Damage Detection for Composite Panels by Frequency-Domain Reverse-Time Migration

Damage in aircraft structures can grow due to cyclical operational loads. Structural health monitoring aims to detect damage and reduce revenue losses. Advanced techniques estimate defect characteristics, enabling informed maintenance scheduling. Among the various techniques developed, guided-wave inspection is particularly effective for high-resolution defect characterization over large areas. In addition, reverse-time migration, a method originally developed to identify reflectors in the Earth, has been effectively adapted for guided-wave applications. Many implementations of this method rely on finite element and finite difference methods to discretely evaluate the migration operator and apply it to ultrasonic data. However, the migration operator can also be precomputed in the frequency domain for more rapid application, provided that Green's functions are known over a specific frequency range. In modern aircraft structural applications, Green's functions must account for the anisotropy of composite materials. A frequency-domain reverse-time migration framework is presented in this paper for damage location and sizing in composite structures, where Green's functions for the out-of-plane displacement of a composite laminate are estimated to locate defects. This guided-wave damage-detection framework is implemented for defect location and sizing in a unidirectional carbon-fabric laminate. To facilitate this, a temporary ``defect'' is introduced by placing a pair of neodymium magnets on opposite faces of the panel to locally deform the material. The location and size of the magnet pair are then estimated using the developed framework.