The applied load and structural reaction force from the fully constrained nodes

[Clark Jo]

Dear all,

I am doing analysis focusing on the structural reaction force. In other words, I want to know the relationship between the structural deflection and its reaction force. 

The loading condition should be uniformly distributed load exerted on a single-span beam. So I chose the load_node set. By increasing the load predefined in a time-dependent series, I get the whole deformation stages of the beam span with fixed boudary conditions at each end.

Then the problem appeared: the structural reaction force (acquired from SPC force) showed increasing-softening-reincreasing phase as the structural deflection increased. however, the external load, namely the load_node set was set monolithicly increasing with time. The external load and the reaction force did not match with each other.

The solution method was selected as explicit solver. So did this mean that the dynamic balance equation was involved? And as a result, there should be inertial force balancing the exertal and reaction force.

I want to know if there has official file explaning such a result.

Thank you very much for your attention, and appreciate for any suggestions!

Regards,

Clark

[James Kennedy]

Dear Clark,

 

A note posted online. See if this is of some help;

 

In LS-Dyna, when you apply an external force to a node that is constrained by a Single Point Constraint (SPC), the reaction force generated by the SPC will exactly match the applied external force in magnitude but act in the opposite direction, ensuring that the constrained node remains fixed at the specified location. 

 

Key points about matching reaction and external forces with SPCs: 

 

Equal and opposite:

The reaction force from the SPC will always be equal to the applied external force on the constrained node, creating a balanced force system.

Force direction:

The reaction force will act in the opposite direction of the applied external force, effectively "pushing back" to maintain the fixed constraint. 

 

How to check this in LS-Dyna: 

 

Output options:

You can use the "*DATABASE_SPCFORC" command to specifically extract the forces generated by SPCs in your simulation results.

Visualizing forces:

By plotting the nodal forces, you can clearly see how the reaction force from the SPC directly opposes the applied external force on the constrained node. 

 

Example scenario: 

 

Imagine a car model where you want to simulate a crash test by applying an impact force to the front bumper.

To fix the car chassis to the ground, you would use SPCs on specific nodes of the chassis.

When the impact force is applied, the SPCs will generate reaction forces on the constrained nodes that exactly counteract the impact force, keeping the chassis fixed in position. 

 

Sincerely,

James M. Kennedy

KBS2 Inc.

October 21, 2024

--
You received this message because you are subscribed to the Google Groups "LS-DYNA2" group.
To unsubscribe from this group and stop receiving emails from it, send an email to ls-dyna2+u...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/ls-dyna2/850b0fcd-24d2-4501-ad7c-e8b95a319f2fn%40googlegroups.com.

[Clark Jo]

Thank you very much James!

For the car crash scenario, I might not agree with that there will be equal force monitored from the SPC points to the impact force, if the car is set non-rigid. Because there should be internal force developed due to the force and the corresponding inertia might be involved as the impact force induced that.

In my case, the beam part inheretly has different reaction in accordance with different deflection stages. So there will be different stages induced for the force balance. At the beginning, the reaction force is equal to the applied node force. However, when the applied force become greater than the capacity of the beam to bear, there will be a unstability being activated. So the inertial force, I think, might be involved, and the SPC force reveals the force consistent to the beam's capacity at this stage. The inertial force balanced the gap between the applied force and the beam capacity.

I checked the general explicit analysis algorithm in ABACUS, but I did not find any handbooks explaining the algorithm adopted in LS-DYNA.

Would you mind sharing some relevant documents that introducing the explicit analysis in LS-DYNA?

Very appreciate your help!

Regards,

Clark

[James Kennedy]

Dear Clark,

 

See if these presentations are of some help:

 

Belytschko, T., and Tsay, C.S., “Explicit Algorithms for Nonlinear Dynamics of Shells,”AMD, 48, ASME, 209-231 (1981).

 

Belytschko, T., “Partitioned and Adaptive Algorithms for Explicit Time Integration,” in Nonlinear Finite Element Analysis in Structural Mechanics, ed. by Wunderlich, W. Stein, E, and Bathe, J. J., 572-584 (1980).

 

Belytschko, T., Lin, J., and Tsay, C.S., “Explicit Algorithms for Nonlinear Dynamics of Shells,” Comp. Meth. Appl. Mech. Eng. 42, 225-251 (1984)

 

Belytschko, T., Wong, B.L., and Chiang, H.Y., “Improvements in Low-Order Shell Elements for Explicit Transient Analysis,” Analytical and Computational Models of Shells, A.K. Noor, T. Belytschko, and J. Simo, editors, ASME, CED, 3, 383-398 (1989).

 

Sincerely,

James M. Kennedy

KBS2 Inc.

November 2, 2024

To view this discussion visit https://groups.google.com/d/msgid/ls-dyna2/8ed5d0cf-6860-4a36-babf-5052cf675500n%40googlegroups.com.