Hello Doug, The answer depends on the construction and the materials used. 1 metal layer?, 2? ... or more? Also the thickness of the structure and of the copper must be accounted for. The smaller the bend radius the greater the strain and the higher the elongation requirement of the copper. If you need a very small bend radius, very thin, ductile copper and thin polymer films are best. Long standing rules of thumb for 35 micron copper and 25 micron base films and coverlayers have been: 3-6 times the thickness for single metal layer with coverlayer 6-10 X for 2 metal layers with coverlayers 15 to 20 or more times the thickness for bonded multilayers (Again this is highly construction dependent.) Use of a forming mandrel is a good idea if you are getting close ot the limits Finite element modeling is useful to get a sense of the limits. Simple 2D models will likely serve. Keep in mind that shock and vibration can make your static design into a dynamic one. Kind regards, Joe