Abaqus dynamic explicit result. Why the result is fluctuating?

Dear Dhanya Krishnan,

The reaction force response shown in your plot seems to exhibit high-frequency oscillations, especially in the early phase of the simulation, which gradually stabilize. This behavior is often observed in explicit dynamic analyses, and the underlying cause can be related to several key factors:

Loading Direction and Application Method: Please double-check whether the applied load is acting in the correct direction (usually negative Y in vertical loading), and ensure it's applied smoothly over a suitable time period. A sudden or improperly directed load can cause artificial stress waves and unphysical oscillations at supports.

Output Request Settings: If the reaction forces are extracted from nodes rather than reference points or sets, small numerical instabilities at individual nodes can amplify. It's preferable to define a Set or use History Output from Reference Points (RP) at supports to get cleaner, averaged results.

Dynamic vs. Static Behavior: Since this is a four-point bending test, the expected response in a quasi-static setup should be stable and smooth. If you're using Abaqus/Explicit, ensure the loading rate is slow enough to minimize inertial effects. A loading duration that's too short leads to dynamic amplification and high-frequency oscillations, which are not representative of static behavior.

Damping Considerations: In case you’re running an explicit dynamic simulation, consider adding viscous damping (e.g., with *DAMPING, ALPHA= or *DAMPING, STRUCTURAL) to suppress unrealistic oscillations.

Mesh Quality & Contact Settings: Check the mesh size at the supports and the contact definitions. Sometimes, overly stiff or improperly constrained support regions can create artificial spikes or reflections in the reaction force history.

Based on the concepts I highlighted in bold above, I summarize and present the following suggestions:

Verify the load application direction and method (Amplitude, Load Step Time, etc.).
Extract reaction forces from a properly defined Reference Point or Element Set, not directly from nodal output unless necessary.
If your analysis is meant to be quasi-static, increase the total time of loading to reduce dynamic effects.
Introduce appropriate damping, especially in explicit simulations.
Confirm that boundary conditions, supports, and constraints correctly reflect physical reality.

Best Regards,

Armin Barzin

Simon Smith

Hi Dhanya,

Is it possible that the fluctuation is real? Might be worth calculating the natural frequency of the concrete beam and comparing this to the frequency of the the fluctuations in your model.

Regards,

Simon

Soheil Gholibeygi

If you use Dynamic/Explicit for a quasi-static problem, you should check and select the proper mass scale. You can check it from energy outputs. Also check the time of the solver. Also try the dynamic/implicit solver with the type of quasi-static.

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