PSI - Issue 75

Robert Goraj et al. / Procedia Structural Integrity 75 (2025) 691–708 Goraj/ Structural Integrity Procedia (2025)

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Fig. 5. Second moment of area dependent Bode plot

The unit of the mechanical stress is dB (according to:1dB = 20log 10 ( σ HCF /1e6)) and the phase shift is plotted in degrees. In the magnitude-part one recognizes two characteristic rails, which correlate to the excitation frequency with the previously discussed dotted lines obtained by the modal analysis. The maximal stress amplitude σ HCF in the analyzed range of I y equals 33.2 dB (45.9 MPa). 3.4. Transient analysis Making use of (3) and (15) one obtains a transient course of the longitudinal stress in the bearing spoke: ( , )= 1 2 ℒ −1 { 3 ( , ) ( )} (22) The peak value of (22)ranges between max[ ( , )] = 643 MPa and max[ ( , )] = 170 MPa. However, after the propeller switch off, σ zz becomes negative and ranges between min[ ( , )] = -270 MPa and min[ ( , )] = -74 MPa. The transient course (22) is presented in one special case later in section 6. 4. Optimal second moment of area Based on material parameters chosen for the spoke (i. e. for stainless steel 1.4542 X5CrNiCuNb16-4) and the temperature ranging from -40°C to +100°C, an optimal second moment of area is found using the parametric model presented in sections 2. is a parameter, at which the degree of material utilization a BK equals to one. The quantity a BK is estimated based on the FKM analytical strength assessment guide line of the Cooperative Research Association for Mechanical Engineering (FKM, Frankfurt/Germany)[30] except for a shape of the S/N curve. The S/N used in the FEM is here changed to a S/N curve, which stays in accordance with the Haibach rule [35], [36]. 4.1. Characteristic stress amplitude The reference point for the fatigue calculation is already defined in section 3. The estimation of the degree of material utilization follows under a consideration of the one-step stress spectrum approach. It follows for the stress amplitude and the stress mean value: | ( )| = max[ ( , )] − min[ ( , )] 2 (23) ̅ ( ) = max[ ( , )] + min[ ( , )] 2 (24) The relations (23), (24) are used for the estimation of the stress ratio: