Eren BillurLocal and Global Formability, Part 1: Effects on the Press Shop
February 8, 2024Comments
In the metal forming industry, material properties receiving frequent attention are yield and tensile strength, and total elongation to fracture, commonly abbreviated as YTE (yield, tensile, elongation). When dealing with a coil or batch of sheets exhibiting a high scrap rate, a metal former typically begins its investigation by looking at YTE using either a tensile-coupon test or examining the material’s mill certificate. In almost all cases, the metal former will find that material falls well within specifications, even when scrap rates are elevated.
The problem of “within-specs/high scrap rate” for steels increased significantly with the introduction of multiphase advanced high-strength steels (AHSS). One mode of failure, characterized by edge cracks around pre-pierced holes or blanked edges, cannot reliably be predicted using traditional methods such as a forming-limit diagram (FLD).
Why Some Cracks Are Unpredictable
During the past two decades, most process and tool designers have relied heavily on sheet metal forming simulations. When trying to assess the risk of crack formation, users of simulation tools often rely on a parameter known as the maximum failure criterion. To calculate this criterion, one must create an FLD that combines the forming-limit curve (FLC, representing a material property—Fig. 1, blue curve), and the strain distribution (representing the process outcome—Fig. 1a, shown with more than 4000 dots). To determine the strain distribution, designers can use simulation software, or analyze the physical stamping using circle-grid analysis or other optical techniques.
The maximum failure criterion corresponds to the strain distribution closest to the FLC. When the failure criterion reaches 1 (or 100%), it indicates necking in the part. As expected, when the criterion exceeds 100%, the likelihood of cracks occurring becomes very high. Most tool designers aim to design a process with a maximum failure criterion of 0.7 (or 70%) for a part, depending on FLC-curve reliability. Despite this margin, the occurrence of edge cracks or splits still can be observed.
To assess the risk of a necking-based failure, three methods find use:
- Applying a safety margin (Fig. 1b), by moving the FLC down by a fixed amount of strain, typically 10%.
- Maximum thinning, the easiest method to check on the shop floor.
- Maximum failure criterion: distance of strain distribution points to the FLC. For example, in Fig. 1, the failure criterion of P1 is the distance O-P1 divided by distance O-F1, corresponding to 0.56 (56%).
Per Fig. 1b, all three points are on the same 10%-safety-margin curve. The thinnest point would be P3, but the maximum failure criterion would be observed on P2. So, although all three methods are used, the most common (and the safest) would be the maximum failure criterion. 
After such observations, researchers began investigating tests to evaluate local formability, or resistance to failure by fracture, including the hole-expansion test and its resulting HER. Another option: bendability, often used to evaluate high-strength low-alloy (HSLA) steels. In some HSLA grades, steel grades considered for roll forming (such as martensitic, CP, DP and armor steels), we commonly see a minimum (inner) bending radius as a function of thickness (bendability = 2T, for example). Since the late 2000s, many European metal formers have used a three-point bending test (also called a VDA bend test) to measure local formability performance. Researchers have found that energy-absorption capacity of press hardened/quenched steels correlate well with sharp radius bending test results. 
Using the various tests and calculations allows engineers to generate a failure forming-limit curve (FFLC). Another failure curve commonly used is the fracture curve (FC). In an FFLC, the X axis shows minor strain and the Y axis shows major strain, just like an FLC. With an FC (Fig. 4), the X axis shows stress triaxiality (a stress calculation beyond the scope of this article) and the Y axis shows fracture strain. 
