Author: Technical Neighbor | Nick_Liu
Using infinite elements can eliminate the reflection of stress waves, allowing for consistent computational results with a smaller model, which greatly saves computational resources and improves accuracy. Infinite element models can be used when local loading has little effect on the overall model or when the problem is unbounded.
ABAQUS 6.14 provides 17 types of infinite elements. Commonly used infinite elements in non-acoustic problems include: CINAX4, CIN3D8, CINPE4, and CINPS4. When using them, simply modify the element type keyword in the input file.
The key to using infinite elements lies in the division of the model region. This article demonstrates the process of establishing a two-dimensional infinite element model through a case involving pressure loading that varies with time and space on a cylinder.
1) Modeling Process
Step-1: Establish a symmetric model and divide it into 3 regions, as shown in Figure 1:
Figure 1 Modeling and Division
Step-2: Set the seed density in the thickness direction of the boundary region to 1, with the same number of seeds in the width direction; the internal region uses element type CAX4R, and the boundary region uses element type CAX4; the mesh division is shown in Figure 2:
Figure 2 Model Mesh
Step-3: Isolate the mesh and adjust the stacking direction, as shown in Figure 3:
Figure 3 Adjusting the Stacking Direction
Step-4: Assign material properties and establish the assembly and dynamic analysis step (Dynamic, Explicit).
Step-5: Apply pressure that varies with time and space at the top of the model through the VDLoad subroutine; simultaneously apply symmetrical and fixed constraints to the model, as shown in Figure 4:
Figure 4 Load and Constraints
Step-6: Create a new Job, output the Input file, and change the element type CAX4 in the Input file to CINAX4.
Step-7: Create a new Job with the modified Input file and submit the calculation.
2) Calculation Results
The calculation results are shown in Figure 5:
Figure 5 Calculation Results
It can be seen that when the stress reaches the white infinite element boundary, there is no reflection, and the result is consistent with the actual situation.
3) Precautions
a. There can only be one element in the thickness direction;
b. It must and can only share one edge with the finite element;
c. The material stacking direction must point towards infinity;
d. Boundary condition settings should avoid over-constraint;
The 2D operation of infinite elements is relatively simple and easy to implement. The 3D operation follows the same process as the 2D operation, but the model division is more complex. Additionally, since ABAQUS/Standard only supports acoustic infinite elements, when performing non-acoustic calculations and needing stable results, it is usually necessary to extend the computation time to obtain approximate stable results. This approach is obviously not ideal when the required computation time is lengthy. However, although Standard does not support non-acoustic infinite elements, it is still possible to import the explicit calculation results into Standard for solving, saving a lot of time.
More details on establishing 3D models, specific import operations, and the use of the VDLoad subroutine will be shared later.
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