just to recap as here i get the feeling that far too many methods have been discussed together. For volumes you are basically using shape matching as in:
http://www.matthiasmueller.info/publica ... _SIG05.pdf
where for now you only have one cluster of vertices right? if so in it's basic formthere is no explicit volume conservation constraint(something is mentioned as a variant at the end of section 4.2) and all the considerations on momentum conservation mentioned in the paper apply.
you maybe interested in having a look at defcolstudio 1.0.0 where a few methods for deformations were implemented:
when using tetrahedral meshes typically the artwork is interpolated inside it by the use of barycentric coordinates.
See for example the following demo where it is possible to switch between art/simulated mesh:
so a tetrahedron is playing a similar role to a cluster in the shape matching approach. For example we can have a cube made up of a few tetrahedra and have a very complicated artwork interpolated inside it.
im not trying to say that one approach is better than the other just pointing out similarities.
the main problem i can see with shape matching is deformableVsDeformable collision detection which could be very slow given the lack of extra information. Also fracture seems a bit tricky, on the good side it can handle very stiff bodies.