Page 42 - MetalForming February 2022 Special Automotive Issue
P. 42

 Metal Matters
By Daniel J. Schaeffler, Ph.D.
Edge Cracking in Advanced Automotive Steels
  Fracture Forming-Limit Curve: Measures Local Formability
Necking Forming-Limit Curve: Measures Global Formability
           In stamped parts, necking describes the local- ized thinning that occurs when strains con- centrate in a narrow region in the body of the formed part. The traditional forming-limit curve defines the major and minor strain combinations that result in a neck. Practitioners design parts and processes to avoid exceeding these limiting strains in production. Simulation tools have become quite adept at predicting regions of formed parts at risk for necking failures. Similarly, plant- floor personnel have decades of experience to draw upon should these issues occur during tool development and production.
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
        However, other failure modes occur during
metal forming operations, primarily fractures that 0 originate at either cut edges or at the outer surface
of bends with tight radii. The ratio of bend radius
to sheet thickness, along with the material strength
and microstructure, controls the latter type of failure. The factors influencing edge fractures are far more complex.
A Greater Risk in Advanced Steels
Fractures at cut edges are not a new phenomenon. Readers of MetalForming magazine know that necking failure is asso- ciated with uniform elongation as measured in a tensile test, while fracture is associated with total elongation. The strains causing necking are lower than those causing fracture. This explains why parts made from conventional sheet metal grades usually do not fail by edge cracking; these grades reach the strains that result in necking failure before attaining the strain levels resulting in cut-edge fracture.
Two scenarios, when combined, put advanced high-strength steels (AHSS) at greater risk for edge or bend fracture. First: With conventional steels, fracture strains are substantially higher than necking strains. A tensile test shows this, where total elongation is twice that of uniform elongation in mild
Dr. Danny Schaeffler, with 30 years of materials and applications experience, is president of Engineering Quality Solutions (EQS) and Chief Content Officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing compa- nies; 4M teaches fundamentals and practical details of material properties, forming technologies, process- es and troubleshooting needed to form high-quality components. Schaeffler is the Metallurgy and Form- ing Technical Editor of the AHSS Application Guide- lines available from WorldAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
 -0.8
-0.6 -0.4
-0.2 0 0.2 Minor Strain
0.4 0.6 0.8
Fig. 1
and some high-strength steels. While fracture strains remain higher than necking strains in AHSS grades, the gap is smaller. We must also examine the affected portion of the steel required for each type of failure. For necking failure to occur, strains through the entire sheet thickness must exceed the necking failure limit. This is not difficult when the failure locates in the body of a formed part, as tensile deformation occurs throughout. In contrast, consider a bend around a tight radius. The highest strains occur at the outer surface, with the strain levels decreasing at locations moving toward the inner surface. Strains exceed the necking failure limit only on the outer surface but not through the thickness,
reducing the likelihood of necking failure at bend radii. There are different requirements for failure by fracture. Here, the necessary strains must exceed the fracture limit only at one portion of the sheet thickness. The highest strains still occur at the outer surface of bends or at cut edges, cre- ating likely fracture-initiation sites. Once a microcrack forms, additional deformation or energy will cause it to grow to the point of failure. So, while fracture strains still exceed necking strains, in AHSS grades the gap is smaller than with conventional steels, with the triggering mechanism easier
to attain (Fig. 1).
Researchers typically describe these differences as either
global or local formability. Global formability characterizes the resistance to necking, important when deformation occurs in the presence of a relatively large and uniform strain field. Local formability refers to fracture resistance in response to deformation concentrated within small and localized regions.
  40 MetalForming/February 2022
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