Understanding the guided waves propagation behavior in timber utility poles

Abstract

Guided stress waves are considered one of the most efficient and reliable techniques that provide sufficient quantitative and qualitative assessment. In this study, we focused on scrutinizing the propagation behavior of guided waves in western white pine timber poles, experimentally, and numerically using COMSOL Multiphysics. Macro fiber composites (MFCs), due to their flexibility and convenience to install on curved profiles, were used to actuate and sense guided waves along the tested specimens. Various solutions for wave mode tuning and characterization have been tested for traction free and embedded boundary conditions. The behavior of propagating wave modes was analyzed and compared in the two boundary conditions tested. Also, the excitation frequency, based on the dispersion curves generated for transversely isotropic timber, was selected to ensure the presence of favorable propagating (for instance longitudinal modes) modes with minimal dispersion. Undesirable wave modes—such as flexural modes (non-axisymmetric)—were eliminated by a ring design composed of multiple MFC actuators coupled around the pole’s circumference. The remaining propagating longitudinal modes and their reflections, such as modes L(0,1) and L(0,2) propagating at nearly 1000 m/s and 800 m/s respectively, were significantly enhanced by the actuation of the ring which could be effectively used for the assessment process. The results demonstrated the complexity of the propagating modes in circular timber structures and the importance of the ring design in the excitation of the selected modes of interest and damping unwanted ones.