玉米秸秆力学特性的离散元建模方法研究

发布时间：2019-06-12 20:27

【摘要】：玉米秸秆作为一种重要的生物质资源,在资源日益匮乏的经济时代,逐渐被大家所重视。由于玉米秸秆具有复杂多样的内部结构,导致其具有独特的个体差异,从而呈现出不同的力学性能,直接影响它的利用。深入研究玉米秸秆各组分的力学性能及其差异,并建立相应的力学模型,探讨外载荷作用下的力学特性变化规律,为其深度综合利用和预处理加工机械设计提供基础的理论依据。针对玉米秸秆的力学特性理论分析和数学建模这一较为困难的问题,在对玉米秸秆的生物物理力学特性进行研究的基础上,利用离散元方法建立玉米秸秆离散元力学模型,包括:玉米秸秆外皮、内穰的剪切和拉伸力学特性离散元模型;单根玉米秸秆的径向压缩、弯曲力学特性离散元模型和玉米秸秆集合体的打捆力学特性离散元模型等。论文围绕玉米秸秆离散元模型的建立,对玉米秸秆物理和力学特性进行了研究,取得的主要研究成果如下:(1)测试了玉米秸秆基本物料特性参数,得到:玉米茎秆的平均直径为16.14±4.05mm,平均外皮厚度为0.9mm,外皮、内穰和整体的平均密度分别为1.12、0.66和1g/cm3,同时用图像处理的方法得到玉米秸秆维管束的横截面直径为0.15mm,用烘干法测得新鲜玉米秸秆含水率为80%,风干玉米秸秆的含水率为8%。(2)建立了玉米秸秆外皮轴向剪切力学特性的离散元模型,同时进行了剪切测试与模拟对比。剪切测试结果表明:外皮平均剪切强度为3.00MPa,平均弹性模量为0.23GPa;外皮轴向剪切强度和弹性模量随节间位置升高而降低;含水率和品种对剪切强度和弹性模量均无影响。模拟结果表明:对玉米秸秆外皮轴向剪切模型加载后模拟的力学特性与实际测试力学特性相似;模拟的破坏过程中的载荷—位移图线与测试实测曲线相一致;获得了外皮维管束间的离散元模型参数,主要参数s-bond和kn的取值范围分别为0.14-0.32 N和1×106-1.2×107N·m-1。(3)建立了玉米秸秆外皮和内穰进行拉伸力学特性的离散元模型,同时进行了拉伸测试与拉伸模型模拟对比。拉伸测试结果表明:秸秆外皮拉伸具有塑性材料的特征,在新鲜状态下,抗拉强度为131.1?48MPa,弹性模量为22.93?13GPa;风干状态下,抗拉强度为113.42?40MPa,弹性模量为15.71?6GPa;玉米秸秆内穰拉伸具有脆性材料的特征,在新鲜状态下,抗拉强度为1.09?0.27MPa,弹性模量为0.06?0.02GPa;在风干状态下,抗拉强度为0.86?0.67MPa,弹性模量为0.12?0.07GPa;玉米秸秆的拉伸强度和弹性模量随节间位置升高而降低,随含水率的增加而增加,拉伸强度受到品种的一定影响,拉伸弹性模量却不受品种影响。秸秆内穰的拉伸强度,新鲜秸秆大于风干秸秆;而对于拉伸弹性模量,则为风干秸秆大于新鲜秸秆;节间位置和品种对内穰的拉伸强度和弹性模量无显著影响。离散元模型模拟结果表明:玉米秸秆外皮和内穰组织的离散元模型轴向拉伸力学特性曲线,以及破坏现象,均与测试特性相似;并分别获得外皮轴向模型主要力学性能参数bondstrength为85.64-131.11 Pa·m-1(新鲜秸秆)和80.53-113.42 Pa·m-1(风干秸秆),pb-kn为4.58×1013-1.05×1014 Pa·m-1(新鲜)和3.79×1013-6.97×1013 Pa·m-1(风干),以及内穰离散元模型的离散元参数kn为0.5×106-1.5×106 N·m-1(风干)和1.6×106-3×106 N·m-1(新鲜),ks为1.0×106 N·m-1,n-bond为5-30 N(风干)和35-50N(新鲜),s-bond为10-20N(风干)和40-50N(新鲜)。(4)建立了单根玉米秸秆的径向压缩力学特性离散元模型,同时进行了径向压缩测试与模拟结果的对比。模型的模拟结果表明:得出得到虚拟的载荷-位移曲线主要包括弹性阶段、破坏阶段和强化阶段,并分析了虚拟的压缩破坏现象和压缩过程中的微观力学响应;将测试结果与模拟结果对比分析,认为模型合理,与实际材料吻合,并通过模型分析出玉米秸秆受压状态时内部结合机理。(5)建立了单根玉米秸秆的弯曲力学特性离散元模型,同时进行了弯曲测试与模拟结果对比。模拟结果表明:弯曲力学特性曲线主要包括弹性阶段和粘弹性阶段,并分析了虚拟的弯曲破坏现象和压缩过程中的微观力学响应;将测试结果与模拟结果对比分析,认为模型合理,与实际材料吻合,并通过模型分析出玉米秸秆弯曲破坏时内部结合机理。(6)建立了多根玉米秸秆的打捆力学特性离散元模型,并进行虚拟打捆模拟与测试结果对比。打捆测试得到:捆扎密度与打捆松紧度的关系为二次曲线关系;秸秆打捆的绳索张力与捆扎直径的关系为指数函数关系。虚拟打捆模拟得到:虚拟打捆力学性能曲线与测试曲线相似;虚拟打捆模拟的载荷与圆捆半径呈现指数函数关系。测试结果与模型模拟趋势相同,表明离散元模型可用于玉米秸秆加工利用机械设计中的物料力学特性模拟。
[Abstract]:As an important biomass resource, the corn straw is gradually being paid attention to by the economic times of the increasingly scarce resources. Because the corn straw has a complex and diverse internal structure, it has unique individual difference, thus presenting different mechanical properties and directly affecting the utilization of the corn straw. In this paper, the mechanical properties and the difference of each component of the corn stalk are studied deeply, and a corresponding mechanical model is established to study the change rule of the mechanical property under the action of the external load, which provides a theoretical basis for the comprehensive utilization of the maize straw and the mechanical design of the pre-treatment process. Based on the research of the physical and mechanical properties of the corn straw, the discrete element method of the maize straw is established by means of the discrete element method, which includes the following steps: A discrete element model of shear and tensile mechanical properties of a single corn straw, a discrete element model of the radial compression of a single