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improving contact between two surfaces
 
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bhishan.kayastha
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Joined: 10 Dec 2013
Posts: 20

PostPosted: Mon Apr 07, 2014 4:27 am  Reply with quote

Hi all, I am simulating a screw being rotated inside a bone. the contact interface between screw thread and bone is defined as frictionless. I would like to know what is the setting required for contact to improve the contact status ?

thanks.

Bhishan kayastha
Flinders University
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ayo.brimmo
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Joined: 01 Jul 2013
Posts: 38
Location: Masdar City, Abu Dhabi

PostPosted: Mon Apr 07, 2014 5:12 am  Reply with quote

Bhishan,

I remember it being repeatedly pointed out here that this model would not work with a frictionless interface. Although I don't totally understand what you are trying to improve in the contact status but if you succeed in doing this you would must probably end up in a nightmare when it comes to force convergence.


Best,
Ayo Brimmo
Masdar, UAE


-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of bhishan.kayastha
Sent: Monday, April 07, 2014 3:28 PM
To: xansys@xansys.org
Subject: [Xansys] improving contact between two surfaces

Hi all, I am simulating a screw being rotated inside a bone. the contact interface between screw thread and bone is defined as frictionless. I would like to know what is the setting required for contact to improve the contact status ?

thanks.

Bhishan kayastha
Flinders University






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Ayo Brimmo
Research Engineer
Masdar Institute of Science and Technology
UAE.
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bhishan.kayastha
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Joined: 10 Dec 2013
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PostPosted: Mon Apr 07, 2014 4:03 pm  Reply with quote

Hi Ayo,

lets say, If i introduce friction in the interface between bone and screw thread, how would i improve the contact status between the screw thread and bone then ? Here is the image of contact status: http://postimg.org/image/cdv68d3kz/ . As you can see in the figure, it shows most of the contacts as sliding and near. I want it to be all sticking. Now, i want to achieve this so that, during rotation of screw, the force gets transferred properly from screw to bone. Now, is there any parameter i can change in contacts setting which improves the contacts status ?


Thanks.


Bhishan Kayastha
Flinders University
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christopher.wright
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Joined: 17 Jun 2009
Posts: 927

PostPosted: Mon Apr 07, 2014 9:11 pm  Reply with quote

On Apr 7, 2014, at 6:03 PM, bhishan.kayastha wrote:

Quote:
lets say, If i introduce friction in the interface between bone and
screw thread, how would i improve the contact status between the
screw thread and bone then ?

You'd improve your contact status as far as the list goes, if you'd
describe our problem completely; pay attention to the comments you've
already received and explain what on earth you actually mean by
'improve the contact status.'

You define the contact parameters yourself then run the problem. What
have you observed from your model that needs improvement and what do
you think you need to do about it? (Contact status BTW, means
whether the contact is made or not made. You don't improve it, you
observe it.)

As has been pointed out, without friction your model won't be in
equilibrium. If you can't get your arms around this notion, make a
free body diagram of the system you're modeling and make a note of
the loading and reactions needed for equilibrium. If you need help
with a free body diagram, discuss the matter with your advisor or
supervision. XANSYS can't help with a problem you can't describe.


Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/



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ayo.brimmo
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Location: Masdar City, Abu Dhabi

PostPosted: Tue Apr 08, 2014 4:01 am  Reply with quote

Bhishan,

Contact status is either open, closed or penetrating so you might have to explain what you are trying to improve here.

Best,
Ayo Brimmo
Masdar, UAE

-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of bhishan.kayastha
Sent: Tuesday, April 08, 2014 3:03 AM
To: xansys@xansys.org
Subject: Re: [Xansys] improving contact between two surfaces

Hi Ayo,

lets say, If i introduce friction in the interface between bone and screw thread, how would i improve the contact status between the screw thread and bone then ?

thanks.


