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inchak at tatasteel.com Guest
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Posted: Wed Jul 13, 2005 4:16 am Post subject: Message No 52 - Bending of bars |
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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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