Element Removal as a funtion of temperature

[ddcaca]

Hi all,
I am new here. I got a question in sequentially coupled thermo-stress analysis and hope your guys could offer me some help.

We are modelling bolted timber joints at elevated temperatures. Joints are loaded in tension and there are high-concentrated stresses at the interaction between bolts and wood holes. Plus, joints are exposed to fire on four sides. Wood is gonna char at 300C and the charred layer is assumed to have no stiffness and strength.

I plan to use the sequentially coupled thermo-stress method. First of all, a pure heat transfer analsys will be conducted. In the sequential stress analysis, material properties are temperature-dependent and the temperature will be loaded as a pre-defined varaiable based on the heat transfer analysis.

The challedge is that the char layer will be formed at the contact between bolts and wood holes. Because of its non-stiffness and strength of char(or very low), the deformation of the char layer in the sequential stress analysis must be severe and there will be a convergence problem at contact surfaces.

So I am looking for a solution to remove those elements of which the temperature is over 300C, prior to the stress analysis, either in Abaqus/Standard or Abaqus/Explict. In this case, bolts will contact with un-charred materials and the stress analysis will be performed without severe deformation of elements.

I looked into User's Manua of Abaqus but I didnt get any idea. Can anybody kindly give me some help? Or if you have some other solutions, please let me know. I will sincerely appreciate that.

Lei

 

[corus]

You could simply have the material strength vary with temperature so that at 300C the strength is negligible.

corus

 

[ddcaca]

corus,
The char layer will be formed at the contact between bolts and wood holes. If those elements at contact surfaces are not removed, the deformation of the char layer must be severe and it is difficult to get convergence.

 

[truckcab]

You can always use the *MODEL CHANGE capability on Abaqus/Standard. I do not have the documentation here at the moment and it was a while since I used the feature. Please look into the documentation about the keyword *MODEL CHANGE In princple you do it this way:

STEP 1
*MODEL CHANGE, ADD
element set
(Just a dummy step in order to get Abaqus to recognise your elements for latter changes).

STEP 2
Your thermal step

STEP 3
*MODEL CHANGE, REMOVE
element set

STEP 4
Your final step with the elements removed.

Hope this helps you out. As I told you, you must check the suntax in the socumentation since I do not have it in my memory at this point.



Live Long and Prosper !

 

[ddcaca]

Hi truckcab,
Thank you for your hints.

The elements that need to be removed are those of which temperature is over 300C. It depends on the thermal step. So, in your STEP 1, we cann't define this element set.

Let's suppose we define those elements in the step after thermal analysis. I don't know how to pick those elements over 300C based on the thermal analysis. Is there a keyword or a subroutine which allows us to determine and define a new element set based on temperature values?