The objective of this research task is to implement a model to make it possible to simulate the fatigue behavior of a helicopter blade by modal analysis. A study of the aerodynamics of the helicopters was made though the determination of the aerodynamics loads applied to the main blade of helicopter and extract their frequencies and eigen modes with crack and without crack. The expressions of the different energies and virtual work from elements of the blade are developed. A finite element model was defined in order to study the vibratory phenomena. This model makes it possible to extract the Eigen frequencies and modes shapes of the blade, and to calculate the stresses which act on the structure.
The Eigen frequencies of helicopter blade were decreased after cracking in the critical zone, and this reduce is nonlinear; however the stress increased with crack propagation. Therefore the modal analysis is an important factor for the detection of fatigue in aircraft structures