Bhishan Kayastha
Flinders University






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| The Online Community for users of ANSYS, Inc. Software |
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| Send administrative requests to xansys-mod@tynecomp.co.uk |
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| Send administrative requests to xansys-mod@tynecomp.co.uk |
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Ayo Brimmo
Research Engineer
Masdar Institute of Science and Technology
UAE.
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bhishan.kayastha
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Joined: 10 Dec 2013
Posts: 20

PostPosted: Wed Apr 09, 2014 4:39 pm  Reply with quote

Hi Ayo,

Sorry for late reply. I have now made the interface bonded. I observed the connection status in the interface using contacts tool in workbench and got the following result:

http://postimg.org/image/9blp72q7x/

As seen in the image, it shows most of the connection is "near" and some of it is "sliding". Now i don't know if i call this contact "open" or "closed". What i think is that, i can get accurate results if i somehow make all the connection to "Sticking". Again, i might be thinking wrong. If i am right, how do i make the connection to "sticking" if its possible.

Bhishan Kayastha
Flinders University
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ayo.brimmo
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Joined: 01 Jul 2013
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Location: Masdar City, Abu Dhabi

PostPosted: Thu Apr 10, 2014 12:23 am  Reply with quote

Bhishan,

{Disclaimer}: Using a bonded interface to model screwing is at your own risk.

"Near" and "Far" are open while "sliding" "sticking" are closed.

It seems like you had already ran the simulation before checking for the contact status. To know if your contact is good enough, check the initial contact (use a contact tool before running). Using bonded interface, it should be all sticking. If not refine the mesh or check your geometry to ensure there's no space between the bodies. Increasing the pinball region could do some help but it's not the best approach. When you are satisfied with that you should switch to frictional contacts to observe the contact status.

Now you claim you have "now made the interface bonded" but the picture you are showing is dated 5th which is two days before you made a post about using frictionless contacts. Plus Bonded interfaces are always together like they were glued; not allowed to separate nor slide. I smell a rat!

For your sake, I hope you:

(1) Are not still trying to use frictionless interface.
(2) Have not now decided to use bonded contacts to simulate screwing.

Best,
Ayo Brimmo
Masdar, UAE


-----Original Message-----
From: Xansys [mailto:xansys-bounces@xansys.org] On Behalf Of bhishan.kayastha
Sent: Thursday, April 10, 2014 3:39 AM
To: xansys@xansys.org
Subject: Re: [Xansys] improving contact between two surfaces

Hi Ayo,

Sorry for late reply. I have now made the interface bonded. I observed the connection status in the interface using contacts tool in workbench and got the following result:

http://postimg.org/image/9blp72q7x/

As seen in the image, it shows most of the connection is "near" and some of it is "sliding". Now i don't know if i call this contact "open" or "closed". What i think is that, i can get accurate results if i somehow make all the connection to "Sticking". Again, i might be thinking wrong. If i am right, how do i make the connection to "sticking" if its possible.






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| Send administrative requests to xansys-mod@tynecomp.co.uk |
+-------------------------------------------------------------+
+-------------------------------------------------------------+
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| Hosted by PADT - www.padtinc.com |
| Send administrative requests to xansys-mod@tynecomp.co.uk |
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Ayo Brimmo
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Masdar Institute of Science and Technology
UAE.
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bhishan.kayastha
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Joined: 10 Dec 2013
Posts: 20

PostPosted: Thu Apr 10, 2014 12:58 am  Reply with quote

Hi Ayo,

Thanks for letting me know. I realized i jumbled up the link to the image. Thats still my old link. And sorry for the confusion caused. Thanks for your reply. The reason i was working with frictionless interface was just a beginning of my work. I wanted to make my model easy to start with. My supervisor had mentioned the shearing force due to screw rotation contributes more part than frictional force. I just wanted to see if my simple model would work or not and then move to making it more realistic and complex later.

Best,

Bhishan kayastha
Flinders University
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