Compared with traditional mechanical connection methods, the glued structure has obvious advantages in terms of high connection efficiency, small stress concentration, wide applicability, and lightweight structure. Silicone adhesive is a high-performance adhesive widely used in aviation, automobiles, electronics, and other industrial fields. It can maintain good bonding strength in high and low temperature environments and has good water resistance, chemical stability and resistance, and corrosion properties. During use, the bonded structure is exposed to environmental external forces, and the interface of the bonded layer will generate internal stresses. Internal and interface stresses may cause debonding defects in the bonded structure, leading to the failure of the entire bonded structure, thus seriously threatening the safety of use. Therefore, for the safe use of adhesive structures, it is of great significance to use non-destructive and accurate methods to conduct research on the mechanical properties of organic silicone.

In this article, the terahertz time-domain spectroscopy system was used to study the stress characteristics of silicone adhesives. Terahertz spectrum detection is a non-destructive measurement method that can avoid damage to the sample. Terahertz waves have high time resolution and good penetration in non-metallic materials. Taking organic silicone adhesives as the research target, a transmission terahertz polarization time-domain spectroscopy system was built. By measuring the transmission spectrum of the adhesive film in the terahertz band, the optical response of the adhesive film in the terahertz band under different stress states was analyzed. The stress optical coefficient of the organic silica gel film was calculated using two parameters: the refractive index difference of the organic silica gel and the spectral retardation information. In addition, a reflective terahertz time-domain spectroscopy system was established to analyze the relationship between the stress of the organic silica gel and the terahertz delay time under different thicknesses by changing the stress state of the organic silica gel film. Terahertz time-domain spectroscopy was used to characterize the stress characteristics of organic silica gel.

Firstly, the stress optical coefficient of the organic silica gel film was calculated using a transmission terahertz time-domain spectroscopy system. By receiving signals through the terahertz detection device, the terahertz time-domain waveforms of the organic silicone film under different stress conditions were obtained (Fig. 3). By calculating the refractive index curves of organic silicone under different forces in the frequency range of 0.2-1.0 THz, the refractive index changed in the range of 1.77-1.87. It can be found that with the increase in tensile stress, the refractive index value of organic silicone adhesive gradually increases (Fig. 5). The refractive index differences of the rubber film before and after stress were obtained in five groups of different terahertz frequency bands, and the stress optical coefficient of the silicone rubber was calculated (Fig. 6). Secondly, the phase difference curves of the adhesive film under different forces in the frequency range of 0.2-1.0 THz were obtained. As the tensile stress increased, the value of the phase difference gradually increased, and the slope changed (Fig. 7). The calculated stress optical coefficient of organic silica gel in the frequency band of 0.2-1.0 THz was 0.18 MPa^-1. Finally, the reflective terahertz time-domain spectroscopy system was used to detect the adhesive film under stress. The terahertz time-domain waveforms under six stress states were measured. It can be seen that as the tensile stress increased, the peak time of the echo on the lower surface of the adhesive film gradually moved forward (Fig. 10). The delay time difference of terahertz spectrum was extracted, and the relationship between stress magnitude and delay time difference was established (Fig. 12). A formula for the relationship between the delay time difference of terahertz time-domain spectroscopy and the tensile stress of organic silicone adhesive was obtained.

We study the stress characteristics of organic silicone adhesives based on terahertz time-domain spectroscopy technology. In terms of measuring the stress optical coefficient of the adhesive, the terahertz time-domain transmission spectrum information is used to establish the organic silicon adhesive in the range of 0.2-1.0 THz. The refractive index difference of the silicone film changes with the magnitude of the tensile stress, and the corresponding stress optical coefficient is 0.18 MPa^-1. According to the mapping relationship between the terahertz time domain spectral time information and the stress on the material, the spectral phase delay method is used to measure the obtained stress optical coefficient of 0.18 MPa^-1, which verifies the feasibility of using terahertz time-domain spectroscopy to characterize the stress optical coefficient. In addition, based on the reflective terahertz time-domain spectroscopy system, the results of the organic silica gel film under different stress states are obtained. Based on the terahertz time domain spectral information, the characterization relationship between the tensile stress and the delay time difference of the adhesive film under the thickness of 2 mm and 3 mm is established respectively. Compared with the theoretical formula, the mean square error of the results is below 0.06, and the correlation coefficient is greater than 0.9. The final formula for the relationship between the delay time difference ΔT of terahertz time-domain spectrum and the thickness dand tensile stress σof the adhesive film is given as ΔT=2.05dσ. The research results indicate that this method can quantitatively characterize the magnitude of adhesive film stress using the delay time difference in terahertz time-domain spectral information. A new non-destructive testing method for measuring the stress situation of adhesive is proposed, further providing a reliable basis for evaluating the stress state of adhesive structural materials.