Enhanced Terahertz Imaging of Small Forced Delamination in Woven Glass Fibre-reinforced Composites with Wavelet De-noising

Abstract

Terahertz(THz) reflection imaging is applied to characterize a woven glass fiber-reinforced composite laminate with a small region of forced delamination. The forced delamination is created by inserting a disk of 25 µm thick Upilex film, which is below the THz axial resolution, resulting in one featured echo with small amplitude in the reflected THz pulses. Low-amplitude components of the temporal signal due to ambient water vapor produce features of comparable amplitude with features associated with the THz pulse reflected off the interfaces of the delamination, and suppress the contrast of THz C-and B-scans. Wavelet shrinkage de-noising is performed to remove water-vapor features, leading to enhanced THz C-and B-scans to locate the delamination in three dimensions with high contrast.

High-precision terahertz frequency modulated continuous wave imaging method using continuous wavelet transform

“…In mathematics, a continuous wavelet transform (CWT) is used to divide a continuous-time function into wavelets. 12 Unlike Fourier transform, the continuous wavelet transform has the ability to construct a time-frequency representation of a signal that offers very good time and frequency localization, 13 it is possible for CWT to detect singularities in a signal by using an appropriate basis function () t  . Based on the formula of the wavelet, the continuous wavelet…”

“…Compared with the Fourier transform, the CWT can handle the timefrequency signal very well. [13][14] Therefore, it is very efficient in complex information processing such as edge and corner detection. The CWT is also very resistant to noise in the signal.…”

Terahertz frequency-wavelet domain deconvolution for stratigraphic and subsurface investigation of art painting

“…Stationary wavelet shrinkage is applied to further attenuate the residual noise. This technique decomposes a 1D signal into the approximation coefficients vector and detail coefficients by convolving with a low-pass filter and a high-pass filter along the temporal axis at each level [20]. Wavelet coefficients with small absolute values can be considered as noise, and wavelet coefficients with large absolute values are regarded as the main featured information of the signal [17].…”

“…Removing the small absolute-value coefficients by thresholding and then reconstructing the signal is expected to produce a signal in which the contribution of noise has been reduced. Detailed information about the stationary wavelet shrinkage can be found in [20]. A typical THz waveform reflected from the painting in our study is shown in Fig.…”

Visualization of subsurface damage in woven carbon fiber-reinforced composites using polarization-sensitive terahertz imaging

“…THz imaging was first applied to the NDE of fiber-reinforced composites in 2006 [1] and has been explored extensively since [2]. For composites reinforced by glass fibers, THz waves can be utilized to characterize both surface and underlying damage and defects, including mechanical/heat damage [3], voids [4], delamination [5,6], intrusions and moisture contamination [7]. Material parameters [8] and fiber orientation [9] in glass fiber-reinforced composites have also been successfully characterized via THz imaging.…”