毛竹材弧形竹片干燥特性研究

对毛竹材弧形竹片的干燥特性进行了研究,结果表明:干燥温度越高,干缩率越大,弦向平均干缩率大于径向平均干缩率;弦向竹青侧干缩率高于竹黄侧干缩率;当含水率降至5%以下时,干缩率趋于稳定;随着干燥温度的升高,弧形竹片依次产生翘曲、皱缩及开裂等现象。     

楠竹竹片基材弯曲构件冷压胶合工艺研究

采用微波加热软化的方式,研究了楠竹竹片基材弯曲构件冷压胶合工艺中各个因子对胶合质量的影响。实验结果表明:随着涂胶量的增加,胶合质量先增大后减少;弯曲胶合后应立即上好夹具进行干燥;干燥温度对胶合质量的影响不显著;干燥后随着陈放时间的延长,胶合质量逐渐趋于稳定;在容易弯曲到规定的半径值的前提下,建议采用较低的含水率竹片进行弯曲件的制作。     

基于微波真空干燥的圆竹材干缩性研究

圆竹材的合理干燥是决定竹材利用质量优劣和寿命的重要环节,圆竹材的微波真空干燥,是一项探索性研究课题。笔者采用微波真空干燥方法,在一定的干燥基准条件下,依据含水率变化曲线,将干燥过程分为加速、恒速和减速干燥三个阶段。研究结果表明,干燥初期随着温度逐渐上升,水分蒸发量逐渐增加,中期恒速干燥阶段,大量液态水转化为水蒸气排出,持续时间长,后期减速干燥阶段含水率较低,主要是结合水的蒸发,含水率变化很小,三个阶段所占干燥时间分别约为20、90 min和40 min;沿壁厚方向干缩率不明显,沿着竹竿高度方向由下向上,干缩率逐渐增大。     

初期干燥对竹展平板变形的影响

竹展平板具有材料利用率高、无需使用胶黏剂、环保等优点,但展平过程中需克服圆竹内应力,易出现翘曲起拱等现象。针对这一问题,以竹展平板为研究对象,采用恒温恒湿干燥箱对竹展平板进行初步干燥(24 h),测试4种不同干燥工艺、刻痕、厚度(7.5、8.5、9.5 mm)、宽度(60、70、80 mm)、长度(400、500、600 mm)对竹展平板含水率、长、宽、厚、拱高变化率的影响。结果表明:厚度对竹展平板平衡含水率的影响明显,竹展平板厚度在7.5 mm以下的平衡含水率明显低于厚度在7.5 mm以上时的平衡含水率,长度和宽度对平衡含水率影响不大。刻痕对竹展平板的干缩具有明显抑制作用,4种干燥工艺中,无刻痕竹展平板的拱高干缩率分别为92.02%、134.38%、22.22%、100.96%;有刻痕的竹展平板的拱高干缩率分别为34.00%、45.38%、11.11%、92.59%。材料长、宽、厚3类尺寸中,宽度对竹展平板的厚度干缩率有较大影响,3种宽度下的厚度干缩率分别为1.29%、1.48%、1.83%;长度对拱高变化率的影响最大,3种长度下的拱高变化率分别为-11.9%、-3.84%、27.78%。长度较小时有利于得到较为平整的展平竹表面。初期干燥过程中保持温度不变而持续降低环境湿度、选用长度尺寸较小的竹展平板以及对竹黄侧进行刻痕处理,可提高竹展平板的干燥质量。     

导热油加热的连续热压干燥机干燥速生杨单板的应用研究

导热油作载热体,在速生杨木单板连续式热压干燥中应用后,效果良好。美洲黑杨生材单板(名义厚度1．7mm)可在2min内于至8％以下的含水率。干单板基本平整、光滑,终含水率均匀,无撕裂。单板厚度干缩率平均5．3％,横纹干缩率4.9％。胶合强度平均0．812MPa,略高于网带对流干燥单板胶合强度0．779MPa。研究结果表明,此法干燥速生杨木单板是可行的。     

初含水率对白橡锯材热压干燥特性的影响

以白橡锯材为研究对象,采用平板热压机对其进行干燥处理,系统研究了初含水率对木材温变特性、干燥速率、干缩特性、干燥缺陷和微观构造的影响规律,探明白橡锯材的热压干燥特性。结果表明:热压干燥是一种高效快速的干燥方法,将初含水率为14%~75%的木材在温度为140℃、压力为0.1MPa的条件下干燥到2%以下终了含水率仅需120~210 min,木材干燥速率随着初含水率的增加而增加;初含水率较高的木材在热压后会产生严重内裂和皱缩缺陷,当木材初含水率降至15%以下时,热压后无内裂缺陷产生,截面变形也明显减小;随着初含水率的增加,木材厚度干缩系数呈增加趋势,而宽度干缩系数则呈下降趋势。通过观察木材的横切面微观结构发现,高初含水率试件的内裂沿木射线生成,其早材大管孔部位可观察到明显压缩。     

