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Lothar Gaul

Lothar Gaul has a degree in Mech. Eng. from FHS Wilhelmshaven and M Eng., Dr.-Ing. and Dr.-Ing. Habil. In Mech. Eng. from the University of Hannover. Along his career he has been Visiting Prof. in various universities worldwide, Full Professor at the Univ. Federal Armed Forces Hamburg, in 1981–1993, where he was also Dean, in 1991-93, Elected Member of Academic Senate of the Univ. of Stuttgart, in 2002-2010, Director of the Institute of Applied and Experimental Mechanics of the Univ. of Stuttgart and Dean of the Univ. of Stuttgart (Process Engineering and Engineering Cybernetics), in 1999-2002. Currently, he is Full Professor of Mechanics at the University of Stuttgart (since 1993). Prof. Gaul has received many awards, the most recent ones being the Thomes Caughey Dynamics Award 2013 at San Diego`s ASME Conference and will receive the Helmholtz Medaille 2015 at the DAGA Conference at Nuremberg, which is the most prestigious acoustics Award in Germany. His professional activities include being a consultant for several companies, member of various professional and scientific associations, author of several patents and member of the editorial board of various scientific journals. He is the author of 3 books, contributed to 7 book chapters, author and co-author of more than 150 journal papers and of more than 200 conference papers. Prof. Gaul has supervised 40 PhD. Students, co-supervised 18 and supervised 8 Post-Doctoral Associates for Habilitation Theses. 

Crack Detection in Overhead Transmission Lines by Ultrasonic Waves 
 Lothar Gaul, Christoff Schaal, Stefan Bischoff 
 Institute of Applied and Experimental Mechanics 
 University of Stuttgart

Ultrasonic waves are widely used tools for non-destructive evaluation of solid structures. Cylindrical structures allow for propagation of guided waves, which have little decay in propagation direction; therefore, a good choice in testing large structures for defects. In order to localize and characterize defects, an exact knowledge of the propagation and scattering properties of the ultrasonic waves is required. These properties can be obtained using the Finite Element Method by modeling a segment of the periodic waveguide with a periodicity condition. The solution of the corresponding eigenvalue problem leads to all propagating modes of the waveguide as well as locally generated evanescent modes.  

The Boundary Element Method is used in combination with the Finite Element Method for defect characterization in periodic 3D waveguides. The mode conversion at defects, such as cracks or notches, can be subsequently described by scattering coefficients. The obtained results allow for identification of suitable wave types and frequencies for structural health monitoring applications. Furthermore, knowledge of the interrelation between reflection and transmission coefficients and defect geometry improve defect detection algorithms. In consequence, measurement data can not only be used to deduce the presence and position of defects but also their geometry, i. e., depth of a crack in the medium.

The reliability and numerical accuracy of the simulation results are verified by comparison with experimental data. In this work, an appropriate crack detection algorithm based on the Hilbert transform is presented. By means of this algorithm, crack localization in multi-wire cables of overhead transmission lines is achieved through a time-of-flight analysis of the wave packets. Crack identification can be performed by evaluating the reflected waves’ amplitudes. The algorithm is fully automatized and differs between wave packets from different waves independently. Its applicability is shown for single cylindrical wires and for multi-wire cables.