Page 89 - MetalForming Magazine October 2022 - FABTECH

Metal Matters By Daniel J. Schaeffler, Ph.D.
Metal Properties: Tensile Strength
Note: This article continues a series on metal properties: Elastic Modulus, August 2022 issue of MetalForming; and Yield Strength, September 2022 issue of MetalForming. Determining material strength
during a tensile test requires
dividing the pulling load by the cross-sectional area of the tensile bar, leading to the units of lb./in.2. Impact- ing the results are whether the calcu- lation incorporates the tensile-bar width and thickness before testing begins, or whether the calculations consider the ever-changing dimensions instead.
Using the initial dimensions of the tensile bar creates the engineering stress-strain curve. The material provider bases the material’s proper- ties, provided on the metal certifica- tions, on engineering units. Because the initial cross-sectional area is a con- stant, any changes in stress reflect changes in the load required to deform the tensile specimen. This stress-strain curve has a characteristic parabolic shape, with the maximum strength defined as the ultimate tensile strength (UTS, TS or Rm). After reaching the ten- sile strength, the curve shape might
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
600 500
Tensile Strength
400 300 200 100
True Units Engineering Units
0
0 5 10 15 20 25 30 35
Strain (%)
suggest that the metal softens. How- ever, it does not, with other aspects of the test at play to explain the down- ward-sloping shape.
While using the initial dimensions of the tensile bar is convenient, it does not reflect what occurs during the test. A tensile load elongates the test sample, with both the thickness and width decreasing. Remembering that metal alloys gain strength with additional deformation from work hardening, the applied load continues to increase. With the cross-sectional area contin- ually decreasing as the test progresses, strength (load divided by the instan- taneous cross-sectional area) continues to rise. Using the ever-changing dimen- sions of the tensile bar creates the true stress-strain curve. Properties incor- porated into metal forming simulations are based on true units, accounting for dimensional changes occurring with deformation.
Engineering vs. True Curve-Shape Differences
During a tensile test, strength and sample dimensions continually change. The applied load required to pull the sample increases, since the sheet metal gets stronger as it deforms
during the tensile test. Concurrently, the sample width and thickness decrease, but this is not factored into the engineering units that only rely on the starting dimensions. Initially, the positive influence of the strength- ening from work hardening exceeds the negative influence of the reduced cross-section; this gives the stress- strain curve a positive slope. As the influence of the cross-section reduc- tion begins to overpower that of the strengthening increase, the slope of the stress-strain curve approaches zero. When the slope reaches zero, the vertical axis (strength) reaches a max- imum UTS; the corresponding strain is the uniform elongation, as necking has not yet initiated.
As shown in the accompanying fig- ure, the true stress always exceeds the corresponding engineering stress, because the instantaneous cross-sec- tion always is smaller than the initial cross-section.
When considering the true stress- strain curve, which reflects the dimen- sional changes occurring on the sam- ple, there is no stress maximum occurring during the test. The material continues to harden and deform at stresses well above the UTS. MF
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Stress (MPa)