Standard Test Method for Tension Testing of Structural Alloys in Liquid Helium

Abstract

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.