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There   are   other   formulas   that   are   used   to   obtain   this bend   allowance   also.   Some   are   fairly   accurate   some   are   not. One   in   particular   also   helps   to   show   that   the   material indeed   stretches.   It   is   called   the   Neutral   line   theory.   The neutral   line   is   an   imaginary   line   that   passes   somewhere close   to   the   middle   of   the   material.   It   is   the   area   where the   material   neither   stretches   nor   compresses   in   the   bend. If   the   material   did   not   stretch   at   all   then   the   neutral   line would   be   at   50%   of   the   material.   I   believe   that   there   is more   stretching   than   compressing   occurring   due   to   the   fact that   the   neutral   line   is   closer   to   the   inside   of   the   bend. Mild   cold   roll   steel   would   be   about   45   percent   of   the material   thickness   if   you   are   air   bending   and   43   percent   for bottom   bending.   The   next   three   pages   show   you   how   to   use   the neutral   line   to   figure   bend   allowance.   In   this   manual   we will   use   the   empirical   formula   to   obtain   the   bend   allowance. Page: 19
NEUTRAL LINE THEORY Note: 43% is usually used for CRS. Neutral line is closer to the inside bend.
Neutral Line theory