Mechanical tubes can bend around fixed or rotating form blocks. Sand or some other particulate matter may have to fill the tube to keep it from collapsing, but the tubes risk another danger as well. If the tube bends too sharply, fractures will form, cracking the material. The tube's minimum bend radius is the smallest radius of a curve that it can form without cracking. Smaller tubes and tubes made from more flexible material can safely bend further.
Consult documentation from your tube's manufacturer to determine the tube's percentage tensile reduction in area. This value describes the extent to which a fracture would affect the tube's cross-sectional area.
Divide 50 by the tube's percentage tensile reduction in area. If the tube, for instance, has a value of 15, divide 50 by 15 which gives you 3.33.
Subtract 1 from this answer which gives you 2.33.
Multiply the result by the thickness of the tube. If the tube is, for instance, 3 cm thick: multiply 2.33 by 3 which gives you 7. Therefore, the tube should form no curve with a radius less than 7 cm.