Why the pile moving upward after the loading ends? 1

[kumara123]

Hello,
I am trying to model a pile penetrate into soil media. I modeled the problem with Abaqus and ran with no errors. I used Axisymmetric elements (CAX4R) to model both pile and soil also introduced the contract between soil and pile. An impact load was applied on top of the pile which lasts for 0.02s and extracted data for 1s. The results showed that the pile is moving downward into soil for around 0.16s after that it is moving upwards for rest of time. Please find the pile tip movement graph. Can anyone explain this to me? Why didn't pile rest at the end of loading?
Thank you.

 

[whitwas]

It looks like a qualitatively correct graph for an object bouncing off a surface with some loss of energy and then flying away. Maybe the soil is mostly deforming elastically? Maybe you don't have enough friction? No gravity? The long linear region suggests that the pile is weightless since it's not falling back down.

Are you sure it's wrong? How much springback do you expect? Surely not zero.

 

[kumara123]

Hi, Thank you very much for your answer.
Yes, it is bouncing off and flying away. I am trying to model a pile penetrate into soil by an impact force. I am following a research paper and they penetrate the pile into soil with same material properties, loading, and contacts. They did not specially mention that how to avoid bouncing off. The soil is modeled as an elastic material with Mhor-Coloumb plasticity. They have introduced Lagrangian-Eulerian analysis.
In this model, I haven't defined gravity for the pile. I defined gravity load for the pile in the explicit step and compare the results. It shows a reduction in bouncing off gradient.Please find the attachment.
If you have any suggestions please let me know.
Thank You.