Page 42 - MetalForming February 2019
P. 42

  The Science of Forming By Daniel J. Schaeffler, Ph.D.
The Importance of Tensile-Test Standards
and Gauge Length
               A
B
C
                                            Elongation, measured in a tensile test, is defined as the increase in length as compared with an initial reference length, called the gauge length. However, the elongation result pertains only to the specific shape of the tensile-test sample and the gauge length over which it is measured. Think of the length-of-line increase that occurs when stamping a blank into your engi- neered part. An increase of 1 in. is more significant if you start with a 5-in. blank than if starting with a 30-in. blank.
Take the example of a tensile bar with the gauge length divided into 10 equal sections (Fig. 1A). As the tensile bar elongates during a test, each of the sections expands. If all of the sections expand uniformly, then it would not matter what initial section that we chose for our reference gauge length (Fig. 1B).
But we know that the greatest strains concentrate near the location that ulti- mately fractures. In the example shown in Fig. 1C, fracture will occur in the center of the gauge length. Because we have chosen a gauge length to encom- pass all 10 zones, we average the strains
Danny Schaeffler, with 30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). 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, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Fig. 1—This tensile bar features a gauge length divided into 10 equal sections (A). If all sections expand equally during elongation (B), choice of the initial section for reference gauge length does not matter. But, the greatest strains concentrate near the location that ultimately fractures (as shown in red in (C)).
over that entire distance. This incor- porates the high-strain areas in the central zone as well as the lower-strain areas at the edges. Using this example, we can visually estimate a 50-percent increase in length when comparing the length after pulling the bar to the initial length.
Instead of taking all 10 sections as the gauge length in our tensile bar, what if we considered only the central two sections (shown in red in Fig. 1C)? Here, we estimate a 100-percent increase in length as compared to the initial sections. We are averaging over a smaller region, so our measured strains increase.
Tensile-Bar Shape
Each nation or region has a body responsible for writing standard test procedures, with the American Society for Testing and Materials (ASTM) responsible in the United States. Other organizations include DIN (Germany), JIS ( Japan) and ISO (international). From each, we can find the appropriate
tensile-bar shape for testing of sheet- metal products. ISO takes the approach of designating Type I, II and III bars, which represent ASTM, DIN and JIS shapes, respectively.
These tensile-bar shapes exhibit sig- nificant differences in width and gauge length ( Table—Differences in Tensile Bars and Fig. 2). Although the ASTM and JIS bars feature similar gauge length, the width of the JIS bar meas- ures twice that of the ASTM bar. The ASTM and DIN bars feature a 4:1 ratio of gauge length to width, where the JIS bar exhibits a 2:1 ratio.
These shape differences mean that elongation performance will change depending on the test-sample standard used, even when testing identical mate- rial. With the combination of the short- est gauge length and widest sample, elongation from JIS bars typically are higher than what would be generated from the other shapes.
Yield strength and tensile strength are not functions of the shape of the tensile bar. The definition of strength:
  40 MetalForming/February 2019
www.metalformingmagazine.com










































































   40   41   42   43   44