Table 3.

Classification of DI according to domain.

Reference	DI Time-Based	DI Frequency-Based	Remark
[99]	Coefficient of CWT, Z = $\frac{\left(\frac{A}{B}+\frac{A}{C}\right)}{2}$ Where A = Fraction of the total energy of the CWT that lies at the centred frequency B = Fraction of the total energy of the CWT that lies at the higher frequency C = Fraction of the total energy of the CWT that lies at the lower frequency		Thickness reduction in a thin plate
[58]	Peak amplitude coefficient, $\beta =\frac{A_2}{A_1^2}$ Where A1 = amplitude of the fundamental wave and A2 = amplitude of the second harmonic		Fatigue crack propagation in aluminium pipe
[27]	The beat wavelength, $L=\frac{2\pi }{K_{A0}-K_{S0}}$ KA0 and Ks0 are the wavenumbers of the fundamental symmetric and antisymmetric		Monitored thickness reduction due to general corrosion activity
[90]	$R_{ref}=\frac{A_{wel}}{A_{dir}}$ Awel = Peak amplitude at the weld Awel = Peak amplitude from a direct source		The severity of damage in the fluid-filled pipe
[101]		$DI=\left|1-\frac{f{d}^{T}fd}{f{b}^{T}fbL}\right|$ fd = the spectral signal frequency response at damage state fbL = the spectral signal frequency response at the undamaged state	Corrosion severity detection in pipeline
[59]		$D{I}_{ij}\left(f_{ex}\right)={\displaystyle \sum _t}\left|V_{ij}^B\left(t,f_{ex}\right)-V_{ij}^D\left(t,f_{ex}\right)\right|$ for i,j = 1~6 where: DIij(fex) = Damage signal differential $V_{ij}^B\left(f_{ex}\right)$ = Baseline signal when pairing the i-th PZT actuator and the j-th PZT sensor at a given excitation frequency (fex). $V_{ij}^D\left(f_{ex}\right)$ = damage signals when the corrosion damage was present at the targeted position of the plate.	Corrosion detection and severity in the plate Aluminium
[61]		Spectral density, $D{I}_2=\frac{\left|A\left(2\omega \right)\right|}{\left|A\left(\omega \right)\right|}$ Where|$A\left(2\omega \right)$| = spectral magnitude of the fundamental frequency |$A\left(\omega \right)$| = Spectral magnitude of the second harmonic frequency	Microscale crack detection in a plate structure