Unbreakable Tube (with a string inside)

This is the third test version of an unbreakable tube. The tube is highly resistant to axial pressure. Accidental radial forces will angle tube's element, but will not break the tube. The string inside the tube will build up a tension similar to "prestressed concrete". Angled tube elements will flip back to their origin, if you use an elastic string. Use a strong string/wire to make the tube manual unbreakable.
Some results so far:

• Longer tube elements are less stable.
• Bending the tube very often will damage the edges of the tube elements. Check the red arrows in one of the pictures.
• The string looses tension after a bending due to less friction of the angled parts.

Things to do:

• design tube elements with a constricted opening on both sides
• design a rounded, more durable shape
• find another tightening mechanism

Update to V3 (improved elements, OpenSCAD optimization)