PSI - Issue 75

698 Robert Goraj et al. / Procedia Structural Integrity 75 (2025) 691–708 Goraj / StructuralIntegrity Procedia (2025) ( )= ̅ ( )−| ( )| ̅ ( )+| ( )| (25) In the considered case the normal stress σ zz changes from tension to compression after the propeller is switched off. This time behavior implies that: max[ ( , )] >0 and min[ ( , )] <0 . Moreover, due to the absence of the propeller trust after the switched of follows: max[ ( , )] > −min[ ( , )] . Consequently, one obtains for the mean stress ratio a following range: −1 ≤ ( )≤0 (26) Following a most applied overload case, in which ( ) remains constant in case of over load during operation (indicated in FKM as “F2”), one obtains for the mean stress correction factor: ( )= 1 1+ ̅ ( )/| ( )| (27) The quantity M σ in (27) is the mean stress sensitivity: = · 10 −3 /MPa+ ≅0.27 (28) where a m and b m are fatigue constants and R m is the material tensile strength (see Table 2). Table 2. Input parameters for geometry definition Parameter Symbol Value Unit spoke roughness R z 100 µm material temperature range T m -40, +100 °C tensile strength R m 1070 MPa yield strength R p 1000 MPa minimum tensilestrength strength R m,N,min 400 MPa fatigue constant for the mean stress sensitivity a M 0.35 MPa fatigue constant for the mean stress sensitivity b M -0.1 - fatigue constant for the roughness factor a R,σ 0.22 - surface treatment factor K V 1 - coating factor K s 1 - fatigue constant for the completely reversed normal stress f w,σ 0.4 - number of load cycles at the knee point of the component constant amplitude S/N curve N D 1E6 - number of allowed load cycles ̅ 1E7 - slope of the S/N curve for N < N D k 5 - material safety factor j F 1.35 - load safety factor j S 1.35 - The mean stress correction factor (27) is plotted (and compared with other correction approaches) in appendix B as a function of I y . A quotient of (23) and (27) yields the I y -dependent characteristic stress amplitude: ,1 ( )= | ( )| ( ) (29) The characteristic stress amplitude varies in the defined range of I y between 135 MPa and 507 MPa. 4.2. Component fatigue limit for a completely reversed stress Demanding the spoke surface roughness R z = 100 µm and using the parameters from Table 2 one obtains the roughness factor: =1− , log( /1µ ) · log(2 / , , )=0.68 (30) Assuming a poor surface treatment (see Table 2) and highest possible notch sensitivity (n σ =1), results in a conservatively estimated design factor: =1/ , =1.47 (31) 8