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.

The translucency of terahertz wave through many polymer composite materials, particularly foams, makes it a good complement to another nondestructive testing (NDT) methods. Terahertz time-domain spectroscopy (THz-TDS) technology can obtain amplitude and phase information simultaneously. It is a powerful tool for nondestructive testing of foam materials to embody the depth information of defects while acquiring defect location, size, and nature. In the process of NDT, the refractive index is one of the important factors that affect the detection effect. In previous studies, the effective refractive index of polymethacryl imide (PMI), a commonly employed foam material, in the THz band is found to be very close to that of air. However, based on systematic measurements of the THz spectra and scattering parameters of PMI with a variety of thicknesses and densities, it is found that THz attenuation in PMI has a strong frequency dependence and that the size of the particles making up the material has a great influence on the attenuation: the larger the size of the micro particles and the faster the attenuation of the THz wave. This paper attempts to explain this phenomenon through combined experimental study and theoretical analysis, supported by the numerical simulation based on the Mie scattering model.

Hydrogen embrittlement and the anodic dissolution mechanism are two important aspects of the corrosion behavior of magnesium alloys. Here, to evaluate the effects of these two aspects on the corrosion failure of magnesium alloys under stress, the stress and corrosion behaviors of the AM50 magnesium alloy in air, deionized water, and NaCl solution after solid-solution (T4) treatment were investigated by X-ray diffraction, scanning electron microscopy, slow strain rate tensile testing, and vacuum dehydrogenation. The as-cast AM50 magnesium alloy was mainly composed of the α-Mg and β-Mg₁₇Al₁₂ phases. After T4 treatment, the amount of the β-Mg₁₇Al₁₂ phase was significantly reduced, and only a small amount existed at the grain boundaries. After T4 treatment, the stress corrosion resistance in deionized water improved, but it decreased in an NaCl environment. Dehydrogenation experiments showed that the effect of hydrogen on the corrosion process was weakened owing to the decrease of the β-Mg₁₇Al₁₂ phase after solution treatment. The effects of hydrogen embrittlement and the anodic dissolution mechanism on the corrosion behavior of the AM50 magnesium alloy under stress were different. In deionized water, the hydrogen embrittlement mechanism played the major role, while the anodic dissolution mechanism played the major role in the presence of Cl⁻ ions.

The performance of a measurement matrix is always the key factor affecting the application of compressed sensing in engineering practice. A measurement matrix designed based on a chaotic map is easy to implement in physical circuits, but the weak chaotic behavior and small chaotic interval of the common one-dimensional chaotic map directly affect the signal reconstruction accuracy. To solve this problem, this paper uses the ratio form of the logistic chaotic map to the simple quadratic chaotic map to improve the sine chaotic map and obtain a new type of compound sine (NC-sine) chaotic map. Its good chaotic behavior and chaotic interval expansion characteristics are verified by a bifurcation diagram, the Lyapunov exponent, and complexity analysis. Based on the NC-sine chaotic map, a dynamic sparse circulant (DSC) measurement matrix with adaptive zero-setting elements is designed. The simulation results show that compared with the sine measurement matrix, the reconstruction success rates of the DSC measurement matrix are increased by 5% and 9.69% on average for a one-dimensional signal when the measurements and sparsity change, respectively. The peak signal-to-noise ratio of the reconstructed two-dimensional signals at different compression rates is improved by more than 0.92 dB on average, and the reconstruction efficiency is higher. The average structural similarity of the reconstructed signals at different initial values is improved by more than 0.027 compared to that of the Gaussian measurement matrix. This matrix can thus be utilized to promote the rate and accuracy of signal transmission.

We carried out systematic measurements of Polymethacrylimide (PMI)’s Terahertz (THz) spectroscopic and scattering parameters with various thicknesses and models. The Terahertz time domain spectroscopy (THz-TDS) would be useful as a nondestructive testing (NDT) tool for PMI. While the index of refraction is very close to that of air, with flat frequency dependency, the attenuation constant exhibits a remarkable strong frequency variation, suggesting a scattering dominated attenuation process. Based on the Mie scattering model, we gave the satisfactory explanation: the THz wave propagating through the PMI suffer scattering from the micro sized particles, and the sizes of the particles (shown to obey a normal distribution) determine the frequency characteristics of the interaction.