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Daniel Schaeffler Daniel Schaeffler
President

Tensile Testing Part 1: Equipment, Samples and Procedures

April 29, 2020


We use tensile testing as a way to characterize a material’s mechanical properties. Products in a variety of industries require this evaluation. In addition to metal forming and fabricating, nonmetal applications that utilize tensile tests include composites (think motorcycle helmets), nylon fibers (fishing lines, for example) and flexible plastic (as with catheter tubes). Each of these materials has characteristics that affect test-sample preparation, fixturing, testing and analysis in ways that differ from sheet metals.
In the most basic description, a tensile test involves gripping a sample at each end and pulling it apart. During the test, we record the pulling force and associated extension of the sample, and analyze the data to determine the material’s strength and elongation.

dogbone sampleWhen testing sheet metal, we machine a test sample to a defined shape, such as that prescribed in ASTM A370—Standard Test Methods and Definitions for Mechanical Testing of Steel Products, and ASTM E8—Standard Test Methods for Tension Testing of Metallic Materials. Although it may be simpler to machine a rectangular test sample, failure can initiate anywhere on such a sample. To promote failure within the center, monitored region of the sample, the standards call for a dogbone-shaped sample with a section of reduced width (see the accompanying figure). 

During a tensile test, the testing apparatus applies load using either a mechanical or hydraulic system. To ensure sufficiently accurate and precise test results, operators must calibrate the load cell, used to measure force during the test, over the loading range appropriate for the tested material. In practice, this means that testing of metals and rubber will require use of different load cells. 

With a stamped metal part, we measure a length-of-line increase over some reference distance, possibly over the full blank or in between two features. The choice of reference distance impacts the calculated percent increase. Similarly, on a dogbone tensile-test specimen, the reference distance over which we measure the extension influences the calculated elongation. This reference distance, called gauge length, is 2 in. (50 mm) on full-size ASTM dogbones. 

To measure sample extension, the most rudimentary procedure involves taking the fractured test specimen, placing the two pieces back together, and measuring the distance between the gauge marks. This process, however, results in low repeatability, and likely will lead to an overestimation of elongation since the pieces will not precisely fit together. Also, this method only determines tensile strength and total elongation after fracture; it does not measure extension throughout the tensile test.

As another way to measure extension, test operators can clip an extensometer to the tensile specimen, which expands during the test. The associated electronics capture the displacement as a function of time, and we use this data, synced with the loading response over time, to create a stress-strain curve. The clip-on extensometer must be appropriate for the test material in order to maintain accuracy and precision. Some plastics can deform to more than triple their length before breaking, where most sheet metals see a 10- to 40-percent increase. 

A third, and more accurate, method to measure extension is use of a noncontact video extensometer or digital image correlation (DIC) to track material movement during the test. Learn more about DIC in the Cutting Edge column, by Dr. Eren Billur, in the February 2020 issue of MetalForming. The stress-strain curve is generated by synchronizing the displacement data captured by these noncontact methods with the load-cell data.

The rate at which test samples are pulled will influence test results. Through yielding, use of a slower pulling rate will ensure that the test acquires a sufficient number of load and displacement readings to accurately determine material properties. The straining rate can be left at this low level for the entire test, but this reduces productivity. Most test locations will quicken the pulling rate considerably after reaching 3- to 5-percent elongation, since test speed has a smaller influence on properties after this point. ASTM E8 lists the required test speeds.

High Strength = More Concerns

Existing load frames may be inappropriate to test higher-strength alloys. Just like the crown of an undersized stamping press may upwardly deflect under high loads, so, too, can the upper support structure of a tensile-test load frame, if it lacks sufficient stiffness. When that happens, the measured loads include both the force to deform the test sample and the force to deflect the load frame. The test lab must abide by the maximum usage requirements set by the test-equipment provider.

Be sure to place the dogbone test specimen in the load frame so that the apparatus will only apply an axial load. Ensure precise alignment of the grips (and their action) with the axis of the dogbone at the beginning of, and during, the test. The impact of bending or twisting is more significant when testing higher-strength materials.

The dogbone is held in place by top and bottom grips attached to the tensile-testing machine. During the test, the grips move apart at a constant rate, with the force and displacement continuously recorded. Be aware that older, lower-capacity machines may not develop sufficient gripping force to prevent slipping when testing higher-strength materials. Avoid the use of pneumatic-actuated flat-faced grips. Instead, use hydraulic-actuated knurl-faced grips.

Words of wisdom: Sample shapes and test procedures will differ based on the material type and product form. Shops that need to send samples to an independent test lab for tensile testing should ensure that the lab is equipped to test their specific material grades. MF

Danny Schaeffler is the technical editor for metallurgy and forming for the upcoming release of the AHSS Guidelines. Contact him to contribute a case study to the document. He is looking for lessons-learned, where things may not have gone smoothly at first, and will give full attribution where requested.

Industry-Related Terms: Alloys, Bending, Blank, Case, Center, Edge, Form, Forming, Gauge, Lines, Point, Tensile Strength
View Glossary of Metalforming Terms

 

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

Technologies: Quality Control

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