Evaluation of the measurement uncertainty during the tensile tests of high-strength bolts

Abstract

A developed methodology for evaluating measurement uncertainty during the tensile tests of metals and alloys is presented. In this connection, the various sources of uncertainty are considered. The object of studies included high-strength bolts made of 40Kh “Selekt” steel, which were subjected to tensile tests according to the GOST 1497-85 State Standard using a UMM-100 universal tensile testing machine. The basic sources of uncertainty in the measurement of relative elongation and reduction were shown to include repeated measurements of relative elongation; errors of measuring the finite length by a vernier calliper and marking the initial length of a calculated section, as well as the measurement error of the tensile testing machine. These also include operator reading error, repeated relative reduction measurements, as well as the error of measuring the sample diameter by a micrometer before and after the tests. During the measurements, temperature deviation was demonstrated to constitute an additional source of uncertainty when the ambient temperature is different from the standard temperature value ((20±5)°C). Assumptions underlying laws describing the distribution of input values were assigned. Tensile tests are shown to be characterised by two components of uncertainty evaluated as types A and B. A mathematical model constructed for measuring relative elongation and relative reduction during tensile tests is presented. The standard uncertainties of input values are evaluated based on the assumed laws of their distribution. A correlation between the final length of the calculated section, the diameter of the sample following the test, and the applied force, is revealed. Expressions for the calculation of sensitivity coefficients, which characterize variations in the output value (relative elongation) depending on variations of input values, were obtained. The total and extended measurement uncertainties were evaluated. Based on the performed studies, a procedure for evaluating measurement uncertainty when carrying out tensile tests of high-strength bolts was described. The evaluation of measurement uncertainty during the product testing was shown to be a rather labourconsuming work. In this regard, the authors propose the development of procedures for evaluating the measurement uncertainty during the tests with their inclusion into regulatory documents for control methods.