# yade-users team mailing list archive

## Re: [Question #681532]: Are polyhedra aggregates of spheres?

```Question #681532 on Yade changed:

Jan Stránský proposed the following answer:
Hi Damiana,

> I am considering using Yade for a project for which I need to simulate
large pieces of ice breaking under certain forces. The idea is to start
with a large polyhedron shape which breaks into smaller polyhedra under
compression forces

One option is, as described, to use one big particle and then split it.
The other option is to use an "agglomerate" approach where the ice is approximated by (possibly spherical) cohesive particles .

In general (as always :-) the proper choice depends on many factors (geometry, loading type, expected failure mode and results, ...)
Could you share some more details about the project?
Why do you want to use Yade/DEM and not another method (FEM)?

> two criteria to establish if the initial polyhedron breaks have been
implemented before: the Mohr-Coulomb and MaxTau principles

the "plain" PolyhedraSplitter [1] is the third one

> how does the algorithm decide in which point the material breaks and
how the crack propagates in 3D?

After a quick look, it seems that all the methods uses b.state.pos (basically center of mass of th epolyhedron) as the point of the split.
What "propagates" mean? Currently there is no propagation, just creating the crack at once.
If you mean the orientation of the crack, it is one or two planes plane(s) based on principal directions of the approximated overall stress tensor (we can point you to the code/references if you want/need).

In general, arbitrary condition/method could be defined.

> What is the internal structure of the polygon? Is it made of spheres
(in this case the sigma-forces would be given by the sphere-sphere
contact forces)?

I assume polygon = polyhedron.
Polyhedron has no internal structure, it is just a polyhedron.
Stress is approximated from contact forces with other bodies using Love-Weber formula [2,3].
The splitting is just "replacing on polyhedron with its sub-bodies", actually modification of dimensions of the original body to be one of its split sub-bodies and adding the other new split sub-body/ies.

cheers
Jan