On the detection threshold for fatigue cracks in welded steel beams using vibration analysis

Abstract

In this paper the influence of a surface fatigue crack on vibration behaviour of tee welded plates was studied and results compared to the influence of machined through thickness cuts on the dynamic response of cantilever beams. Comparison of experimental data with two and three dimension finite element modelling results was also carried out and discussed. Detection threshold using natural frequencies shift was found and compared to literature.The influence of naturally grown fatigue cracks on the oscillation frequencies was analysed and compared to two and three-dimensional numerical modelling results. An experimental detection threshold was found using the variation of the first mode natural frequencies.The power density spectra of the acceleration signals were analysed. Crack shape was found important for the detection threshold of the method. It was found that small size semicircular fatigue cracks do not noticeably modify the beam natural frequency but generate integer harmonics of the vibration modes that provoked stresses at the samples surface affected by the crack. The harmonics found were found to be generated by the breathing behaviour of the fatigue crack. A detectionthreshold using this spectral analysis was found to be lower than those found by using frequency shifts. The results obtained showed the ability of the experimental technique to detect cracks from a size of 0.6% of the cross section area, at 55% of the fatigue lifespan, 35% earlier than the other method studied.