Effects Of Decay On Section Bending Properties Of Poplar Wood And Trunks Of Standing Trees

Decay is a quite common phenomenon during trees growing process due to the effects of surroundings and external factors.Decreases of wood mechanical properties caused by decay have very nagetive impacts on the security of trees,even result in dangerous accident and enormous economic loss.Therefore,in order to evaluate the effects of decay on mechanical properties of trees,poplar trees,which were common broadleaf species in urban forest ecosystem,were chosen as the object of study,mechanism exploration indoors,test study outdoors and FEM(Finite Element Method)simulation analysis were combined for research in this thesis.During indoor experiments,brown-rot decay was cultured;changes of mechanical properties of decayed poplar wood and the changing mechanism were analyzed.In outdoor tests,both nondestructive testing technology and trees static pulling system were used to detect poplar standing trees,and the effects of inner decay in trunks on the elastic modulus of trees were analyzed.The FEM software ANSYS was also used to model trunk structures of decayed poplar trees,and then the effects of decay degree and decay position on the bending properties of tree cross-section were also anlyzed.Based on these research ideas,the effects of decay on the section bending properties of poplar wood and trunks of standing trees could be studied systematically from micro level to macro level.Main research contents and research methods included:? Cultivation of brown-rot decay and mechanical properties testing in laboratory.Standard specimens made from sound poplar sapwood were infected with brown rot fungus for 12 weeks and sampled once a week to determine wight loss,modulus of elasticity(MOE),modulus of rupture(MOR),impact bending strength(IBS)and compressive strength parallel to grain(CS?).Then the effects of different degrees of decay on mechanical properties were compared and analyzed.? Study on the mechanism of changes of mechanical properties in brown-rot decayed poplar wood.At first,the microstructure of decayed slice samples was observed.Then the chemical compositions of wood powder were determined.And the relationship between the microscopic chemical properties and the mechanical properties was analyzed.? Quantitative detection of inner decay of poplar standing trees.Three kinds of methods which were ERT(Electrical Resistance Tomography),Resistograph and growth cone were used to detect the inner decay of poplar trees.And the quantitative method of high accuracy for determination of decay degree was selected out by comparative analysis.?Poplar trees static pulling test.The static pulling test was conducted on the sample trees,and then the deformation rules under pulling force of different sections were compared.The elastic modulus of each section was calculated and the relationship between elastic modulus and section decay degree was determined by correlate analysis and regression analysis.The initial elastic modulus of decayed section was estimated according to the regression model between healthy section's elastic modulus and its mean resistance,and then the relationship between section decay degree and loss of elastic modulus was analyzed.? Mechanical simulation analysis of decayed poplar standing trees.The mechanical models of poplar trunk with different decay degrees and different decay positions were established by ANSYS software.The maximum stress and loss of section bending modulus were analyzed to determine the effects of decay degree and decay position on the bending properties of poplar sections.The research results showed that the color of poplar specimens showed a change from dark yellow to dark brown with the extension of degradation time.Surface of the specimens was completely decayed with crisp,soft and easily desquamated material at the 11th week and the 12th week.During the 12-week brown-rot treatment,the maximum weight loss of poplar specimen was about 25%.Mechanical properties of poplar wood under different decay degree were significantly different(P<0.01).MOE,MOR,IBS and CSu were negatively correlated with wood weight loss significantly(r =-0.986?-0.826?-0.897 and-0.985,repectively).The losses in IBS and MOR increased logarithmically with the degree of decay,whereas the losses in MOE and CS?increased slowly with linear trends.Changes on mechanical parameters were much more remarkable than the changes on wood mass during the decay process.The ranking of the 4 mechanical parameters for the response speed to decay and the degree of the influence of decay were IBS>MOR>MOE>CS?.During the process of brown-rot decay,changes of chemical compositons were the fundamental reason for the decrease of mechanical properties in poplar wood specimes.The degradation of hemicelluloses caused a fast and significant decrease in IBS(r = 0.910,P<0.01)while the subsequent removal of paracrystalline cellulose resulted in MOR loss(r =0.884,P<0.01).The variation of MOE was in correlation with the cellulose content(r =0.995,P<0.001).A decrease in CS? was due to the slow deterioration of crystalline cellulose(r = 0.965,P<0.001).In the decay tests of poplar standing trees,loss of resistance LR,which was calculated according to Resistograph curves,has the best effect to quantitatively characterize the inner decay of poplar trees by compared with the weight loss of wooden core which was considered as the true value(R2 = 0.822,r = 0.91,P = 0.000).Seventeen sections of the total fifty tested sections were healthy,while thirty-three sections were decayed.There was a higher correlation between the elastic modulus E of healthy section in poplar standing trees and its average resistance FR(r = 0.874,P = 0.000),so the elastic modulus of healthy section could be estimated according to the linear regression model E = 0.052FR-2.143(R2 = 0.823,P = 0.000).As for decay sections,the higher the degree of decay were,the smaller the modulus of elasticity was and the more severe the losses of elastic modulus were.Initial elastic modulus of decayed section was estimated by using the regression model of 95%confidence between E and FR,and the loss of elastic modulus has a significant positive linear correlation with resistance loss LE-2.361LR(R2 = 0.861,r = 0.722,P = 0.000)The stress on concentric decayed section was symmetrical,but with the increase of decay degree,the maximum compressive stress increased obviously.When r/R<0.6,changes of the compressive stress was not obvious,and there was no significant difference between the compressive stresses of rotten bodies with different heights.It indicated that concentric decay had not yet caused a bad effect on cross-section.From r/R = 0.7,the compressive stress increased sharply,and the stress of higher rotten body was larger than that of the shorter one,indicating that the effects of decay on section stress was not only related to the decay degree,but also in connection with the volume of the decayed body in whole tree trunk,and the greater the degree of concentric decay was,the more obvious effect of decay volume had.The value of r/R equal to 0.7 could be considered as the threshold for safety assessment of concentric decayed sections.If the value of r/R was less than 0.7,section was regarded as safe.On the contrary,if the value of r/R was higher than 0.7,section was regarded as in danger.The stress on eccentric decayed section was asymmetrical,the compressive stress at decayed side was significantly larger than the tensile stress,and the neutral axis was offseted to the tensile side.With the increase of decay degree,both the maximum compressive stress and loss of the section modulus of bending increased.For sections with the same degree of decay,the greater the degree of eccentricity was,the worse the effect on the bending properties of section was.For one section of a standing tree,the severer the eccentricity degree was,the lower the risk thresholds were.