Jack find that many students habitually turn on large deformation when doing simulations, regardless of whether the analyzed problem actually involves large deformation. To help everyone better understand large deformation, this article briefly explains the basic concept of large deformation.
We know that simulation has three major nonlinearities:
1. Material nonlinearity
2. Contact nonlinearity
3. Geometric nonlinearity
The so-called turning on large deformation means activating geometric nonlinearity. Conceptually, large deformation typically refers to the situation where a material or structure experiences deformation that is relatively large compared to its original dimensions during the loading process. In such cases, geometric nonlinear effects need to be considered, meaning that deformation significantly changes the geometric shape of the structure, thereby affecting the distribution of stress and strain.
Take the following figure as an example (from ANSYS Help):
This shows the direction changes of acceleration, element nodal forces, and element surface loads before and after structural deformation when large deformation is not considered. From the figure, it can be seen that the directions of acceleration and nodal forces do not change with structural deformation – the directions are fixed. Once the structural deformation is large, this situation may introduce significant errors.
Generally speaking, the following situations require consideration of geometric nonlinearity:
1. Large strain: If the structure or component experiences strain greater than 5-10% during loading, large deformation effects usually need to be considered. Common examples include rubber materials, plastically deformed metals, and some composite materials.
2. Large displacement: When the displacement of a structure or component is not negligible compared to its characteristic dimensions, large deformation should be enabled (this ratio can refer to code limits on deflection). Examples include membrane structures, cables, long-span steel beams, roof shells, etc.
3. Rotation and bending: Some structures experience significant rotation or bending during loading, which leads to geometric nonlinearity. For example, a long cantilever beam subjected to a large lateral load at the free end will bend and rotate significantly.
4. Contact problems: When there are significant contact, friction, and separation phenomena, large deformation needs to be considered to accurately simulate changes in contact conditions.
5. Buckling analysis: For buckling problems, large deformation analysis is needed to capture post-buckling behavior. This is particularly important in thin-walled structures (such as thin plates and shells).
In ANSYS APDL, large deformation can be enabled using the NLGEOM command. In Workbench Mechanical, simply turn on the large deformation option in Analysis Settings.
To compare the difference between the two, take the following Kiewitt single-layer lattice shell as an example. The loading diagram is shown below. It can be foreseen that as the applied force increases, the deformation characteristics become more and more obvious, and the influence on the geometric stiffness and force direction of the structural system also becomes larger and larger.
Below are the maximum displacements of the structure under different loads:
It can be seen that as the load gradually increases, the impact of large deformation on the structural system becomes increasingly significant. If it reaches a certain level, its influence cannot be ignored!
In practical operation, Shuige believes that the decision to turn large deformation on or off can be considered from the following aspects:
1. When not considering large deformation, check whether the deformation of the component or system exceeds code limits. If it does, large deformation should be considered;
2. Conduct two analyses with typical load cases – one with large deformation and one without. If the error between the two exceeds the engineering tolerance of 5%, large deformation effects should be considered in subsequent analyses;
3. For flexible structures such as cables, membranes, etc., turn on large deformation without hesitation;
4. When studying the ultimate bearing capacity of a structure, large deformation should be turned on.
The above is my humble understanding of large deformation, for your reference only!
If you find this useful, welcome to repost and share!
No comments yet