transmission area and these aggregates serve as part of the force chain; otherwise, the main force transmission area is deleted.

[0047] After deletion in step 5, the 4304 main force transmission areas between aggregates in this embodiment are as follows: Figure 5 shown.

[0048] Step 6. Determine the force chain. Specifically, for each main force transmission area that is retained after being deleted in step 5, determine the unit with the largest absolute stress value, use the centroid of the unit as the representative force transmission point of the main force transmission area, and connect the representative force transmission point with the centroid of the aggregate that transmits force in the area obtained in step 5, thus establishing a force chain. Check the number of aggregates contained in each force chain, and remove force chains with less than three aggregates.

[0049] Figure 6 This is the force chain of the asphalt mixture microstructure in this embodiment, which reflects the force transmission path between aggregates. The analysis results of the force chain characteristics (for example, force chain length, force transmission efficiency) can be used as a basis for evaluating the performance of the skeleton structure, thereby guiding the aggregate grading design of the asphalt mixture.

Claims

1. A method for identifying the mesoscopic structure force chain of asphalt mixture based on finite element analysis, characterized in that: Here are the steps: Step 1. Use finite element software to obtain the mechanical response of the asphalt mixture microstructure under external loads, specifically: use finite element software to establish a finite element model of the asphalt mixture microstructure, and obtain the stress response of the asphalt mortar unit under the external load of the model; Step 2. Use the local detection method to detect the units with stress concentration in the asphalt mortar, specifically: (2.1) For any asphalt mortar unit, take the centroid of the unit as the origin, and set all the asphalt mortar units whose centroids of the surrounding units are less than a preset distance threshold D from the origin as the local detection area of ​​the unit; if the stress of the detected unit is greater than α times the average stress of the local detection area where it is located, α≥1, then it is determined that the unit has stress concentration; (2.2) Using the method in step (2.1) to test each asphalt mortar unit, all stress concentration units in the asphalt mortar are obtained; Step 3. Group the asphalt mortar stress concentration units obtained in step 2 to obtain the main force transmission area of ​​the aggregate; specifically: (3.1) In the asphalt mortar stress concentration unit, starting from any unit, find the unit that shares the same node with the unit, and continue to search for the unit that shares the same node with the newly found unit until no unit can be searched, and the searched units are automatically divided into a group; (3.2) After obtaining a group of asphalt mortar stress concentration units, continue step (3.1) in the remaining ungrouped units until all asphalt mortar stress concentration units are grouped, and each group of asphalt mortar stress concentration units represents a main force transmission area; Step 4. Define the extensive contact area of ​​aggregate as the asphalt mortar unit that shares the same node with the aggregate unit, and determine the extensive contact area corresponding to each aggregate accordingly; Step 5. For each main force transmission area obtained in step 3, if it is a subset of the intersection of the extensive contact areas of at least two aggregates, these aggregates transmit force through the main force transmission area and these aggregates are part of the force chain; otherwise, delete the main force transmission area; Step 6. Determine the force chain, specifically: for each main force transmission area retained after deletion in step 5, determine the unit with the largest absolute stress value, use the centroid of the unit as the representative force transmission point of the main force transmission area, connect the representative force transmission point with the centroid of the aggregate that transmits force in the area obtained in step 5, and thus establish a force chain; detect the number of aggregates contained in each force chain, and remove force chains with less than three aggregates.

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