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Subject: Re: Superelement in ANSYS/Substructuring
Author: Christopher Wright
Date: 1991-02-01 01:42:00

>If you deliberately keep mismatching mesh densities some nodes
>of the finer mesh remain free and hence the results will be erroneous.
This depends on the problem. The St Venant principle governs
substructures and sub-models. If the loading imposed on the cut surface
of the sub model is statically equivalent to that imposed by the
surrounding structure in the full model, then the results at points more
than a couple of thicknesses from the cut won't be affected by the
mismatched node densities. I suspect that if your model isn't linear, or
at least isn't linear in the range of the sub-model, you're out of luck
with either substructuring or sub-modeling.

> submodelling is the best bet.
Maybe I'll quibble a little with this too. Sub-modelling works real well
to get local stresses, but again, when non-linearities are present or
when there are doubts about the interpolation of boundary displacements
between connection points with the full model, it gets iffy. In any
event, both sub-modelling and substructuring both depend on the St Venant
principle for accuracy. And in both cases you're using the modeled
response (either stiffness or displacements resulting from the stiffness)
of a large region to provide loading to a smaller connected region, using
the assumption that the stiffness of the detailed sub-model and the
submodelled region are equal.

I'd also have qualms about predicting crack propagation using a sub-model
with displacement controlled loading rather than a substructure with
force controlled loading. I'm sticking my neck out here because I don't
really know what the user is trying to accomplish, but intuitively I'd
think the strain energy/external work relationships are different in the
two situations, affecting crack growth.

I confess I've never been able to convince myself how external loads
should be applied to a sub-model if such loads exist over the
sub-modelled region in the full model. On the one hand the loading
present on the sub modelled portion must be reflected in the cut boundary
displacements and therefore the load needn't be re-applied--seems as if
the loading would be applied twice. OTOH intuitively you want all the
loads carried by the sub-model that were present in the full model, so
the external loads _should_ be applied. Or maybe the sub-model is always
so stiff by comparison with the full model that it doesn't make any
difference. I've sworn to check this out very carefully before I do a
loaded sub-model, but the occasion hasn't arisen. Maybe some day.

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

Posts possibly associated with message #20740AuthorDateScore
20651Superelement in ANSYS/Substructuringsbordas@2001/01/30 
20655Re: Superelement in ANSYS/SubstructuringMark Rodamaker2001/01/30 
20659Re: Superelement in ANSYS/SubstructuringK.S. Raghavan2001/01/30-10
20724Re: Superelement in ANSYS/SubstructuringChristopher Wright2001/01/31 
20730Re: Superelement in ANSYS/SubstructuringK.S. Raghavan2001/02/01 
20735Re: Superelement in ANSYS/SubstructuringJason Husband2001/02/01 
20740Re: Superelement in ANSYS/SubstructuringChristopher Wright1991/02/01 
20742Re: Superelement in ANSYS/SubstructuringOsman Buyukisik2001/02/01 
20747Re: Superelement in ANSYS/SubstructuringChristopher Wright1991/02/01 
20768Re: Superelement in ANSYS/SubstructuringK.S. Raghavan2001/02/02 
20783Re: Superelement in ANSYS/SubstructuringChristopher Wright2001/02/02