FCGR in Helicopter Blade

The blade of a helicopter rotor during a scheduled inspection has been found affected by a tiny surface crack on the side put in traction the wing lift, as schematized in Fig. 11.39. Its dimensions are a — 0.3 mm depth and Co = I 4 a — 0.42 mm length. The thickness of the metal, an aluminum alloy type 

The two non-dimensional functions F and Q, whose expression are given in Appendix A, depend on the crack size a as shown in the diagram of Fig. 11.40. This means the explicit integration of the Paris-Erdogan equation is rather difficult. It will be used the finite difference integration approach. 

Since the stress is varying from a minimum of 26 MPa to a maximum of 75 MPa, the stress amplitude Ac = 75 — 26 = 49 MPa is applied with an -ratio 

The integration of the Paris-Erdogan Eq. (10.10) by the finite differenced method (see example 1 of Sect. 10.10) is done dividing the possible crack growth interval Aa = 4.5 — 0.3 = 4.2 mm into 30 equal parts in each of which the and the F and Q factors can be considered constant. Results are shown in Table 

To become a through-wall-crack there will be necessary 1.7 10 cycles. Considering that the rotor angular velocity is 200 rpm, it will take 

---