PSI - Issue 22

S.M. Xie et al. / Procedia Structural Integrity 22 (2019) 353–360 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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5) When the bearing capacity x a , y a ,  a and v a are less than or equal to 1, the fatigue strength of the weld is evaluated to meet the standard requirements, otherwise the local structure needs to be optimized and recalculated until the requirements are met; 6) The stress factor of the welded joint is determined by the stress bearing capacity , and the stress state level is determined. 2.2 Proceduralization of evaluation methods The fatigue evaluation program based on DVS1608-2011 standard nominal stress is written in APDL language. The analysis program is mainly composed of four sub-modules, and the steps of implementation process are as follows: 1) Weld classification module: according to the type and bearing condition of the welded joints, the weld is divided into different accuracy grades, and the weld notch curve index is determined; 2) Node parameter extraction module: extract the six stress tensors of the weld joints of all fatigue calculation conditions in the global coordinate system from the ANSYS calculation result *.rst file. 3) The stress parameter calculation module: calculates the stress ratio and the stress amplitude in the three directions of the evaluation point, and determine the allowable fatigue strength and load-bearing degree of the evaluation location, etc; 4) Calculation result output module: the fatigue analysis result such as the stress amplitude, fatigue strength value and bearing capacity of the weld joint is evaluated and stored in the form of a *.txt file by creating a macro file statement *CFWRITE. 3. Stress state level assessment method based on IIW-2008 standard The IIW-2008 standard provides 57 S-N curves of aluminum alloy materials, covering dozens of welded joint types. Fatigue grade (FAT) is used to calibrate welded joints with the same fatigue sensitivity, which also meets the requirements of BS EN15085 for welding structure. 3.1 Nominal stress assessment method The technical route for determining the stress state level of aluminum alloy welded joints based on the IIW-2008 standard is as follows: 1) Under the condition of fatigue load, the magnitude and direction of the principal stress of the welded joint are evaluated by finite element simulation calculation; 2) The maximum of the principal stress values of the evaluated position under all fatigue conditions is determined as the maximum principal stress max  , and the corresponding direction cosine ( x n , y n , z n ) is taken as the basic stress vector direction; 3) Multiplying the stress tensor matrix of the evaluation point by the basic stress vector column vector to obtain the stress column vector  s  (the stress vector acting on the plane where the normal vector is the basic stress vector), then project it into the direction of the basic stress vector to get  N  , and the calculation is shown in equation (3). The minimum value of the evaluation point  N  under all fatigue conditions is determined as min  ;

(3)

4) By using the formula of stress variation range (4), the stress variation amplitude of the evaluation point is obtained;