The effect of moisture content and temperature on the propagation characteristics of guided waves in timber utility poles-numerical and experimental validation

Abstract

Ultrasonic-guided waves (GWs) have shown a high potential to be applied to the structural integrity of timber utility poles to detect and assess distinct types of defects. Yet, there are many challenges associated with this method, which may hinder its application, including the orthotropic nature of the timber, the presence of natural cracking, and the effect of environmental factors such as temperature and moisture content (MC) on the propagation of GWs. This study aims to scrutinize the effect of MCs ranging from 0 to 24%, and temperatures between − 20 and 100 °C on the propagation characteristics of GWs (longitudinal and circumferential) in cylinder timber structures excited over a range of frequencies between 10 and 20 kHz, experimentally and numerically. The numerical analysis was carried out using COMSOL Multiphysics, and Macro Fiber Composites were used to excite and sense the GWs. The anisotropic nature of timber poles was modeled using a transversely isotropic behavior. The results showed that the effect of timber’s temperature and MC on the GWs should be assessed simultaneously. Three-dimensional maps were generated to present the relationship between various wave modes, temperature, and MC. The work observed a larger critical role of MC than the temperature on the propagating GWs and timber’s material properties. The effect of temperature is more critical when timber’s MC increases above the fiber saturation point (FSP) due to high stiffness variations. This is seen as the group velocities of the longitudinal and flexural waves (as well as the bulk wave) shifted more at MCs above FSP than below FSP. When MC varies above FSP with no temperature variations the velocity difference is negligible with 1.5% due to complete timber saturation at all temperature values. The results showed that the change in the mode velocities in dry wood is not significant with varying temperatures, as the stiffness changes by 0.07%. Experimental validation, for the numerical results, showed low differences for the bulk wave, and flexural modes were within [7.1 15]% and [1.3 4.7]% for the 2nd flexural branch at 12.5 kHz, respectively. Based on the above results, the environmental conditions can highly impact the GWs characteristics in timber structures, hence this should be carefully considered in the application phase.