| With close-celled, high strength to weight ratio, high ductility, flexible design possibilities,long-term stability, and other attractive features, advanced foam composite materials, such as Polymethacrylimide(PMI) and Polymethacrylimide(PVC) aviation foam core are applied to the construction of many important parts in a number of high-tech fields. However, in the process of production and service, there will be various defects in the foam core materials and the sandwich structures, which will have potentially dangerous consequences. Therefore, it is imperative to develop nondestructive testing(NDT) technologies to ensure the safe and long-term deployment of the foam core materials and its sandwich structures. However,certain special properties of the foam materials render the traditional nondestructive testing methods ineffective. The terahertz(THz) radiation’s semi-transparent nature for the foam materials is complementary to the conventional technologies. Domestic and foreign research institutions have carried out a few researches of the relevant technical issues, which are especially attached great importance by NASA, after the 2007 Space Shuttle accident, but the reported results are limited to Polyurethane(PU) based materials and structures. The preparation and application of the advanced foam core materials have brought a new research topic to THz-NDT. Based on the high performance aircraft grade PMI, PVC and foam core sandwich structures as the focus of our research, we have carried out systematic studies in the THz-TDS-NDT technology for the four main defects, which provide basic data and important technical foundation for the wide ranging and practical applications of the THz-NDT in materials.The main investigations are as follows:(1) The experimental study on the terahertz spectrum of advanced aviation foam core materials.PMI foam and PVC foam are currently widely used for high performance foam core inaviation field due to their many excellent characteristics. They are the main core material of the sandwich structure in the aviation and aerospace field, with performances that are obviously better than that of the honeycomb sandwich structure. Therefore, in this thesis, we choose these two types of air bubble as the research target, and the THz-TDS technology is used to study the terahertz spectral characteristics. THz transmission rate, refractive index,absorption coefficient and other parameters are extracted. In the frequency band of 0.1~1THz,the refractive index of the 10 mm thick 51HF/WF and 71HF/WF PMI foams sample are about1.0271, 1.0379, 1.0412 and 1.0549, and the 5mm thick HT61/81/110/131 PVC foam sample is about 1.0219, 1.0255, 1.0375 and 1.0445, respectively. An experimental phenomenon has been found that the relationship between the absorption coefficient of the foam core materials and the stop frequency of the terahertz pulse is related to the diameter of the micro structure inside the foam.(2) The incident terahertz pulse energy loss in the foam with different inside micro structure diameters.In order to further verify the experimental results in the terahertz spectral characteristics investigation, the PMI foam core materials with the same thickness and different inside micro structure diameters are selected as the experimental samples. Based on the classical Mie scattering theory of electromagnetic wave, the scattering model combined with the foam micro structure diameters distribution function is established. By comparing the numerical simulation with the actual test data, it is concluded that the energy loss of the bubble is mainly derived from random scattering and not from material absorption. The larger the inner micro structure diameter of the foam core, the higher the scattering intensity, and the greater the loss.This conclusion lays the foundation for the further analysis of the foam composite structure with THz NDT.(3) THz NDT of the foam-aluminum structure.Aluminum alloy skin is commonly used in the aviation field. In this thesis, the structure of foam aluminum is used as the research sample. Taking into account the strong reflection of terahertz wave on polar materials, the four kinds of pre-defects include the bonding, inclusion,void and crack are studied by using THz-TDS reflection mode. The results show that the degrees of debonding between the aluminum sheet and the foam are related to the changes ofthe main reflection wave front peak. The greater the degree of debonding, the longer the delay between the wave front and the main reflection peak. The degree of debonding was significantly associated with this feature. When the time delay features are used in the THz-TDS reflective imaging, the results show a significant change in the grayness of the figure, the degree of debonding zone can be up to 44 ?m. The distinction between inclusion and debonding defects mainly depends on the phase change of the main reflection wave,which is related to the relative refractive index of the foam, the inclusion and the air. The void defects imaging is characterized by a bright center and dark edges. The thickness of the foam at the void center can be calculated by the time delays. In reflection mode, due to the strong scattering of terahertz waves on the PMI foam surface, the energy consumption is large.Therefore, the effects of voids position on the detection results is mainly presented as the void,which are closed to the substrate, where the reflection is stronger, the image is clearer, and it is easier to be detected. The characteristic of the crack is the modulation of the amplitude of the reflected wave. By means of background scanning and removal of noise, the absorption rate imaging method has been applied to improve the detection resolution. This method has a better contrast ratio than other methods.