PhD Defence by Marie Brøns "Vibration-based Estimation of Bolt Tension"

A wide range of engineering structures are held together by bolted joints. Bolted connections become an increasing problem when pushing the limits of size, efficiency, and reliability of structures. The confidence in bolt tension needs to improve to help allow for new strides in optimized engineering structures and lower maintenance costs. Current technologies and procedures are not very accurate, and considerable efforts and means are employed to ensure the proper bolt tension and to prevent failures. Critical bolted joints are especially of interest. Failures of these are inadmissible, as they may lead to dangerous situations, even structural collapse, which can result in significant loss in revenue and potential personnel and environmental damages.

The sound from a bolt struck by a hammer is different if it is tight than when it is loose. The sound difference indicates that the dynamics of the bolt-structure system changes with bolt tension. This thesis investigates the possibility of using a vibration-based approach to estimate bolt tension more accurately, easier, and at a lower cost than by using conventional tools. The work is split into three levels of complexity; one bolt, two bolts, and multiple bolts, all undergoing thorough testing, modeling, and analysis. This division follows throughout the thesis.

Transverse natural frequencies of a tensioned beam depend on tension. The first part of the project presents different beam models, designed to gain insight into tension’s relation to natural frequencies. A single bolt can be simplified and modeled as a beam in tension, with general stiffness boundary conditions. Expanding to a two-beam model, representing two adjoining bolts, can provide insights into potential inter-bolt coupling phenomena. For an alternative perspective, simulations of a real structure are conducted with a FEM model, from the commercial software COMSOL Multiphysics.

The second and crucial part is the experimental investigations. Dynamic testing of different bolts is conducted applying several excitation techniques, different post-processing, and substantial repetition to investigate reproducibility. The experimental investigations establish which features are useful for vibration-based bolt tension estimation. The models are fitted to measurements data, obtaining good agreement.

The third part presents two techniques for bolt tension estimation. The first provides rough estimates of bolt tension but is easy to apply. The second method combines nonlinear regression with a new smart vibration test procedure, providing accurate estimates of bolt tension and boundary stiffness.

The final part applies the proposed methods to real measurements on tightened bolts, and estimate the bolt tension. The analysis and proposed methods lend confidence in the applicability of a vibration-based estimation technique.

Main supervisor:
Associate Professor Jon Juel Thomsen, DTU Mechanical Engineering

Professor Alexander Fidlin, Karlsruhe Institute of Technology, Germany
Dr. Dmitri Tcherniak, Hottinger Brüel & Kjær

Associate Professor Jan Becker Høgsberg, DTU Mechanical Engineering
Professor Olivier Thomas, Arts et Métiers, Lille, France
Professor Simon Andrew Neild, University of Bristol, United Kingdom

Associate Professor Niels Aage, DTU Mechanical Engineering


fre 02 okt 20
13:00 - 17:00


DTU Mekanik



The Technical University of Denmark
Building 421/074 - limited number of seats assigned via signup to

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