A study on the behaviour of guided wave propagation in utility timber poles

Publication Type:
Thesis
Issue Date:
2014
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Timber is a widely used engineering material because of its availability and good engineering properties. The round timber is suitable for electricity poles, wharfs, piles, bridge piers, etc. There are nearly 7 million utility poles in the current network in Australia, in which around 5 million timber poles are used for distribution of power and communications. The utility pole industry in Australia spends about $40~$50 million annually on maintenance and asset management to avoid failure of the utility lines. Each year about 30,000 electricity poles are replaced in the eastern states of Australia, despite the fact that up to 80% of these poles are still in a very good serviceable condition. In addition, with discovery of scour problems in bridge foundations in the past 30 years, a study on the USA’s national bridge stock showed that out of approximately 580,000 highway timber bridges in the National bridge inventory, about 104,000 of these bridges had unknown foundations depth. Therefore, a reliable non-destructive evaluation technique is essential for the condition assessment of timber poles/piles to ensure public safety, operational efficiency and to reduce the maintenance cost. Different types of non-destructive tests (NDT) were developed during the last decades to evaluate the embedment depth and the quality of materials of embedded structures. Some of these methods have also been utilised for timber piles or poles. However, the extent of knowledge developed on non-destructive tests for timber piles is far from adequate and the effectiveness and reliability of current NDTs are questionable due to uncertainty on materials, structures and environment. In addition, one dimensional assumption is usually considered while dealing with timber poles/piles which is insufficient to reflect the actual behaviour of stress wave propagation in the columnar structures. Also, the anisotropic behaviour of timber and the effects of environment are not taken into account in numerous conventional non-destructive evaluations (NDE) that leads to errors regarding the condition assessment of timber poles. Waves propagating along a pile/pole include different clusters of waves, called guided waves (GWs). In GW, the velocities of a wave (such as phase velocity, group velocity, energy velocity) become a function of frequencies (i.e. wave dispersion behaviour) and displacement magnitude varies when waves propagate along the pole. Besides, GWs that have the same frequencies possess shorter wavelengths than their counterpart of conventional surface wave. Hence, it is possible to detect smaller sized defects with a guided wave technique than a surface wave technique. Hence, it is essential to model the actual three dimensional behaviour of wave propagation inside the timber pole instead of one dimensional assumption, and the environmental factors in conjunction with the actual timber pole situation is necessary to be addressed before suggesting an experimental set up and verification. This thesis investigates the GW propagation inside the timber pole using an analytical, one semi analytical and one numerical method. The actual GW equations are solved analytically considering the timber as both isotropic and transversely isotropic material to emphasize the importance of modelling timber as an anisotropic material. Some parametric studies are also carried out to show the effect of the diversity in material properties of timber on the stress wave propagation. Also, the dispersion curves, mode shapes, contribution of different branches of longitudinal and flexural waves in a signal are presented in order to propose a suitable input frequency and number of cycles, the distances among the sensors, the location and orientation of sensors, etc. Although the analytical GW solution can offer a number of suggestions for the experimental set up, the time domain results cannot represent the actual boundary conditions due to the complexity involved in solving the partial embedment of soil that reflects the actual field behaviour. Besides, the impact location and orientation cannot be implemented in the analytical GW solution. Accordingly, a semi analytical method, namely, Spectral finite element method (SFEM) is employed to model the timber pole with the actual boundary conditions together with the impact location and orientation to illustrate the propagation of different kind of waves and branches. Even though SFEM can explain both the dispersion curves and time domain reconstruction, the dispersion curves are only accurate up to a certain frequency. Further, the three dimensional behaviour is unavailable in SFEM as this method cannot present the wave propagation in the circumferential direction. To overcome this issue, a numerical technique is implemented using the Finite Element method, and based on the signal obtained from this method, the three dimensional behaviour is explained which is then utilized to separate different kind of waves. Beyond that, two popular advanced signal processing techniques are applied to the numerical signals to compare the efficiency of these two approaches leading to determining the wave velocity and the embedment length of the timber pole.
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