corn straw, a discrete element model of a bending mechanical property and a bundling mechanical characteristic discrete element model of the corn straw aggregate, and the like. The paper studies the physical and mechanical properties of the maize straw, and the main research results are as follows: (1) The basic material property parameters of the corn straw are tested, and the average diameter of the corn stalk is 16.14-4.05mm. The average density of the outer skin is 0.9mm, the average density of the outer skin, the inner layer and the whole is 1.12, 0.66 and 1 g/ cm3 respectively, and the cross section diameter of the corn straw vascular bundle is 0.15 mm by the method of image processing, and the water content of the fresh corn straw is 80% by the drying method, and the moisture content of the air-dried corn straw is 8%. (2) The discrete element model of the axial shearing and mechanical properties of the corn straw outer skin is established, and the shear test and the simulation comparison are also carried out. The results of the shear test show that the average shear strength of the skin is 3.00 MPa, the average elastic modulus is 0.23 GPa, the axial shear strength and the elastic modulus of the outer skin decrease with the increase of the position of the internode, and the water content and the variety have no effect on the shear strength and the elastic modulus. The simulation results show that the mechanical characteristics of the simulation after loading the axial shear model of the corn straw outer skin are similar to that of the actual test mechanics, and the load displacement curve in the simulation is in line with the measured curve of the test, and the discrete element model parameters between the outer sheath and the pipe bundle are obtained. The values of s-bond and kn are 0.14-0.32 N and 1-106-1.2-107 N 路 m-1, respectively. (3) The discrete element model of the tensile and mechanical properties of the outer skin and the inner surface of the maize straw was established, and the comparison of the tensile test with the tensile model was also carried out. The tensile test results show that the tensile strength is 131.1-48MPa, the elastic modulus is 22.93-13GPa in the fresh state, the tensile strength is 113.42-40MPa, the elastic modulus is 15.71-6GPa, and the tensile strength of the corn straw is characterized by the brittle material. in the fresh state, the tensile strength is 1.09-0.27MPa and the elastic modulus is 0.06-0.02GPa; in the air-drying state, the tensile strength is 0.86-0.67MPa, the elastic modulus is 0.12-0.07 GPa, the tensile strength and the elastic modulus of the corn straw are reduced along with the increase of the position of the internode, and the elastic modulus is increased with the increase of the water content, The tensile strength is affected by the variety, and the tensile elastic modulus is not affected by the variety. The tensile strength of the straw in the straw is higher than that of the air-dried straw, and for the tensile elastic modulus, the air-dried straw is larger than the fresh straw, and the internode position and the variety have no significant influence on the tensile strength and the elastic modulus of the inner layer. The simulation