A modal approach to identify fatigue damage in threaded connections of large scale tubular structures in a resonant bending test rig

The current work aims at using a modal approach to detect fatigue damage of threaded connections in large scale tubular structures. A full scale resonant bending fatigue test rig has been developed to test at first a plain pipe with notches. Two kinds of vibration testing are used for fatigue crack detection. Forced vibration tests (input-output) are performed before and after the fatigue test, in which input signals from forced excitation as well as output signals from accelerometers and dynamic strain gauges are both recorded. Whereas, the ambient vibration test (output only) follows the fatigue experiment. The modal parameters of the structure can be determined from the acquired data using subspace identification algorithms, i.e. the Reference-based Combined Deterministic-Stochastic Subspace Identification one for the forced vibration tests and the Reference-based Stochastic Subspace Identification one for the ambient vibration test. An observed change in the extracted modal parameters implies the degradation of the bending stiffness when a fatigue crack initiates. The modal approach demonstrates to be a viable and reliable way to detect fatigue cracks in large scale tubular structures. This potentiality makes vibration based testing a complement to conventional approaches like e.g. global deflection control; moreover, it is rather simple to implement and can be used for long-term monitoring.