V. V. Nazarov

ANALYSIS OF TWO METHODS FOR CALCULATING THE ULTIMATE STRESSES OF CREEP AND CREEP RUPTURE PROCESSES

It is well known that structural elements and parts operating in a loaded state at high temperatures can be subjected to irreversible deformation in time. The calculations of operating life (based on the corresponding mechanical models) may require the mechanical characteristics of the material, ones of which are starting creep stress and rupture stress (ultimate creep stresses) for a particular material at a given temperature. These mechanical characteristics cannot be determined from the diagram of the mechanical state of the material; they are measured in uniaxial tensile tests of cylindrical specimens at a constant tensile stress over time. The complexity of such tests will require alternative methods of calculating the desired mechanical characteristics. In this paper, instead of special tests, it is proposed to calculate the desired mechanical characteristics from approximations of secondary creep and creep-rupture strength. To this end, we have considered two fractional power-law functions with four material parameters (S. A. Shesterikov and M. A. Yumasheva, 1984), two of which have the physical meaning of starting creep stress and rupture stress: approximation of the nominal stress dependence of strain rate for secondary creep (1) and approximation of the nominal stress dependence of rupture time (2). The calculations used experimental data obtained for titanium alloy VT6 at 650 °C on cylindrical specimens, 5 mm in diameter and 25 mm in the base length. The parameters of the fractional power approximations were calculated by the iterative method, termed the Generalized Reduced Gradient Method (Microsoft Excel), under the condition of a minimum total error of the discrepancy between the experimental data and the approximating curve in the corresponding logarithmic axes. The analysis of approximation errors (1) and (2) has shown that error (1) is less than (2) and that the ratio of rupture stress to the starting creep stress is greater. It is concluded from the analysis of the results that it is better to use approximation (1) for calculating ultimate creep stresses.

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