(4) THz NDT of the glass fiber foam sandwich structure.First, the glass fiber foam sandwich structure is simulated and the defect is preset. Due to the wave permeability, combined with a large number of experimental data, with the glass fiber as the carrier rocket skin, glass fiber foam sandwich structure has been designed, four kinds of defects are prepared, and i.e., inclusions, voids, debonding, and crack have been preset. Secondly, the terahertz spectrum characteristics of the extrusion modified nylon, GFRP,PTFE composite material have been studied before the test. The results show that the refractive index of PTGFR and PTFE composite material are very close, with a value about2.2, which is far greater than the refraction index of PTFE coating, which makes the debonding analysis similar to the case of foam-aluminum structure. Then the THz NDT experiment was carried out on the pre-defects of the glass fiber foam sandwich structure. The modes of transmission and reflection are both used. The terahertz wave is incident from the surface without defects. The results show that the terahertz pulse scanning transmissionimaging detection effect is good for debonding(PTFE coating), voids, cracks, but the inclusions(extrusion modified nylon) detection effect is not obvious. The characteristics information, which was acquired from the time domain window extended, in the reflect mode,can significantly improve the detection effect of debonding and inclusion defects. The gray value of stretching effect is better, can realize the debonding degree, the thickness of the inclusion of distinction. The test results can meet the requirements of quantitative detection.But for the void and crack defect, even using the feature spectrum imaging in the reflection mode, the effect improving is still limited. Although the effect of the absorption coefficient imaging method is more obvious, the operation is relatively complex. Therefore, the void and crack detection are more suitable for THz-TDS transmission scanning imaging mode.(5) The design of super resolution reconstruction algorithm based on the WDHMM and its application in THz NDT.The SR algorithm based on WDHMM denoising is studied and designed. Starting with the analyses of the ways for resolution improvement, combining with the mechanism of THz-TDS pulse imaging, the WDHMM of multiple low-frequency and high-frequency defect characteristic images are established, and the optimization of wavelet coefficients is realized.The resolution improvement of the glass fiber foam sandwich structure was achieved by the nonlinear interpolation method. Based on the visual effect, the WDHMM SR algorithm is better than the other general SR imaging and noise reduction methods.In this doctoral dissertation, some innovative ideas about the THz-NDT of foam materials and their sandwich structures have been proposed and applied. They include:(1) The terahertz spectral characteristics of various types of PMI, PVC aerospace grade foam core material was first studied, the relationship between foam density and refractive index and the internal micro structure diameter effect to the terahertz wave loss has been obtained.(2) The relationship between the foam internal structure diameter and the terahertz wave transmission loss was first detailed analyzed. We have obtained result that the loss is mainly caused by the Mie scattering but not the absorption and the numerical simulation has been used to confirm this.(3) The relationship between the degree of skin debonding and the change of the wavefrontof the foam aluminum structure is studied. The relationship between the degree of debonding and the phase shift difference of the defect free is fitted. Using this feature imaging, the accuracy of the detection of debonding can reach 44 ?m, which breaks the limitation of the common imaging methods.(4) A systematic study on the application of THz-NDT in the multiple defects of the glass fiber foam core sandwich structure has been carried out for the first time. By adjusting the time delay, it can overcome the bottom layer flaws detection difficulty due to that obtained the primary reflection wave only.(5) For the first time, a super resolution reconstruction method is proposed, which combines the WDHMM denoising algorithm with the bi-quadratic polynomial interpolation method for THz-NDT imaging. The imaging effect is improved obviously, which provides a new idea to improve the detection ability of small defects in thick foams using THz-NDT technology. |
PDF Full Text Request