Blast/explosion load (Pressure/time) in WB

[pietjepuk]

Hi,

we want to perform a blast load calculation on a structure. Can this be done with Ansys Workbench ?
We think it should be a Transient Dynamic Analysis, but we can only find this option in Ansys classic.

 

[pitizone]

Hi,

I'm not an expert but I don't understand the difference between a permanent and a transcient dynamic analysis? Why don't you use a permanent analysis with ansys classic?

 

[pietjepuk]

Hi,

We want to know if it is possible to do it in Workbench, because we already have a 3D model imported in Design modeler. It is a transcient analysis, because the factor time is included.
The blast load is a pressure load in 0,2 sec.
So we want to know what steps to perform in Workbench. At this point we only see a thermal transcient analysis.

 

[Drej]

I think it depends on the version of Workbench you are using. The easiest thing to do is to do the basic creation of your model in WB (meshing and basic materials) and then write (or transfer) this model into Classic.

In Classic you can easily do a blast load analysis (ANTYP,TRANS), and apply the pressures with respect to time using a TABLE (*dim) and refer to this in your SF command by:

SF, Nlist, Lab, VALUE, VALUE2

and enclose the TABLE name in the "value" within "%" characters.

For example, if your TABLE is called BLAST then:

SF, Nlist, PRES, %BLAST%

see the help file for Nlist.

Be warned that this type of analysis is generally very expensive (time consuming) since I'm guessing you will need to undertake a non-linear analysis. If it's a purely linear blast load assessment, then of course it is much less expensive.


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[pietjepuk]

Ok, thanks, we will try that. I will let you know the results.

Regards,

 

[Transient1]

If your using ANSYS workbench with a mechanical license you can set up a transient problem easily using a flexible dynamic analyis. You can apply the pressure load versus time. However, what is transient about a pressure load increasing versus time? I don't have experience with this type of analysis.

I would think it is easily solved using a static analysis and you can get the stress at each time increments. I can possibly think of a couple of reasons why you might want to perform this analysis using a transient type analysis, but can you clarify for me why the approach is that way?

 

[pietjepuk]

Actually, we will use the flexible dymamic analysis. We are settting that up right now.
When the factor time is involved, isn't it always a transient type analysis ?
I don't know what you mean doing a static analysis with time increments.

 

[pietjepuk]

We have done the following calculations:

1. Static structural analysis:
- A constanct force from the inside of the tank (contents)
- A constante pressure of 0.3bar on the outside of the tank

2. Flexible dynamic.
- A constanct force from the inside of the tank (contents)
- A pressure load of 0,3 bar, in 0,2 seconds (triangular), 0,1 sec. to max, 0,1 sec. to zero. This is the actual blast load.

The results are almost the same. We thought that the resulsts form blast would be different (higher values). The results do not exceed the yield strenth, so we are not doing a non-lineair analysis.

 

[cbrn]

Hi,
Pietjepuk, in FE terms, a full-transient analysis is a succession of static analyses with the addition of all the inertial effects. Or, on the other side, if a transient phenomenon (time-dependent) has negligible inertia effects, then it can be simulated as a succession of static states.
It's really not surprising that a full-transient analysis of a blast (a typical case where the inertial factors are negligible) does not provide sensibly different results from a static analysis. This is true because you had already determined that you would not have to analyze deflagration or such (in this case, very complicated analyses involving fracture mechanics would have had to be built...). But, by extension, you had all means to decide "a priori" that a full-transient analysis would be pointless, in this case...

Regards