NON-DESTRUCTIVE EVALUATION FOR NEW WOODEN POLES QUALITY ASSESSMENT
Authors: Jean-Luc Sandoz, Dr. Ing. Yann Benoit, Dr. Ing.
PROVISORY VERSION
INTRODUCTION The selection of wood poles used as supports for telecom or power overhead lines is visually operated. National or companies standards define the wood poles mechanical classes based, for each species, on some material singularities (knots, slope of grain, annual rings...) and intrinsic properties (length, diameter, weight...). If this visual grading can be considered as a first approach for wood poles grading, this method is subjective and is not sufficient for the overhead lines asset management. Indeed, due to the large variation of wood mechanical properties within a same species, a subjective grading can let some weak components to be installed in a network. By snowball effect, a weak component can be fatal to the entire network, causing many technical, human or financial troubles. After 15 years experience of non destructive technologies for both new and in-field wood poles, CBS-CBT can present concretely the advantages of selecting the best poles to be installed in the networks in terms of maintenance costs and reliability aspect.
WOOD MATERIAL Wood is produced by the forests and its growing conditions affect strongly its mechanical performances. Even from a same species, the mechanical properties can vary from 1 to nearly10. Concerning the wood poles, this variation can be confirmed thanks to destructive tests presented by the figure 1. 240 new wooden poles from the same species (Spruce-Fir) have been broken until failure (Cantilever protocol). The Cantilever bending strength (MOR) has been obtained for each broken pole. If the lowest mechanical property is around 10 N/mm2, the strongest one is up to 80 N/mm2.
Figure 1 shows the allowable strength limits for both, Switzerland (15N/mm2) and France (18.3N/mm2). The figure underlines the fact that some new wooden poles are weaker than the codes expected values.
Figure 1:
240 new wooden poles tested until failure to determine the Cantilever bending Modulus of Rupture (Spruce and Fir).
NON DESTRUCTIVE TECHNIQUE FOR NEW WOOD POLES GRADING In order to evaluate the initial mechanical properties of new wooden poles, a non-destructive technique using ultrasonic can be used. This method, developed in the 80ies, has been transferred into the Sylvatest Duo technology. Thanks to two transducers, an emitter and a receiver, the ultrasonic velocity is measured and is translated into mechanical performances. Figure 2 presents the concept of the ultrasonic measurement for a new wood pole.
Sylvatest Duo
O E
Mechanical performances of the pole
Transmitter
Wood pole
Receiver
Figure 2:
Scheme of a non-destructive measurement using ultrasonic (Sylvatest Duo) for new wood pole mechanical performances.
The correlation between the measured ultrasonic velocity and the mechanical properties has been obtained, for each species, by destructive tests as illustrated by the figure 3.
Spruce - Fir (CH+F)
90 Cantilever bending strength [N/mm2] 80 70 60 50 40 30 20 10 0 4000
y = 0.0222x - 61.595 R2 = 0.5493 n=60
4200
4400
4600
4800
5000
5200
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