Why is the Blender soft body physics permanent deformation not working? At the moment I'm working on an auto crash scene. I've set up a good rigid body sim and perented a low poly car to it so that I can run a soft body sim on it and then use it to deform the high poly model. 

But whatever I do the soft body sim will not have permanent deformation, even though my plastic value is at 100.





At the moment I'm working in Blender 2.79b. I really hope someone can tell me why there is no permanent deformation and what I can do about it.
In order for the Soft Body 'plastic' setting to have a meaningful effect you need to disable the 'goal' for those vertices - otherwise the 'goal' will simply pull the vertices back to their original positions.

However, in your situation it probably isn't as simple as disabling the 'goal' since the soft body will then be subject to other forces such as gravity so you'll need to create a Vertex Group of just those vertices that would not be expected to move (eg, the wheels) that the rest of the mesh can be supported upon. In the Soft Body Goal settings you can then leave the 'goal' enabled and set the Vertex Group to limit the goal to only the desired vertices (eg, the wheels that would be resting on the floor).

This can produce the following :