Discussion:
Implicit and Explicit main difference?
t***@yahoo.com
2014-01-21 08:43:36 UTC
Permalink
Greetings,


May i know the main difference of Implicit and Explicit analysis? Is't will affect the result of analysis? and what the step or how we select either implicit or explicit analysis...


Thanks for the helps.


Regards,
Ng
***@yahoo.com
Dave Lindeman
2014-01-21 23:09:09 UTC
Permalink
The Abaqus Theory Manual or a proper finite element methods textbook (or
even Google) will do a much better of explaining the differences. But,
in a nutshell:

The difference is in the time integration scheme (implicit vs.
explicit). In an implicit dynamic analysis the code is solving the
equation [M]{a} + [C]{v} + [K]{u} = {F}, and updating the displacement
and velocity using a Newmark integration scheme. This is stable and
accurate for relatively large time steps. In an explicit dynamic
analysis the code is basically just solving {a} = [M]^-1 {F}, and
updating the velocity assuming acceleration is constant over the time
step. This method is only stable for very small time steps (on the
order of the smallest value of the ratio of characteristic element
length to speed-of-sound in the material of the element). So, it is
typically only practical to use explicit dynamic procedures when the
phenomena being simulated is very short in duration (like an impact
event). Implicit dynamic methods are used for simulating longer
duration events (e.g., looking at the vibration of a structure following
an impulse load).

These are generalizations, of course. Some people use explicit dynamic
methods to simulate quasi-static problems, and obviously there's a
regime where either could be used.

Regards,

Dave Lindeman
Lead Research Specialist
3M Company
3M Center 235-3G-08
St. Paul, MN 55144
651-733-6383
Post by t***@yahoo.com
Greetings,
May i know the main difference of Implicit and Explicit analysis? Is't
will affect the result of analysis? and what the step or how we select
either implicit or explicit analysis...
Thanks for the helps.
Regards,
Ng
ted_kingdom
2014-01-21 23:27:51 UTC
Permalink
The main difference is that explicit scheme has a stability region. Hence,
you have to control your PNEWDT varible in UEL subroutine (if you write
one), so it does not exceed this region. Implicit schemes are usually
unconditionally stable (they will converge regardless the time step).
It doesn't affect the results. Both schemes have to give you correct
solution.
Step depends on your task and constitutive law. In principle, you have to
make a decision what is easier for you: more complicated formulation of
constitutive law approximation or definition of stability region.



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