The standard guided bend fixture using a 1 1/2 inch diameter mandrel is intended to bend carbon steel specimens that are 3/8 inch thick and results in elongating the specimen 20% along the convex surface. When the welded coupon is thicker than 3/8 inch thick, the code allows the side bend in lieu of the standard face and root bend.
Think about the clearance between the plunger and the die of the typical guided bend testing machine. If one tried to perform a face bend on a 1 inch thick specimen, it simply would not work, i.e., there isn't sufficient clearance between the plunger and the die. The side bend allows the laboratory to cut 3/8 inch wide specimens from the welded coupon to eliminate the need for time consuming, expensive machining. If the laboratory prefers, they can machine each bend specimen to the required 3/8 inch thickness so the specimen can be bent as a face of root bend. It that route is taken, the laboratory must machine the face bend specimen to a thickness of 3/8 inch by removing material from the root surface. If the root bend is performed, the face surface must be machined to reduce the thickness to 3/8 inch.
It is easier and more efficient to simply cut side bend specimens and eliminate all the machining discussed in the preceding paragraph.
Be aware that the bend diameters used to evaluate the welded coupon is dependent on the base metal properties, the filler metal properties, and the thickness of the welded coupon. For example, a welded coupon consisting of 6061-T6 base metal welded with 4043 filler metal must be machined to 1/8 inch thick and bent with a mandrel that is about 16 times the thickness of the coupon. The reason is that the welded specimen has very poor properties of elongation. The very large bend diameter is needed to prevent breaking the specimen. As a matter of fact, because the HAZ is overaged, it is recommended that a wrap-around bending machine be used instead of a standard plunger and die bending fixture.
In a similar manner, high strength steels require a larger bend diameter because the high strength steel has lower elongation properties than the lower strength steel. The applicable code with dictate the proper bend diameter to be used.
There is a potential for a laboratory to use the incorrect bend diameter if they do not pay close attention to the code requirements and the material properties of the welded coupon. It is for that reason I always include the bend diameter used when the test results are reported. It allows the client to "audit" the bend test to ensure the proper test was performed.
Best regards - Al