ASTM E8 | ASTM E8M Standard Test Method for Tensile Testing of Metallic Materials

Important information is provided for specimen preparation, to ensure that the machining process and the subsequent specimen preparation do not influence the material, since this could in turn affect the results of the tensile test.

There can be a wide range of shapes for tensile specimens. ASTM E8/ASTM E8M lists standard flat specimens for sheet metals and thin sheet metals, for tubular products, for special specimen grips, and standard round specimens for other metal products, and specifies the corresponding initial gauge lengths to which all strain values refer. With a few exceptions, all dimensions that are needed for specimen preparation are specified, or else minimum dimensions are indicated.

Both the specimen shape and the specimen machining process are defined in the ASTM E8 test standard. Specimens must be prepared in such a way that the material properties are not influenced. All areas that were strain hardened through cutting or punching during the specimen machining process must be processed accordingly, if they have an effect on the specimen properties.

Products with a constant cross section (profiles, bars, wires, etc.) and cast specimens

(e.g. cast iron, nonferrous alloys) can be tested without processing. Preferred specimen shapes according to ASTM E8 are flat specimens (shouldered-end) and round specimens.

Particular focus is on the test speed. ASTM E8 and ASTM E8M support five different ways of specifying test speeds. They are designated as

• (a) Specimen strain rate,
• (b) Specimen stress rate,
• (c) Crosshead speed,
• (d) The elapsed time for completing part or all of the test, or
• (e) Free-running crosshead speed (rate of movement of the crosshead of the testing machine when not under load).

For determination of the so called yield properties, that is the yield strength, yield point elongation, and offset yield, in general all characteristic values related to the change of material behavior from elastic to plastic, it is important to define a suitable control of the test speed. Because in the case of metallic materials, these characteristic values can be significantly dependent on the actual test speed, and therefore the test speeds must be maintained within specified tolerances. ASTM E8 and ASTM E8M take this into account with three different control methods. They are designated as Method A, B and C.

All figures for the three methods described below are to be used unless product standards or standards for special applications specify other values.

If determination of the yield strength and offset yield have been completed (or do not have to be determined) and a specimen elongation of more than 5% is expected, the test speed can be increased to 0.05 up to 0.5 in./in./min (oder mm/mm/min*)). This specification refers to either the initial gauge length of the measuring extensometer or the initial parallel length of the specimen. The test speed is therefore indicated as a strain rate. At this test speed, all other characteristic values for the tensile test are then determined according to ASTM E8/ASTM E8M.

Method A is based on the increase in tensile stress during load application.

• In the linear elastic part of the tensile test, that is at the very beginning of the test, the rate of stress application must be between 1.15 and 11.5 MPa/sec (this corresponds to 10000 and 100000 psi/min).
• However, it is clearly stated in ASTM E8 and ASTM E8M that these specifications and method do not imply that the stress increase should be held constant up to the point of plastic behavior or that closed loop control of the force increase may be applied beyond the linear elastic range.

Method B is based on the control of the strain speed or strain rate (strain rate control) during the load application.

• The testing machine keeps the closed loop strain rate constant by using an extensometer to continuously provide strain values that are used to calculate the precise strain rate.
• The standard specifies a strain rate of 0.015 ± 0.006 in./in./min or mm/mm/min, which corresponds to a tolerance of 40% when setting the strain rate.