This is not a concern in dynamic analysis because of the presence of the inertial term. The equilibrium equations, which are elliptic partial differential equations, need sufficient Dirichlet boundary conditions to have unique solutions. The resulting stiffness matrix is singular and the stationary solver fails to converge.Īnother way of looking at this problem is from the mathematical side. As such, there are multiple solutions to the problem that differ from each other by a rigid body motion. This is inconsistent with the equilibrium assumption. If constraints are not provided, the out-of-balance forces want to move or rotate the object. In 3D, we have three displacements and three rotations to constrain. In 2D, we need to prevent two displacements and one rotation. Reaction forces come from constraints on displacements and rotations. To this end, the reaction forces have to balance the applied forces. Whereas the object is free to deform, it is not free to move or rotate as a rigid body. In a stationary or quasistatic structural analysis, we are looking for an equilibrium solution. Kinematic Constraints in Structural Analyses Today, we showcase the Rigid Motion Suppression feature in the COMSOL Multiphysics® software, which you can use to automatically figure out the constraints you need. However, it is not trivial to provide constraints that do not induce artificial stresses. Often, this result comes from a lack of displacement constraints. You provide the heat fluxes and temperature constraints on the boundaries, compute, and get a convergence error. (Means entire body is subjected to tensile loading).Say you want to compute thermal expansion and stresses in an object. (Pressure load is not applied on all faces). I just want to apply pressure on any two parallel faces. So in my case inside a cube I need to introduce ellipse with different orientaions and apply pressure load on one face then observe stress distribution. This pic is simulation result from literature. Load steps are 1) THERMAL COOLING from 1326 to 20Ģ) Mechanical tensile loads on faces parallel to YZplane So I would like to give Generalized BC which will solve my problem for all cases. Instead I would like to build full model and Observe stress distribution after cooling with different interior gemetries. In such a cases I cannot cut the whole cube into 1/4th and apply BC. Inside objects has different material properties and outside object has different properties. I would like to see stresses at the interface by introducing different gemetry types inside the cube.(sphere, ellipsoid with different orientations etc) After running for this simple part later I need to change the geometry without symmetry.Įxample I thought of building is shown in the fig. This could be achieved by selecting the faces on the +Z side of the quarter blocks and using a Remote Displacement, set Z = 0 leaving the other five DOF Free. There is one DOF left to constrain, which is motion along the Z axis. Apply a Displacement BC on the newly cut faces of Y = 0 leaving X and Z Free. Delete half again, keeping the 1/4 blocks on the +Y side. It will be convenient to make a plane parallel to the XZ plane that goes through the center of the half blocks and split the blocks again. That allows you to apply the pressure load on just one side and have a BC to push against. The cut faces get a Displacement BC of X=0 and Y, Z Free. Use a plane parallel to YZ that goes through the center of the blocks Split the blocks using that plane and delete one half, keeping the blocks on the +X side. Since your geometry and the loads have two planes of symmetry, you can use that in the constraints. Yes, you have correctly applied the kinematic mount for the thermal load, but since you want to apply a pressure to the faces, you can't use three single nodes.
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