Page 45 - MetalForming magazine • December 2022

Metal Matters By Daniel J. Schaeffler, Ph.D.
Metal Properties: Uniform Elongation
Tensile Strength
Yield Strength
Engineering Stress (ksi or MPa)
Diffuse Necking Begins
F F AB AB
CD CD
F
Local Necking
Slope = Modulus of Elasticity
F
AB
C D
F
AB
C D
FF
F
Engineering Strain (%)
Uniform Elongation deformation from beginning of forming through necking
Total Elongation at Fracture deformation from beginning of forming through splitting
The elongation value found on metal certs relates to its ductility to the point of fracture. Certs show this as “elongation at fracture” or “elongation after fracture,” depend- ing on the test method used. These values differ from the focus of this arti- cle—uniform elongation.
During a tensile test, the dogbone- shaped sample undergoes a reduction in the cross-sectional width and thick- ness as its length increases in tension. A stress-strain curve plots the material’s response to the applied load and result- ing elongation. The shape of this curve shows a peak at the maximum engi- neering stress, called the tensile strength. This characteristic shape results from the opposing effects of the work hardening and the reduction in cross-sectional area that occurs when the sample deforms in tension.
The beginning of the curve slopes upward due to work hardening effect outweighing that from the cross-sec- tional area reduction. Starting at the tensile strength, reduction in the cross- sectional area of the test sample over- powers the work hardening and the slope of the engineering stress-strain curve decreases. A diffuse neck forms
Dr. Danny Schaeffler, with 30 years of materi- als 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 com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler is the metallurgy and forming technical editor of the AHSS Application Guidelines available from Worl- dAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Fig. 1—This engineering stress-strain curve illustrates how the shape of a dogbone ten- sile-test sample changes as the test progresses.
at this maximum engineering stress, typically found in the middle of the sample in the reduced-width portion of the gauge length.
The elongation where the maxi- mum stress occurs is called “uniform elongation.” In a
tensile test, uni-
during the test, with the gauge region highlighted in blue. Through uniform elongation, the cross-section has a rec- tangular shape. Necking begins at uni- form elongation, and the cross-section no longer is rectangular. All subsequent
form elongation is the percent increase in gauge length occurring at peak stress relative to the initial gauge length. For exam- ple, if the gauge length at peak stress measures 59 mm and the initial gauge length was 50 mm, uniform elongation is (59- 50)/50 = 18 per- cent.
The diagrams in Fig. 1 illustrate how the tensile bar changes shape
Fig. 2—Strain evolution of the 201 points on the DP980 tensile- test specimen exhibits divergence beginning before uniform elongation—counter to conventional thinking. Reference: T.B. Stoughton, “The DIC Revolution in Metal Property Characteriza- tion,” Great Designs in Steel 2021; available from www.steel.org/ steel-markets/automotive/gdis/2021-gdis-presentations/
42 MetalForming/December 2022
www.metalformingmagazine.com