Issue 48

B. Chen et alii, Frattura ed Integrità Strutturale, 48 (2019) 385-399; DOI: 10.3221/IGF-ESIS.48.37 396 fatigue strength assessments for base metal, butt weld and other weld forms. In order to evaluate the fatigue strength reliability of the frame, the function expressions of NGSFLD are given in Eqn. (8), Eqn. (9) and Eqn. (10). ( ) ( ) ( ) ( ) ( ) ( ) =     = + −     = + −   −   = − −   −   = − −    = −    345 197.5 345 0.68 206 107.2 197.5 2 345 345 107.2 345 345 107.2 1.30 206 107.2 197.5 2.2 345 197.5 345 y x y x x y x x y x y x x y x x (8) ( ) ( ) ( ) ( ) ( ) ( ) =     = + −     = + −   −   = − −   −   = − −    = −    316.3 243.7 316.3 0.68 148 150.5 243.7 2 345 345 150.5 345 345 150.5 1.30 148.2 150.5 243.7 2.2 318.9 243.7 316.3 y x y x x y x x y x y x x y x x (9) ( ) ( ) ( ) ( ) ( ) ( ) =     = + −     = + −   −   = − −   −   = − −    = −    316.3 269.7 316.3 0.68 133.68 160.5 269.7 2 345 345 160.5 345 345 160.5 1.30 137.75 160.5 269.7 2.2 401.8 269.7 316.3 y x y x x y x x y x y x x y x x (10) Fatigue strength reliability calculation of control point Control point as the maximum point of fatigue strength in the deterministic model, its fluctuation under uncertainty variables directly reflects the minimum fatigue strength reliability of the frame. Therefore, the accuracy of fatigue strength reliability analysis of control point provides theoretical support for ensuring the overall fatigue reliability of the frame, reducing design redundancy and lightweight. Traditional fatigue strength analysis of bogie frame is based on observing whether the evaluation points are within the GSFLD or not, and for the evaluation points beyond the NGSFLD, if the stress amplitude does not exceed 20% of the corresponding yield stress, the fatigue strength design requirements are considered to be satisfied. Because of the excessive security redundancy, this method allows the existence of evaluation points exceeding the standard. However, the excessive safety redundancy makes the bogie frame designed meet the standard requirements, but it is too bulky and does not meet the current goal and demand of lightweight structure. Based on this, the reliability-based fatigue strength analysis method proposed in this paper perfectly solves the above problem. The control points are calculated by Eqn. (3) and (4), and the probability of control points within the fatigue limit diagram is calculated by combining the function expression of GSFLD. The limited state equation of bogie frame can be defined as = − ( ) X Z Y f x (11) The fatigue strength reliability of bogie frame is given by = n R N (12) where Y is the Y value of the control point obtained from the important sampling, ( ) X f x is the NGSFLD function corresponding to the X value of the control point, n is the number of control points at  0 Z and N is the total number of control points.