Daniel Schaeffler Daniel Schaeffler
President

Metal Properties: Total Elongation

November 1, 2022


Metal certs document the tensile properties of your purchased coils. Elongation is one measure of ductility, as denoted by the E in the YTE abbreviation. This article discusses elongation at fracture and elongation after fracture, with both distinct from uniform elongation—a topic for a future article.

Higher elongation values are associated 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 material grade significantly influences this correlation. Deformation on the outer bend surface is comparable to what occurs during tensile testing. Comparing 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 quality produced at the manufacturing facility. 

Although seemingly a simple concept, more goes into determining the elongation value than many realize.

Tensile Dogbone Standard Shapes

1-dogbone-tensile-testThe tensile-test sample, called a dogbone due to its characteristic shape, contains a section of reduced width typically 60 to 120 mm long. All measurements for elongation occur within the central portion of this reduced section, called the gauge length. Elongation calculations are expressed as a percentage change in length from a gauge length of either 50 or 80 mm 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 at a length of 80 mm. Compared to the 50-mm gauge length of the ASTM A50 sample, the A80 shape averages strains over a larger area, leading to reduced values.

A related example uses length-of-line calculations on metal stampings. With knowledge of the starting dimensions of a flat metal blank, tool and die professionals lay a flexible tape on the part and measure the dimension after forming. This tape indicates the amount that the line lengthened, making it possible to calculate the percentage change in length. A small feature on the stamping may not cause a dramatic length-of-line increase relative to the entire blank, but locally it could be quite significant. We commonly see one to two points lower elongation measured on DIN ISO II A80 samples as compared to the ASTM ISO I A50 shape. Similarly, the elongation on the JIS #5 ISO III shape measures one to two points higher than the ASTM dogbone, owing to the wider shape of the JIS samples.

Ensure that the metal cert documents test results using the same testing standard as required in your engineering calculations.

Determining Elongation on Dogbones

2-broken-edge-dogbone-tensile-test-elongation-after-fractureASTM standards allow for the use of several methods when determining elongation. Advanced systems use a noncontact approach that tracks the movement of pixels projected onto the deforming test sample and determines elongation at fracture. This represents the most accurate approach, but likely requires the greatest capital investment. Other companies use a manual clip-on precision extensometer that expands as the test sample elongates. 

To avoid damaging the electronics or even the unit when the dogbone halves break apart, some companies remove this extensometer prior to the point of fracture. Lacking the ability to capture the dimensions at fracture, technicians fit the faces of the two broken halves together, and manually measure the distance between the gauge marks to determine elongation after fracture. The edges of the two specimen halves rarely fit together perfectly (Fig. 2), leading to an overestimation of actual elongation.

Computer algorithms based on the load-displacement data acquired during the tensile test must incorporate consistent definitions for calculating elongation independent of how the sample fractures and the appearance of the broken faces. ASTM standards define elongation at fracture as the strain associated with a drop in load of 10 percent, measured after reaching the maximum load during the test. MF

Industry-Related Terms: Alloys, Blank, Die, Dimension, Ductility, Form, Forming, Gauge, Point, Surface, Tape
View Glossary of Metalforming Terms

 

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

Technologies: Materials

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