Page 42 - MetalForming June 2019
P. 42

  The Science of Forming By Daniel J. Schaeffler, Ph.D.
Intro to Strain Analysis: Evaluating the Impact of Allowed Tensile-Property Variation
Earlier this year, this column described the steps involved in measuring strains and creating a forming limit curve (FLC) and thin- ning limit curve (TLC), and detailed the importance of using the curves for your specific product rather than generic ones found online or in old brochures. Here, we learn to evaluate the impact of measurement uncertain- ty and the allowed variation within a metal grade.
The Marginal Zone
FLCs represent the boundary between strains associated with neck- ing failure and strains not at risk for necking. FLCs are created from testing a discrete coil having unique proper- ties. The strains on an engineered part must be lower than the forming-limit strains for robust stamping. The stamp- ing is formed under unique condi- tions—lube amount and distribution, binder and ram tonnage, and blank placement all vary within allowable limits.
Resolution, accuracy and precision
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 •
E-mail or
of each measurement leads to some uncertainty in the FLC shape and placement, as well as in the strains measured on the stamping. Recogniz- ing the imprecise nature of the testing and measurements, companies employ a buffer between the failure zone and safe strains. Strains falling within this marginal zone highlight regions on the panel that might be at risk of necking due to small measurement errors or changes in metal flow. Companies should require all strains to plot below the marginal-zone limit as a condition of tooling buyoff.
Companies should require all strains to plot below the marginal-zone limit as a condition of tooling buyoff.
The choice of how large to make the size of the marginal zone involves balancing risk tolerance, process con- trol, tooling-development budget and timing constraints. Use a small mar- ginal zone only when you have tight control of the process and of the incoming sheet metal, or if you prefer to shorten tooling-development timing and are willing to absorb the risk of encountering production stamping issues.
Conventional wisdom favors a 10- percent safety margin, where an absolute 10 percent is subtracted from the major-strain value associated with every point on the FLC. Consider an alloy where FLC0 plots at 40-percent major strain and 0-percent minor strain. Here, the marginal limit curve, at its lowest point, exhibits 30-percent
major strain and 0-percent minor strain.
Other companies use a 10-percent safety margin where 10 percent indi- cates a percentage of the major-strain value. Consider the same alloy where FLC0 plots at 40-percent major strain and 0-percent minor strain. In this case, the marginal limit curve at its lowest point exhibits 36-percent major strain and 0-percent minor strain. The smaller marginal zone means that more strains can be considered safe, leading to faster tooling buyoff. However, this means a greater risk of strains crossing into the failure side of the curve with small changes in metal flow.
Worst Case
Sheet metal properties vary based on how the sheet is produced. Inputs such as chemistry, processing temper- atures and rolling-thickness reductions must be controlled within defined lim- its in order for the mill to produce a specific grade. Changes in any input parameter cause fluctuation in output parameters such as strength and formability. Portions of the rolling oper- ation occur at speeds in excess of 30 miles/hr., leading, understandably, to minor differences throughout the coil.
The specifications to which you order your sheet metal allow for vari- ation. When specifying minimums or ranges in chemistry, hardness or tensile properties, you acknowledge that one batch may not be identical to the next. Accounting for this variation when using only one or two shipments dur- ing pre-launch tooling tryout is a chal- lenge addressed by evaluating a part with certain property assumptions.
FLCs are created experimentally by deforming test samples of different shapes and determining critical strains, above which show as a neck on the sample. The boundary defining the
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