| With the rapid development of China's aviation industry,new high-performance civil aircraft composite materials play an increasingly important role in the aviation field,and its excellent performance has been fully verified and generally recognized.However,due to the particularity of civil aircraft composite materials and its structure,any quality hidden danger or defect will lead to the safety hidden danger of aircraft.For aircraft in service,bird strike,hail strike and maintenance tool strike are easy to cause damage in composite components.Such damage is usually called low-velocity impact damage,which is difficult to observe from the surface and needs the assistance of other detection equipment.Therefore,it is necessary to explore the visual and quantitative nondestructive testing technology for the low-velocity impact damage defects in civil aircraft composite materials to eliminate the potential safety hazards of aircraft.(1)In this paper,the flash thermography was used to test the low-velocity impact damage defects in civil aircraft composite materials.In the process of defect extraction,for the defects in the original infrared image,such as lack of detail,edge blur and low contrast,the corresponding image enhancement and noise suppression were designed.Firstly,the image was enhanced by the Otsu double threshold method and histogram equalization method,and then the background and noise of the image were suppressed by Wiener filtering.Finally,the defects were segmented and extracted.In addition,a quantum genetic algorithm was introduced to improve the efficiency of threshold calculation.The results showed that the image processing method designed in this paper can better enhance the display of defects in the image and make the defect extraction more accurate and fast.Compared with the ultrasonic C-scan results,the relative recognition error of the defect extraction results is less than 5%,which meets the damage detection requirements of the active aircraft carbon fiber reinforced composite.(2)In this paper,the reflective terahertz time-domain spectroscopy technology was used to test the low-velocity impact damage defects in civil aircraft composite materials.The spectral characteristics of the defect detection data obtained in the time-domain and frequency-domain were compared and analyzed,and the imaging analysis of the time-domain,power spectral density and absorption coefficient was carried out according to the spectral characteristics of the defect.The results showed that terahertz time-domain spectroscopy technology can effectively distinguish the low-velocity impact damage in the carbon fiber composite laminates.In the time-domain,the spectrum of defects is attenuated and delayed compared with that of non-defects.In the frequency-domain,the power spectral density of defects is lower than that of non-defects,while the absorption coefficient of defects is higher than that of non-defects.In general,the results of time-domain imaging are better than those of frequency-domain imaging.Based on the local characteristics of the amplitude difference of defects in time-domain,the best results of defect imaging can be obtained.After defect extraction and area calculation,the relative recognition error of defect extraction results is less than 4% compared with the results of ultrasonic C-scan,which can realize the quantitative detection of low-velocity impact damage of civil aircraft composite materials.In a word,both flash thermography and terahertz time-domain spectroscopy technology can be used to test the low-velocity impact damage defects in civil aircraft composite materials.In the detection of low-velocity impact damage defects,The flash thermography needs to overcome the problems existing in its original infrared image,and the image processing method is worth further study.The terahertz time-domain spectroscopy technology has shown better detection ability for low-velocity impact damage defects,which has more research value and application potentials in the future. |
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