Page 45 - MetalForming November 2022
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 Metal Matters
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 dimen- sions 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, mak- ing it possible to calculate the percent- age change in length. A small feature on the stamping may not cause a dra- matic 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 dog- bone, owing to the wider shape of the JIS samples.
Ensure that the metal cert docu- ments test results using the same test- ing standard as required in your engi- neering calculations.
Determining Elongation on Dogbones
ASTM 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 deter- mines elongation at fracture. This rep- resents the most accurate approach, but likely requires the greatest capital investment. Other companies use a manual clip-on precision extensome- ter 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 bro- ken 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 per- fectly (Fig. 2), leading to an overesti- mation of actual elongation.
Computer algorithms based on the load-displacement data acquired dur- ing 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
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