Theories And Factors Of Wood Moisture Content Testing Based On Stress Wave Technology

Wood high moisture content and distribution detection are unsettled issues in the wood drying and wood processing. In this thesis, the effects of moisture content, basic density, fiber proportion, temperature and length on the timber longitudinal stress wave velocity were studied using defect-free specimens of poplar (Populus cathayana), urophylla(Eucalyptus grandis X E.urophylla), beech (Fagus longipetiolata) and larch (Larix gmelinii) wood. The models of longitudinal stress wave velocity were established and validated. The effects of moisture content and tangential angle on the stress wave velocity of wood horizontal were discussed. And the effects of wood moisture content distribution on the stress wave two-dimensional tomographic images were also discussed. They provided a theoretical basis for wood moisture content and distribution detection basing on stress wave technology.The main results and conclusions were obtained as follows:(1) Two models of longitudinal stress wave velocity were established with the moisture content below or above fiber saturation point (FSP), respectively. The results showed that the models were suitable for actual values withthe errors less than8.2%. The model below FSP was more suitable than the model above FSP.(2) The longitudinal stress wave velocities decreased with increasing moisture content, and there was a good negative linear relationship. Below FSP, the rate of velocity variation was more than two times of that above FSP.(3) For the species with a constant moisture content, from bark to pith, the stress wave velocities of poplar and beech wood increased, and the velocities of urophylla and larch wood decreased. The difference in Larch wood was around13%, and the difference of poplar, beech and urophylla were all around20%. This was due to the anisotropy of natural lumber, which were corrected by the relative velocity.(4) For the same species, there was a linear relationship between stress wave velocity and basic density. However, as to the different species, the linear relationship between the velocity and basic density was not obvious. Velocities slightly decreased with the increasing temperature of wood in the range of25℃to100℃, but the variation was only about3%. The velocity was constant in the length range of200-1000mm.(5) There was an linear relationship between stress wave velocity and poplar fiber proportion. There was a negative linear relationship between the new parameter of corrected velocity (divided the stress wave velocity by the fiber proportion) and moisture content. Specifically, for the case of below FSP, the rate of change of corrected velocity was more than twice of that above FSP. It is belieived that the corrected velocity could be used to measure the moisture content of poplar wood as the data of fiber proportion was available with measurement before drying.(6) Both radial and tangential stress wave velocities in the cross section of wood inccreased with decreasing moisture content. A good negative linear relationship between velocities and moisture content below FSP was found. Above FSP, the velocities were slightly varied. Velocities increased quickly with the increasing of the angle when the tangential angle was less than60degrees, while velocities had a small increase when the tangential angle was more than60degrees.(7) The two-dimensional tomographic stress wave image on the cross section of wood is feasible to reflect the variation of wood moisture content. However, so far, it was difficult to quantitatively and qualitatively describe the distribution of moisture content.