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Message No 52 - Bending of bars

 
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies.    www.sefindia.org Forum Index -> E-Conference 20th June 2005
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Author Message
inchak at tatasteel.com
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PostPosted: Wed Jul 13, 2005 4:16 am    Post subject: Message No 52 - Bending of bars Reply with quote

Dear Mr. Rupen Goswami
I complement you for asking a very good question which is reproduced below:
QUOTE
Further, now with TMT bars becoming common in use, is it okay to continue
to
use the recommended bending radius (originally developed for normal steel
bars), or are these numbers already validated through experiments for TMT
bars? This question arises because the TMT bar is now a non-homogenous
cross-section with a softer core, as against the previous bars that are
homogenous across the cross section.
UNQUOTE

My response to your question:

TMT bars can easily be bent following the same mandrel diameters as
prescribed in IS:1786. Infact TMT bars are inherently capable of taking
much sharper bends than what is advised in IS:1786. At Tata Steel, we have
successfully subjected a 32 mm Tiscon bar (!) to a close 180 degree bend
without any mandrel and the two sides touching each other without any
visible crack appearing on the bend. However, we do not recommend such
sharp bends in construction practice because of excessive work hardening of
the outer fibre as was very rightly pointed out by you. The reason for the
ability of TMT bars to take a sharper bend lies in the fact that the outer
fibres comprise tempered martensite, an extremely crack resistant
microstructural constituent in steel.

While on this subject I would also like to point out a very common
misconception that has been expressed through your statement "TMT bar is
now a non-homogenous cross-section with a softer core, as against the
previous bars that are
homogenous across the cross section". I presume by previous bars you mean
the conventional CTD bars, which are also, by no way, homogenous. You can
easily appreciate that when a bar is twisted to improve its strength by
cold working, the fibre at the center undergoes zero deformation and the
strength remains at the as-rolled value. On the other hand the fibre at the
surface undergoes the maximum deformation and therefore, maximum
strengthening. As a result there is also a gradient of strength from center
(minimum) to the surface (maximum) for CTD bars.the only difference the
outermost fibre - the one that relly matters for initiation of cracks -  is
highly cold worked for CTD bars and, therefore, has a low ductility, while
the outermost fibre  for TMT bars has martensite tempered at above 550 Deg
C and is, therefore, highly  ductile.
Trust this clarifies the matter.
With regards

Indranil Chakrabarti


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