• PDF / 844,457 Bytes
• 7 Pages / 594 x 792 pts Page_size
• 23 Downloads / 220 Views

DOWNLOAD

REPORT

---

PROPAGATION OF A FATIGUE CRACK ON THE SURFACE OF SIDE FRAME OF A FREIGHT BOGIE UNDER IRREGULAR OPERATING LOADS D. V. Rudavskyi,1 М. S. Shefer,1 and Yu. І. Kanyuk1,2

UDC 620.19

We study the kinetics of propagation of fatigue cracks on the surface of side frame of a rail car with regard for the stochastic nature of the operating load. Its dependence on the initial geometry of the crack is established. Moreover, under given loading conditions, we determine the shape of semielliptic contour of the fatigue crack characterized by the highest growth rate into the depth of the frame wall. We also formulate a mathematical model suitable for the evaluation of the residual service life of the side frame containing a surface cracklike defect with given initial area. Keywords: side frame, fatigue crack, stress intensity factor, probability distribution function, residual service life.

Note that the cast side frames of freight bogies are made of 20GFL, 20GL, or 20GTL low-alloy steels. The requirements to their production are regulated by the corresponding normative documents; in particular, by the technical specifications of the manufacturer. According to these requirements, the service life of the cast frames should be close to 30 yr. However, in practice, they often fail after much shorter periods of operation and, according to the statistical data, most frequently for the first 2–3 years of operation. This is mainly connected with the defects of casting in the most dangerous zones of cast beams mainly near mechanical stress concentrators, which promote the initiation of fatigue cracks even under standard loading conditions (Fig. 1). In the process of fatigue fracture of metal structures, it is possible to distinguish the following two main stages: The stage of initiation of a fatigue crack (incubation period) N i and the period of its growth up to critical size N s , which is determined by the residual life of the structural element containing the crack (Fig. 2). According to the existing standards of fatigue testing of cast components of the bogies of rail cars, it is necessary to determine only the first stage, i.e., the incubation period, according to the Palmgren–Miner rule [1] of linear accumulation of fatigue defects in the process of operation. There are numerous publications [2–4] devoted to the prediction of reliability and fatigue strength of side frames within the framework of the indicated approach. At present, there are almost no efficient computational methods aimed at the evaluation of the residual service lives of cast frames for the second stage of the process (Fig. 2). However, on the basis of the existing methods, it is possible to determine the maximum admissible sizes of the cracks and cracklike defects and substantiate the required periodicity of the corresponding diagnostic measures aimed at prevention of the unpredicted failures of cast frames. The aim of the present work is to construct a new mathematical model of growth of surface fatigue cracks and develop, on the basis of this model, a computation