This discussion centers on the implementation of boundary conditions (BCs) in Finite Element Analysis (FEA) software, specifically addressing the use of displacement and force BCs simultaneously. It confirms that controlling the degrees of freedom (DoF) for individual nodes is supported in various FEA software, such as Abaqus and Ansys. The conversation highlights that while applying conflicting BCs can lead to solver warnings, the software typically allows for the computation of results without errors, depending on the order of BC application. The importance of understanding the specific FEA software's documentation is emphasized for effective implementation.
PREREQUISITESEngineers, FEA analysts, and researchers who are working with boundary conditions in Finite Element Analysis, particularly those using Abaqus or Ansys for complex simulations.
the number of DOFs is fixed in a model, =2 in a 2D plane problem.Baluncore said:Can you control the “degree of freedom” of nodes in your model ?
Controlling the DoF of individual nodes is supported in some FEA software.feynman1 said:the number of DOFs is fixed in a model, =2 in a 2D plane problem.
Ansys, abaqus etc. I don't know how “anisotropic” or “directional properties”, "zero friction" have sth to do with this topic. How do you control DOFs?Baluncore said:Controlling the DoF of individual nodes is supported in some FEA software.
Search in the documentation for “anisotropic” or “directional properties”.
Maybe consider a "zero friction" layer between the material and the boundary.
Rather than considering generalities, it might help if you identified the particular FEA software package you are using.
many thanks but is there a specific example or demonstration of how this works as a response to my OP?FEAnalyst said:In FEA software it's pretty straightforward. You can apply boundary conditions to individual or grouped nodes (only CAD-embedded FEA modules are limited to geometry-based selection) and control each DOF (3 translations for 3D solid elements or 3 translations and 3 rotations for 3D shell or beam elements). Usually, you can specify local (user-defined) coordinate systems for boundary conditions, including cylindrical systems. There are also special constraints (called SPC and MPC that allow you to define more complex relations for degrees of freedom, for example, you can make selected DOFs for two nodes equal).
You would have to focus on particular software. Personally, I recommend Abaqus where features like that are very well implemented. Abaqus documentation is very comprehensive and contains multiple examples for all options, including MPCs.feynman1 said:many thanks but is there a specific example or demonstration of how this works as a response to my OP?
When applying conflicting BCs, namely applying a displacement and force BC along the same direction on the same boundary, the software still can compute without an error message. Does that vary from software to software? In this case, which one will be overridden, displacement or force condition?FEAnalyst said:Sure, since those are separate DOFs, it's not a problem. In fact, such situations are quite common - for example, consider a block sliding on a surface. You can push the block towards the surface (force in the vertical direction) and slide it on this surface (prescribed displacement in the horizontal direction). This load and boundary condition can be applied to the same region of the block (i.e. its top face). In fact, it wouldn't be a problem to apply force and prescribed displacement to the same region and in the same DOF but it just wouldn't make much sense. The only thing that you can't do is apply conflicting boundary conditions - for example, fix the surface in one direction and make it move by 2 mm in that direction at the same time. In such a case the solver will give you an error message regarding redundant BCs.
What I meant by conflicting constraints, was limited to boundary conditions (prescribed displacements). Loads are somewhat different in FEA. Boundary conditions influence the left side of the equation while external forces form the right side of it: $$[K] \lbrace u \rbrace=\lbrace f \rbrace$$ where: ##[K]## - stiffness matrix, ##\lbrace u \rbrace## - displacement vector (from it we get the solutions), ##\lbrace f \rbrace## - external forces vector.feynman1 said:When applying conflicting BCs, namely applying a displacement and force BC along the same direction on the same boundary, the software still can compute without an error message. Does that vary from software to software? In this case, which one will be overridden, displacement or force condition?
Right! Then perhaps the final FEA result depends on the sequential order of how the software loads the displacement and force BCs, that is the former (displacement/force) load will be overridden by the latter (displacement/force), and perhaps the software won't regard this as an error. Is it possible to investigate this sequential order, while I doubt software will release this trivial detail?FEAnalyst said:What I meant by conflicting constraints, was limited to boundary conditions (prescribed displacements). Loads are somewhat different in FEA. Boundary conditions influence the left side of the equation while external forces form the right side of it: $$[K] \lbrace u \rbrace=\lbrace f \rbrace$$ where: ##[K]## - stiffness matrix, ##\lbrace u \rbrace## - displacement vector (from it we get the solutions), ##\lbrace f \rbrace## - external forces vector.
Yes, redundant features are sometimes treated this way in FEA software. However, this does not apply to such important stuff as boundary conditions - the program does not perform the analysis and shows an error so that the user realizes that he has defined conflicting boundary conditions. Redundancy is allowed only in cases when removal of unnecessary definition (the second one, solver leaves the one that’s first in the input file) won’t have significant impact on the solution. And even then there is a warning (but not error) message in output files.feynman1 said:Right! Then perhaps the final FEA result depends on the sequential order of how the software loads the displacement and force BCs, that is the former (displacement/force) load will be overridden by the latter (displacement/force), and perhaps the software won't regard this as an error. Is it possible to investigate this sequential order, while I doubt software will release this trivial detail?