Fusion of Modal Strain Energy for Health Monitoring of Aircraft Structures

An early prototype of the Vertical Stabilizer Assembly (VSA) of the Shuttle Orbiter was modal tested at healthy and damaged states to study vibrational nondestructive damage evaluation in aerospace structures. Frequency Response and Coherence functions were collected with a Laser Vibrometer at 84 points when the healthy and damaged VSA was shaken with a continuous random force from 0 to 300 Hz. The measurements were used to extract the resonant frequencies and modal shapes for the healthy and damaged states. After pairing of the mode shapes between the healthy and damaged states through the Modal Assurance Criterion and an analysis of the differences in the frequency response functions (FRFs), the strain energy densities of the elements due to modal deformations were determined through a finite element model. The energy densities were normalized so that the total energy of the structure be the same for the mode pairs of the undamaged and damaged sates. The differences in the modal strain energy densities between the healthy and damaged structures provide features that allow for the localization of damage. This is achieved by implementing fusion techniques that combine competing and complementing information obtained from the energy density differences of several mode pairs. This paper evaluates the detectability and performance of two fusion methods to localize damage in the VSA. These are averaging and Bayesian fusion. The performance is based on detectability and the number of false calls