| The Chinese painted mural has a long history and is a gem of the splendid culture of China for five thousand years.It has profound of historical value,scientific value and cultural value.Ancient murals have suffered long-term erosion from natural environment or intentional or unintentional vandalism,such as empty drums,soda,nails and cracks.Time went by,these diseases will further accelerate the spread and will eventually cause irreparable damage to the murals.So the protection and restoration of these endangered ancient murals is very urgent.The analysis and evaluation of diseases is an important part in the research,protection and restoration of cultural relics.It is necessary to investigate and understand the current and disease status of the mural on painting and structural level(the ground layer or support).By doing this,we sift the protective materials methods and develop scientific and reasonable methods for protection and restoration.Due to the non-renewability and fragility of murals,the non-destructive principle must be followed when they are deteced and analyzed.In the inner layer structure detection technology of murals,the traditional tapping and visual method rely on the subjective experience of cultural relic workers.X-ray and thermal wave imaging technology can not obtain the internal layer structure information of painted murals.Moreover,they have potential damage to murals.Coherent tomography has limited detection depth and is unable to detect diseases in the ground layer of the mural.Ultrasonic non-destructive testing has strong penetrating ability,but generally requires a coupling agent.If high-sensitivity ultrasonic sensors were used,the ultrasonic echo signals can be directly obtained.So the internal structure information of the object can be detected by tomographic imaging,which is a non-destructive testing method,to realize the disease of the ground layer in the painted murals..Based on the non-destructive detection of ground layer disease in painted murals,in this paper we study high-sensitivity fiber-optic ultrasonic sensors and build an air-coupled ultrasonic tomography system to realize the two-dimensional and three-dimensional imaging on empty disease in the ground layer of the mural model.The main research content and results are:(1)The diaphragm type Fabry-Perot(FP)interferometric fiber-optic ultrasonic sensor was studied.The relationship between the sound pressure sensitivity of the FP interferometric fiber-optic ultrasonic sensor and the Young's modulus of the film forming the FP cavity was studied theoretically and experimentally.Structural optimization and performance improvements were both made.(2)Using a low Young's modulus gold film to fabricate an FP-structured fiber-optic ultrasonic sensor,the signal-to-noise ratio under air coupling was measured.The signal-to-noise ratio at 300 k Hz,1 MHz and 3 MHz was 68.18 d B,50.03 d B and38.64 d B,respectively.(3)We ralize the hollow structure and efflorescence in ground layer of mural model,by using the air-coupled sensor.The imaging results show that the thickness of the ground layer is 0.1880 cm,0.6210 cm and 0.3730 cm,respectively.The error rates were 5.64%,8.73% and 3.83%,respectively.The thickness of the efflorescence area in the ground layer was about 2.2710 cm,and the error rate was 13.9%.The results indicate that the diaphragm type Fabry-Perot interferometric fiber optic ultrasonic sensor can realize the fault imaging of the inner layer structure of the mural,which can provide service and support for the protection and repair of the painted mural artifacts. |
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