A Strain-Based Experimental Methodology for Measuring Sectional Stiffness Properties of Composite Blades

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Sectional stiffness properties of composite rotor blades, such as the axial, torsional, and bending stiffness, play an important role in the design and analysis of rotorcraft, as they impact the predicted rotor dynamics, structural loads, and stress and strain fields. Multiple numerical tools exist for predicting the sectional stiffness properties based on the cross-sectional geometries and materials; however, rotor blades are often made of composite materials and require complex manufacturing procedures, making it challenging to provide an accurate prediction of the inputs needed for these numerical tools. This dissertation therefore focuses on developing an experimental technique for calculating the sectional stiffness properties, based on the measurement of a detailed strain field using digital image correlation.

Two primary features distinguish the developed methodology from currently existing techniques. First, this method is based upon measured strain fields, as opposed to measurements of displacements or frequencies that are traditionally used. The strain field provides a description of the local deformation of the blades, thereby allowing measurements of the sectional stiffness properties to be made at discrete spanwise locations along the blade and providing the capability to predict changes in properties due to features commonly associated with helicopter rotors, such as twist or taper. Second, this method can be used to calculate the full cross-sectional stiffness matrix based on a combination of experimental measurements and a numerical warping function, as opposed to only a subset of these properties that are typically measured in displacement or frequency based approaches.

The developed methodology is first validated using numerical results from 3-D FEA for cross-sections that have received extensive study in literature. A test setup is then developed for experimental implementation of the proposed method and applied to five different beams. The material properties and geometries of these beams were selected to place an emphasis on the unique capabilities of the method, including the measurement of elastic coupling stiffness components and spanwise variations in the stiffness properties, with results compared against analytic solutions and predictions from SectionBuilder. A detailed uncertainty analysis is also implemented for estimating the impact of measurement errors on the stiffness properties.