Reference-free fatigue crack detection, localization and quantification in lug joints

This work focuses on fatigue crack detection, crack tip localization and quantification in lug joints using a reference-free method. In many practical applications the environmental conditions in which a structure is operated vary over time. Sensor signals, thus, collected for the damaged state cannot be compared directly with the baseline because a change in the signal can be caused by several factors other than structural damage. Therefore, reference-free methods are needed for damage detection. Two methods have been discussed in this paper, one with collocated sensors and the other using matching pursuit decomposition (MPD) to detect waves undergoing mode conversion from fatigue crack tip. The time of flight (TOF) information of Lamb wave modes undergoing mode conversion, along with their respective velocities, is used to localize the crack tip and ultimately find the extent of crack. The efficiency of reference free methods to detect fatigue cracks is demonstrated on aluminum plate samples and lug joint samples. Direct mode converted waves are used for plate samples and mode converted waves reflected from the edges are used for lug joint samples for crack localization study. Comparison of crack lengths obtained from experiments with crack lengths calculated through reference-free technique are found to be in good agreement.