Stress Analysis of Composite Rotor Blades

The design of Composite Rotor Blades requires the analysis of tridimensional stress states including interlaminar stresses. Despite the powerfulness of modern computers, standard tridimensional finite elements approximations of the entire rotor blade are not yet considered feasible, because of the high degree of accuracy required in the material properties of the blade cross section. As a consequence, the problem is generally formulated in two different consecutive steps. The first step considers the stress analysis of the blade cross section. This is modeled as a two dimensional continuum, and the following analyses are performed: - Eigensolution analysis of self equilibrated modes and of the diffusion lenghts; - Particular solutions under prescribed stress resultants. The second step is mainly devoted to the dynamical behavior of the entire blade. Here the blade is usually considered as a one dimensional continuum. Under this approximation, in general, the following computations are performed; - Trim solution under steady flight conditions; - Linear stability analysis of Floquet's type. The subdivision of the problem into two steps is equivalent to the metod of separation of variables first proposed by De S. Venant which, as it is well known, is exact only for slender, straight and untwisted beams under applied loads to the edges, but it is also applicable to curved, twisted and swept beams undergoing small strains. The present discussion focuses on the cross section analysis. This is usefull even if the overall behavior of the beam is geometrically non linear, as it happens in helicopter blades.