Self-reference broadband local wavenumber estimation (SRB-LWE) for defect assessment in composites

Abstract

Local wavenumber estimation (LWE) applied to a full wavefield response is a powerful approach for detecting and characterizing defects in a composite structure. However, the narrowband nature of the traditional LWE techniques brings several challenges for application on actual test cases.This study proposes a self-reference broadband version of the LWE technique. The broadband vibrations are injected using low-power piezoelectric actuators (sine sweep signal) or using pulsed laser excitation in the thermoelastic regime. The out-of-plane velocity response of the surface is recorded using an infrared scanning laser Doppler vibrometer. The dispersive Lamb wave behavior, corresponding to the damage-free base material, is identified from the broadband vibrational response. Using the identified dispersion curves (i.e. self-reference approach), a Lamb mode passband filter bank in the wavenumber-frequency domain is constructed. Searching for the maximum bandpower density in function of the assumed material thickness provides a robust estimate of the effective local thickness of the tested component, and as such yields a detection and evaluation of damage.The performance of the self-reference broadband LWE algorithm is demonstrated on aluminum plates with various flat bottom holes, as well as on cross-ply CFRP aircraft components with a stiffener disbond and barely visible impact damage. Compared to the traditional narrowband LWE approaches, the proposed self-reference broadband LWE method allows a higher level of automation, removes the need for a priori knowledge on the material and/or defect properties, and results in an improved characterization of defects.