Development of a vibration-based non-destructive testing method for in-service utility poles

Jalali Nodoushan, Mohammadhadi (2021) Development of a vibration-based non-destructive testing method for in-service utility poles. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Wooden utility poles for electric networks are widely used, with approximately two million poles in North America. A reliable and cost-effective non-destructive testing (NDT) method is necessary for strength evaluation during the life span of the poles. To improve an emerging modal testing-based NDT method, this thesis develops a novel method to measure natural frequencies and damping ratios of poles even though they are connected to conductors. This thesis makes contributions to the wood pole NDT state of the art in two major areas – analytical and numerical modeling of pole-cable systems, and frequency-domain decoupling methods to identify pole properties despite their connection to cables. Improved analytical models of the “cable-beam” system were developed in order to understand the coupled vibration behavior of the system. Bending stiffness and sag of the cable were considered in the modeling and the effects of them on vibration behavior of the system was studied. Two-dimensional and three-dimensional dynamic models using bond graph method were developed for vibration of stranded cables and vibration of the cable-beam system. The models were verified by experiments in free and forced vibration. The second contribution area is the development of substructure decoupling-related methods to decouple the beam frequency response function (FRF) from the assembled cable-beam system. The FRF of the beam was obtained as an independent substructure after decoupling analysis and the FRF was compared to the directly measured FRF from modal testing of the beam substructure. A good agreement showed that the substructure decoupling method can be used to filter out the effects of cables from the assembled system in cabled structures, assuming that all points in the system are accessible for measurement. An FRF-based finite element model updating was then developed to overcome the practical limitation of accessing some measurement points in the field. The FRFs of accessible points were used as a basis for updating the FE model and then the FRFs of inaccessible points were obtained from the updated (optimized) FE model. A substructural damage detection was also developed for the systems that consist of a few substructures but only the main (target) substructure is susceptible to damage. In the developed method, FRF of the main substructure is first obtained using the substructure decoupling method and then FRF-based finite element model updating is used for damage detection, localization and quantification. The method was successfully able to identify location and magnitude of damage, which was modeled as a localized reduced stiffness due to material degradation or cracking. Finally, in support of the larger ongoing NDT research project, full-scale pole modal testing was done in the field. In-ground pole FRF’s with and without cables were generated, for future use in model validation. In-ground poles without cables were subjected to modal testing, and then brought to the lab for modal and destructive testing. The differences in the resulting FRF’s allow future simulation-based prediction of the foundation properties, and the destructive tests have added to the research group’s database for correlation of modal properties and pole strength.

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