The paper considers the processes related to the specimens and loading device in the tensile testing of metals in strain-controlled mode, including under the conditions of cryogenic temperatures up to 4.2 K. It is demonstrated how the potential elastic energy accumulation and its subsequent relaxation in the development of the specimen strain affect the process of kinetics. The strain rate dependencies on the strain value and stiffness factor (relation between the values of stiffness of the specimen and the machine) are obtained. Noteworthy is that the values of initial and nominal strain rates can differ by an order of magnitude in case of the machine compliance. To obtain the strain rates that are similar for different machines at the initial stage of the process, the formula is proposed allowing one to calculate the required nominal rate. At the temperatures lower than 30 K there is a dramatic increase of the influence of the machine stiffness, dimensions and shape of the specimens on the obtained characteristics, which requires the special measures in the process of testing. Some international and national standards are considered. It is shown that, at present, the process of standardization for tensile testing of metals is inadequate, and the requirements of the current regulatory documents are the minimum, which are not in agreement with the development paces of testing equipment. To significantly enhance the level of accuracy of the obtained mechanical characteristics, it is required to employ the urgent measures as the machine compliance limitation, as well as the decrease of the dimensions range for the specimens together with the standardization of the permissible range of the stiffness factor, and the required selection of the nominal strain rate considering the latter.
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Notes
Assessment of inaccuracy requires the determination of the material response to the strain rate. “For some types of materials, one can use some limited information” and examples given in the standard.
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Translated from Problemy Prochnosti, No. 3, pp. 145 – 152, May – June, 2018.
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Vorob’ev, E.V. On the Accuracy in Determination of the Mechanical Material Characteristics at Low Temperatures. Strength Mater 50, 487–492 (2018). https://doi.org/10.1007/s11223-018-9993-6
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DOI: https://doi.org/10.1007/s11223-018-9993-6