Void and crack detection of polymethacrylimide foams based on terahertz time-domain spectroscopic imaging

Abstract

Terahertz reflection imaging is considered as a potential diagnostic tool for the investigation of polymethacrylimide foam defects. Using terahertz time-domain spectroscopy (TDS) and detection methods based on terahertz spectroscopic analysis, the reflection imaging results of different thicknesses of polymethacrylimide foam with two kinds of detections (cracks and voids) are studied. The samples (Degussa Rohacell WF71) are planar slabs of polymethacrylimide foams with thicknesses of 35 mm, 60.5 mm and 10 mm. It is found that the same kinds of polymethacrylimide foam defects with different foam thicknesses have similar spectral characteristics, with marked differences only in the amplitude and phase of the reflected wave. In view of this, we focus our study on the defective spectral characteristics of one of the foams (35 mm thickness). The characteristics of void and crack defects are analyzed in the time domain, which is based mainly on the variation of the reflected waveform. In particular, the imaging and clear identification of voids of less than 2.4 mm in diameter, fine cracks (0.3 mm wide), and the quantification of defects can be readily achieved using the terahertz non-destructive testing technique described here.

Research on Crop Water Status Monitoring and Diagnosis by Terahertz Imaging

“…And each pixel of the image corresponds to the peak-to-peak amplitude of the transmitted THz pulse in time domain at one exact point of the leaf surface. It can indicate the obviously enhanced contrast difference [30]. However, in frequency domain, the image obtained based on dispersive coefficient, absorption coefficient, and/or other physical parameters of each pixel can usually yield a relatively better imaging [31].…”

“…For the time-domain imaging, each pixel in the image corresponds to the signal waveform of THz pulse in time-domain on one certain point of the leaf. It can reflect the obviously enhanced contrast difference [30]. While the frequency-domain pattern imaging based on the absorption coefficient, dispersive coefficient, refractive index, or other physical parameters of each pixel can usually yield a relatively better imaging [31].…”

Temporal and Spatial Variability of Water Status in Plant Leaves by Terahertz Imaging

“…For the time-domain imaging, each pixel in the image corresponds to the signal waveform of THz pulse in time-domain on one certain point of the leaf. THz time-domain spectral imaging can reflect the obviously enhanced contrast difference [27]. While the frequency-domain pattern imaging based on the absorption coefficient, dispersive coefficient, refractive index or other physical parameters of each pixel can usually yield a relatively better imaging result [32].…”

“…The absorption and dispersion coefficients, refractive index and other physical parameters can be employed as the signals of pixels to perform THz imaging [27]. Previously, THz imaging has been proved to be a useful complementary technology for nondestructive testing [28].…”

Terahertz Scattering and Spectroscopic Characteristics of Polymethacryl Imide Microstructures

“…Therefore, it is necessary to develop nondestructive testing technology to ensure the safety of foam core materials and sandwich structures. However, the lightweight, low-density, and highly porous nature of PMI foam materials make X-ray almost transparent to foam materials and defects, and the defects imaging of voids or inclusions are ineffective with X-ray CT. On the other hand, THz wave is translucent to these materials, yet provides enough contrast even in foam-based structures, showing great potential in non-destructive testing of such materials [2]- [4]. Research on related technologies has been carried out by major research establishments at home and abroad, especially NASA, who, because of the grave impact of the 2003 shuttle accident, has embarked on a comprehensive program of using THz for NDI of polyurethane foams.…”