Study On Visualizing The Electrothermal Healing Process Inside Polymer Composites With Micro Damage

With the continuous development of composite materials,high-strength and lightweight composite materials are widely used in various high-end occasions,such as aerospace,rail transit and other fields.However,the complex production process and application environment will cause the early damage of composites and gradually evolve into defects that seriously threaten the safety of composites in service.The structural failure caused by the defects of composites will cause huge losses to people’s lives and property.Therefore,the detection of material defects,especially the timely detection and repair of early damage,is of great significance to the service safety of composite materials.The early damage of composite materials often has the characteristics of small size and high concealment.At present,the accuracy of conventional non-destructive testing technology is only millimeter-level,which can not meet the accuracy requirements of early damage detection,and there is a lack of an effective means to characterize the evolution of composite defects in the repair process,which can guide composite materials to complete the precise repair of early damage.Based on the above problems,a visual monitoring method of composite damage detection and repair process based on line scanning spectral domain optical coherence tomography system（LF-SD-OCT）is proposed in this thesis.The main research contents include:（1）Based on the basic principle of spectral domain optical coherence tomography,an inverted LF-SD-OCT experimental system is designed and built,and the experimental system is calibrated by using a 1 mm checkerboard calibration plate and a glass slide with known thickness.The experimental conclusions indicate that the system has a depth resolution of 5.20 μm,a depth range of 3.36 mm,a lateral resolution of 6.02 μm,and a field of view width of 5.80 mm,which verifies that the method has a micron-level tomographic resolution and can be used for high-precision imaging and damage detection of internal sections of materials.（2）The internal defects of translucent silica gel,light-cured resin and transparent ethylene-vinyl acetate copolymer（EVA）were imaged by LF-SD-OCT system.The conclusion show that the method can successfully detect the air gap,slag inclusion,delamination and other defects in the material,and the results based on the calibration experiment can quantitatively show the size and location of the defects,and can judge different types of defects according to different amplitude characteristics.（3）Based on the phase contrast technique,the differential phase of composite materials in the cooling field was measured continuously by using the LF-SD-OCT system.The abnormal phase change at the defect of uneven curing in the light-cured resin is found by the method,so that the position of the defect is located,and the dewinding phase is calculated by a branch-cut method to obtain the deformation field of the material.It can be found that if there is a heterogeneous material in a homogeneous material,due to the difference of its thermal expansion coefficient,there will be a sudden change at the heterogeneous place in the phase field.（4）LF-SD-OCT was used to dynamically monitor the complete process of electro-thermal repair of internal damage in EVA material.The evolution process of defects in the repair process was observed from the tomographic images of the sample.The repair process was visualized and the relevant characterization parameters were calculated by digital image processing technology.Finally,the phase change field and displacement field of the repair process were characterized from the mesoscopic point of view by phase contrast technology,and the corresponding characterization relationship between the related amplitude and phase change characteristics of the early damage under electrothermal action and the early damage melting repair process of thermoplastic resin was studied.The results show that this method has superior resolution and dynamic measurement ability.It provides a new idea for guiding the precise repair of composite materials.