Research On Detecting Method And Equipment For Moisture Content Field And Strength Assessing Of Waterlogged Archaeological Wooden Relic

Since being highly hydrophilic than sound wood,waterlogged archaeological wooden relics(WAWRs)generate more uneven moisture content and moisture induced stress field with the variation of relative humidity,which may lead to strength degradation even structural failure of WAWR.Therefore,it is very important for long-term preservation and repairation of WAWR by studying the detecting method of moisture content field,and analyzing the rule of strength degradation of them.However,because of limitations of environment,size,cost and other factors,the current moisture content detecting technologies are still difficult to be applied on WAWR.Consequently,by regarding Chinese fir as the research object,a surface deformation measuring method of wooden members based on digital image correlation(DIC)and wood texture was raised.Then,combining the orthotropic constitutive model of moisture induced stress with the measuring method,a moisture content field detecting method based on DIC and strength degradation model of Chinese fir was proposed,and a detecting system for moisture content filed of WAWR is designed based on above researches,which can provide the technical support for structural health monitoring of WAWR.The main research results of this paper are as follows:1.By proposing a surface deformation measuring method of WAWR based on DIC and wood texture patterns,the moisture content filed detection scheme was established.After analyzing the intensity feature and distribution of wood texture,a speckle quality assessing method using mean directional intensity derivative(MID)was raised based on error analysis theory of DIC.Then,through performing sub-pixel displacement numerical experiments and four-points bending test of wood beams,the measuring accuracy of displacement and strain field using wood textures were clarified that the relative errors of wood texture patters are smaller than 5%.The results show that the speckle quality of wood texture provides a good precision for detecting surface deformation of WAWR.2.On the basis of moisture induced deformation under the effects of humidity and loads,the influences of moisture diffusion and viscoelastic creep on moisture content and moisture induced stress were discussed,and then an orthotropic constitutive model of moisture induced stress field in Chinese fir was established.Using the parameters in the constitutive models determined by moisture diffusion tests,compression and creep experiments,the numerical simulation of Chinese fir under humidity was conducted.The results show that the simulated moisture induced strain field is conformed to the mechanosorptive experimental results,which indicates that the proposed orthotropic constitutive model can provide an accurate mechanical model to analyze the moisture content and moisture induced stress field of Chinese fir.3.After analyzing the variation of moisture induced strain on the diffusion surface of Chinese fir under humidity,a moisture content field detecting method based on DIC was proposed(MC-DIC).The 2-D and 3-D detecting models of moisture content field were deduced using the constitutive model of moisture induced stress field in Chinese fir,thus the moisture content distribution in the wood component can be determined based on the iteration method.After verified the proposed mathematical models by numerical simulations,the moisture content field of Chinese fir during adsorption experiment was measured using MC-DIC method.The detecting results show that when the variation of average moisture content is smaller than 1%,MC-DIC method produces the absolute deviation lower than 0.5% with the coefficient of determination over 0.95.Therefore,the proposed MC-DIC method can provide an effective method to detect the moisture content field of WAWR non-contact and nondestructively.4.Considering the strength variation of Chinese fir under the effect of humidity which was analyzed in artificial aging tests,the evaluation formulas of accumulative damage under various humidity were raised based on the Gerhards accumulative damage model,and the strength degradation model of Chinese fir component considering the gradient variation of moisture content field was established.Later,the radial damage distribution of the FE model of Chinese fir board component under varied humidity was predicted using Comsol Multiphysics.By regarding the maximum radial damage as evaluation indicator,the radial strength degradation was plotted,which offered an accurate prediction model and assessment method for strength assessment of WAWR.5.A moisture content field detecting system for WAWR was designed using MC-DIC method,and the detecting software platform was developed based on Matlab.This detecting system can measure the full-field surface deformation of WAWR in real-time,and realize moisture content field analysis and strength assessment of the key measuring points.Regarding Song Dynasty shipwreck excavated from Quanzhou Bay as the research object,the moisture content field and strength degradation of the wood component on the shipwreck were measured using this detecting system.The measured results show that the average moisture content varies from 9.73% to 7.47%,and the radial strength of the wood component reduces approximately 1%/100 years in the last 600 years.By comparing with shipwreck component and artificial aging test results,the assessing results of this detecting system are verified to be rationality,which provide the technology and data support for the preservation and repairation of WAWR.