Characterization of the guided wave propagation in simplified foam, honeycomb and hollow sphere structures

In recent years, researches about the capability of ultrasonic guided waves for detection of damages in cellular lightweight structures have been done. Besides the experimental studies, numerical approaches utilizing the finite element method are used to investigate the propagation and the interaction of guided waves in such complex material systems. To reduce the computational efforts, simplified models based on a homogenization technique are often used. The complicated heterogeneous cellular mid-core is simplified to a simple orthotropic layer modeled with eight-node 3D brick elements. However, it has been reported that this approach has specific limitations. The compact design of the simple homogeneous layer does not interact with the ultrasonic guided waves in a similar way as the real extended cellular structures do. These limitations are more obvious for the specific range of frequencies where the wavelength are smaller than the characteristic length scale of the cellular structure and the ultrasonic guided waves are more influenced by the microstructure. Therefore, a new kind of simplification approach based on geometrical parametric study is suggested in this paper. In this new kind, instead of material homogenization the main aim is to simplify the structural geometry. The proposed method has been used to simplify different cellular structures. The results of ultrasonic guided wave propagation are compared with the complex geometrical models. In order to prove the proposed approach, the wave propagation in a simplified hollow sphere sandwich plate is compared with experimental results. In addition, an application example of the proposed simplification approach for structural health monitoring in a hollow sphere structure is provided. Moreover, the experimental results for structural health monitoring gained by laser scanning vibrometry are presented and it is shown that the wave field interacts with damages. Finally, a computer tomography of the plate is also made in order to visualize debondings between the core material and the cover plates