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Fatigue damage identification in threaded connection of tubular structures through in-situ modal tests
Bui, T.T.; De Roeck, G.; Van Wittenberghe, J.; De Baets, P.; De Waele, W. (2011). Fatigue damage identification in threaded connection of tubular structures through in-situ modal tests, in: Materials and structures in construction and design. Sustainable Construction & Design, 2(2): pp. 207-216
In: (2011). Materials and structures in construction and design. Sustainable Construction & Design, 2(2). Ghent University, Laboratory Soete: Ghent. ISBN 9789490726010. , more
In: Sustainable Construction & Design. Laboratory Soete of Ghent University: Ghent. ISSN 2032-7471, more

Available in Authors 
Document type: Conference paper

Author keywords
    fatigue, four-point bending test, in-situ modal test, modal analysis, threaded connection

Authors  Top 
  • Bui, T.T., more
  • De Roeck, G., more
  • Van Wittenberghe, J., more

Abstract
    The use of threaded connection is a valuable alternative to conventional welding in tubular constructions, e.g. pipelines, drill pipes and deep water risers. Those applications are normally exposed to environmental hazards – wave induced vibrations, temperature changes, etc. – as well as subjected to severe service conditions. A classical way to determine fatigue strength in many engineering situations is to calibrate material models by means of ad hoc designed experiments. Unfortunately, it is very difficult in this case even under laboratory conditions due to complicated stress and strain states in the contacting tapered helical thread surfaces of the connection. Therefore, a classical four-point bending fatigue test setup has been built with a real pipe specimen of 3.75 meter long, consisting of two standard API pipes connected by a threaded coupling, under unsymmetric (non-zero average) load control cycles. A complete vibration study has been carried out based on input-output modal tests for the entire period of the fatigue experiment. Input excitation is due to hammer impact and responses are recorded by accelerometers and by reusable dynamic strain gauges. The measured modal strains from the dynamic strain gauges allow for direct calculation of the modal curvatures, rather than deriving approximately from acceleration information. By comparing the measured modal parameters with those of a numerical model of the same structure in undamaged condition, damage detection, localization in the coupling and quantification are possible. This study leads to the following conclusion of practical use: the recent advancement in modal analysis, i.e. the reference based input-output combined deterministic-stochastic subspace identification, makes it possible to identify the structural modal properties from in-situ modal tests, which are performed while the fatigue test is ongoing. In this way the fatigue test is uninterrupted to avoid the problem of stress and strain disturbances happened in un-reversing load cycles test.

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