Vyatka State University (Kirov, Russia):

A. V. Udalov, Cand. Eng., Associate Prof., Dept. for Materials Science and Fundamentals of Design, e-mail: a.v.udalov1960@gmail.com

The size effect or “indentation size effect” (ISE), which manifests itself in a change in the readings of the instruments when measuring the hardness of the test material depending on the indentation conditions, greatly complicates the improvement of materials manufacturing technologies. The article provides an overview of the main signs of the manifestation of the ISE and its description models for indenters of various shapes with a constant degree of loading. The aim of the work is to determine the criterion responsible for the manifestation of the ISE when the test load changes during the measurement of hardness by a ball of constant diameter, as well as the optimal indentation conditions, providing real values of the hardness of the material that are closest to the true values. Research Methods. The influence of the test load on the manifestation of the ISE was studied using the example of measuring hardness of an exemplary measure 114НВ of steel 10 with a ball with a diameter of 15.1 mm. The obtained experimental dependence of the indentation depth on the test load was approximated by a power-law function on the basis of which the following main process parameters were determined for various load values: the degree of deformation of the material in the deformation zone; material hardness; the specific work of plastic deformation and the relative change in the resistance to deformation of the material with increasing load. Results and discussions. An analysis of the results showed that the increase in HB hardness with an increase in the test load at a constant ball diameter is due to a corresponding change in the material deformation resistance in the deformation zone under the indenter. Actual hardness values are most consistent with true values if the ratio of the imprint diameter to the ball diameter is 0.375. The developed methodology and the results obtained can be used in the practice of determining the mechanical characteristics of materials by the hardness indentation method.

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