Interest rates have been artificially manipulated to very low levels and really only have one way to go now and that is up. When rates rise, bond holders get slapped with principal losses because older lower rate bonds are not worth as much as newer higher rate bonds. Once rates begin to rise, however, is probably not the best time to head for the bond exits, as there will be a mad stampede of other bond holders seeking escape at that same time and only so many can fit through the exits at one time.
In any case, this is NOT a place where anyone who wants to preserve their wealth should be. Holding US Treasury paper is the financial equivalent of holding a lighted stick of dynamite without knowing just how long the fuse is or how quickly it is burning. I would very strongly urge anyone holding US Treasury paper, perhaps in their retirement account, to sell it immediately.