Fatigue crack detection and localization using reference-free method

This work focuses on fatigue crack detection, crack tip localization and quantification in plate like structures using a reference-free method. In many practical applications the environmental conditions in which a structure is operated do not remain same 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 a 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) of these mode converted waves along with their respective velocities are further used to localize the crack tip and ultimately find the extent of crack. Both these approaches were used to detect fatigue cracks in aluminum plates made of 6061 alloy. These samples were instrumented with collocated piezoelectric sensors and tested under constant amplitude fatigue loading. Crack tip localization was done from the TOF information extracted for mode converted waves using MPD. The crack lengths obtained using this reference-free technique were validated with experimental crack lengths and were found to be in good agreement.