Fatigue-induced damage is often progressive and gradual in nature. Fatigue is often
deteriorated by corrosion in ageing structures, creating maintenance problems, and
even causing catastrophic failure. This ushers the development of structural health
monitoring (SHM) and nondestructive evaluation (NDE) systems. Recent advent of
smart materials applicable in SHM alleviates the shortcomings of the conventional
techniques. Autonomous, real-time, remote monitoring becomes possible with the use
of smart piezoelectric transducers. For instance, the electro-mechanical impedance
(EMI) technique, employing piezoelectric transducers as collocated actuators and
sensors, is known for its ability in damage detection and characterization. This
article presents a series of lab-scale experimental tests and analysis to investigate
the feasibility of fatigue crack detection and characterization employing the EMI
technique. This study extends the work by Lim and Soh  to incorporate the
phases involving crack initiation and critical crack. It is suggested that the EMI
technique is effective in characterizing fatigue induced cracking, even in its incipient
stage. Micro-crack invisible to the naked eyes can be detected by the technique
especially when employing the higher frequency range of 100–200 kHz. A quick
and handy qualitative-based critical crack identification method is also suggested by
visually inspecting the admittance frequency spectrum.
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