Contact Clearance

Hello everyone,

I would like to learn the duty of 'clearance' in PCONT card. In Hypermesh help it says: 'Prescribed initial gap opening between master and slave, irrespective of the actual distance between the nodes.'; but that does not explain anything for me.

Could anybody tell me in a detailed way, what happens when i use clearance section in PCONT card for contacts.

Thanks in advance.

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Altair Forum User

Hello,

it is really good described at comment 11:

'Prescribing CLEARANCE overrides the default contact behavior of calculating initial gap opening from the actual distance between Slave and Master.

CLEARANCE now becomes the distance that Slave and Master have to move towards each other in order to close the contact.

Negative value of CLEARANCE means that the bodies have initial pre-penetration.'

If you set clearance to 0, then the initial contact is closed. It is also true, e.g. if the distance between the shells is more then the sum of the both half thicknesses.

Pay also attention to the warnings at the comment 11 !!

Best regards,

Mario

Altair Forum User

Hello,

it is really good described at comment 11:

'Prescribing CLEARANCE overrides the default contact behavior of calculating initial gap opening from the actual distance between Slave and Master.

CLEARANCE now becomes the distance that Slave and Master have to move towards each other in order to close the contact.

Negative value of CLEARANCE means that the bodies have initial pre-penetration.'

If you set clearance to 0, then the initial contact is closed. It is also true, e.g. if the distance between the shells is more then the sum of the both half thicknesses.

Pay also attention to the warnings at the comment 11 !!

Best regards,

Mario

Thank you for your reply Mario. I have already read comment 11, but i still don't understand something. I am so new to contact algorithms, so apologise me if I am asking a very easy question.

The thing I wonder is: Is the contact mechanism working like this: there are 2 contact surfaces (slave and master), there is a distance between them (for example 5 mm), if I do not use clearance, they will not see each other until they move 5 mm to each other. If I use a clearance of 5, they will immediately get into contact. Is that right?

If I am wrong, please explain the mechanism for me with the way I explained. I mean in a non-formal way /emoticons/default_smile.png' alt=':)' srcset='/emoticons/smile@2x.png 2x' width='20' height='20'>

Thanks again.

Rahul_P1

Atakanerg

When you generally define contacts, there is a distance the slave and master must move to achieve contact, this is the initial gap opening. If it is 5mm as in your example then they will not contact until they move 5mm towards each other, totally, so the first part of your statement is correct.

when you use a clearance, say of 4 mm then even though the actual calculated distance between the master and the slave is 5 mm they will behave as if in contact with 4 mm itself, clearance overrides initial gap value.

Altair Forum User

Atakanerg

When you generally define contacts, there is a distance the slave and master must move to achieve contact, this is the initial gap opening. If it is 5mm as in your example then they will not contact until they move 5mm towards each other, totally, so the first part of your statement is correct.

when you use a clearance, say of 4 mm then even though the actual calculated distance between the master and the slave is 5 mm they will behave as if in contact with 4 mm itself, clearance overrides initial gap value.

Thank you Rahul for your explanation. I understood 'initial gap' and 'clearance' terms. But there are 2 more term that is not really clear in my mind.

1. GPAD : '“Padding” of master or slave objects to account for additional layers, such as shell thickness, etc. This value is subtracted from contact gap opening as calculated from location of nodes.' - The thing I wonder about this is : Hypermesh Help says if we use 'THICK' selection for GPAD, it accounts for shell thicknesses, but shouldn't it already account for the thickness? If we turn back to the example I asked before, we have two shell components (one slave one master), initial gap is 5 mm, shell thicknesses are 2 mm for each. As I understood from the above explanation, If I do not turn GPAD on, they will get into contact when they move 5mm totally. If I choose THICK for GPAD, then they will get into contact after 3 mm of movement. But what changes between these situations? (I think the shell thicknesses should be taken into consideration in default)

2. SRCHDIS: 'Search Distance' - What is the important point about search distance? Why should I change that value? And what must be the best search distance for different situations - is there a criteria?

Thanks for anybody who helped and will help.

Altair Forum User

Hello,

i dont understand, how i assign the property with the card image PCONT? Is it use like other properties: rightklick/assign?

Altair Forum User

Hi,

try the following:

1. open your CONTACT card with the card editor

2. switch 'Property Option' from 'Property Type' to 'Property Id'

3. choose your PCONT card at PID

Best regards,

Mario

Altair Forum User

Hello,

i dont understand, how i assign the property with the card image PCONT? Is it use like other properties: rightklick/assign?

Please do not mess up the topic.

Rahul_P1

I suggest the excellent chapter on contacts in the free ebook from the altairuniversity website

Practical Aspects of Finite Element Simulation

Altair Forum User

I suggest the excellent chapter on contacts in the free ebook from the altairuniversity website

Practical Aspects of Finite Element Simulation

Thanks Rahul.

Jens_22368

Hi,

i don't really understand how clearance is working during an linear optimization. For my understandig the stiffness matrix is assambled before the first analysis starts. Therefore, the contact statuses have to be defined.

Lets assume a simple example: a rigid impactor collides with a plate, because a force is applied on the rigid impactor in direction of the plate. The initial geometrie is set, such that the distance of impactor and plate is 1mm. Contact elements between impactor and plate are created, but the contacts are open. Clearance is 0.05 mm

Now the analysis is carried out with open contacts. Because of the small stiffness of the open contact elements the impactor moves towards the plate and the distance reduces to 0.5.

Obviously the defined clearance is exceeded and the contacts have to be closed. But how does this work?

Is the Anlysis rerun with closed contacts?

in short: How does the solver determine if the clearance is exceeded and when does the update of the contacts take place (especially during an optimization with several iterations and analyses)?

Thanks Jens