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I am trying to use cell fracture to create a destructible building, but the simulation instantly causes all the rigid body pieces to fly outwards. I tried using both convex hull and mesh as the collision shape, but the same thing happens. Any advice? The scale is quite large (25x35x160m), which I imagine would be enough for blender to process. I am expecting the pieces to just fall down but instead I get this effect:

Frame 1: Freshly cell-fractured pieces with rigid bodies applied. Freshly cell-fractured pieces Frame 2: Every piece instantly explodes outwards Every piece instantly explodes outwards

I tried cleaning up every shard with a "Merge by Distance".

I've also tried applying the rigid body to just a singular shard, which should just let it fall since there is nothing to collide with -- instead it goes flying out in a random direction. I don't know what this indicates.

How do I fix this? Here is the file: https://we.tl/t-XYmZwCpPUr

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  • $\begingroup$ I can't seem to find what is broken there. But no matter what, if I open a new file, take a cube and scale it to your dimensions (and apply the scale), then do the Cell Fracture etc. everything works fine. But in your file, even creating it new with a new cube, it always results in an explosion... $\endgroup$
    – Gordon Brinkmann
    Commented Aug 16, 2022 at 10:53
  • $\begingroup$ I found a solution to fix this behavior, but I don’t know what specifically broke in the first place: after performing cell fracture, go to Edit mode —> select all —> mesh —> merge by distance; then, delete the physics world and remove all rigidbodies; finally add all rigidbodies back. I’m guessing there was bad topology in the cell fractured pieces that got cached into the physics world. $\endgroup$
    – xyzhou
    Commented Aug 17, 2022 at 0:30
  • $\begingroup$ I really don't know. I don't think it's the topology of the fractured pieces, it's the Rigid Body World itself. You see, I deleted all pieces, took the original building and pressed X > Limited Dissolve in Edit Mode, so that only a tall cube with 8 vertices remained. Then I subdivided it and ran Cell Fracture again: the same exploding pieces. The same with a completely new cube, but the old Rigid Body World. Only deleting the Rigid Body World and making it new helped, so something messed up the physics before - I just couldn't find any traces what went wrong. $\endgroup$
    – Gordon Brinkmann
    Commented Aug 17, 2022 at 5:46

3 Answers 3

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Apart from all strange things we discussed in the comments, you can "repair" the simulation without starting it all new. There seems to be a problem with the existing Rigid Body World collection and also the collision shape Mesh in your file.

To make the simulation work with the existing fractured pieces you have to do the following:

  1. Select all fractured pieces, e.g. by right-clicking on the "frac" collection in the outliner and choosing Select Objects.
  2. Go to the Physics properties and set Shape to Convex Hull (the default settings are okay).
  3. From the viewport menu choose Object > Rigid Body > Copy from Active. Now all pieces should use the Convex Hull shape.
  4. Switch to Scene Properties > Rigid Body World > Settings and change the Collection from "Rigid Body World" to "frac" (where the pieces are contained).
  5. Now go to Cache > Bake or Bake All Dynamics.

If you play the animation afterwards, you will see the pieces no longer explode. At this moment they will simply fall down, to change that to interaction with the floor either put the object "ground" inside the "frac" collection or make a new collection to which all rigid bodies are linked and choose this in the Rigid Body World.

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Had a similar issue with regular rigid bodies on a bunch of smaller objects.

Found out that that everything "exploding" was caused (in my case at least) by one of the objects being set to animated while the others were not.

This only applies to active type of rigid bodies, passive ones are not affected.

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Simply moving the object close to the ground fixed it for me. The explosive action stopped and gravity started acting normal again.

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