Diffusive subsurface propagation of Rayleigh mode induced by embedded ultrasonic waveguide

Abstract

Surface degradation in the wood surface has generated unwanted variation in the ultrasound and reduced the efficacy of ultrasonic based non-destructive evaluation (NDE). This study proposes a novel alternative by inserting a waveguide into the wooden medium to eliminate the impact of surface variation on the characteristics of the ultrasonic propagation. This technique creates a small circular insertion of 2 cm depth with 5 mm in diameter, which has been accepted by many utility inspection firms for structural health monitoring (SHM) of wooden utility grids. This work develops a closed-form analytical solution of the elastodynamic problem using the proposed embedded waveguide technique. The resulted solution based on the Navier's formulation discovered two interfering wave modes embedded in the displacement field. These two modes termed fast and slow induce diffusive displacement propagation as a function of the Poisson's ratio of the medium. The analytical result also shows a greater elastic energy penetration as a function of the insertion depth. Resulted wave characteristics are different from the traditional contact-based Rayleigh excitation. A developed numerical model using the semi-explicit differential-algebraic equation (DAE) technique with both steady-state and transient load conditions is demonstrated. The simulated displacement fields are analyzed and compared with the analytical result.