# Non linear quasi-static analysis using gap elements

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Hi all,

I was running a model in Optistruct 13.0 dealing with a two step non linear quasi-static analysis of two plates coming into contact. In the first step my goal is to make sure the two plates don't come into contact; let's say my initial gap is 5 mm, one plate is pushed down by a force of 100 N, the other one is fixed at the base. At the end of this step the two plates should have minimum gap between them, in other words no load should be transferred from one to another. After the second step in which I apply a force of 1000 N, the two plates obviously come into contact, resulting in a load transfer to the spcf applied at the base of the fixed plate, since friction between the plates is activated. I tried using a gap element in order to simulate the problem but no matter how I change the values into the pgap card (for example I first put a value of Kb equal to 25 N/mm to make sure the pushed plate didn't reach 5 mm of displacement with a force of 100 N.... didn't work, then I put it equal to 0 and I obtained the same result ), at the end of the first step the gap of 5 mm is always reached, resulting in load transfer. How can I avoid this situation in order to make it work properly? I'll attach the .hm file so you can check it. Thanks for any replies, hope someone will help me with this matter!

model_with_gap.hm

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Why don't you set initial distance as 0mm and then apply 1000N load? there was a similar example in hw solver help

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HI @Dirk76

Do you mean something like this?

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On 6/18/2018 at 10:10 AM, Prakash Pagadala said:

HI @Dirk76

Do you mean something like this?

after the first iteration with 100 N, the two plates don't come in contact? then it's what I need, since everytime I apply different load values the two plates reach the initial gap.

If it's the case, would you share the model? Thanks.

On 6/18/2018 at 10:01 AM, tinh said:

Why don't you set initial distance as 0mm and then apply 1000N load? there was a similar example in hw solver help

that's because my main goal is to have the two plates in contact only after the second step of the analysis, so I need to set the problem in this way.

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HI,

So you want the blocks to come in contact in 1st subcase and not in 2nd subcase?

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On 6/26/2018 at 1:11 PM, Prakash Pagadala said:

HI,

So you want the blocks to come in contact in 1st subcase and not in 2nd subcase?

My goal is to have them in contact during second subcase. If the initial gap is 5 mm and during the first subcase I apply 100 N, the two plates shouldn't come in contact... this is why there's some mistake in the model. If you try to increase the gap, with that contact (or different, I tried various solutions) formulation, no matter what, the plates will come into contact.

A model that work with these initial conditions should be made in a way that the gap only reduces of a finite quantity < initial gap, so that increasing the load in the following substeps the gap should reduce to 0.

When you tried running this model, you obtained these results?

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Hi,

I will share the model files soon.

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