Fabien,
"So Q : I study a tube with several internal reinforcements. How I can see the stress on the reinforcements inside the tube ?"
You can follow:
1] Perform analysis, get results you want.
2] While the results are shown, from File > Save Copy As, save as dwf file format.
3] now a dwf file should be open in Autodesk Design Review [ADR]
4] in ADR, you can cut the part/assembly with a slicing phane, which should allow you to show the results of your interest inside the tube. Note it shows surface results, not solid internal results. Again, the max/min value usually should be in the surface, in your case, the surface of the reinforcements inside the tube.
Hope it helps,
Gary Tang
Autodesk Inc.