Damage Identification of Chordwise Crack Size and Location in Uncoupled Composite Rotorcraft Flexbeams

Abstract

The appearance of cracks and delaminations in composite rotocraft flexbeams can lead to degradation in flapwise and lagwise performance of the rotor blade. In addition, cracks in composite rotorcraft flexbeams under cyclic loading can result in rapid fatigue failure of these elements. A novel detection strategy, which attempts to use structural waves to locate damage in the form of a crack, is evaluated in this work. Previous approaches have focused on modal response and damage detection methods. These techniques are incapable of detecting small defects or flaws in structures. However, a continuum mechanics description of the local structural dynamics provides greater resolution in the ability to detect small flaws. Such a model is employed here to determine the location and depth of a crack by examining the frequency domain properties of the scattering coefficient. The magnitude and phase of this parameter contains properties that capture the size and location of the crack. The modeling technique can also be applied to beams under rotation. The additional centrifugal force is modeled as an increase in the stiffness and the analysis is repeated. The analysis captures the trends observed experimentally. A least squares optimization is performed to determine both crack depth and location. Analytical predictions are confirmed on an experimental [0/⁹0]s uncoupled graphite epoxy flexbeam of approximately 16 inches in length. Analytical and experimental results indicate that this approach is able to locate the crack and approximate depth under centrifugal loading. The model predicts the location within 7% and approximates the depth to within 20% under various rotation rates.