Bend Testing

Tensile testing cannot be used to determine bendability, since these are different failure modes. Failure in bending is like other modes limited by local formability in that only the outermost surface must exceed the failure criteria.

ASTM E290^A-26, ISO 7438^I-8, and JIS Z2248^J-5 are some of the general standards which describe the requirements for the bend testing of metals. In a Three-Point Bend Test, a supported sample is loaded at the center point and bent to a predetermined angle or until the test sample fractures. Failure is determined by the size and frequency of cracks and imperfections on the outer surface allowed by the material specification or the end user.

Variables in this test include the distance between the supports, the bending radius of the indenter (sometimes called a pusher or former), the loading angle which stops the test, whether the loading angle is determined while under load or after springback, and the crack size and frequency resulting in failure.

For automotive applications, the VDA238-100^V-4 test specification is increasingly used. Here, sample dimension, punch tip radius, roller spacing, and roller radius are all constrained to limit variability in results. Figure 1 shows a schematic of the test.

Figure 1: Schematic of Bend Testing to VDA238-100, with Bending Angle Definition.

This video, courtesy of Universal Grip Company,^U-5 describes the support rollers in the VDA238-100 test.

Calculation of the bending angle is not always straightforward. Bending formulas such as that shown within VDA238-100 assume perfect contact between the sheet metal and the punch radius. However, experimental evidence exists showing this contact does not always occur, especially in AHSS grades.

Figure 2 presents one example testing DP600 where the punch radius is larger than the radius on the bent sheet, leading to a physical separation between the punch and sheet.^L-12

This physical separation also has implications for standardized bendability characterizations. A common measure of bendability is the punch radius to sheet thickness ratio, r_PUNCH/t. In higher strength grades where this punch-sheet-liftoff is likely to occur, this may lead to an overestimation of how safe a design is when the punch radius may be measurably larger (less severe) than the tighter, more extreme radius actually experienced on the sheet.

Figure 2: DP600 After Testing to VDA238-100. Note punch radius is larger than radius in bent sheet resulting in separation.^L-12

Furthermore, bending tests do not always result in a round bent sheet shape and constant thickness around the punch tip, especially when testing 980MPa tensile strength steel grades and higher which have low strain hardening capability. Figure 3 shows pronounced flattening and thinning of the sheet below the punch tip after bending, occurring primarily on the side opposite the punch stretched in tension. Efforts to replicate this phenomenon in simulation have failed, since the underlying mechanism is not yet fully understood.

Figure 3: Flattening and Thinning Behavior after Bending.^L-12

Results from bend testing are typically reported as the smallest R/T (the ratio between the die radius and the sheet thickness) that results in a crack-free bend. Many steel companies report minimum bend test limits for various grades and certain automakers include minimum bend test requirements in their specifications as well. Different steel companies and automakers may have different bend test methods and/or requirements, so it is important to understand those requirements and procedures to better match the material characteristics with the customer’s design and process expectations. The test methods could involve a bend of 60°, 90°, 180° as well as various radii, die materials, speeds, etc.

Figure 4 shows etched cross sections of different grades bend to either 0T (fold flat) or 0.5T radii for reference purposes.

Figure 4: Etched cross sections of various grades.  Top row, left: 0T bend of DP350/600;  Top row, right: 0T bend of HSLA450/550;  Bottom left: 0.5T bend of TRIP 350/600;  Bottom center: 0T bend of TRIP 350/600;  Bottom right: 0.5T bend of DP 450/800.^K-1