Health monitoring of timber poles using time-frequency analysis techniques and stress wave propagation

Stress wave propagation can be effectively used as a non-destructive testing technique for the condition assessment of timber utility poles. Stress waves can be generated by applying a transverse impact close to the ground level of the pole, within the comfortable reaching height of the inspectors. The material behaviour of timber, the presence of natural imperfections and soil-pole interaction generate complexity in the propagation and reflection of generated transverse stress wave. Furthermore, the nonstationary nature of the reflected stress wave creates difficulties in identifying the defect features (i.e., location and severity of the defect) in the time domain. However, the time-frequency representation (TFR) can reveal local features of a reflected stress wave signal in both time and frequency domains simultaneously and hence, it can be effectively used to extract information to assess the health of timber poles. This paper presents a review of widely used TFR techniques in structural health monitoring and evaluates the applicability and efficiency of those methods in the context of structural health monitoring of timber utility poles. The signals collected from laboratory pole experiments and in-situ poles within power distribution networks along with numerical simulations were used to evaluate the performance of different TFR. A numerical simulation of the pole-soil system with different defect types and levels was carried out using the finite element method. Finally, the effect of different defect types, locations and applied impacts on the performance of different TFRs were evaluated.