Split Hopkinson Bar Contact Issue

[Nicholas Daras]

Hi All

I am attempting to simulate a compression Split Hopkinson Bar in LS-Dyna. I have managed to successfully propagate a stress wave through the input bar, reflect the stress wave off the free-end and then have 'infinite elements', in the form of a non-reflecting boundary, absorb the reflected stress wave. However, when I try to get that stress wave to propagate through into an output bar, I cannot record any force/stress readings in the output bar. I believe the issue is related to the contact as in a previous iteration of this simulation, prior to implementing the 'infinite elements', I was able to get the stress wave to propagate through into the output bar using a AUTOMATIC_GENERAL contact. However, the 'infinite elements' require a TIED_SURFACE_TO_SURFACE contact to operate correctly and as a result I cannot use the AUTOMATIC_GENERAL contact. I have tried a number of different contact types between the two bars and I have tried using different combinations of segment sets, part IDs and node sets, but I believe an AUTOMATIC_SURFACE_TO_SURFACE contact would be the most applicable in this case, but I am open to suggestions.

I have attached my keyword file for reference.

Apologies for the long post, I just wanted to give as much context as possible. Please do let me know if other information might be of use. Thank you all in advance for your assistance; it really is much appreciated!

Kind Regards and Thanks

Nick

[l...@schwer.net]

There is no mention in the User Manual description of *BOUNDARY_NON_REFLECTING “requiring” a TIED_SURFACE_TO_SURFACE contact to operate correctly? All that is required is a segment set.

 

I assume you are using *BOUNDARY_NON_REFLECTING to shorten the length of the split Hopkins bars (input & output). Why not use axisymmetric shell elements to model the round bars and thus be able to include the full length of the bars with minimal degrees of freedom. Also, using a ratio of element sizes away from the specimen arear between the bars will save DOFs.                   --len

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[Nicholas Daras]

Hi Len

Thank you for your response, it is much appreciated!

Firstly, you are correct regarding the  *BOUNDARY_NON_REFLECTING not “requiring” a TIED_SURFACE_TO_SURFACE contact to operate correctly. However I have had to use the extra row of elements at the end of the bars, using the bulk modulus in those elements, as opposed to the elastic modulus I use in the bars themselves, to have the boundary completely remove reflected waves by from the free ends of the bars. The contact between those extra elements (which I refer to as 'infinite elements') and the bars requires a TIED_SURFACE_TO_SURFACE contact. Apologies for the lack of clarity in that regard.

Secondly, your assumption regarding the shortening of the bars is correct. I am unable to use symmetry as I will ultimately be putting a specimen with complex geometry (the cross-section of a bone shaft) in between the bars. I am not too sure what you mean regarding 'a ratio of element sizes away from the specimen' as I believe I have tried to implement that in my current setup, but I could be misunderstanding what you mean, so apologies for that.

I have attached a screenshot of my simulation to give a bit more clarity as to what the setup looks like with the 'infinite elements' and the shortened bars. The yellow and blue parts are the infinite elements, while the red and green parts are the input and output bars respectively.

Thank you once again in advance for your assistance.

Kind Regards and Thanks

Nick

[James Kennedy]

Dear Nicholas,

 

Perhaps of some interest:

 

The *MAT_002 material model (data included) was employed to represent the S2-glass/vinyl

ester composite material:

 

Hossain, M.K., "Dynamic Simulation of Split Hopkinson Pressure Bar (SHPB) for Composite

Materials Using LS-DYNA", Senior Project, Department of Mechanical Engineering, University

of Nevada Las Vegas, Las Vegas, Nevada, December, 2003.

 

http://www.egr.unlv.edu/~bj/MEG_795_E_Methods/PDF_Files/Kamal%20SHPB.pdf

 

Finite Element modelling of a Split-Hopkinson Pressure Bar (SHPB) test on fine quartz sand was

carried out using LS-DYNA in order to assess whether *MAT_005 could replicate the results from

experimental tests, which would enable a more detailed investigation of the stress state in the sand

specimen. Quasi-static test data was used to select the input data for the material model, and the

model SHPB was set up to replicate the experimental conditions. The results showed that *MAT_

005 replicated the volumetric response provided as input data, but failed to predict the shear

response observed in the quasi-static experiments. This was found to be due to the model treating

the shear modulus as a constant rather than it increasing with strain, a feature which makes the

*MAT_005 unsuitable for modelling SHPB tests on sand:

 

Barr, A.D., Clarke, S.D., Petkovski, M., and Rigby, S.E., "Modelling Split-Hopkinson Pressure

Bar Tests on Quartz Sand", The Annual Postgraduate Research Student Conference - 2015,

pp. 38-42, University of Sheffield, Sheffield, United Kingdom, April, 2015.

 

http://www.gruppofrattura.it/ocs/index.php/gigf/aprsc2015/paper/viewFile/12157/11545

http://eprints.whiterose.ac.uk/85101/1/Barr%20et%20al.%20(2015)%20Modelling%20split-Hopkinson%20pressure%20bar%20tests%20on%20quartz%20sand.pdf

 

Numerical Simulation model: SHPB setup is modeled in hyper mesh and simulated in LS-DYNA

Loading pulse is analyzed to validate the model.

 

Bagaria, M.K., “Experimental and Numerical Simulation of Split Hopkinson Pressure Bar Test on

Borosilicate Glass”, Master’s Thesis, Mechanical Engineering, Michigan Technological University,

2019.

 

https://digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=1941&context=etdr

 

The research presented in this thesis, is not to test materials at high strain rates, rather it is to study

the wave shaping testing techniques in split Hopkinson bar by using finite element code LS-DYNA

and also to study the tensile testing methods of SHPB.

 

Tasneem, N., “Study of Wave Shaping Techniques of Split Hopkinson Pressure Bar Using Finite

Element Analysis”, Master’s Thesis, Department of Mechanical Engineering, Wichita State

University, Wichita, Kansas, December, 2005.

 

https://soar.wichita.edu/bitstream/handle/10057/2341/t05035.pdf

 

Sincerely,

James M. Kennedy

KBS2 Inc.

October 2, 2023

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[l...@schwer.net]

Nicolas –

 

I am going to respond to you directly (privately) as I doubt your modeling issues are of interest to many. Anyone that is interested can send me an email to be cc’ed on future responses.                          --len

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