Page 35 - MetalForming February 2020
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    UPCOMING EVENTS
  MANAGEMENT DEVELOPMENT ACADEMY
Class 10 - Session 1 | February 16-18, 2020 | Nashville, TN
          FORMING OUR FUTURE
March 4-7, 2020 | Honolulu, HI
  AUTOMOTIVE PARTS SUPPLIERS CONFERENCE
April 28-29, 2020 | Detroit, MI
  FABTECH MEXICO
May 12-14, 2020 | Mexico City, Mexico
    FABTECH CANADA
June 16-18, 2020 | Toronto, Canada
  INTERNET OF THINGS EXPERIENCE FOR MANUFACTURERS
June 22-23, 2020 | Chicago, IL
    3D METAL PRINTING TECH TOUR AND EXPERIENCE
July 22-23, 2020 | Chicago, IL
      MEXICO METALFORMING TECHNOLOGY CONFERENCE
August 19-20, 2020 | San Luis Potosi, Mexico
 For more information regarding these events, please visit pma.org/events.
 Cutting Edge
Longitudinal and width strains can be calculated directly, and when known allow calculation of thickness strain from volume constancy, and then the instantaneous cross-sectional area. Thus, we can calculate the true strain when we know the force at this time increment (􏰁 =F/A).
As an example, consider the results of an experimental study (see the accompanying figure, page 32) with a bake-hardenable steel, 220-MPa yield strength. Testing with a mechanical extensometer revealed a 28-percent uniform elongation (Ag) and 35-percent
Digital image
correlation data can
reduce the error in
stress calculation due
to extrapolating the
tensile data.
total elongation (A80). In true strain, these numbers correspond to 0.25 and 0.30, respectively, so a true stress-strain curve with the extensometer only gives valid data until 0.25 true strain (the red curve in the figure). Conversely, testing with DIC enabled measuring true strain until 0.81.
Still, most steel standards rely on extensometer data with a given initial gauge length (50 or 80 mm). However, DIC data can reduce the error in stress calculation due to extrapolating the tensile data. Further, the maximum DIC strain can be used to predict local formability problems such as hole- expansion ratio.
Next time, I’ll discuss using DIC for developing a forming limit curve and for conducting a bulge test.
1 In a recent effort to find out the oldest tensile test machine still in serv- ice, a hydraulic tensile test machine from 1876 was selected.
2 Commonly known as BH220. HC220B according to EN10268—similar to SAE J2340-210B. MF
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