lagging behavior of DOF on transient Analysis

[avillan]

I'm running a tansient analysis on a pressure vessel that is loaded with a huge and sudden load above its normal loading value. The huge load takes place on a time scale on the order of microseconds (E-6) and is modeled as a gradual curve reaching the maximum value and then returning to the original value of normal loading. I've noticed that the maximum DOF is not reached at the time of maximum load which suggests a lagging between DOF and applied Force. At the end of the curve (with no load above normal) the DOF's still show a growing behavior. I'm trying to extend the simulation period to see the subsequent behavior of DOF by adding load steps with the same end value of the load at different subsequent times. I cannot get convergence at any of this subsequent steps (small pivot problem). Any ideas?

 

[cbrn]

Hi,
the simulation of deflagration is, I believe, still extremely tricky with FEM...
However, here's just some considerations:
- perhaps it would be better to run a Plastic Collapse analysis (as per ASME VIII-2 App.4, 4-136, or as per EN-13445 which is more precise and complete) and compare the "extreme" load of explosion with the limit load
- in case of extremely-short transients, there will be a lag between load introduction and structural response because of the inertial (internal) effects
- you are probably running into a non-equilibrated force field caused by the "explosion", which seems to indicate that the vessel would collapse (but I'm not sure of it)

There may be guidelines in the ASME and EN Norms as regards explosion. I can not tell more, because even if I deal with pressurized recipients, they are filled with water and so they can not explode!!

Hope this helps,
regards