Learning Forum

[Henrik Wittberg]

Hello,
I'm currently working on a thesis project and I'm trying to approximate a contact stiffness in order to compare the results of my simulation to those from a self written MATLAB routine in which I specify a contact stiffness as a concrete value. I'm using shells and what I can find is k = alpha*K*A / (max shell diagonal) where k is the contact stiffness, alpha is a penalty scale factor, K is the bulk modulus and A is the segment area.

Assuming this is the right way to calculate it and provided I use a mesh with equally sized elements (that is, I know the max shell diagonal), how do I find alpha and A? The contact I'm looking at is a turbine blade edge coming into contact with the surrounding stator wall surface.

Thanks in advance for the help.

[Bernd Hochholdinger]

Dear Henrik,

you can find the formulas for the calculation of the penalty contact stiffness in the LS-DYNA Theroy Manual section 29.3 (Penalty Method).

Please be aware that the contact stiffness depends on the contact settings SOFT=0/1/2.

For SOFT=0:

Regards,

Bernd

 

[Henrik Wittberg]

Hello Bernd,

Thanks for your reply. I feel what you answered is basically what I already stated...

I don't know how to find that value since I don't know how to treat the parameters involved, such as fs and A in your equation. That is what I need answers to. Is there not any way to find what contact stiffness Dyna used in a simulation, in an output file of some sort?

Thanks again

[Bernd Hochholdinger]

 

Well, … A ist the area of the shell segment – you already had stated that yourself. You can use a postprocessor like LS-PrePost to measure the area of a shell element or calculate it within your Matlab script based on the nodal coordinates that define the shell element.    

And alpha or fs in equation 29.19 is 0.1 as stated in the screenshot above!

The contact stiffness is not output by LS-DYNA.

 

[Henrik Wittberg]

Understood, thank you. And I agree with alpha/fs being stated by you above. What I'm confused about is what is meant by "segment". As I stated I have an edge coming into contact with a wall surface, so in that context, what is the segement area? Does this context matter?

Thanks again

[Bernd Hochholdinger]

 

The term “segment”  is used for surfaces of a body. In your context that simply means “shell element”.

For a standard penalty contact (SOFT=0/1)  “nodes” are checked, if they penetrate an “element/segment”.

So in your case the nodes of the “shell edge” are contacting the wall “elements/segments”.

The above formula is using the properties of the “segment” that is in contact with a “node”.

For further information on that please refer to the LS-DYNA theory manual, like the section shown below:

 

 

 

[Henrik Wittberg]

This was a valuable answer. I'm going to take this information with me and see if I can advance in my work. Thank you for your time, I greatly appreciate it.

Kind regards