The Effect of Grains Orientation, Moisture, and Temperature on the Propagation of Guided Waves in Timber Structures towards Integrity Assessment

Abstract

Timber is a natural material widely used in various engineering applications, such as in construction and infrastructure due to its availability in nature, durability, integration into the environment, being renewable, and favorable mechanical properties. A very common application is in timber utility poles. Timber poles are prone to several types of defects such as fungus within the embedded part where moisture is high, and that leads to complete material degradation of the heartwood region (heart rot). Any detrimental change in the material properties firmly affects its performance and might yield structural failure. Therefore, it is essential to assess the effect of temperature and moisture on the timber material characteristics, in turn, assess its structural integrity. The application of ultrasonic guided waves (UGW) proven to be a promising technique in structural assessment due to their ability to propagate in complex structures and over long distances. The propagation characteristics of UGWs are highly impacted by the mechanical properties of the medium they are propagating in, especially when used for applications such as timber utility poles. The scope of this work first focuses on understanding the effect of moisture content (MC), temperature, and grain orientation on timber’s viscoelastic properties. Dynamic mechanical analysis (DMA) is employed to measure the viscoelastic properties (storage modulus, loss modulus, and loss factor) of timbers with 0% MC to above the fiber saturation point (FSP) over -20ºC to 100ºC for longitudinal, tangential, and radial specimens. Also, timbers with elevated MCs are considered, having >100% MC and are studied over +20ºC to 100ºC. Following this, the effect of the viscoelastic properties under critical moisture and temperature conditions on the UGWs propagation were scrutinized. This work concentrates on dominant UGWs that are effective and sensitive in structural health monitoring (SHM) of timbers using contact and non-contact techniques. Numerical methods were used to explore a wide range of MCs and temperature on the UGW (longitudinal and circumferential) propagation in timber utility pole over a range of frequencies and are validated experimentally. The numerical analysis is carried out using COMSOL Multiphysics, and Macro Fiber 2 Composites (MFCs) are used to excite and sense the GWs. Furthermore, the effect of heart rot on the propagation characteristics of UGWs is investigated, with the aim of identifying and localizing internal damage of varying sizes. In this experiment, various actuation configurations are explored, including single actuation, 4-ring actuation, and an 8-ring actuation, since the MFC ring can tune and improve the propagating wave modes in particular longitudinal modes and cancel the flexural modes. The DMA results show that the storage modulus, loss modulus, and tan δ decrease with increasing temperature and MC, and the reduction is larger transversely than longitudinally due to the effect of grain orientation. The effect of MC is more pronounced on the viscoelastic properties than temperature. Studying the effect of MC on GWs shows that as timber’s MC increases, the propagating GWs arrive in longer times and reduced velocities as a result of timber’s lower storage modulus. The A0 mode proved to be the dominant GW. The prevailing effect of MC is also observed in numerical results, in influencing both the propagation of GWs and the material properties of timber. Whereas the temperature shows significance influence when timber’s MC increases above the 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 (BW)) shifted more at MCs above FSP than below FSP. Lastly, varying sizes of internal damage are identified and localized experimentally using the reflection of L(0,1) mode actuated using 4-ring and 8-ring. The MFC ring actuation is able to tune the propagating signal, and the L(0,1) mode converged and arrived in a more stable and analyzable signal. 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. From the results, the UGW shows high promise in terms of characterizing viscoelastic behavior, with high sensitivity along various grain orientation. Also, UGWs have a high potential for investigating and localizing internal damages in timber utility poles.