Significance and Use
4.1 Tension tests provide information on the strength and ductility of materials under uniaxial tensile stresses. This information can be useful for alloy development, comparison and selection of materials, and quality control. Under certain circumstances, the information can also be useful for design.
4.2 The force-time and force-extension records for some alloys tested in liquid helium using displacement control are often serrated (1).3 Serrations are formed by repeated bursts of unstable plastic flow and arrests. The unstable plastic flow (discontinuous yielding) is a free-running process occurring in localized regions of the reduced parallel section at higher-than-nominal rates of strain with adiabatic heating. Examples of serrated stress-strain curves for a typical austenitic stainless steel with discontinuous yielding are shown in Fig. 2.
4.3 A constant specimen temperature cannot be maintained at all times during tests in liquid helium. The specimen temperature at local regions in the reduced parallel section rises temporarily above 4 K during each discontinuous yielding event (see Fig. 2), owing to adiabatic heating. The number of events and the magnitude of the associated drops in magnitude of force are a function of the material composition and other factors such as specimen size and test speed. Typically, altering the mechanical test variables can modify but not eliminate the discontinuous yielding (2-4). Therefore, tensile property measurements of alloys in liquid helium (especially tensile strength, elongation, and reduction of area) lack the usual significance of property measurements at room temperature, where deformation is more nearly isothermal and discontinuous yielding typically does not occur.
4.4 The stress-strain response of a material tested in liquid helium depends on whether force control or displacement control is used (3). Crosshead displacement control is specified in this standard since the goal is material characterization by conventional methods. The possibility of a different and less favorable material response must be taken into account when data are used for design in actual applications subject to force-controlled conditions.
Scope
1.1 This test method describes procedures for the tension testing of structural alloys in liquid helium. The format is similar to that of other ASTM tension testing standards, but the contents includes modifications for cryogenic testing that require special apparatus, smaller specimens, and concern for discontinous yielding, adiabatic heating, and strain-rate effects.
1.2 To conduct a tension test by this standard, the specimen in a tensile cryostat is fully submerged in normal liquid helium (He I) and tested using crosshead displacement control at a nominal strain rate of 10−3 mm/mm/s or less. Tests using force control or high strain rates are not considered.
1.3 This test method specifies methods for the measurement of yield strength, tensile strength, elongation, and reduction of area. The determination of the Young’s modulus is treated in Test Method E111.
Note 1: The boiling point of normal liquid helium (He I) at sea level is 4.2 K (−269 °C or −452.1 °F or 7.6 °R). It decreases with geographic elevation and is 4.0 K (−269.2 °C or −452.5 °F or 7.2 °R) at the National Institute of Standards and Technology in Colorado, 1677 m (5500 ft) above sea level. In this standard the temperature is designated 4 K.
1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. See Section 5.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