results of the discrete element model show that the mechanical characteristic curve of the axial tensile and mechanical properties of the discrete element model of the corn straw outer skin and the inner-shell tissue are similar to those of the test characteristics. And the main mechanical property parameters of the outer skin axial model are 85.64-131.11 Pa 路 m-1 (fresh straw) and 80.53-113.42 Pa 路 m-1 (air-dried straw), and the pb-kn is 4.58-1013-1.05-1014 Pa 路 m-1 (fresh) and 3.79-131013-6.97-1013 Pa 路 m-1 (air-dried). And the discrete element parameter kn of the internal discrete element model is 0.5-106-1.5-106N 路 m-1 (air-dried) and 1.6-106-3-106N 路 m-1 (fresh), ks is 1.0-106N 路 m-1, n-bond is 5-30N (air-dried) and 35-50N (fresh), s-bond is 10-20N (air-dried) and 40-50N (fresh). (4) The discrete element model of the radial compression and mechanical properties of a single corn straw is established, and the comparison of the radial compression test with the simulation results is carried out. The simulation results of the model show that the virtual load-displacement curve mainly includes the elastic phase, the failure stage and the strengthening phase, and the virtual compression failure phenomenon and the micro-mechanical response in the compression process are analyzed, and the test results are compared with the simulation results. It is considered that the model is reasonable and is in good agreement with the actual material, and the internal binding mechanism of the corn straw under pressure is analyzed through the model. (5) A discrete element model of the bending and mechanical properties of a single corn straw is established, and the comparison of the bending test with the simulation results is carried out. The simulation results show that the curve of bending mechanics mainly includes the elastic phase and the viscoelastic phase, and the virtual bending failure and the micro-mechanical response in the compression process are analyzed. The results of the test and the simulation results are compared and analyzed, and the model is considered to be reasonable and consistent with the actual material. And the internal binding mechanism of the corn straw bending and destruction is analyzed through the model. And (6) establishing a discrete element model of a bundling mechanical property of a plurality of corn straws, and comparing the virtual bundling simulation with the test result. The relationship between the binding density and the bundling diameter is the quadratic curve, and the relationship between the rope tension and the binding diameter of the straw bundle is an exponential function. The virtual bundling simulation results in that the virtual bundling mechanical property curve is similar to the test curve, and the load of the virtual bundling simulation is in exponential function relation with the radius of the round bundle. The result of the test is the same as that of the model, which indicates that the discrete element model can be used to simulate the mechanical properties of the material in the mechanical design of the corn straw processing.
【学位授予单位】：西北农林科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ914.3

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