Experimental and Numerical Methods for Detection of Voids in Wood Poles Using Ultrasonic Testing: Fid-Bau Portal

Experimental and Numerical Methods for Detection of Voids in Wood Poles Using Ultrasonic Testing

Tallavo, Fernando / Pandey, Mahesh D. / Cascante, Giovanni

Utility wood poles are extensively used to support electrical-transmission and distribution lines in North America. Wood poles are exposed to extreme weather conditions, making them vulnerable to internal deterioration resulting in loss of pole strength, which can compromise the reliability of energy supply to customers. Ultrasonic testing is a nondestructive method that has been used for evaluation of the internal condition of in-service wood poles. However, the current assessment criterion is mostly based only on the first arrival of compression waves. This paper presents experimental and numerical methods for the detection of voids in wood poles using ultrasonic waves; the wood is considered as an orthotropic material. This investigation involves ultrasonic testing of a red-pine pole without and with a void. Changes in both compression wave velocity and wave attenuation are used to identify the presence of a void. An orthotropic finite-element model, calibrated with experimental results, is used to study the propagation of ultrasonic waves in a cross section of red-pine poles. Experimental and numerical results show that for a void–pole diameter ratio of $ø_{void} / ø_{pole} = 0.2$ and frequency of 50 kHz ( $λ = 1.7 ø_{void}$ ), the wave velocity across the pole decreases an average of 20%, whereas the wave is attenuated by a factor of 10. Thus, the consideration of wave attenuation in addition to wave velocity is critical to improve the reliability of current methodologies. The compression wavefront shows a triangular shape before reaching the pith of the wood pole, after which it shows a circular shape, as in the case of an isotropic material. In addition, the surface wave shows displacements mostly in the radial direction instead of the typical retrograde ellipse in an isotropic medium.