Compensate or Countermeasure? Taking Sidewall Curling Seriously
In this blog post Akshay Wankhede, Application Engineer at AutoForm, describes the causes for the springback phenomenon known as the “sidewall curl”, how its occurrence on stamped panels can be identified early through simulation of full production process, and how it can be counter-measured well ahead of die tryout. Read this post to understand the challenges of the sidewall curl effect for Advanced High-Strength Steels (AHSS).
Revisiting Sidewall Curl
The issue of the sidewall curl is a common phenomenon in sheet metal stamping which is observed on the side wall of a panel, typically following the drawing operation. This distortion results in dimensional variation, creating challenges in assembly operations, affecting productivity and subsequent part quality. Naturally, it is critical that you identify the sidewall curl ahead of time to eliminate acute problems down the road. The easiest way to recognize when a sidewall curl is occurring would be to look for any progressive angle change on the sidewall or flange area as shown in Figure 1 below.
Sidewall curl issues arise as soon as the panel comes out of the tool, where it is “free” to springback. It occurs in those areas where the strain distribution is not uniform.
To better understand the sidewall curl issue, it is important to dig deeper into the basics of the cause-effect relationship of all springback phenomenon.
In general, the elastic return effect on a metal is due to the residual stress that remains after the forming forces are removed (tool opening) or after the draw panel has been trimmed.
In the sheet metal stamping process there are three main types of stresses that can be applied to a generic section of the panel: Membrane Stress, Pure Bending Stress, and Superposed Bending Stress.
1. Membrane Stress: occurs when a specimen is uniformly stretched along its thickness beyond the elastic limit (yield point) and then allowed to relax; stresses and elastic strains are both fully released. The residual elastic strains left in the material cause the specimen to springback.
Consider the sketch in Figure 2 where the stress applied to the specimen by Force F generates a uniform stress along the thickness s0. When the force is removed the specimen springs back by the elastic strain el. Quantitatively, the elastic return is directly proportional to the applied stress and inversely proportional to the Young’s modulus E.
2. Pure Bending Stress: commonly occurs when the sheet metal is bent over a radius (the radius of the wipe post during flanging for instance). In this case there is a large difference of stress and in strains between the outer and the inner layer of the sheet.
Let’s assume we have a flange-up operation as shown in Figure 3. If we take a look at the stress and relative strain of the layers along the thickness of the sheet, we can see that there is a neutral portion in the cross-section of the specimen that remains under “pure” elastic strain while the external and internal portions are in the plastic domain of the stress-strain curve; the inner layers are compressed (negative strain) while the outer layers have positive strain.
The layers in the elastic portion try to go back to their original length but they are unable to completely recover original shape due to the plastically deformed portion, so the final position of the sheet is the one generated by the equilibrium of these two moments. The elastic relief moment is what causes the flange to springback by a certain angle or distance, as noted in Figure 3.
3. Superposed Bending Tension: occurs when the material is stretched and bent over a radius at the same time. Because of the stretching, the strains on the inner and the outer layers are not very different and if stretched enough, they have the same direction (all strained). Therefore, when the elastic stresses are released, the residual stresses are reduced and are more uniform. In such cases, springback is more stable and produces a lower dimensional deviation (See Figure 4).
These conditions show that springback is caused from the strain deviation between the layers of the panel. The arising sidewall curl is a consequence of strain variation not only through the material thickness, but simultaneously along the length of the sidewall. A sidewall curl issue can also be treated as a type of springback deformation resulting from successive bending and unbending when the sheet metal is drawn over a die-radius or through a drawbead. Materials such AHSS (because of their high tensile strength) and aluminum alloys (because of their low Young’s Modulus) usually show more springback and sidewall curl problems.
Why Take Sidewall Curl Issue So Seriously?
Springback is challenging to avoid, but there are techniques that can minimize its negative effects. Other contributions in this AHSS Insights blog (see references at the end of the blog) describe using darts and beads to lock in the residual stresses and produce dimensionally accurate panels. In addition, by incorporating effective tool morphing strategies, springback can be compensated to purposefully produce dimensionally accurate panels within close tolerances. Unfortunately, sidewall curl is very difficult to fix with morphing compensation, which makes achieving a dimensionally accurate panel almost impossible to achieve. Therefore, it becomes extremely important to eliminate the sidewall curl before morphing the tools to compensate for springback during engineering prior to finishing the tools.
It is possible to examine the springback and sidewall curl risk by using simulation software tools such as AutoForm. Springback is a consequence of differences between the stresses in the layers of a sheet metal, with greater differences leading to increased sidewall curl. Evaluate bending moments in the proper orientation: If the critical radii are perpendicular to the rolling direction, then you must consider transverse properties.
Design and process changes that address springback and sidewall curl can be proven out during simulation, making it a cost-effective and efficient approach which should be incorporated into the part development process.
The Influence of Different Material and Sheet Thicknesses
Different grades do not behave the same way under all forming conditions. Higher strength of the incoming steel means higher forming tonnage, which leads to greater differences between the top and the bottom layers of the panel. This in turn leads to greater springback and in some cases greater sidewall curl. Increased thickness typically reduces both springback and curl.
The same crossmember panel shown in the pictures above has been used to investigate the effect of sidewall curl for different materials and different thicknesses. Five different materials of 1.6 mm thickness ranging from low carbon steel to high strength steel including aluminum have been used to study this effect, shown in Figure 5.
DP600 was also tested for a thicker gauge of 2.0 mm and 2.5 mm to study the effect of thickness on sidewall curl. A 5XXX series aluminum alloy is included showing the increased springback associated with the significantly lower Young’s Modulus (Elastic Modulus) characteristic of automotive aluminum alloys.
Springback cannot always be completely compensated; issues such as the side wall curl, oil canning, large springback magnitudes, and the lack of robustness (repeatability) need to be addressed through improvements to process and/or product. It is important to become aware as early as possible of the potential for any such issues. The earlier this awareness is achieved, the higher the chances to look for appropriate countermeasures.
Waiting to address springback and sidewall curl in tryout is a poor strategy. A thorough study of the springback phenomenon and associated conditions by simulating the full process allows for greater understanding of potential springback related challenges. The sooner potential issues are discovered, the greater likelihood that appropriate countermeasures can be successfully deployed.
To learn more about springback management, you may want to have a look at the following links:
AutoForm Blog: AutoForm’s Springback Compensation Best Practices