Predicting springback with FEA/FEM methods
March 10, 2011
The increased use of advanced high-strength steels (AHSS) have caused many fabricators to rethink the way they bend metal. AHSS have a relatively high springback associated with it, so much larger overbend angles are generally required. Rotary bending technology is a suitable way to product that overbend angle.
The use of thinner and stronger steel materials, commonly referred to as downgauging, is becoming more and more common within the metal fabricating industry, especially for applications requiring weight reduction.
These steels are referred to broadly as advanced high-strength steels (AHSS), but they can be divided further into two main subcategories:
While the weight reduction with these materials is significant, it comes at a price. Although regarded as having good formability characteristics, AHSS materials can present challenges in piercing, trimming, and forming operations that typically do not occur with conventional low-carbon-alloy mild steels.
One particular challenge in processing AHSS is producing accurate and consistent bend angles. With its relatively high springback, AHSS often requires much larger overbend angles—in other words, the material must be bent well beyond the desired final position to compensate for the elastic springback that occurs as the tooling disengages.
Low-alloy steels typically need 3 degrees of overbend, whereas AHSS materials may require 7, 10, or even 15 degrees of overbend, depending on the actual yield strength of the material and the geometry of the bend. A certain minimum bend radius is required to prevent cracking of the material but, in general, the larger the bend radius, the greater the resulting springback angle. With conventional wipe tooling, a linear bypassing motion is all that's available, so the necessary overbend angle in the part is achieved simply by "mashing" or "crowding" the material at the bend. The amount of overbend that can be achieved by this method is very limited.
Rotary bending (see Figure 1) is a better way to produce the required overbend and gain tight control over the resulting angle after the bend has been completed (see Figure 2). Rotary bending is, in essence, a rotating-fulcrum process. The rotary bender contacts the workpiece in only two places—at the hold-down/clamping point, and somewhere along the leg which is to be bent. The rotary bender's rocker serves as the fulcrum point, and it rotates accordingly as the part is bent by the force of the press.
In the past iterative physical bending of test samples usually was required to ascertain the correct amount of overbend angle needed for a new and unfamiliar material. Today, however, with finite element modeling (FEM) and finite element analysis (FEA) methods, it's possible to analyze the stress and strain conditions in the material and predict the amount of springback, which permits a reasonable initial estimate of the required overbend angle. In some cases, the overbend can be determined without the need for physical bending tests (see Figure 3).
To validate this approach, Anchor Danly engineers collaborated with researchers at The Ohio State University to compare several FEM/FEA scenarios with test data from the bending of actual material samples (see Figure 4).
Two different scenarios were considered in determining the theoretical springback angle: One employed an elastic modulus value of 150 gigapascals (GPa), and the second used a value of 210 GPa. This dual-path approach was in keeping with the findings of several researchers who have observed that elastic modulus decreases significantly during unloading, or springback, with increasing prestrain (overbending).1,2
With heavy-gauge and high-strength materials, it's sometimes necessary to use a spring-loaded pressure pad when clamping the workpiece, and the amount of clamping force required is a function of the material thickness, tensile strength, and bend radius. High-strength materials usually require more generous bend radii to prevent splitting or cracking; larger bend radii, in turn, usually result in greater springback, which means more overbend is required.
FEM/FEA methods also can be used to ascertain changes in springback angle caused by variations in material thickness and fluctuations in mechanical properties. The use of these methods to predict rotary bending results can contribute to a quicker tooling build cycle.
In addition to its suitability for use in processing advanced materials, rotary bending technology also can be used for press brake applications as an alternative to folding, wipe-forming, and air-bending operations. The technology can increase bend angle consistency, decrease tooling wear and galling, and minimize part marking. For soft materials that are especially prone to marking or scratching, a Delrin® insert can be installed on one or both lobes of the rotary bender's rocker, or the entire rocker can even be made of Delrin.
Many different bend geometries are possible with rotary bending technology used in press brakes, including standard 90-degree bends, oversquare/undersquare bends, "hat" bends, channel bends, Z bends, and short-leg bends. In addition, working tonnage sometimes can be reduced by as much as 50 percent. A formula for calculating the required force for a rotary bending application is shown in Figure 5.
In a press brake application, it's good design practice to keep the rotary bender's axis of rotation centered within the plane of the press brake's ram. With heavy-gauge materials, a thrust heeling provision should be included in the tooling design whenever possible (see Figure 6) to prevent transferring excessive reaction force to the guide elements of the press brake machine itself; this would also be good practice with heavy-gauge wipe-forming operations. In either case, some clearance may be required for any backgauge system that might be in use.
Although the rotary bending device shown in Figure 6 is mounted to the tooling, which is affixed to the ram of the press brake, the rotary bender also can be located on the lower, stationary portion of the tool. Care should be taken (such as with hydraulic crown compensation) to avoid any problems with ram or bed flex that could possibly cause the rotary bender to also flex and, subsequently, bind.
To adjust the overbend angle positively, the rotary bender can be moved closer to the anvil in the horizontal plane. To decrease the overbend angle, the rotary bender can be moved away from the anvil. Note that rotary benders always should be heeled with a key or "pocketed" for thrust absorption.
Figure 7 presents a guideline for setting the K dimension on standard 90-degree bends in cold-rolled steel. When AHSS is bent, however, the amount of overbend required should be determined either through FEM/FEA methodologies or by performing actual test bends. Heavy coining generally is not recommended with any rotary bending operation, and shimming the rotary bender downward to create more overbend angle can result in severe damage to the bender. Don't do it.