LSdyna-Impact vs time problem

[Samkhp]

Hi guys,

I am using Ansys/LS-Dyna to simulate behavior of a rock bolt jointed concrete

under impact load. I have attached a schematic of the model for your consideration. I have used rigid material for the impactor, MAT_159 for concrete and grout, MAT_24 for the rock bolt, MAT_003 for steel confinement wrapped around the concrete. I used Automatic_Surface_TO_Surface contact between each part as follows:
Concrete to concrete (Joint faces)
Grout to Rock bolt
Concrete to grout
Concrete to steel confinement
Steel confinement to Impactor
The geometry was imported through NASTRAN file built in strand7 software.
I have been running this model with no problem in terms of workability and getting results such as Total energy, internal energy and sliding energy and the results seem ok in comparison with the experimental results. However, the impact load vs time can not be driven and I have no idea why. I actually getting the impact results from the impactor to confinement as well as confinement to concrete contact but the results don't look fine. The results show some peaks which are basically due to the impact, and no load bearing is shown in the graph. What I mean is that when the impactor hit the steel confinement, I expected a compressive force to be implemented into the system but from the impact results this can't be seen. only some bounces can be captured. However, through confinement to concrete contact, the bounces are still there but there are some load bearing are seen.
I would be grateful if you could advise what the problem could be.

 

[FEA way]

Can you attach some pictures showing mesh and stress distribution in this model ? What are the boundary conditions ? Have you tried with bonded connection between the parts ?

 

[IceBreakerSours]

I do not think I have understood the model set-up or the question/issue at hand. What you mean by "..impact load vs time can not be driven.."? How are you driving the model? Is it an impact load or disp/vel/acc history?

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[Samkhp]

.

The boundary conditions can be seen from the attached pic.

Three concrete blocks with 150x300x150 mm3 dimensions are attached to each other with an insertion of a rock bolt fixed with a nut at the end. A hammer impacts on the middle of the section with a define velocity of 6.5 m/s (Initial- Velocity generation). Only middle section is supposed to move downward vertically to create a shear force at the joint face. As I mentioned before, the whole system works good and the only problem is the Impact load vs time graph problem. Impact load vs time is the graph that defines the amount of force created in the system and resisted by the rock bolt. The experimental results shows a very big peak at the time of impact which is known as inertial force and after that there are some oscillation which basically is the resistance load from the rock bolt and then failure. However, in the numerical simulation, instead of having one big peak and some oscillation, we have many peaks which means the hammer is bouncing on the system and between each peak there is no load bearing or no oscillation. So my question is how to fix this bouncing action of the hammer, if it is the hammer problem.

 

[IceBreakerSours]

Can you share the predicted plot as well? I imagine you are running an explicit job and, if that is the case, noise in the response is expected especially during impact.

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[Samkhp]

Sure

Attached are the impact load vs time for the impact from the impactor to the confinement and the confinement with the concrete

 

[IceBreakerSours]

Can you share the contact card? In this case, you want some "stickiness" in the specific contact formulation.

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[Samkhp]

 

[Samkhp]

What kind of contact are you referring to ?

 

[IceBreakerSours]

Try *CONTACT_AUTOMATIC_NODES_TO_SURFACE with IGAP=1 for the contact that is separating.

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[Samkhp]

Thank you so much for you reply, I used Automatic

_Nodes to surface contact for the impactor and the confinement which is the plate wrapped around the concrete sample and it is the medium that impactor hits. I attached the results.

 

[Samkhp]