摘要
木材干燥应力应变是制定最佳木材干燥工艺的理论依据,干燥应力模型可以用数学的方法来描述木材干燥应力的产生原因和发展规律,是实现干燥应力控制技术的理论基础;因此,木材干燥应力模型的研究具有重要理论意义和实际生产指导意义。
本文以30mm厚马尾松板材为研究对象,把干燥过程中板材实际干缩分为自由干缩应变、瞬时弹性应变、粘弹性应变、机械吸附应变四部分,根据差分原理,对板材干燥时间进行离散,建立马尾松板材干燥应力数学模型,用模型解释了板材内部某层的自由干缩与板材实际干缩之差是该层产生干燥应力的原因;还用模型解释板材干燥过程中的应力变化规律、残余应力产生的原因及粘弹性应变和机械吸附应变对干燥应力的消除作用。模型验证实验表明,本文提出的干燥应力模型可以准确描述木材干燥应力应变变化规律;但此干燥应力模型不能计算和解释调湿处理过程中的干燥应力。
本文突破传统干燥应力定性分析方法,用数学方法对马尾松板材干燥应力应变规律进行深入研究,研究表明:粘弹性应变和机械吸附应变很大程度上释放了干燥应力。由于内部各层含水率依次低于FSP开始产生自由干缩应变,而受到其它层的制约,板材将由表层到芯层依次产生拉应力。在干燥过程中瞬时弹性应变最小,粘弹性应变与机械吸附应变较大。各层粘弹性应变与瞬时弹性应变比值近似为常数2.49;而机械吸附应变与瞬时弹性应变的比值波动较大,最大机械吸附应变与对应时刻的瞬时弹性应变比值为8.73～45.34;由于机械吸附应变的叠加,在应力转向前机械吸附应变比粘弹性应变大,应力转向后由于机械吸附应变正负叠加,可能使机械吸附应变小于粘弹性应变。
本文设计制作了一种电阻应变计式木材干缩的传感器,此传感器实现了木材干缩和粘弹性应变的连续测量,从而为木材力学性能和木材干燥应力应变研究提供一种新测量方法。本文还提出一种新的干燥应力连续测量方法,用此法测量马尾松板材表层临界干燥应力,并用表层临界干燥应力作为控制参数对30mm厚马尾松板材进行干燥,结果表明:此法比常规干燥快7.5h,干燥效率提高13.89%,干燥过程中未发现板材表裂,干燥质量达国家二级质量标准。
本文还得到了马尾松木材物理力学性能,为马尾松木材的深入研究奠定了理论基础。研究表明:温度对马尾松木材全干缩率和自由干缩系数无影响,在FSP以下自由干缩系数与含水率无关。马尾松木材弦向粘弹性系数不受载荷变化的影响,但随含水率和温度的变化而变化;马尾松木材弦向机械吸附系数不受载荷和含水率变化的影响,但随温度的升高而增大。
Wood drying stress and strain is the theoretical basis of making the optimum wood drying schedule. The drying stress model can describe the cause of generation of wood drying stress and it's developing regularity by mathematical method, and it is the theoretical basis which can accomplish the control technology of wood drying; therefore, it is theoretically important and practically significant to study wood drying stress model.The object of this paper was 30mm thick Pinus massoniana boards. The actual shrinkage during the process of drying was divided into four components, including free shrinkage strain, instantaneous strain, viscoelastic strain and mechano-sorptive strain. The mathematical model of drying stress of Pinus massoniana boards was established by diverging wood drying time, the difference between the free shrinkage of one layer in the board and the actual shrinkage of the board could be described using the model, which was the cause that drying stress generates in that layer. The model could also explain the changing regularity of stress, the cause of generation of residual stress during drying and the relievable effect of viscoelastic strain and mechano-sorptive strain on drying stress. The experiment of model verification indicated that the drying stress model presented in this paper could describe the changing regularity of drying stress and strain of wood during drying, but the drying stress model couldn't calculate and explain the drying stress during process of conditioning.In this paper the traditionally analytical method of drying stress was broken through, the study on drying stress and strain of Pinus massoniana board by mathematical method showed that viscoelastic strain and mechano-sorptive strain released the drying stress to a significant extent Because the moisture content of inner layers was orderly below FSP during drying, free shrinkage strain occured in the board, tensile stress took place orderly from outer layer to core layer. The instantaneous strain was the smallest during drying, viscoelastic strain and mechano-sorptive strain were larger. The ratio of viscoelastic strain to instantaneous strain of all layers was 2.49; but that of mechano-sorptive strain to instantaneous strain was in the large range of fluctuantion which was between 8.73 and 45.34; Because of its superimposition, the mechano-sorptive strain was larger than viscoelastic strain before stress reversal, but after that, mechano-sorptive strain might be smaller than viscoelastic strain.In this paper a wood shrinkage sensor of resistance strain was designed and made. The sensor realized the continuous measurement of wood shrinkage and viscoelastic strain and it provided a new measuring method for the study on the mechanical property and drying stress and strain of wood.A new method of continuous measurement for drying stress was also presented in this paper
and the critical drying stress of the surface layer of Pinus massoniana board was measured by that method. Pinus massoniana board of 30mm thick was dried by regarding the critical drying stress as controEing parameter. The results showed that the drying period was 7.5h faster than that of conventional drying, the drying efficiency was improved by 13.89%. The surface check was not found out during drying, and the drying quality could come up to National second degree drying quality standard.The physical and mechanical properties were obtained in this paper and the theoretical basis of profound research for Pinus massoniana board was established. The research indicated that temperature had no effect on the ratio of total shrinkage and the coefficient for free shrinkage, the coefficient for free shrinkage was irrelevant to moisture content below FSP. The shrinkage coefficient for Pinus massoniana board in tangential direction was not affected by the change of load, but it changed with the change of moisture content and temperature; the tangential mechano-sorptive coefficient was not affected by the change of load or moisture content, but it increased with the increase of temperature.
引文
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