苎麻茎秆力学模型的试验分析

为了提供苎麻收割、剥制机械研究设计的力学参数及理论基础,论文对苎麻茎秆的几何形状进行假定,运用复合材料力学理论建立苎麻茎秆力学模型,利用微机控制万能试验机对苎麻茎秆各向(轴向、径向)及各组分(木质部、韧皮部和茎秆整体)进行拉伸、压缩、弯曲等力学试验,获得力学参数数据,并通过数据的分析计算获得苎麻茎秆力学模型的全部弹性参数。同时,试验结果表明,苎麻茎秆在轴向拉伸中表现更多为木质部的承载作用,其韧皮部与木质部的粘附力不能阻止韧皮部沿木质部表层滑移;苎麻茎秆径向弹性参数测量值和通过各组分弹性参数值计算获得的计算值接近,苎麻茎秆径向符合复合材料的特性。

Experimental analysis on mechanical model of ramie stalk

Abstract: Mechanical cutting and fiber peeling are important parts in the ramie production process, and its working process is the process of interaction between rigid body (machine) and flexible body (ramie stalk). To make the developed harvester and peeling machine to meet the working requirement of high quality, efficiency and low consumption, the premise is to get mechanical characteristic parameters of crop, to have a fundamental understanding of ramie harvesting and fiber peeling mechanism. Based on this, in order to provide the mechanical parameters and theoretical basis for the research on cutting and peeling ramie, experimental analysis on the mechanical model of ramie stalk has been made in the paper. The crop object for experimental analysis was the third crop of Zhongzhu No.1 planted in Xianning Ramie Comprehensive Experiment Station of China Agricultural Research System (Bast fiber crops), the acquisition time was on November 2nd, 2013, and the test was conducted in Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture, during November 5th to 15th, 2013. During the experimental research process, the geometric model for ramie stalk was assumed firstly. The cross section of ramie stalk was similar to the circle, which could be respectively divided into central medulla, xylem, phloem and green husk layer. The mechanical property in the central medulla and green husk layer might be ignored. The paper abstracted and simplified the geometrical shape, and presumed that the cross section was round with certain diameter. The material, diameter and wall thickness dimension of each fraction were even. After ignoring the central medulla and green husk layer, the geometrical shape of ramie was assumed as hollow pipe, made up of different kinds of materials including xylem and phloem. Secondly, composite mechanical theory was employed to construct mechanical model of ramie stalk. Elastic parameters that should be obtained from experiment were decided according to the assumption of ramie geometrical shape and experimental equipment conditions, as well as the calculation formula of each elastic parameter. Thirdly, PC-controlled universal testing machine was applied to make mechanical test, such as tensile, compressive and bending, to ramie stalk from each direction (axial direction and radial direction) and to each part (xylem, phloem and the whole stalk), to acquire mechanical parameter data. The universal testing machine used in the experiment was WDW-10 PC-controlled electronic universal testing machine with test power range 5 kN, and the accuracy of its force sensor and displacement sensor were both maintained within ±0.1%. In tensile test, xylem, phloem and stalk samples were 20 groups of rectangle samples about 80 mm long, 6-7 mm wide, material thickness thick. Plate fixture was used, and the clamped position was covered by gauze for protection. Pre-tightening force when starting was less than 5 N, test loading rate was 5 mm/min. Then, axial tensile stress-strain curve of each sample group was obtained. In compressive test, xylem, phloem and stalk samples were 10 groups of rectangle samples about 10- 11 mm long, 5-6 mm wide, material thickness thick. Compressive test briquetting was used. Thus, radial comprehensive stress-strain curve of each sample group was obtained. In bending test, xylem and stalk samples were 10 groups of samples with the length of about 120 mm. The sample cross section was tube-shape, and external and inner diameter was decided by material conditions. Sample was placed between the support and pressure head of three-point bending test fixture. Span of the support was 80 mm, pre-tightening force when starting was less than 5 N, and test loading rate was 5 mm/min. Radial bending force-deformation curve was obtained. At last, all elastic parameters of ramie stalk mechanical model were obtained with analysis of test data. The radial compressive elasticity modulus were 9.73 MPa (stalk), 10.34 MPa (xylem) and 8.03 MPa (phloem); the axis tensile elasticity modulus were 466.17 MPa (stalk), 320.92 MPa (xylem) and 2409.22 MPa (phloem); the radial bending shear modulus were 39.77 MPa (stalk), 69.02 MPa (xylem) and 33.80 MPa (phloem), the axis torsional shear modulus were 3.74 MPa (stalk), 3.98 MPa (xylem) and 3.09 MPa (phloem), the Poisson's ratios of plane XZ of each parts was assumed to 0.3, and the Poisson's ratios of plane XZ and the Poisson's ratios of plane XZ were equal, the values were <0.0269 (stalk), <0.0451 (xylem) and <0.0043 (phloem). Meanwhile, the test results of paper showed that the axial tension of ramie stalk played the load-bearing role much more in the xylem, and the adhesive force in the phloem and xylem was incapable to prevent the phloem from sliding along the surface of xylem; the measurement value of elastic parameter in the radial direction of ramie stalk was closed to the calculated value gained by calculating the elastic parameters of each fraction, and the radial direction of ramie stalk could match the characteristics of composite materials. In conclusion, the assumption of the composite model adopted in the experiment is reasonable, and experimental analysis results can be applied into subsequent related researches.