An Ultrasonic Scanning System for Damage Detection in Composite Built-up Panels by Inversion of Guided Wave Dispersion Curves

This paper proposes an ultrasonic scanning system for the inspection of stiffened composite panels used in modern aircraft construction. Conventional ultrasonic scanning systems usually track individual features of the propagating waves (e.g. amplitude, velocity, etc. at specific frequencies). The proposed scanning system, instead, is able to track the full dispersion curve across a significant frequency range, and, through an inversion process, to identify the elastic constants of the panel at each scanning position for an enhanced inspection. The proposed scanning system utilized a "single-input-dual-output" (SIDO) scheme whereby ultrasonic guided waves are excited by an impact and detected by two air-coupled ultrasonic sensors. At each scanning point, the system extracts the phase velocity dispersion curves of the panel via a phase spectral analysis of the measured waveforms. The measured dispersion curve is then fed to an inversion algorithm that identifies the composite engineering constants through an optimization loop. The Semi-Analytical Finite Element (SAFE) method is used as the forward model in the inversion procedure. Validation experiments were performed on a stiffened skin-to-stringer CFRP panel with impact-type damage present on the hat shape stringer cap. The system showed an ability to detect the internal damage with access only to the outer skin. The ability to track the elastic constants of the test part is quite relevant to the ultimate goal of determining the part residual strength using the ultrasound scanning system.