New methodology for NDT assessment of in-service wood poles

Abstract

Wood poles are extensively used in North America to support electrical lines and their service life is usually between 35 and 50 years. Nevertheless, factors such as temperature, moisture, bacteria or fungi may induce internal decay in the poles. The existence of decayed areas result in a reduction of the strength of a pole and sudden failures may occur. To ensure the reliability of the electrical network, the condition of the poles is assessed in maintenance programs usually through visual and sounding inspection. These methods have the disadvantage that they are subjective, they provide a local evaluation of the condition, and are unable of detecting early decay. This paper summarizes the results of a research project that included the development of a new methodology to assess the condition of wood poles based on ultrasonic waves, the construction and calibration of the testing device, numerical modeling of wave propagation in wood, laboratory and field tests. Methods based on changes of wave velocity have been used before for the evaluation of wood. However, these methods have not taken into account that wood presents different elastic properties in the longitudinal, radial and tangential directions. To perform a reliable evaluation using ultrasonic waves, it is essential to consider the variability of stiffness and mass density, as well as the effect of moisture content and temperature on wave parameters. The proposed methodology considers wood as an orthotropic material, takes into account the variability in the elastic and mechanical properties, and the effect of temperature and moisture content in these properties. The methodology uses two statistical indices: the overall dissimilarity index (ODI) and the condition rating index (CRI). ODI describes the overall condition of a pole in terms of ultrasonic measurements and estimates the deviation with respect to the expected values for a sound pole. Laboratory destructive testing was performed to correlate the CRI with the expected remaining strength ratio; which is used to determine the end of-life (EOL) of an in-service wood pole. The method has the advantage that is nonintrusive, provides quantitative measurements of the internal condition of wood poles and allows detecting early decay in wood poles.