Robust non-destructive inspection of composite aerospace structures by extraction of ultrasonic guided-wave transfer function in single-input dual-output scanning systems

Ultrasonic guided-wave testing is widely utilized for damage detection in plate-like structural components, including composite aircraft fuselage and wing panels. Many guided-wave tests involve transducer scanning to cover finite areas, a task that is most effectively performed by non-contact wave transduction means. The most common guided-wave test implementation consists of a "single-input single-output" scheme. The single-input single-output scheme leads to a transfer function that is convolved with the particular frequency response of the transducers and that of the transducer-to-structure paths (in both excitation and detection). These responses can be unknown or generally variable, especially in non-contact scanning systems and impact excitations. This article proposes a "single-input dual-output" scheme for ultrasonic guided-wave testing in scanning systems that is based on a deconvolution operation. The single-input dual-output scheme better isolates the structural transfer function that is the only property affected by the presence of possible damage. The single-input dual-output scheme was applied to two guided-wave scanning systems under development to detect impact-type damage in stiffened skin-to-stringer panels representative of modern composite aircraft construction. The results demonstrate the dual-output technique and also shed some light on the role of the different frequency bands for the detection of damage at different locations of the skin-to-stringer assembly.