Diffusive subsurface Rayleigh mode induced by embedded ultrasonic waveguide

Irregularity on wood surface has generated unwanted variation in the stress wave propagation. In turn, it reduces the efficacy of ultrasonic based non-destructive evaluation (NDE) for wooden structures. This study proposes a novel alternative by inserting a waveguide (2 cm depth with 5 mm in diameter) into the medium to excite subsurface oscillation. This work develops a closed-form analytical formulation of the elastodynamic problem using the proposed embedded waveguide technique. The analytical solution is based on the classical Navier’s formulation. The result produces two interfering wave modes embedded in the displacement field. These two phasors are termed fast and slow modes. They induce apparent 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. This finding is different from the conventional contact-based Rayleigh excitation used in the metallic medium. A developed numerical model using the semi-explicit differential-algebraic equation (DAE) technique is discussed. The simulated displacement fields are analyzed and compared with the analytical result.