More than one way to skin a cat, M.B. Filling the tubing to be bent with sand will force the pipe to retain its round shape as it is being bent. The outside of the bend is stretching while the inside is constricting. The forces bear against the ends, to a point, but following the path of least resistance exit outward at the apex of the arch forming a kink. This is easily demonstrated with a simple coffee stir stick, on of those little red plastic straws. Place it between the index finger and thumb and squeeze. Of course it bends but soon kinks in the center. Repeat with the straw now filled with sand and you will see it can be bent father before it fails.