| How to use wood resource efficiently and enhance product quality,have become the very important and challenging subject for forestry and timber industry in our nation.However,decay is one of the most important factors that cause heavy loss of forest resource,and it can reduce wood utilization.So studying the wood nondestructive testing becomes quite necessary.The incipient decay of living trees can be detected by electric resistance method;therefore,the losses caused by decay will be reduced by detecting and addressed the decay of standing trees as early as quickly.It can provide the scientific evidence of health evaluation not only for living trees but also for ancient and famous trees to enhance the nondestructive testing theory and develop the advantages of electrical resistance method.It also can provide important reference for optimal bucking and reasonable processing.So it has a great theoretical value and broad applicative prospects to research on the mechanisms and implementation of application of resistance theory.Four typical wood in northeast of China,Populus ssuriensis,Fraxinus mandshurica,Pinus koraiensis and Larix olgensis were selected as the research objects to conduct a series of the wood electrical resistance tests.Then,the mechanisms of the influence of decay on the wood resistance was further analyzed,the effects of temperature and moisture content(MC)on electrical resistance of healthy and decay wood were systematically analyzed by indoor and outdoor tests,and comparative analysis of electrical resistance method and other common nondestructive detecting technologies were carried out through experiment.Firstly,in the laboratory,the wood specimens were infected by rot fungi and different decay degree wood specimens were obtained to study the impact of wood decay on electrical properties.The change of microstructure was studied by the scanning electron microscopy and energy spectrum analysis.The ion concentration of decay wood specimens has been detected by ICP-OES.The variations on main biochemistry components of wood were analyzed during the decay process.Secondly,aimed at four kinds of wood,the variation rule of electric resistance in healthy and decay wood was studied when the temperature ranges from-20? to 200C and moisture content ranges from 0%to above 100%.Based on this,the integrated influence of temperature,moisture content and decay was analyzed.Third,aimed at four kinds of wood the effect of the temperature variation was studied through outdoor experiments in different seasons.Finally,through indoor and outdoor experiments,electrical resistance method,stress wave techniques and resistograph.were made comparisons in order to find out the appropriate technique matched with specific condition of wood.The main research results showed that:(1)Decay had significant influence on electrical resistance of four kinds of wood,the electrical resistance decreased rapidly after infected two weeks,and over time,it turned to slow down.For the relationship between decay degree(Es)and electrical resistance variation(Ed),when Es<30%,Ed had a significant positive correlation relationship with Es(Populus ussuriensis:R=0.76,p<0.01,Fraxinus mandshurica:R=0.83,p<0.01)for Populus ussuriensis and Fraxinus mandshurica wood.There existed a log curve between Ed and Es for Pinus koraiensis and Larix olgensis wood,Ed had the positive relevance to Es(Pinus koraiensis:R=0.679,P<0.01),Larix olgensis:R=0.81,P<0.01)in the curve models.(2)Microstructure analysis of different degree decay wood showed that the growing hypha destroyed the microcosmic appearance and structure of four kinds of decay wood.Naturally there were individual differences among different wood samples,but the destruction of rot fungi to wood cell wall got more and more serious as the decay degree deepened.ED AX and ICP-OES analyses showed the metal elements content and the total concentration of metal elements of had been on the increasing trend as a whole over time of decay,there was remarkable positive correlativity between decay degree and ionic concentration,and all the correlation coefficients were larger than 0.7;FTIR analyses indicated that holocellulose absorption peak wear off with the decay became serious,relative crystallinity of samples decreased after decayed,however,there was not a significant correlation between electrical resistance and crystallinity of four decay wood.(3)At different moistures,all the logarithm values of electrical resistance of four kinds of healthy wood increased as the temperature decreased;when moisture content(MC)was more than 25%,the logarithm values of electrical resistance had an abrupt jump near the freezing temperature;the main reason for the jump was that the phase transformation of free water in wood occurred at that temperature.At different temperature,the logarithm values of electrical resistance had the same pattern as the MC changed in all of four kinds of healthy wood.When the MC was below fiber saturation point,the logarithm values of electrical resistance significantly decreased as the MC increased;when the MC was above fiber saturation point,MC had little effect on the logarithm values of electrical resistance.(4)With the temperature and MC changed,the change regular of the logarithm values of electrical resistance of decay wood was similar to that of healthy wood,that is,the logarithm values of electrical resistance also had an abrupt jump near the freezing temperature.At the same temperature or at the same MC,the logarithm values of electrical resistance of decayed wood was a slightly less than that of healthy wood in the same specie samples.(5)With the seasonal temperature changed,the mean of the cross section electrical resistance of four healthy and decay standing trees was very different among the four seasons of the year,especially the electrical resistance increased remarkably when the temperature downed to the freezing point in winter.At different moistures,as the temperature increased,the logarithm values of the cross section electrical resistance decreased rapidly at the beginning and then maintained stable,and the logarithm values of electrical resistance had an abrupt jump near the freezing temperature;at the same temperature,the logarithm values of the cross section electrical resistance of healthy wood was a slightly higher than that of decayed wood.At different temperature,the MC had little effect on the logarithm values of electrical resistance.(6)For the defect recognition in wood,electrical resistance tomography(ERT)was a bit better than stress wave imaging technology(SWIT);for the holes defects in wood,ERT was more sensitive than SWIT,but SWIT had better identification capability of defect outlines than ERT.(7)three NDT methods were able to make different degree estimation for the degree of decay of standing trees;In different decay degree,when Es<30%,Ed had a strong positive correlation with Es(R=0.823,P<0.01),when 30%?Es<50%,Ey have a significant positive correlation relationship with Es(R=0.658,P<0.01),and when Es>50%,Ez have a more positive correlation relationship with Es(R=0.914,P<0.01),so electric resistance tomography can show better diagnosis than the other two for incipient decay of standing trees,while stress wave imaging method can significantly use in the middle stage of decay testing,and resistograph can be used in the different stages of decay,and can significantly use in the serious stages,however,resistograph can cause micro-damage to wood.It is suggested that each technique can be employed in the practical internal decay testing for standing trees according to the decay stage and operational conditions. |
PDF Full Text Request