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• Metal Formability Info         ⇓⇓
  • MetalForming magazine and the Precision Metalforming Association announce
       a new publication, Tool & Die Authority, containing exclusive technical information
       and timely news by industry experts for tool and die professionals. Danny
       Schaeffler of EQS is the sheet metal forming contributor to this monthly newsletter.
       Read the introductory issue here, containing his article
          "Measuring Accurately During Circle Grid Strain Analysis"   ⇒
  • Intro to AHSS: Part I-Grades   ⇒
  • Intro to AHSS: Part II-Processing   ⇒
  • Stamping 101: Materials Selection   ⇒
  • Forming Advanced High Strength Steels   ⇒
  • Introduction to Thickness and Circle Grid Strain Analysis
      Click here for a sample page    ⇒
            ⇓⇓    Section Topics     ⇓⇓
       Strain Calculations
       Deformation Modes
       Forming Limit Curves (FLC)
       Forming Limit Diagrams (FLD)
       Safety Margin (Also Known As Safety Factor)
       Tools & Techniques for Thinning Strain Analysis (TSA)
       Tools & Techniques for Circle Grid Strain Analysis (CGSA / CGA)
       Thinning Strain Calculation Example
       Measuring Strains Using a Calibrated Mylar Strip Ruler
       Measuring Strains Using Automated Grid Reading Techniques
       Plotting Strains on A Forming Limit Diagram
       Failure Thinning Limit Curve
       Failure and Marginal Thickness Determination
       Bead Correction Factor
       Measuring Small Strains Distributed Over Large Areas
       Using A Reference Panel to Check Production Process Robustness
       Thickness Strain Analysis for Statistical Process Control
       How Strain Analysis Techniques Affect Part Safety Assessment
  • Stuart Keeler's ...   The Science of Forming, Volume 2      ... Sponsored by EQS   ⇒

• Do you know the ...???         ⇓⇓
  • difference between IF, EDDS, and VDIF steels? ⇒ NONE!!!
    Steel is, by minimal definition, an alloy of iron and up to 2 percent carbon
    (if it is more than 2 percent, the alloy is cast iron). Carbon is small enough
    to fit into the interstices of a primarily iron matrix, making it an
    "interstitial element" in steel. If the steel alloy has an ultralow carbon level
    (typically less than 50 parts per million), most of these gaps will not be
    occupied and, as such, can be called interstitial-free (IF) steel. These
    primarily ferric (iron), very formable IF steels are extra-deep-drawing
    steel (EDDS). Achieving this low carbon level does not occur using conventional
    steel processing. Instead, the molten steel must be put under a vacuum that
    decarburizes it by removing carbon monoxide, as well as other gases like
    hydrogen and nitrogen. This process is called vacuum degassing, and it is
    done in the production of vacuum degassed interstitial-free steels (VD-IF).
  • The correct way to use the Mylar strip is to measure from the center-width
    locations of the boundary line around the circumference of the now-deformed
    ellipse. Measuring from inside-to-inside or outside-to-outside is wrong!
    With a fuzzy ellipse boundary line (old stencil, poor gridding technique, etc.),
    it is not hard to make measurement errors more severe than just measuring
    the outside-to-outside dimensions of a crisp circle/ellipse.
    The width of the line forming the boundary of an etched circle is about 0.008",
    which is also the thickness of the lines of the Mylar strip commonly used to
    measure the deformed ellipse manually(the diverging railroad tracks).
    If you are measuring inside-to-inside of a ellipse that was formed after starting
    with a 0.100" diameter circle that was stretched 20% in one direction, you’ll
    measure the major axis as 0.120"-0.008", or 0.112 inch, which is 12% on the
    major strain axis. You’ll also run the risk of measuring the minor strain wrong at
    0.100"-0.008", or 0.092 inch, which corresponds to a minor strain of -8%.
    Similarly, if you are measuring the outside-to-outside dimensions, you’ll wind up
    with 28% on the major strain axis and + 8% on the minor axis.
    A poor technique can turn a correct (20%, 0%) reading into anything between
    (28%,8%) and (12%,-8%)! Unless you are measuring from exactly the center-
    width position on the line making up the circumference of the circle/ellipse,
    you can get vastly different results, confusing the strain analysis interpretation.
    importance of a crisp grid pattern and proper grid measuring technique? ⇒
  • Think about pulling a bar in tension. Load divided by cross-sectional area is
    force, or stress. But what cross section are you considering? Before starting
    to pull, the bar had a known cross-section of (lets say) 0.5" wide x metal
    thickness. It's easy to measure these, since it is your starting material.
    At any load, the engineering stress is the load divided by this initial cross-
    sectional area. While you are pulling, the length increases, but the width and
    thickness shrink. At any load, the true stress is the load divided by the cross-
    sectional area at that instant. Unless thickness and width are being monitored
    continuously during the test, you cannot calculate true stress. It is, however,
    a much better representation of how the material behaves as it is being
    deformed, which explains its use in forming simulations. In circle grid
    analysis, engineering strain is the % expansion of the circle compared to the
    initial diameter of the circle.
    The relationships between engineering values and true values are:
          σ = s (1+e)            ε = ln (1+e)
    Where "s" and "e" are the engineering stress and strain, respectively, and
    "σ" and "ε" are the true stress and strain, respectively.
    difference between true stress/strain and engineering stress/strain? ⇒
  • Answers are being posted - let us know what else you want to see!!