Detection of fatigue crack initiation and crack growth in tubular T-joints using modal analysis

Modal responses of fatiguing tubular T-joints were determined using frequency response function approach. Strain gauges and accelerometers were used to measure the acceleration and strain frequency response functions; about forty strain gauges and seven accelerometers were located at salient points to monitor the responses. From the measured responses it was observed that the antiresonant regions and the quasi-static regions of the FRF curves showed considerable changes during the crack initiation and early part of crack growth regimes, depending on whether the cracking was displacement/force controlled. An effort was also made to determine whether it was possible to sense the presence of fatigue cracks from strain measurements made from gauges located far away from the zone of cracking; some significant results have been obtained from this aspect of the study. The results obtained from three tubular joints are compared and salient findings presented in this paper. End result of this study is to utilize model updating procedures and predict detailed crack profiles and locations from the combination of results obtained from measured experimental responses and finite element analysis.