How do I increase the thickness of the chunks of a fractured mesh?

Hi. I have created a destructible mesh and when it fractures, the chunks have no thickness. The object is made entirely of stone. How to fracture it properly ?

It seems that there were problems with the normals of the mesh. Some of them were facing inwards. In this case, UE4 doesn’t assign a second material slot for the inside, so it appears that the mesh has only the outer layer.
I reexported the mesh from Blender, this time pressing Ctrl-N to solve the normals problem. Now it works.