Fatigue Crack Growth Analysis on a Rotor Blade Under Forced Response

This paper focuses on the fatigue crack growth resulting from an aeroelastic behavior of a fan blade when operating under upstream distortion. The forced response of the first bending mode of the blade due to the inlet distortion is analyzed and the mechanical stability of blades is investigated. The forced response of one blade is evaluated using an uncoupled approach. In this approach, the forced response is calculated in four steps. First, the modal analysis is obtained using Finite Element (FE) calculations. In the second step, the aerodynamic damping is obtained by performing the CFD (Computational Fluid Dynamics) simulation for a single rotor blade with a prescribed harmonic forced motion. The next step is the estimation of the excitation forces when the unsteadiness of the inlet flow has a frequency close to the eigenfrequency of the blade. In the end, by solving the equations of equilibrium forces of the structure, the forced response is computed. Afterwards, a fatigue crack growth analysis is performed. The crack is assumed to initiate in the area of the maximum principal stress. The crack is inserted into the FE model using the Extended Finite Element Method (XFEM) [1, 2] which is implemented in an in-house plugin “Morfeo-crack” for Samcef (commercial finite element analysis software package). This method allows for easily inserting a crack while minimizing the difficulties inherent to the mesh adaptation since the crack does not need to be explicitly meshed. The calculations are performed under the Linear Elastic Fracture Mechanics hypothesis. Finally, the crack path as well as the lifespan are estimated.