Abstract
In this study, vibration based non-destructive testing (NDT) technique is adopted for assessing the condition of in-service timber pole. Timber is a natural material, and hence the captured broadband signal (induced from impact using modal hammer) is greatly affected by the uncertainty on wood properties, structure, and environment. Therefore, advanced signal processing technique is essential in order to extract features associated with the health condition of timber poles. In this study, Hilbert–Huang Transform (HHT) and Wavelet Packet Transform (WPT) are implemented to conduct time-frequency analysis on the acquired signal related to three in-service poles and three unserviceable poles. Firstly, mother wavelet is selected for WPT using maximum energy to Shannon entropy ratio. Then, the raw signal is divided into different frequency bands using WPT, followed by reconstructing the signal using wavelet coefficients in the dominant frequency bands. The reconstructed signal is then further decomposed into mono-component signals by Empirical Mode Decomposition (EMD), known as Intrinsic Mode Function (IMF). Dominant IMFs are selected using correlation coefficient method and instantaneous frequencies of those dominant IMFs are generated using HHT. Finally, the anomalies in the instantaneous frequency plots are efficiently utilised to determine vital features related to pole condition. The results of the study showed that HHT with WPT as pre-processor has a great potential for the condition assessment of utility timber poles.
Highlights
Timber is a commonly used industrial material due to its accessibility and wide variety of mechanical and physical properties, such as higher strength to weight ratio, electrical, and heat insulation and having a simpler and faster installation process [1]
Empirical Mode Decomposition (EMD) is capable of decomposing a complex multicomponent signal into nearly mono component signals which are referred as intrinsic mode functions (IMFs)
The mean value of upper and lower envelope is calculated which is denoted as m1(t), and the difference between the signal x(t) and m1(t) is the first component of shifting process denoted as h1(t) [38] as follows, x(t) − m1(t) = h1(t) where h1(t) should satisfy the condition of IMF but ideally that is not the case. h1(t) is considered as new signal and the whole shifting process is repeated until h1(t) satisfy the condition of a true IMF, denoted as c1(t)
Summary
Timber is a commonly used industrial material due to its accessibility and wide variety of mechanical and physical properties, such as higher strength to weight ratio, electrical, and heat insulation and having a simpler and faster installation process [1]. Use of these methods are limited due to high cost, limited computational efficiency, and complexity in the operation [4] To overcome these limitations, variety of non-destructive testing (NDT) methods have been investigated and developed by the engineers and researchers in the past two decades to provide accurate information on the condition of timber pole/pile structures such as wave based, and vibration based, NDT methods. Vibration based method has been utilised for non-destructive evaluation of timber poles to assess the underground and top part of the pole since it has higher propagating energies.
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