Low frequency ultrasonic guided wave propagation through honeycomb sandwich structures with non-uniform core thickness

Honeycomb core sandwich structures (HCSSs) are used prevalently in the aerospace and marine industries. Use of guided wave-based inspections has attracted much attention in the recent past for facilitating the NDE of such structures. This paper presents the numerical and experimental analyses of the guided wave propagation through a tapered HCSS, with a primary motive to comprehend and further utilize the alteration of local wave properties as a function of the varying thickness. To this end, a low frequency fundamental antisymmetric mode is used as an interrogation signal. The numerical analysis is performed using 3D finite element simulations, while the experiments are performed on the portion of a helicopter blade with GFRP face sheets and aluminium core. The generation of the global guided waves (GGWs) as well as continuous variation of the local wave properties (wavenumber, group velocity, and phase speed) are ascertained both numerically as well as experimentally. Least-square polynomial fits are presented for the variation of group velocity along the propagation direction. A method of predicting the time of flight utilizing a non-uniform group velocity is outlined. Using this method and the polynomial fits, predictions of time-of-flight of the fundamental antisymmetric mode are made and corroborated with the observed values. Further, to circumvent the need of the prior knowledge of the group velocity variation in predicting the time-of-flight, a generalized expression of the variation of the group velocity is developed by employing the adiabatic nature of the GGW. The present analysis and the proposed methods can find their potential use in the wave-based NDE of tapered HCSS.