Issue 48

B. Chen et alii, Frattura ed Integrità Strutturale, 48 (2019) 385-399; DOI: 10.3221/IGF-ESIS.48.37 395 without safety factor is adopted to more accurately evaluate the fatigue strength of the frame. Since the cause of stress concentration factor is complex, it is taken as a fixed value. f k =1.1 is selected according to the stress concentration coefficient factors diagram provided by BS7608 standard (Guide to fatigue design and assessment of steel structures) and the S-N curve of steel under different stress concentration factors given in the book of fatigue life prediction of structural. The fatigue limit -1 N  of the frame and the fatigue limit -1  of the material are calculated to be 237.8MPa and 261.6MPa, respectively. Tab. 10 shows the turning point coordinates of the GSFLD. In Tab. 10, the closed 8-sided region formed by ABCDEFGHA is a GSFLD, with turning points at each point. Among them, A and E are the symmetrical cyclic fatigue strength under given fatigue life N; C and G are the turning points of the tensile yield limit and the compressive yield limit in transverse and longitudinal coordinates, which are equal for general metal materials. B, F, D and H are turning points to control the limit state when Goodman correction is carried out considering the average stress. In the coordinates of each point,  − 1 N is the symmetrical cyclic fatigue strength under given fatigue life N;  b is the ultimate tensile strength; yt  is the yield tensile strength;  yc is the ultimate tensile strength. Combined with Tab. 10 and mechanical properties of the frame material, a new Goodman-Smith fatigue limit diagram (NGSFLD) is obtained, as listed in Fig. 12 and Fig. 13. Turning point Mean stress  m Stress amplitude  a Turning point Mean stress  m Stress amplitude  a A 0  − 1 N E 0  − − 1 N B      − − − − 1 1 yt N b b N  yt F   − − 1 N yc  − yc C  yt  yt G  − yc  − yc D      − − − − 1 1 yt N b b N ( )        − − − + − − 1 1 1 2 b N yt b N b N H   − − 1 N yc   − − 1 2 N yc Table 10 : Goodman-Smith plot of turning point coordinates Figure 12 : Comparison of Goodman-Smith fatigue limit diagrams Figure 13 : Goodman-Smith fatigue limit diagram without considering the safety factor The black line in Fig. 12 is GSFLD without safety factors, and the red line is the fatigue limit diagram specified in ORE B12/RP17 standard. Fig. 12 shows that the newly defined GSFLD is broader than the ORE B12/RP17 standard definition, that is, it is more conservative to use the GSFLD defined by ORE B12/RP17 standard to evaluate the fatigue strength of bogie frame. It is not only simple and universal to evaluate the fatigue strength of the frame conservatively in engineering practice, but also to ensure the reliability of the product. However, conservative anti-fatigue design can no longer meet the requirements of rail vehicles which are increasingly pursuing lightweight. Therefore, the redundancy of design can be reduced by using the NGSFLD drawn in Fig. 13 for fatigue strength analysis. Fig. 13 contains the types of