The active application in the practice of testing the indentation methods, in particular to measure the physical and mechanical properties of metals, polymers, biological technologies demands to development techniques for the measurement error estimation. At the same time existing traditional measurement error evaluation system, based on the using of the reference blocks, is not always suitable for use in testing and research laboratories. The aim of this work was development the technique for estimating the indirect measurements error of materials physical and mechanical characteristics that can be applied in practice and based on the existing standards. Checking of the proposed approach using the experimental values of the hardness and elastic modulus obtained during static indentation for various metals.It is shown that since the initial information about the material is an indentation curve representing the dependence of the load versus penetration depth of the indenter into the material tested, then it is better to confirm the metrological characteristics of the indentation measuring devices using the applied force and achieved displacement, but to estimate the accuracy of determining the properties through the error of indirect measurements. The equations for calculating the hardness and modulus of elasticity are derived. It allows to determine the component value most influencing the error magnitude. The calculation of error on the base of the value of boundary of a random and non-exclusive systematic error was carrying out.The advantage of the developed technique is the fact that the measurement of the physical and mechanical characteristics is based on the experimental data and does not require the creation of the additional metrological assurance. The proposed approach seems appropriate to extend for the determination of the measurement error of other characteristics: the yield point, the strain hardening exponent, creep, relaxation, determined by the indentation methods.
Phenomenological models for describing the process of straining of bitumen-mastic insulation used for protection of the natural gas pipelines from electrochemical corrosion is proposed. The viscoelastic characteristics of insulation at different temperatures and after aging are determined. It is shown that the viscosity and stiffness coefficients determined on the basis of the accepted rheological model of the Maxwell viscoelastic body can be used to assess the degree of degradation of insulation.
The metrological problems of measuring the physic and mechanical characteristics of materials by dynamic indentation are considered. It is shown that the estimation of measurement error demanding the creation of the reference blocks is ineffective due to the wide variety of controlled materials and a wide range of changes in their properties. A technique has been developed for evaluating the accuracy of measurements based on the errors of individual parameters included in the calculation equation, i.e. by determining the error of indirect measurements. The technique is based on the estimation of the boundaries of the random error of the measured characteristics of the material and the non-excluded systematic errors of the parameters that are used for the calculations of needed characteristics. The results of experimental studies are presented, indicating that due to the different character of the dependencies of hardness and elastic modulus, the error in measuring the elastic modulus exceeds the error in measuring hardness. In addition, it was found that the error in measuring the characteristics of materials by the dynamic indentation method exceeds the measurement error by the static indentation method and can be reduced by increasing the accuracy of the equipment used for the registration of impact process. The obtained values of the physic and mechanical characteristics of the materials and the values of the measurement error show that the dynamic indentation method can effectively solve the problem of non-destructive testing of hardness, elastic modulus, and strain hardening exponent of metals and products with an appropriate error.
The possibility of using dynamic indentation method for measurement the elastic and strength properties of polymer products obtained by additive synthesis using the SLA-technology is considered. The sensitivity of the method to changes in hardness, tensile strength, and elastic modulus of products obtained by different printing modes with a thickness of the cured layer of photopolymer resin of 100, 50, and 25 microns has been estimated. A comparison is made of two main methods for calculating the physical and mechanical characteristics of a material according to the data of its impact loading diagram: an adapted classical method of mechanics of contact interaction, considering the geometric parameters of the deformed region of the material, and a method based on the energy characteristics of shock interaction. It was found that the highest sensitivity of the dynamic indentation method to changes in the properties of the additive polymer, depending on the thickness of its hardened layer, is provided when using an energy computational model for evaluating the properties of the material. The results obtained are the basis for the methods of non-destructive testing of polymer products of additive manufacturing by the method of dynamic indentation. The implementation of these techniques in portable measuring equipment is an alternative to standard destructive tests and will allow obtaining reliable data on the properties of the controlled material without the need to manufacture special witness samples.
Study of the stress field in a plastic imprint and around it is of great practical importance. Processes similar to indentation are used in shot blasting to harden the surface of materials and generate compressive stresses in the surface layers. The purpose of this work was to study the change in the stress-strain state in the area of the plastic imprint with increasing load, in the transition from small to large deformations, as well as to study the change in stress at different strain rates.X-ray diffraction method was used to study the field of residual stresses generated on the surface of a plastically deformed region – in the zone of an imprint formed when a spherical indenter is pressed into the metal. An analysis of the change in the stress distribution with increasing load in the range of plastic imprint depths of 10–60 µm for steels and aluminum was made. Influence of the loading rate on the change in the values of residual stresses under normal contact of colliding bodies was studied. It is shown that the stress distribution has a complex character with areas of compression and tension of the metal and is determined by the ratio of the indentation depth to its diameter.The obtained experimental data make it possible to determine the choice of optimal modes of shot blasting, including for increasing the endurance limit of products.
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