Investigation of 3-D Strains on Highly-Twisted Swept Tip Composite Rotor Blades

This paper presents an investigation of the effects of swept tip on highly twisted composite rotor blades. The focus is on detailed three dimensional (3-D) strain measurement. Both vacuum chamber tests and 3-D finite element method (FEM) analysis were carried out in the investigation. Strain distribution on three sets of in-house fabricated composite blades: straight, twisted, and twisted-swept were studied using strain gauges in a rotating environment in a vacuum chamber. A set of aluminum beams were also tested as an elementary baseline to aid in fundamental understanding. A prediction study of 3-D strain patterns were conducted by X3D: a 3-D structural dynamic solver. Correlating measured strains with the predictions showed overall good agreements. Evidences of strong lead-lag bending moment counteracting the axial extension are found in both aluminum beams and composite blades. Strain patterns of aluminum beams and composite blades have some superficial similarities, but they are not the same. An in-plane shear strain concentration is an unique phenomenon in the swept tip composite blades.