【作者】 刘昊；

【导师】 高建民；

【作者基本信息】 北京林业大学 ， 木材科学与技术， 2014， 博士

【摘要】 木材高含水率段以及含水率分布检测是木材干燥和木材加工中要解决的重要问题。本论文以青杨、巨尾桉、山毛榉和落叶松四种木材的无缺陷试件为试验材料,研究了含水率、基本密度、纤维比量、温度、长度对木材中应力波纵向传播速度的影响,建立了应力波纵向传播速度理论模型并进行了验证。分析了含水率和弦向角对木材横向应力波传播速度的影响以及木材横向的含水率分布对应力波断层图像的影响。为用应力波检测木材含水率及含水率分布提供了理论基础。主要成果和结论如下：(1)分别建立了木材含水率在纤维饱和点以上和以下时的应力波纵向传播理论模型并进行了验证。结果表明,模型理论值和实际值吻合度较好,二者最大误差小于8.2%。在纤维饱和点以下时,理论模型与实际值的吻合度更高。(2)应力波在木材纵向上传播时,传播速度随含水率的下降而增加,且在纤维饱和点上下分别与含水率之间存在较好负相关线性关系。纤维饱和点以下时,传播速度的变化率是纤维饱和点以上时的2倍以上。(3)含水率相同时,从树皮到髓心,青杨和山毛榉木材上的传播速度逐渐增加；巨尾桉和落叶松木材上的传播速度逐渐降低。落叶松木材上差幅为13%左右,青杨、巨尾桉和山毛榉木材上的差幅为20%左右。这是由于木材各向异性造成的,这种差异可以用相对速度参数进行修正。(4)同一树种内,传播速度与木材基本密度线性正相关。在不同树种上,传播速度与基本密度的线性关系不明显。在25℃到100℃的范围内,传播速度随温度的升高而下降,但变化幅度只有3%左右。试件长度在200-1000mm范围内,传播速度不变。(5)青杨纤维比量与应力波传播速度之间存在极好的线性关系。在纤维饱和点上下,新参数修正速度(等于传播速度除以纤维比量)与含水率之间存在较好的负相关线性关系。在纤维饱和以下,修正速度随含水率的变化率是纤维饱和点以上时的2倍以上。木材纤维比量在干燥前可以测出,因此修正速度可以用来检测青杨木材的含水率。(6)应力波在木材横向上传播时,径向和弦向传播速度都随含水率的降低而增大。在纤维饱和点以下时,传播速度与含水率线性负相关,当含水率高于纤维饱和点时,传播速度变化幅度较小。传播速度随弦向角增加而增大,当弦向角小于60度时,应力传播速度随弦向角的增加明显增大；当大于60度时,传播速度增幅较小。(7)木材横截面的应力波二维断层图像能够在一定程度上反映含水率的变化,但目前还难以对含水率分布进行定性和定量表征。更多还原

【Abstract】 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.更多还原