Application of Lamb wave and its coda waves to disbond detection in an aeronautical honeycomb composite sandwich

In this paper, a disbond detection method for aeronautical honeycomb sandwich composites using Lamb waves is proposed. Due to the scale mismatching between the wavelength and the panel thickness, the honeycomb core cannot be homogenized to a single layer medium in wave propagating model with short wavelength. Waves can leak through the honeycomb core and be reflected back and forth between the top and bottom interfaces, which forms the coda waves inside the structure. As direct waves and coda waves have distinct propagation behaviors, the corresponding amplitude and velocity features exhibit a different evolution trend varying accordingly to disbond sizes and interface positions. With the experimental pre-knowledge of wave velocities, the local power spectrum density (LPSD) is employed for energy dissipation representation and a windowed cross-correlation method is addressed for time delay estimation, thereby providing quantitative identification for disbond assessment. By judging the damage feature evolution trends of direct and coda waves, disbond underneath both skins can be further detected. Hence, classifying disbond with one-side accessing measurements is realizable. The experiment was carried out on a honeycomb sandwich composite plate cut from the Airbus A330 elevator. Two pairs of PZTs were bonded symmetrically on the skins to illustrate the disbond characterizing method. The result verified the sensitivity of direct and coda waves to the disbond size and the interface position. Instructions for texting scheme and feature extraction in honeycomb structure health monitoring is provided accordingly.