【作者】 边明明；

【导师】 殷亚方；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2011， 硕士

【摘要】 木材与木质材料的力学性能在很大程度上取决于其内部结构特征。为了更好的了解和掌握木材力学性能与其微观结构的关系,本论文采用自主研发的原位检测平台,研究了不同条件下木材受连续压缩时力学性能及其微观结构的实时变化情况,并着重分析了两者之间的相互关系。本论文首先归纳了原位检测方法在木材组织力学水平上的应用,分析了现有原位检测技术存在的问题及可实现的解决方法;随后重点介绍了自主研发的原位检测平台的构架及实施方式、图像采集系统等重要部位的工作原理;并借助具体试验,综合分析了该平台可实现的技术特点;然后通过该原位检测平台,以人工林杉木(Cunninghamia lanceolata)小试样[5mm(L)×X mm(R)×5mm(T)]为研究对象,系统研究了不同生长轮早材在连续横纹径向压缩时的力学行为及其微观结构变化;并进一步研究了不同加载速度、温度和含水率条件下,早材在连续加载时的力学性能及其与内部微观结构变化的关系。本论文的主要研究结论如下:(1)自主研发的原位检测平台包括两个系统,一是加载系统,二是图像采集系统,两个系统通过时间参数并行统一。图像采集系统能在连续加载的过程中自动聚焦并实时拍摄木材变形的照片,并借助于测量与计算软件对照片进行定量分析,从而实现木材的力学性能及其微观结构变化的动态检测。(2)杉木各生长轮间早材横纹径向抗压屈服强度(σc)和横纹径向抗压弹性模量(Ec)从心材到边材呈逐渐减小趋势。不同部位(心材、边心材交界和边材)间σc与Ec存在显著的差异性。对三个部位(心材、边心材交界和边材)连续受压产生首次屈服永久变形(塑性变形)的微观结构位置进行定量研究结果表明:第7生长轮与第11生长轮出现首次屈服变形的位置一致,均在距被加载表面第15-20层早材细胞处(平均距离分别为643μm和689μm);而第17生长轮在距被加载表面第5-10层早材细胞处(平均距离为153μm)产生首次屈服变形。不同生长轮间早材微观结构的不同影响其横纹抗压力学性能在径向方向上的变化。(3)在不同加载速度(1mm/min,10mm/min和50mm/min)条件下,早材产生首次屈服变形的位置与被加载表面之间平均距离分别为689μm,166μm和23μm。不同加载速度下早材小试样的σc与Ec均差异显著,与试样产生首次屈服变形的位置有关。低速加载下(1mm/min),木材内部最脆弱部位首先产生屈服变形,得到的σc与Ec相对较小,平均值分别为3.16MPa和45.79MPa;较高加载速度下(10mm/min和50mm/min),木材内部非最脆弱部位首先产生屈服变形,得到的σc与Ec相对较大,平均值分别为3.71MPa和56.04MPa,4.51MPa和75.53MPa。(4)在不同温度(-20OC、20OC、60oC、80OC和120OC)条件下,早材σc与Ec的变化趋势一致。在负温度下(-20OC)得到σc与Ec均比正温度下的σc与Ec大。一般来说,随着温度的升高,σc与Ec逐渐减小。同时,不同温度下σc与Ec差异比较显著。较低温度(≤60oC)下与较高温度(≥80 oC)下,早材的破坏方式不同。温度≤60oC时,趋于刚性屈服;温度≥80OC时,趋于柔性屈服;不同温度(-20OC、20OC和60OC)下,出现首次屈服变形的位置相同,均为距被加载表面第17-20层早材细胞处(距被加载表面的平均距离分别为667μm、689μm和643μm),表明在较低温度(≤60OC)范围内,温度的变化并未改变木材内部最脆弱部位的位置,但最脆弱部位木材的力学性能发生变化。(5)在不同含水率(绝干、气干与饱水状态)条件下,早材σc与Ec差异均显著。σc与Ec随着含水率增加呈逐渐减小的趋势。由绝干到气干状态,早材产生首次屈服变形的位置未发生变化,均在距被加载表面第17-20层早材细胞处(距被加载表面的平均距离分别为664μm和689μm);在饱水状态下,加载过程中压出的水分影响微观结构的拍摄和检测。在不同含水率条件下,木材产生首次屈服变形时的破坏方式不同。在绝干和气干条件下,趋于刚性屈服;而在饱水条件下,趋于柔性屈服。在绝干与气干条件下,含水率变化并未改变木材内部最脆弱部位的位置,但最脆弱部位木材的力学性能发生变化。综上所述,本研究通过自主研发的原位检测平台,实现了连续加载条件下木材微观结构变化的实时检测,并较好解释了不同条件下木材横纹径向压缩力学性能变化的解剖学原因,为连续加载条件下木材力学行为的有效检测和预测提供了技术基础和理论依据。更多还原

