The usual mode of bending is curvature around a straight-line radius (Figure 1). Through the thickness is a gradient of strains from maximum outer fiber tension (the outermost surface) through a neutral axis to inner fiber compression (the surface closest to the punch or bend axis). No strain occurs along the bend axis in the direction parallel to the bend axis, and therefore is in plane strain. The discussion below considers only bulk deformation, and excludes the implications of any edge effects. Bend testing procedures are linked here.
When sheet metal flows through draw beads or over the die radius into the punch opening, it is bent, straightened, and in the case of draw beads re-bent in the opposite direction. The net strain after this process may be relatively small. However, each of the sequential bending and unbending steps strain hardens the sheet metal, which reduces the ability for further deformation of the metal in subsequent operations.
Deformation at the outer surface during three-point bending depends on the stretchability capacity of the metal. The failure strain in the bend is related to the total elongation of conventional steel, but AHSS grades with multiphase microstructures such as DP and TRIP experience shear fracture that severely reduces the bendability before failure occurs. A higher total elongation helps sustain a larger outer fiber stretch of the bend before surface fracture, thereby permitting a smaller bend radius. Since total elongation decreases with increasing strength for a given sheet thickness, the minimum design bend radius must be increased (Figure 2).
The ratio of punch radius to sheet thickness, or the r/t ratio, allows for calculation of the amount of elongation on the outermost surface. This value can be compared against the total elongation of the metal as determined in a tensile test, or against the minimum elongation value allowed in the specification. If the part geometry will not allow for sufficient elongation for the selected metal grade, then either the part, process, or steel grade must change. [Note that this is not a perfect assessment, since elongation in a tensile test is measured relative to a 50 or 80 mm gauge length, which is likely different than the dimensions of the bent section.]
For design and springback control, usually a smaller r/t ratio is desirable. However, this may not be suitable in terms of formability. Increased material strength usually is associated with a reduction in total elongation, which in turn means a successful bend requires a larger r/t ratio.
For equal strengths, most AHSS grades have higher total elongations than conventional HSLA steels. However, several AHSS grades have limited local formability based on their microstructure, and may be at risk for cracking during edge expansion.
Cracking in production stamping conditions at stress levels below what is predicted with Forming Limit Diagrams may be attributed to these local formability failures. As an illustration, physical bend tests and simulations were performed for both HSLA and DP780 steels.S-11 The HSLA global formability failure aligned with simulation predictions (Figure 3), and was accompanied by a visible neck (Figure 4). In contrast, the DP780 showed no visible neck at the failure site (Figure 5) and no correlation between the simulation and actual test results (Figure 6).
Like hole expansion, bending limits in AHSS products are further lowered by shear fracture associated with the interfaces between the ductile ferrite and the hard martensite phase in the microstructure. This reduction becomes more severe as the strength increases, since increasing strength is achieved by increasing the volume of the hard martensite phase. More about shear fracture is found here.
One way to address springback involves the use of rotary benders. Rotary benders transfer the vertical movement of a press stroke into a precise, rotary forming motion. A rocker or rotating die can simultaneously hold, bend, and overbend the sheet past 90° to counter material springback (Figure 7).
Use rotary bending tooling where possible instead of flange wipe dies. Rotary bending allows for easy adjustment of the bending angle to correct for changes in springback due to variations in steel properties, die set, lubrication, and other process parameters. In addition, the tensile loading generated by the wiping shoe is absent.
There are four sequential steps to the process:
1) Downward pressure from the rocker clamps the part with the bending lobes before bending starts
2) Induced rotation of the rocker bends material around the anvil
3) The rocker bends the sheet metal past final angle to compensate for springback
4) The rocker releases the sheet metal to allow springback to desired angle
Using rotary benders to roll darts into the part during bending provides another way to reduce springback and stiffen the part (Figure 8).