Fatigue damage identification in threaded connection of tubular structures through in-situ modal tests

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