Page 44 - MetalForming November 2022

Metal Matters By Daniel J. Schaeffler, Ph.D.
Metal Properties: Total Elongation
12.5 mm
50 mm
20 mm
80 mm
25 mm
50 mm
ISO I = ASTM
ISO II = DIN 20x80
ISO III = JIS #5
Metal certs document the tensile properties of your purchased coils. Elongation is one meas- ure of ductility, as denoted by the E in the YTE abbreviation. This article dis- cusses elongation at fracture and elon- gation after fracture, with both distinct from uniform elongation—a topic for a future article.
Higher elongation values are asso- ciated with metal alloys better able to form deeper draws without necking or splitting. Elongation also influences other mechanical behavior. Usually, higher elongation and lower yield strength trend together, but the mate- rial grade significantly influences this correlation. Deformation on the outer bend surface is comparable to what occurs during tensile testing. Compar- ing elongation values between different grades may provide a good estimate of their relative bendability. Alloys with higher elongation values accommodate greater rollover before fracture with some benefit in cut-edge ductility. Stronger influences include the microstructure and strength level of the sheet metal, and the cut-edge qual- ity produced at the manufacturing facility.
Dr. Danny Schaeffler, with 30 years of materi- als and applications experience, is president of Engineering Quality Solutions (EQS) and chief content officer of 4M Partners. 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 is the metallurgy and forming technical editor of the AHSS Application Guidelines available from Worl- dAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Fig. 1—Standards-writing organizations each have differing shape requirements for ten- sile-test-sample dogbones. Images for Figs. 1 and 2 courtesy of Billur Metal Form.
Although seemingly a simple con- cept, more goes into determining the elongation value than many realize.
Tensile Dogbone Standard Shapes
The tensile-test sample, called a dogbone due to its characteristic shape, contains a section of reduced width typically 60 to 120 mm long. All meas- urements for elongation occur within the central portion of this reduced sec- tion, called the gauge length. Elonga- tion calculations are expressed as a percentage change in length from a gauge length of either 50 or 80 mm
Fig. 2—During tensile testing at some compa- nies, technicians remove a clip-on extensometer prior to the point of frac- ture, then fit the faces of the two broken halves together and manually measure the distance between the gauge marks to determine elon- gation after fracture. The edges of the two speci- men halves rarely fit together perfectly, lead- ing to an overestimation of actual elongation.
long. For example, if pulling the sample results in a gauge-length increase to 60 mm with a length before testing of 50 mm, then the elongation calculates as 20 percent.
Standards-writing organizations such as ASTM, DIN, JIS and ISO each have requirements for tensile-testing procedures, calling out the shapes of the test-sample dogbones (Fig. 1).
These shape differences in sample width and gauge length influence the measured elongation values. The DIN shape, referred to as A80, features the longest gauge length. Here, the strains are averaged across an area beginning
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