An ultrasonic scanning system for the inspection of composite stiffened panels from elastic constant identification via inversion of guided waves

Abstract

This paper describes an ultrasonic scanning system for the inspection of stiffened composite panels used in modern aircraft construction based on elastic constant identification. Conventional ultrasonic scanning systems track individual features of the propagating waves (e.g. amplitude, arrival time, etc.) to detect damage by tracking the associated wave scattering. An ability to map the elastic constants of the composite part can greatly enhance the sensitivity of the scan to structural damage that can be detected and quantified by a corresponding reduction in the stiffness components. Elastic constants can be also correlated to residual strength. The proposed scanning system utilizes a “single-input-dual-output“ (SIDO) scheme whereby ultrasonic guided waves are excited by an impact and detected by two air-coupled ultrasonic sensors. The phase velocity dispersion curves of the waves are experimentally measured under a single excitation at each scanning line using a phase-spectrum technique. These curves are then inverted using a Semi-Analytical Finite Element (SAFE) method as the wave propagation forward model and Simulated Annealing as the optimization routine. This approach yields estimates of several elastic effective constants of the composite panel at each scanning line. These constants are then related to impact damage in stiffened stringer-skin composite panels.