Ultrasonic guided waves defect signatures for damage identification in composite aerospace structures

Damage growth and tolerance are major drivers to determine the frequency and cost of inspections and the lifespan of structures. The detection and identification of damages together with the assessment of structural integrity by non-destructive evaluation (NDE) methods is of fundamental importance and requires analysis towards effective solutions. Among the numerous NDE techniques, ultrasonic guided waves (UGWs) are acknowledged as one of the most effective tools to identify damage in composite structures, offering promising results and research opportunities for their nonattenuative nature and complex behavior. Generating from a well-defined interaction of ultrasonic waves with bounded media, UGWs become “naturally” supported from the structure itself and show high susceptibility to interference on a propagation path (i.e. damage or boundary) providing many advantages as large-area coverage, crosssectional area inspection, sensitivity to multiple defects and cost-effectiveness. The implementation and interpretation of UGWs, though, are very challenging, due to the infinite set of mode-frequency combinations to probe structures with. The propagation of UGWs into composite materials (multilayered, anisotropic), aircraft geometries (thin uniquely shaped components, i.e. curved skin, stiffeners, etc.) and interaction with composite impact damage (3D, highly heterogenous, load-structure dependent) add layers of complexity, as wavemode conversion and coupling, and nonanalytical solutions. More studies are needed to advance knowledge in UGWs interaction with specific composite damage modes to fully exploit the potential of this advantageous yet complex inspecting modality. Building on the experimental results of UGWs inspection by transfer function extraction in damaged aircraft panels, this work focuses on the analysis of UGWs measurements to identify impact damage signatures in a variety of impact damage energy levels and on two panel locations.