Detection of fatigue weld cracks using optical frequency domain reflectometry-based strain sensing

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for the early identification and location of fatigue cracks in welds in steel tubular test specimens. This involved subjecting two welded tubular specimens, equipped with surface-mounted optical fiber sensors, to a resonant bending load. We continuously measured the strain distributions in the test specimens without any interruption throughout the entire duration of the test. The measurements were carried out with a spatial resolution of 2.6 mm and a strain resolution of 1 mu epsilon. The test samples were designed so that the fatigue cracks initiated from the inner surface of the pipe at the weld root and propagated through the wall thickness towards the outer surface. The fatigue tests were performed under two stress ranges. In Sample 1 (higher stress), we successfully detected and located the strain concentration region at approximately 5% of the specimen's lifetime before the breakthrough. For Sample 2 (lower stress) the initiation of the hotspot was detected around 27% of the specimen's lifetime before the breakthrough. We demonstrate that our method allows for on-the-fly detection and location of fatigue cracks originating from the inner surface of the specimens' wall.