【Abstract】 Mechanical properties of wood and wood products are highly depended upon the interior structural features. Based on this theory, this paper introduced research methods and tendency of testing on wood mechanical properties, then introduced the applying of in-situ inspection on wood tissue mechanical level and analyzed the existing and unsolved problems of the present in-situ inspection system. A self-developed in-situ inspection platform had been developed, and then the structures of the platform and its implement methods were descried in detail. The mechanism of auto-focus photo-shooting system was given in the paper. By means of wood mechanical tests, detailed descriptions of the platform on how to achieve the variable technical characteristics had been obtained, and technical advantages of this platform were comprehensive analyzed.By virtue of the self-developed in-situ inspection system and earlywood simples in each growth ring of Chinese fir, wood mechanical behavior variations under continuous compressive loading and auto-focused in-situ inspection on microstructure characteristics of wood deformation had been systematically analyzed. Meanwhile, through the change of outer environment conditions, i.e. loading rate, temperature and moisture conditions, variations on mechanical properties in these situations were obtained, and quantitative analysis on microstructure variances were investigated with the help of the measuring and calculating software. The main conclusions of the paper are as follows:(1) The in-situ inspection system can be divided into two parts, one part is the loading system, and the other part is the picture-collection system. The two parts can be combined by the parameter of time. The picture collection system can automatically take photos of materials’surface deformation during the loading process. Meanwhile, by means of the quantitative calculation and measuring on the photos, the system can directly and precisely achieve the simulations of wood mechanical properties. (2) By using early wood samples in each growth ring, the result that yield strength and modulus of elasticity under radial compression are decreasing from heartwood to sapwood. Significant variations were observed among mechanical properties of different parts, i.e. heartwood, transition wood and sapwood. The in-situ inspection on wood micro-structural deformation and quantitative analysis results showed that the first yield positions of both heartwood and transition wood occurred at 15-20 cell layer of earlywood (643μm and 689μm from the surface of loading compression, respectively), while the first yield position of sapwood occurred at 5-10 cell layer of earlywood (153μm from the surface of loading compression).(3) The yield strength and modulus of elasticity at different loading rates are relevant to the first yield position. Loading rates of 1mm/min, 10mm/min and 50mm/min was adopted respectively for mechanical testing of typical softwood of Chinese fir (Cunninghamia lanceolata). The results indicated that the variations of wood mechanical performance at different loading rates were generated from the different position of the first yield point (689μm, 166μm and 23μm, respectively); the in-situ inspecting system could accurately characterize the variations of microstructure deformation. Furthermore, the microstructure characteristics could be used to explain mechanical behavior of wood suffered from different loading rates.(4) Mechanical properties, i.e. yield strength and modulus of elasticity, have the same tendency with the variation of temperature conditions. Mechanical properties at negative temperature are higher than that of at positive temperatures. Generally speaking, yield strength and modulus of elasticity are decreasing with the increase of temperature. The fracture behaviors are different at relatively low (≤60OC)and high temperature(s≥80OC).At relatively low temperatures (including -20OC,20OC and 60OC), the fracture behavior is tend to in a rigid way; While at relatively high temperatures (including 80OC and 120OC), the fracture behavior is tend to in a flexible way. At different temperatures(-20OC,20OC and 60OC), the first yield position is similar, which occurred at 15-20 cell layer of the earlywood (667μm,689μm and 643μm from the surface of loading compression, respectively), which inferred that the temperature had not affect the most vulnerable area in the sample.(5) The yield strength and modulus of elasticity are variable at different moisture conditions. Generally, the yield strength and modulus of elasticity are decreasing from fully dry to air-dry and then to fiber saturate condition. The first yield positions were similar at air-dry and full-dry moisture conditions, which occurred at 15-20 cell layer of the earlywood (664μm and 689μm from the surface of loading compression, respectively); while at firber saturate moisture condition, the free water in cell walls or cavities was compressed out which made it impossible to see the fracture surface.The fracture behaviors are different between the relatively low (fully-dry and air-dry)and high moisture conditions(fiber saturate).At relatively low moisture conditions, the fracture behavior is tend to be in a rigid way; While at relatively high moisture condition, the fracture behavior is tend to be in a flexible way.Above all, the self-developed inspecting method can help us understand and master the microstructure characteristics of wood deformation under continuous compressive loading, and can explain the variations of mechanical properties during continuous compression. The system can realize the effective anticipations of wood mechanical properties suffered from various compression conditions, and can quantitatively analyze the variance of wood microstructure characteristics.更多还原