PSI - Issue 75

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

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Using (31) one obtains the component fatigue limit for completely reversed stress: = , / , = 291 MPa where f W, σ is the fatigue strength factors of completely reversed normal stress. 4.3. Degree of material utilization The S/N curve of type 1 used in the FEM is modified according to the Haibach rule. It follows for the fatigue strength factor [35], [36]: =( ̅ ) −(2 −1) =0.774 (33) where N D = 1E6 is a number of load cycles at the knee point of the S/N curve and ̅ is a number of allowed load cycles accordingly. In the current dimensioning ̅ is set to 1E7 (cf. Table 2). The quantity k in (33) is the slope of the S/N curve for N < N D . Making use of (29), (32)and (33)follows the cyclic degree of material utilization: ( )= ,1 ( ) , / (34) where j D is the overall safety factor. In the considered case j D consists of the material safety factor j F = 1.350 and the load safety factor j S = 1.045 (see Table 2). = =1.411 (35) Whiles the chosen j F corresponds to FKM recommendation for a severe consequence of failure and a regular inspection, j S is an arbitrary choice including the calculation accuracy. The Fig. 6 shows the cyclic degree of material utilization as a function of the second moment of area in the range between and . (32)

Fig. 6. Cyclic degree of material utilization

Each dot in Fig. 6 corresponds to a single calculation of a BK for a chosen I y . For I y = 5.4 cm 4 an inflexion point of a BK is to be seen in Fig. 6. This I y results in the natural frequency f 2 being close to the excitation frequency of the propeller unbalance f exc (cf. results in section 3). Finally, for = = 6.9 cm 4 one implicitly obtains (under a chosen safety factors) an optimal material utilization i. e. a BK = 1. 5. Definition of the spoke geometry Four parameters of the spoke geometry (length L , depth H , width b , web thickness a – see Fig. 1) are constant values listed in Table 1. In the current work L corresponds to the half of the outer stator diameter of the EM. It enables a relatively convenient connection to the aircraft. H is chosen slightly smaller than the axial dimension of existing bearing arrangement and b promises giving sufficient space for enabling the mounting pad assembly. Making use of the previously estimated it follows according (6) to for the distance between fillets: h = 28.9 mm.