Well, I suppose TRex might have been able to 'flip' a
small Ankylosaur, but a full-grown one would have been
impossible to flip given the respective centres of
gravity and body mass - for one thing the clubtail
wouldn't exactly be standing still and cooperating, and
the downside to the TRex - being flipped itself or
dragged so that it winds up falling and possibly
breaking its leg/hip would have been too devastating -
any TRex that tried to make this a winning strategy
would have probably been weened out of the gene pool
more quickly than others.
I strongly suspect that the only Ankylosaur that a Trex
would be interested in would have been a juvenile that
was less dangerous and easier to bite through (recall
last years BBC special which concluded that the
Ankylosaur's knobly skin would not have been strong
enough to prevent a full-grown TRex from biting through
it) - I could see a TRex chomping down on a smaller
Ankylosaur's head/neck area, away from that nasty tail,
and either crushing it or simply holding on til the
animal bled to death - an interesting parallel to that
idea are a few skull fossils of Glyptodonts that show
two perfect canine holes on the top, indicating that a
big cat bit straight through the Glyptodont's head -
apparently the only vulnerable part of the critter.