The second type of the bending test equipment with a mandrel is a standard device in which hardened bearings on the matrix are replaced by rollers. This helps to reduce the frictional force acting on the specimen when bending, and allows you to perform bending with less effort. The last of the common devices for bending tests with a mandrel is called the wrapping device. It is named so because the sample is bent by winding it around the fixed mandrel.
In some qualification tests of low-carbon steels, it is required that the sample to be folded around the mandrel with the diameter four times that of the sample.
Thus, the 3/8 inch thick sample should be folded around the 1 ½ inch diameter mandrel. This method gives a 20% elongation along the outer surface of the bent sample. If you use the smaller mandrel, the elongation will be higher.
In the control samples for qualification tests in which the weld metal is significantly stronger than the base metal, instead of the smooth folding around the mandrel, the sharp kink in the base metal region adjacent to the weld seam may form. If the folding device is not available, the samples can be taken for longitudinal folding instead of standard cross-folding specimens. The weld is parallel to the longitudinal axis of the sample for the longitudinal bending.
To avoid inaccuracies in the results of any bend test, the samples should be carefully prepared. The grinding or stripping traces with the emery cloth on the tensile surface of the sample should be directed towards the bend so that there are no lateral grooves (stress concentrators) that can lead to premature failure of the sample. To relieve stress in the corner areas of the sample, they should be rounded. The samples cut from the control sections of the pipes and intended for bending tests can grind the plane on the side that comes into contact with the pusher, and thus eliminate the possibility of bending in the direction perpendicular to the bending direction.