ASTM E21 – Elevated Temperature Tension Test on Metals

ASTM E21 requires optimal and precise testing machine alignment when applying the load on the gripped specimen. The testing machine and specimen grips must load a precisely machined specimen in such a way that the maximum bending strain does not exceed 10% of the axial strain. It should be noted that the verification of these conditions under elevated temperature is not possible in practice and can therefore be performed at room temperature. In this context, reference is made to ASTM E1012.

When selecting an extensometer, it is recommended to use the ASTM E83 standard to meet the sensitivity and accuracy requirements. To determine the offset yield (Rp0.1 and higher), a Class B-2 extensometer is required. It is important to note that requirement compliance should be verified under elevated temperature conditions. One of these tests can be the determination of the slope of the stress-strain curve, which should be within 10% of the modulus of elasticity (nominal value) of the tested material.

For strain measurement, ASTM E21 explicitly addresses the positioning of specimen markings of the knife edges of the extensometer and of the temperature sensors. If for these specifications, there is a need to deviate from the standard, this must be reported and in the case of acceptance tests, it must be agreed on in advance.

The ASTM E21 standard requires that the selected extensometer is able to determine the strain rates. In addition, it is important to note that the determination of the modulus of elasticity is not included in this standard and that test conditions under rapid heating or high strain rates are not considered. It is important to note, however, that the characteristic values indicated in units of inches and pounds are in accordance with the standard.

When determining yield point and offset yield values, the ASTM E21 standard requires that a strain rate of 0.005 ± 0.002 in./in./min or mm/mm/min (corresponding to 0.000083 ± 0.000033 /sec) be maintained within the reduced parallel section.

After the yield point and offset yield have been determined, the test speed can be increased to 0.05 ± 0.01 in./in./min or mm/mm/min (corresponding to 0.00083 ± 0.00017 / sec). The test speed (strain rate or crosshead speed) must be recorded.

After yield point and offset yield determination, and when it has been confirmed that the crosshead speed is stable within the above tolerances, the crosshead speed in in./min or mm/min can also be calculated from the reduced parallel section (inches or mm) by multiplying by 0.05. This applies to all specimen shapes described in ASTM E8/E8M.

Example: If the reduced parallel section (previously also referred to as test length) is 4 inches long, then a crosshead speed of 0.2 in./min must be set; and for a reduced parallel section with a length of 100 mm, a crosshead speed of 5 mm/min. must be set.

If a complete stress-strain curve is to be recorded, then the test speed of 0.005 ± 0.002 in./in./min. or mm/mm/min. can be maintained throughout the entire tensile test.
If determination of the yield point and offset yield is not required, the use of an extensometer is also not absolutely necessary. The crosshead speed then has to be set so that a strain rate of 0.05 ± 0.01 in./in./min or mm/mm/min can be achieved during the tensile test.

In order to take the behavior of the test arrangement into account in the initial part of the stress-strain curve for determining yield point and offset yield, the set value for the strain rate may be corrected slightly upwards. A specific reference is provided in ASTM E21. Beyond the yield point and offset yield, the uncorrected set value must be applied again.

Manual strain at break measurement is carried out at room temperature. The measurement of the distance between the previously applied gauge marks should be made to the nearest 0.01 inch or 0.3 mm.

Manual measurement to determine the reduction of area is also performed at room temperature. The ends of the fractured round specimen must be fit together carefully and the minimum diameter is measured to the nearest 0.01 inch or 0.3 mm. If the fracture cross section is not round, make sufficient measurements to establish the reduction of area.

The contents of the test report are listed and described in detail. It is important that the test report reflects the test and measurement conditions as accurately as possible. In the case of strain at break, explicit reference is made to the need for specification after measurement at break and measurement after break. Also which initial gauge length is referred to: for rectangular specimens (e.g., 2 in. or 50 mm) and very important for round specimens with 4x diameter or 5x diameter.