冀北山地油松木材干燥特性的研究

采用百度试验法研究了油松木材干燥特性。结果表明:初期开裂等级为1级;截面变形等级为2级;瓦弯变形为1.4 mm;平均体积干缩率为11.49%,差异干缩为1.14;干燥速度等级为1级。根据干燥特性相关数据,初步拟定了25～30 mm厚油松木材干燥基准。     

杨木单板连续式热压干燥的研究

本文以生长在苏北地区的美洲黑杨单板（名义厚度１．７ｍｍ）为试材,在连续式热压干燥机上进行了生产型试验研究,采用高温导热油为载热体,热板温度１９２℃,热板压力０．０９ＭＰａ（前段）至０．１８ＭＰａ（后段）。研究结果表明：采用高温连续式热压干燥的方式辅以适当的“呼吸”周期,可在２ｍｉｎ内将生材单板（初含水率１４９％）干至８％以下的终含水率。干单板基本平整、光滑,终含水率均匀,无撕裂。单板厚度干缩率平均５．３％,横纹干缩率４．９％。胶合强度平均０．８１２ＭＰａ略高于网带对流干燥的０．７７９ＭＰａ,结果说明此法干燥人工林场木单板是可行的。     

微波加热软化竹片弯曲工艺研究

采用微波对5 mm厚的楠竹竹片进行软化实验,研究了微波功率、微波处理时间、试件初含水率等因素对软化效果的影响,并采用正交实验对竹片微波软化进行了工艺优化。研究结果表明:微波加热对竹片有良好的软化作用,当微波功率为500 W,微波处理时间为4 min,试件初含水率为90%时楠竹竹片软化效果最好,其弯曲半径可达5 cm。     

短周期工业材干缩率和干燥应变规律的研究

研究了短周期工业材在干燥过程中木材表层和内部各层干缩率及弹性和塑性应变的变化规律。结果表明,基准硬度水平对应变值的大小和最大变就值的到达时间有显著影响。干燥前期,木材表层伸张弹性应变最大值随着温度和平衡含水率的增加而减少,木材表层塑性应变则随平衡含水率的增加而增加。正确调节基准初期阶段的平衡含水率水平,能有效地控制干燥前期应变最大值,从而达到控制干燥质量的目的。     

Digital image analysis of radial shrinkage of fresh spruce (Picea abies L.) wood

Contact-free digital image analysis was performed of the radial shrinkage of fresh, fully saturated small spruce wood beams. An experimental test set-up was developed to ensure constant distance from the charge-coupled device camera to the sample surface as well as constant climate and light conditions during the whole experiment. Dimensional changes were observed immediately after the drying process began. An unexpected distinct effect could be observed which could not be explained by drying surface layers only. After a fast initial radial shrinkage a slowing down of the dimensional changes occurred at high mean moisture contents. A complete interruption of any dimensional changes followed. Finally, a recovery from shrinkage was even observed. It is assumed that strong negative pressure occurred in the fully saturated capillaries owing to dehydration which led to additional dimensional changes. As a consequence, the break of the water column and aeration in these capillaries finally resulted in a recovery period in the shrinkage rate due to the pressure release. After this effect, the dehydration was characterized by a phase of fast and almost linear shrinkage due to drying surface layers. Finally, the shrinkage slowed down to zero when reaching equilibrium moisture content.     

龙竹竹材的微波干燥特性研究

对龙竹竹材在微波干燥下的干燥特性进行了研究。结果表明:微波功率和竹材构造对干燥特性有显著影响;竹材径向干缩率大于弦向干缩率;纵向干燥速度大于弦向和径向;竹材水分排除主要沿纵向,其次为弦向;竹材试件尺寸大小对竹材干缩率没有影响,试件长度对干燥速度有较显著影响。     

模糊自整定PID切换控制在木材干缩力控制中的应用

针对木材干燥过程中因干缩不均导致开裂等干燥质量问题,提出将模糊控制和PID控制相结合构成模糊自整定PID切换控制,以有效控制干缩力。在大误差时采用模糊控制,使实际干缩力值尽快达到设定值,提高干燥速度;在小误差时采用模糊自整定PID控制,使实际干缩力值与设定值基本保持一致,避免产生内裂。验证试验设置介质温度分别为70、80、90℃,相对湿度为30%,结果显示:松木试件在干缩力值分别达到最大值338、424、398N后开始减小,说明木材产生了开裂使干缩力得到释放;在同等试验条件下,采用切换控制后干缩力值在达到预设值后没有减小,保持稳定不变。MATLAB仿真与验证试验结果均表明:模糊自整定PID切换控制可以实现对干缩力的有效控制。     

Constitutive equation of wood at variable humidity and temperature

Summary Theoretical analysis of the effects of variations in moisture content and temperature on the creep of wood is presented. Thermodynamics of the processes of diffusion of water in wood microstructure is discussed and distinction is drawn between macrodiffusion and microdiffusion. The constitutive relation for steady states of moisture content and temperature is formulated on the basis of Maxwell chain model whose viscosity coefficients depend on moisture content and temperature. It is shown that the apparent acceleration of creep due to simultaneous drying (or wetting) as well as heating (or cooling) may be modeled as additional, stress-induced shrinkage (or swelling) and stress-induced thermal expansion (or contraction), described by shrinkage and thermal expansion coefficients that depend on the absolute values of the rates of pore humidity and temperature. Certain other sources of irreversibility of creep are also discussed.Thanks are due to Forest Products Laboratory of the U.S. Department of Agriculture, Madison, Wisconsin, for funding a large part of this work under subcontract FP-81-0389. Thanks are also due to John J. Zahn, research engineer from this laboratory and the subcontract monitor, for his numerous valuable discussions during the progress of the work.     

