ELM	Extreme learning machine
EMD	Empirical mode decomposition
FFT	Fast Fourier transform
GPR	Ground penetrating radar
HHT	Hilbert–Huang Transform
IE	Impact Echo
IRT	Infrared thermography
MASW	Multichannel Analysis of Surface Waves
NDE	Nondestructive evaluation
PC	Principal component
PCA	Principal component analysis
RMS	Root mean square
SASW	Spectral Analysis of Surface Waves
SL	Supervised learning
SSL	Semi supervised learning
STFT	Short-time Fourier transform
a	Plate width
b	Plate height
Cp	P-wave velocity
DT	Transpose matrix
E	Young’s modulus
f	Frequency
h	Plate thickness
t	Time
v	Poisson’s ratio
${\nabla }^2$	The two-dimensional differential Laplace operator
β	Shape factor
μ	Average
$\rho$	Mass density per unit area of plate surface
λ	Eigenvalue