Daniel Schaeffler Daniel Schaeffler

Intro to Strain Analysis: Evaluating the Impact of Allowed Tensile-Property Variation

May 22, 2019

Earlier this year, this column described the steps involved in measuring strains and creating a forming limit curve (FLC) and thinning 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 uncertainty and the allowed variation within a metal grade.

The Marginal Zone

FLCs represent the boundary between strains associated with necking failure and strains not at risk for necking. FLCs are created from testing a discrete coil having unique properties. The strains on an engineered part must be lower than the forming-limit strains for robust stamping. The stamping is formed under unique conditions—lube amount and distribution, binder and ram tonnage, and blank placement all vary within allowable limits.

Resolution, accuracy and precision of each measurement leads to some uncertainty in the FLC shape and placement, as well as in the strains measured on the stamping. Recognizing 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.

The choice of how large to make the size of the marginal zone involves balancing risk tolerance, process control, tooling-development budget and timing constraints. Use a small marginal 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 indicates 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 temperatures and rolling-thickness reductions must be controlled within defined limits 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 operation 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 variation. 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 during pre-launch tooling tryout is a challenge 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 limit of successful forming in most low-carbon steels can be determined via a shortcut where the lowest point on the FLC, FLC0, can be calculated as a function of only sheet thickness and the strain-hardening exponent, n-value, measured in a tensile test.

Instead of calculating FLC0 based on test properties of the evaluated tryout coil, use the minimum-allowed part thickness and the minimum n-value associated with the chosen grade. This worst-case forming limit represents the lowest strains that result in necking failure on any shipment received within the grade specification. If the measured strains on the part fall below this threshold, stamping success should be independent of tensile-property variability.

Changing the steel vendor may lead to forming differences. Each company will supply a product that meets the specification, but likely occupies a different range within the allowable tolerance. Use the grade’s minimum n-value and minimum part thickness to reduce the effect of different suppliers on forming conditions.

This approach cannot be used with alloys where the FLC does not predictably depend on mechanical properties measured in a tensile test. Here, use a larger marginal zone to account for the influence that sheet metal property variation has on forming behavior. Standard guidance does not exist, so work with your material supplier regarding your specific challenges.

Low-End Properties?

It is easy to plug in grade-minimum n-value into the FLC0 formula, but the task becomes more complex when working with forming-simulation programs that require inputs of multiple tensile-test parameters. Some specifications list minimum-allowed values for yield strength, tensile strength, total elongation, n-value and r-value. Inexperienced simulation analysts may use these minimum values for convenience to project worst-case conditions. Reality is not that neat. Low yield strength usually can be found on products with high elongation and n-value. Complicating matters, most specifications do not contain maximum allowed values of elongation, n-value and r-value. Choose simulation inputs to reflect realistic combinations of formability parameters.

Want to learn more about different sheet metals and formability? Plan to attend PMA’s Sheetmetal Formability of Steel, Aluminum and Stainless Steels seminar in Grand Rapids, MI, June 11-12. Visit www.pma.org to register or contact Marianne Sichi for information. MF

Industry-Related Terms: Case, Forming, Point, Ram, Tensile Strength, Thickness, Tolerance, Alloys, Blank
View Glossary of Metalforming Terms


See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Quality Control


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