# An Improved Hilbert-Huang Transform for Vibration-Based Damage Detection of Utility Timber Poles
## 1 Introduction
Despite some successful application of using natural frequencies as a feature for vibration-based NDT, extensive research proved that natural frequency alone is a poor indicator of damage for the condition assessment due to its low sensitivity to damage especially when severity is low. [PERSON] and [PERSON] [10] determined the first two natural frequencies of six damaged timber beams and compared against the same of the intact ones. Their results showed that first natural frequency is not appropriate for identifying low-level damage. Numerous researchers proposed that mode shapes and their derivatives are more consistent and better indicator than natural frequencies as they have local information that makes them capable to detect multiple cracks. Moreover, they are less effected by environmental change, for instance, temperature [23]. However, limitations exist in mode shape-based methods. Firstly, a dense array of sensors is required to obtain mode shapes and secondly, they are more susceptible to noise compared to the natural frequencies. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Methods** & **Consistency** & **Accuracy** & **Cost Effectiveness** & **Limitations** \\ \hline Visual inspection & Not consistent & Limited & Cost effective & Highly subjective to the experience of the asset inspector \\ Sounding & Not consistent & Limited & Cost effective & Underground section or top part of the pole is unreachable by asset inspector \\ Drilling & Not consistent & Limited & Cost effective & Underground section or top part of the pole is unreachable by asset inspector \\ Radiography and X-ray & Consistent & High & High cost & Limited to local examinations \\ Ultrasonic tomography & Consistent & High & High cost & Limited computational efficiency \\ Vibration based & Consistent & High & Cost effective & Small damage cannot be detected \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the reviewed non-destructive testing (NDT) methods for this study. Features that utilise natural frequencies and mode shapes together [24], or derivatives of mode shapes; for example, mode shape curvature [25], modal flexibility [26], modal strain energy [27; 28] are found to be effective for damage identification. [PERSON] et al. [10] proposed a modified damage index method for locating damage in timber beams by utilising first five mode shapes of the structure and their derivatives attained from the experimental modal analysis. According to the researchers, the modified algorithm is reliable and economical with few anomalies. [PERSON] and [PERSON] [23] used a damage indicator derived from the difference between mode shapes with and without damage using discrete Laplace transform for detecting multiple damage in timber beams. The damage indicator was sensitive for designed damage scenarios. However, it was unable to assess the severity of damage quantitatively. Moreover, the method was also computationally and economically expensive, since it required 43 impact points for each set of data in the modal tests. [PERSON] et al. [29; 30] adopted a damage index method for a one-dimensional (1-D) system that utilises modal strain energy to detect local damage and decay in timber beams using first two flexural modes. The proposed method found to be effective for single damage scenarios. However, it yielded limited accuracy for the multi damage scenarios. Nevertheless, these dynamic properties are affected by orthotropic behaviour of timber material, various species of wood used for utility poles, changing length and diameter of poles through the distribution network, difference in climate and soil condition, and presence of additional restraints, transformers, cross-arm, and conductors. Furthermore, environmental factors including temperature and moisture content result in difference of material properties even for the same timber species and geometric location.