温度对人工林落叶松木材干缩及密度的影响

通过在不同的恒定的温度场中的试验,研究分析了人工林长白落叶松(Larix olgensis A.)木材干缩率、干缩比、心边材干缩差异及基本密度在不同温度条件下的变化规律,为合理地制定木材干燥工艺基准提供科学理论依据。     

竹材热压干燥过程中的水分迁移特性研究

研究了竹材热压干燥过程中的水分迁移特性.结果表明:在整个干燥过程中,前期含水率降低较快,后期含水率降低较慢.竹材平均干燥速度与次表层竹材的干燥速度相近;在含水率较高的干燥初期,水分迁移的阻力在竹材表面,水分迁移主要靠毛细管张力作用;在含水率较低的干燥后期,水分迁移的阻力在竹材内部,水分迁移主要以扩散方式进行,干燥速度取决于木材内部水分移动的速度.竹材热压干燥过程中的水分移动,主要受温度梯度和含水率梯度的共同作用.     

Advantages of prefreezing for reducing shrinkage-related degrade in eucalypts: General considerations and review of the literature

Summary Difficulties associated with the drying of ash eucalypts including collapse and internal checking, are discussed briefly. Prefreezing is one method that has been used successfully as a pretreatment for the drying of both hardwoods and softwoods from temperate and tropical regions.Prefreezing has produced marked reductions in shrinkage, collapse and drying degrade of the heartwood in the following species: California redwood, black walnut, black cherry, tanoak, toon, bamboo, and eucalypts. Little or no collapse reduction has been observed in New Zealand red beech, Pacific madrone, white birch, sitka spruce, and white ash. Limited response has been observed for numerous other species notably red oak and white oak.Reduced drying time in response to prefreezing has been observed in jarrah, karri, black walnut, Asian oak, toon, and California redwood; in Pacific madrone and tanoak the drying time increased. Not all species which respond with a reduction in shrinkage show reduced drying rates.Prefreezing wood at -20°C appears to be the most practicable temperature, although some species respond better at lower temperatures. However, in all cases, it is critical to ensure that the wood freezes and remains frozen for a number of hours. Indications are that the effect is retained for days to weeks and that the length of time of freezing need not exceed 12–24 hours.A number of explanations have been put forward to explain the behaviour of prefrozen wood. It is suggested that the main mechanism responsible for reduced shrinkage is due to the migration of moisture from the cell wall onto frozen lumen water. The moisture loss from the cell wall produces a cold shrinkage; water to ice transformation leads to an expansion of liquid water in the lumen, thus imparting a compressive stress to the cell wall, which together with the moisture loss, make the cell more rigid, and therefore likely to shrink less. There is some evidence that certain types of wood extractives migrate into the cell wall during freezing and may play a role in the reinforcement of the wall. Reduced shrinkage after prefreezing has also been attributed to a reduction of the plasticising effect of wood extractives in wood dried at higher temperatures and low humidities; this effect does not occur at low temperatures.Many suggestions and discussion from Dr. W. E. Hillis are gratefully acknowledged     

Wood as a natural smart material

The dominant feature of artificial smart materials is the “shape memory” effect. This phenomenon is based on frozen strains (FS). They were detected in wood fastened specimens during drying in the early 1960s. The integral law of wood deforming under loading and moisture content and/or temperature changes was subsequently formulated. This law takes into account the forming of FS. It was applied for the calculation of wood drying stresses. Stress memory and strain memory effects for wood were discovered. Wood has the ability to recollect the type of loading (tension or compression) which it had undergone. The difference between the free and restrained shrinkage is named “frozen shrinkage” (FSh). In calculations of drying stresses, it is more justified to use the FSh concept than “mechano-sorptive creep” (MSC). The MSC phenomenon is observed at cyclical change of moisture content in loaded wood. “Hygrofatigue” that reduces wood stiffness plays the main role in this process.     

黄毛青冈等四种硬木干燥特性的研究

对黄毛青冈、高山栲、滇石栎和栓皮栎等四种硬阔叶树材的构造、密度、全干缩率、弦、径向干缩变化、用百度法和气干等进行了观察和试验。结果表明四种木材导管内含有丰富或较多的侵填体,密度甚大,全干缩率大,弦、径向干缩比差异也大,水分传导性差,干燥速度缓慢,干燥时易发生表裂、内裂、翘弯和扭曲等现象,属难干材。通过试验,提出了克服干燥缺陷的一些措施,为制定四种木材干燥工艺提供了依据。