Calculation model for the evaluation of tired defect development in the freight wagon side frame

Abstract

The reliability and safe operation of mechanical elements of rail transport is an important and relevant scientific and technical issue since high-loaded units and elements of its chassis are exposed to prolonged operation and their failure can lead to damage with catastrophic consequences. To prevent the possible failure of such objects, there is a necessity for a reliable estimation of their residual life. Among the cast parts of the freight car trolley, the side frames are one of the most loaded elements, which take on dynamic loads that cause vibrations of the unsprung parts of the freight car. The side frame of the 18-100 trolley, as a typical representative of a number of trolleys of freight cars, does not have a sufficient margin of resistance to fatigue and survivability, so it is sensitive to some deviations and defects (different wall thickness, sinks, and pores, residual stresses) that are detected during the operation process. Based on the energy approach of the mechanics of materials fatigue failure, the calculation models for estimating the dynamics of the development of crack-like defects under the action of operational load are developed in this work. The calculation models constructed using modern information technologies, and the software developed for their numerical implementation allow predicting the impact of irregular cyclic loading and complex stress on the growth of surface fatigue crack in the side frame of the carriage, which significantly brings the problem closer to real operating conditions. Numerical calculations were performed using a computer program of our own development in the Python programming language. At the first stage of the program functioning the spectrum of amplitudes of irregular cyclic loading is built, at the second - the program module of numerical solution construction for systems of usual differential equations of the proposed mathematical model of fatigue defect development is started. Calculations of the fatigue crack growth dynamics taking into account the action of shear stresses in the section with the crack of the side frame showed a slight effect of shear stresses on the residual durability of the frame. It is demonstrated that the dynamics of surface crack development significantly depend on its initial geometry. The proposed calculation method for determining the initial shape of the surface crack with a minimum period to critical growth can be effectively used to predict the residual durability during the technical diagnosis of the running gear parts of railway rolling stock. Keywords: wagon side frame; fatigue crack; probability distribution function; load amplitude spectrum; numerical method.