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木材横纹压缩变形恢复率的变化规律与影响机制
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摘要
本研究对中国和日本产的五个树种——大青杨(p.ussuriensis)、落叶松(L.gmelini)、杉木(Cunninghamialanceolata)、日本柳衫(Cryptomeria japanica)、日本厚朴(Mangnolia obovata)进行横纹压缩,采用四种物理方法处理,保持其压缩后的尺寸,并通过恢复处理,研究了各种方法和处理条件对压缩变形固定效果的影响及各种处理方法的作用机理,在详细测量分析处理后试件的抗胀缩率ASE、重量损失率、密度、变色、力学强度性能指标的基础上,对影响压缩变形固定效果的各因子及各处理方法对处理材物理力学性能的影响进行了分析,确定压缩实体木材较合理的生产工艺及参数,为压缩木的实际应用提供理论依据,并提出压缩木的研究前景。对今后该类树种压缩木的实际生产具有一定的指导意义。
     实验中主要采用了四种方法:1常压冷却法、2热处理法、3水蒸气前处理法、4水蒸气后处理法。选用我国和日本资源较丰富的五种低质针、阔叶树材作为代表,在压缩实验中试件尺寸采用10×30×30毫米,压缩率在50%左右。在强度实验中试件尺寸采用150×15×5毫米。试验支点为100毫米。测量破坏时的力与变形,计算出抗弯强度(MOR)和抗弯弹性模量(MOE)。抗胀缩率和重量损失率实验采用5×30×30毫米的试件,与压缩实验中的试件一起处理,测量其处理前后的尺寸及重量,计算抗胀率和重量损失率。通过对实验结果图表的分析发现,总的来说木材在经过十几分钟的高温高压水蒸气处理、或几个小时、十几个小时的热处理之后,变形会基本被固定,而常压冷却固定效果不显著。在热处理和水蒸气处理过程中强度会随时间的延长而降低,热处理条件下弹性模量的变化趋势大体是先升后降;水蒸气前处理、后处理条件下基本是随着处理时间的延长弹性模量下降。色差、抗胀缩率和重量损失率随热处理和水蒸气处理时间的延长而增加。常压冷却法中杉木压缩变形恢复率最低,为29%;日本厚朴压缩变形恢复率最高,为65%左右。热处理法中日本柳杉压缩变形恢复率最低,为2.1%,日本厚朴压缩变形恢复率最高,为11.6%左右。水蒸气前处理法中大青杨压缩变形恢复率最低,为-3%左右;杉木压缩变形恢复率最高,为6.7%左右。水蒸气后处理法中大青杨压缩变形恢复率最低,为-3%左右;落叶松压缩变形恢复率最高,为11.6%左右。通过对木材动态热力学特性的试验研究,分析了热处理、水蒸气处理后落叶松和大青杨木材的损耗角正切、损耗模量、储存模量等动力学特性参数的变化特点,进一步揭示了半纤维素、木素在处理过程中的变化规律及对压缩变形固定的影响。
     根据木材横纹压缩应力—应变关系、木材流变学特性等理论对木材压缩及变形固定常用方法对形成变形永久固定的主要原因进行了探讨,根据前人对横纹压缩变
    
    形的永久固定理论及本实验中使用的四种物理处理方法在处理过程中对木材细胞壁
    基质物质的变化和表现出的力学特性的影响进行了分析,结合国内外最新研究成
    果,认为形成木材压缩变形永久固定的主要原因是木材内部变形应力充分释放和内
    聚力的有效形成。通过对木材细胞壁基质物质木质素、半纤维素等的含量及在各种
    方法处理中的软化和分解程度在五个树种之间的差异进行分析,发现半纤维素降解
    是释放内应力的主要因素,是降低压缩变形恢复率的一个主要方面,但过度分解会
    造成木材力学强度的下降:木质素的软化、流动、及冷却后内聚力的形成是降低压
    缩变形恢复率的另一主要方面。变形固定过程中的热量传递和色差的改变也在一定
    程度上揭示了压缩变形固定与能量释放间的某种内在联系。
This thesis studies transverse compressive deformation recovery rate and recovery mechanism of five representative species in China and Japan (Ussuri Poplar, Larch, Common Chinese Fir, Japanese Cedar, Whiteleap Japanese Mangnolia) by four physical treatment methods. It stresses on the influence of each treatment method on compressive deformation fixation as well as their impact mechanisms. ASE, WL, density, change of colors, mechanical strength property of processed samples is tested. On the basis of above, it analyzes the influence of factors which affect the result of compressive deformation's fixation on physical & mechanical properties and puts forward the reasonable process parameters for compressive solid wood, which is a theoretical guidance on manufacturing of compressive wood. Moreover, it gives the research prospects of compressed wood in future. Therefore this research has great significance.
    Four physical treatment methods employed in the experiment are usual-pressure-cooling treatment, heat treatment, water steam treatment before compression and water steam treatment after compression. In compressive experiment, the sample size is 10×30×30mm. The compression set is about 50%. In strength property experiment, the sample size is 150×15×5mm. The span is 100mm. Samples destructive strength, deformation are measured. MOR and MOE are calculated after that. In ASE and WL experiment, the sample size is 5 × 30 × 30mm. Samples size and weight are measured after treatment. ASE and WL are calculated later. By analyzing experiment data, we find that compressive deformation would be fixed after more than ten minutes' high temperature, high-pressure treatment or a few hours' heat treatment, while the effect of usual-pressure-cooling is not remarkable. During the process of heat treatment and water steam treatment, the mechanical strength property will go down with time and change of colors. ASE and WL wil
    l go up with time. On the condition of heat treatment, MOR will first go up then go down with time. On the condition of water steam treatment before and after compression, MOR will go down with time. During the usual-pressure-cooling treatment, the recovery rate of Common Chinese Fir is 29%, which is the lowest, while that of Whiteleap Japanese Mangnolia is 65%, which is the highest. During the heat treatment, the recovery rate of Japanese cedar is 2.1%, the lowest one, while that of Whiteleap Japanese Mangnolia is 11.6%, the highest one. During the water steam treatment before compression, the
    
    
    recovery rate of Ussuri Poplar is -3%, the lowest one, while that of Common Chinese Fir is 6.7%, the highest one. During the water steam treatment after compression, the recovery rate of Ussuri Poplar is -3%, the lowest one, while that of Larch is 11.6%, the highest one. Through thermodynamic experiment study, loss angle tangent, dump modulus and loss modulus of Ussuri Poplar and Larch samples after heat treatment and water steam treatment are analyzed.
    According to the theory of stress-strain relationship of transverse compressive deformation and wood rheological character, it studies the factors about deformation permanent fixation caused by different methods, and analyzes the component change of wood cell wall caused by four physical treatment methods in this experiment. According to the newest research achievements, it is thought that the absolute release of deformation stress and the effective formation of cohesion inside wood are the main reasons of compressive deformation's permanent fixation. Through analyzing the contents of lignin, semi-cellulose, their softening and degradation degree of each treatment, we discover that the degradation of semi-cellulose is both the main reason of inside stress release and a main aspect of decreasing the compressive deformation's recovery rate. Lignin's softening, flowing and the formation of cohesion is another main aspect that decreases compressive deformation's recovery rate. Excessive degradation would cause t
    he decrease of wood mechanical strength property.
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