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Reliability Research of Laminated Veneer Lumber Mechanical Properties and Nondestructive Testing

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Tutor: HuYingCheng
School: Northeast Forestry University
Course: Wood Science and Technology
Keywords: laminated veneer lumber(LVL),mechanical properties,nondestructive test(NDT),reli
CLC: TS653
Type: PhD thesis
Year:  2013
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Abstract:
In order to investigate mechanical properties and reliability of laminated veneer lumber (LVL), the longitudinal transmission, longitudinal vibration, flexural vibration, etc non-destructive testing (NDT) methods, composite material mechanics analysis, computer simulation and static mechanical tests were utilized. The main contents including:different compression ratio LVL made from birch, poplar and pine, different assembly pattern LVL made from birch and poplar, the pine LVL of different thickness veneer and the poplar and pine LVL in different relative humidity. The presuppositions model of LVL mechanical properties were found. The feasibility that ANSYS program analyze mechanical properties of LVL were discussed, The linear relationship between the dynamic modulus of various non-destructive testing and static bending modulus of elasticity (MOE) and modulus of rupture (MOR), the reliability of various non-destructive testing methods and LVL mechanical properties with different factors influence were analyzed.For different compression rate, the effects of compression rate on the birch, poplar and pine LVL mechanical properties were investigated and the reasons were analyzed for that. For the mechanical properties of laminated veneer lumber, the composite material mechanics were utilized to analyze LVL mechanical properties. The effects of tree species and compression ratio on the reliability of LVL were analyzed. The reliability of LVL was analyzed with different factors under different loads using the method of advanced first order and second moment (AFOSM). For different assembly pattern, LVL panels made from poplar and birch veneers were tested for mechanical properties. The effects of the assembly pattern on the MOE and MOR of the LVL with vertical load testing were investigated. Three analytical methods were used:composite material mechanics, computer simulation, and static testing. The reliability of the different LVL assembly patterns was assessed using the method of Monte-Carlo. For different veneer thickness, the effects of veneer thickness on the pine LVL mechanical properties were investigated and the reasons were analyzed for that. The linear relationships between various dynamic modulus and MOE or MOR were analyzed. And the reliability of various NDT was analyzed. For different relative humidity, the effects of relative humidity on the poplar and pine LVL mechanical properties were investigated and the reasons were analyzed for that. The effects of relative humidity on linear relationships between various dynamic modulus and MOE or MOR were analyzed. And NDT regression analysis results were compared with the fuzzy mathematical analysis and the classical mathematical analysis results.The results were summarized as follows:(1) For LVL of different compression ratio:The composite material mechanics analysis method can be used to analyze the effect of compression ratio on mechanical properties of LVL The theoretical results of MOE and MOR of LVL could be obtained from formula between mechanical properties parameters and compression ratio, its analysis results were consistent with the statics experimental results. The MOE and MOR of birch, poplar and pine LVL were increased with compression rate increasing when the compression rate was not very large, and it accord with the mechanical model was found in this paper. There were some horizontal cracks and some small compression failure were found through the microscopic structure analysis, so the mechanical properties reduced, the major reson was that the pressing load was larger during hot pressing. The reliability of three species LVL were not consistent in different compression rate and loading direction.(2) For different assembly pattern:Three analytical methods were used to analyze the mechanical properties and reliability of LVL:composite material mechanics, computer simulation, and static testing. The MOE and MOR results of theoretical analysis and computer simulation of each LVL assembly pattern were very close to the test results. The composite material mechanics and ANSYS simulation analyses were appropriate for the LVL mechanical property analysis. When the veneer dosages were the same, the MOE of each LVL assembly pattern was very close, and the serviceability limit state reliabilities were close as well. The strength limit state reliability was different, however, with the reliability of LVL with birch veneers on the outer surface being greater. Although the veneer dosages were the same, the strength of LVL with birch veneers on the top and bottom were much greater than the strengths of LVL with poplar veneers on the top and bottom. A good assembly pattern can improve the utility value of wood and provide a better strength-to-price ratio.(3) For the studies of different thickness of veneer LVL:Four NDT methods were used to test the mechanical properties of LVL, including the longitudinal transmission, longitudinal vibration, flexural vibration(in and out plane). The linear correlation between various dynamic modulus results and MOE or MOR were strong, the linear correlation coefficients of vertical and parallel load were much larger than the threshold of1%sub-bit. The method of best linear correlation was flexural vibration in same direction with the static test loading.Therefore the NDT methods can be used for MOE and MOR test of pine LVL with different thickness of veneer in this study. The theoretical result of LVL mechanical properties index that was obtained using the regression equation between NDT results and static results was not reliable. But its reliability was improved as choice of regression analysis equation of the95%confidence interval lower limit, and it can meet the engineering requirements basically. The MOE and MOR of pine LVL decreased with increase of veneer thickness.(4) For LVL in different relative humidity environments:The linear correlation between various dynamic modulus results and MOE or MOR were better, except the longitudinal transmission on pine LVL with vertical load, when hereinbefore four NDT methods were used to test poplar and pine LVL mechanical properties in different relative humidity environments, the NDT methods can be used for MOE and MOR test of poplar and pine LVL in different relative humidity. The size relationship of each NDT method fuzzy neartude was same with the size relationship of correlation coefficient, the fuzzy neartude can be as a reference for the feasibility analysis linear regression results on LVL NDT. The mechanical properties of poplar and pine LVL decreased with relative humidity increased, the most obvious decrease occurred from60%to70%. The mechanical properties of poplar LVL decreased in21%~37%and pine LVL decreased in25%~41%, as relative humidity increased from40%to70%. The effect of relative humidity on mechanical properties of pine LVL was greater. The origin of effect of relative humidity on LVL mechanical properties was the change of wood, because the strength of veneer changed as relative humidity increased, but the shear strength of layer was no decrease.
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