Page 43 - MetalForming Magazine September 2022

Metal Matters
By Daniel J. Schaeffler, Ph.D.
Metal Properties: Yield Strength
Yield strength (YS) as reported on metal certs comes from the stress-strain curve generated during a tensile test. However, many details influ- ence the reported value. Specifications describing YS calculation procedures must account for differing yield- ing behavior seen in metal alloys, as well as allow for use of the methods to describe YS common in multiple industries.
Strong interatomic bonds hold atoms together. When a punch first contacts a sheet metal blank, the applied forces are low enough that the blank returns to its flat shape when punch direction reverses. At the atomic level, the force applied by the punch stretches the bonds without breaking them. The lack of broken bonds means that the atoms can return to their neutral position after removal of the force. During this elastic deformation, there is a linear relationship between the applied stress and the metal deformation, measured as strain. The slope in this region of a stress-strain curve is the elastic modulus.
Tensile Strength (TS or Rm)
Yield Strength (YS or Rp0.2)
0.2%
Engineering Strain (%)
Forming an engineered stamping requires sufficient force to break these bonds and cause permanent plastic deformation. Once bonds start to break, the in-process stamping cannot return to its original flat shape. The applied stress and resultant strain no longer are linearly related; each increment of additional loading leads to greater deformation.
Definitions
YS is the stress level at which the relationship between stress and strain no longer is linear. In practice, challenges exist with interpreting exactly where this non-linearity begins. Many tensile-test laboratories use automated algo- rithms to determine YS, making a precise definition critical for repeatable and reproducible interpretations.
There are two definitions commonly used. First: the 0.2-
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 forming technical editor of the AHSS Application Guidelines available from WorldAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
Email ds@eqsgroup.com or Danny@learning4m.com
percent offset YS (Fig. 1). Users, or more likely the computer algorithm, create a line parallel to the line which defines the elastic modulus but shifted to the right by 0.2 percent on the horizontal strain axis. The stress where this offset line meets the original curve becomes the YS, sometimes abbreviated as Rp0.2.
The second technique involves drawing a vertical line at the 0.5-percent strain value and extending the line until it intersects the stress-strain curve (Fig. 2). This determines the yield strength at 0.5-percent extension under load, abbre- viated as Rt0.5. These techniques result in similar but not identical YS values.
Continuous vs Discontinuous Yielding
Instead of a smooth transition from elastic to plastic behavior represented by the continuous yielding curves shown in the first two figures, many steel and aluminum alloys instead exhibit discontinuous yielding (Fig. 3). Here, the stress-strain curve first reaches an upper yield point fol- lowed by a load drop to a lower yield point that extends at an approximately constant value for an amount of strain called yield-point elongation (YPE), before resuming the characteristic shape of the stress-strain curve, due to work hardening. YPE results from the formation and movement of Lüders bands, sometimes referred to as stretcher strains. Understanding the atomic interactions help explain these concepts.
Fig. 1—0.2-percent offset yield strength, determined by offset of a line parallel to the modulus line by 0.2-percent strain.
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Engineering Stress (ksi or MPa)