Clarification on MPC Force vs Applied Force in Nonlinear Transient Analysis with RBE2 and Frictional

Shivaraj_1

Description:

I am performing a Nonlinear Transient Analysis in OptiStruct and would like clarification on the interpretation of MPC forces and force equilibrium.

Model Setup

• Load applied through an RBE2 spider.
• Force magnitude: 250 N.
• Force applied at the independent node of the RBE2.
• Loading direction: Normal to the indenter surface.
• Contact type: Frictional contact between indenter and structure.
• Friction coefficient: 0.3.
• Multiple SPC constraints are present on the structure.
• Analysis type: Nonlinear Transient.

Observations

At the time step where the full 250 N load is applied:

• Applied force = 250 N
• MPC force at the RBE2 independent node = 237.5 N
• Sum of SPC reaction forces = 237.5 N

Therefore,

Applied Force = 250 N
MPC Force = 237.5 N
Total SPC Reactions = 237.5 N
Difference = 12.5 N (5%)
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Also, the MPC force and total SPC reaction force are nearly identical.

Questions

1. In a nonlinear transient analysis, should the MPC force at the RBE2 independent node be equal to the applied force, or is a difference due to inertial/contact effects expected?
2. Is it correct to interpret the 12.5 N difference as:
   • inertia force,
   • contact/friction force contribution,
   • damping contribution,
   • numerical residual/convergence imbalance,
   • or a combination of these effects?
3. Since the MPC force and total SPC reactions are both 237.5 N, does this indicate that the load transfer through the RBE2 is correct?
4. Applied Force =
   Internal Force +
   Contact Force +
   Inertia Force +
   Damping Force
   Show more linesfor a nonlinear transient analysis?
5. Is there any output request available to directly evaluate inertia forces or perform a global force balance check for transient contact analyses?

this is where force is applied (on independent node of RBE2).
it is connected to main assembly by frictional contact it acts like impactor

 

Rajashri_Saha

Hi

You are quite correct for the explanation for difference in MPC force and applied load.

In nonlinear transient dynamics, the solver balances external loads with inertia, damping, contact, and stiffness effects over time. The force reported at a constraint or RBE2 independent node is the internal reaction needed to enforce the rigid connection at that instant. This reaction often differs from the nominal applied force due to transient and contact effects, which is expected and indicates proper accounting for these dynamics.

OptiStruct provides a Grid Point Force Balance output (GPFORCE) that reports forces and moments at grids. By enabling GPFORCE in both H3D and text formats, you can verify that applied loads are balanced by reactions and internal forces for each subcase

Thanks

Rajashri