莲仁力学特性参数测定及挤压破碎特性试验

为探讨莲仁在挤压载荷作用下的破碎机理,该文用压缩试验方法对莲仁相关力学参数进行了试验研究。首先,用万能试验机对莲仁试样进行了弹性模量和抗压强度测试,测得5种不同含水率莲仁在纵向、横向2个方向上的弹性模量、抗压强度及应力-应变曲线;测试结果表明,莲仁弹性模量、抗压强度的纵向值均大于横向值,且弹性模量、抗压强度均随莲仁含水率增大而显著减小;用回归分析方法建立了莲仁弹性模量、抗压强度与含水率关系的回归方程,得到了合格莲仁(含水率小于11%)的最小弹性模量为37.12 MPa,最低抗压强度为5.12 MPa。其次,研究了莲仁整体挤压时裂纹的产生规律,测得了5种不同含水率莲仁在平压与侧压2种受载方式下的挤压极限载荷;测试结果表明,莲仁的挤压极限载荷随含水率的增大而减小,对于相同含水率莲仁,侧压比平压更容易引起莲仁破碎;用回归分析方法建立了挤压极限载荷与含水率的回归方程,得到了合格莲仁(含水率小于11%)的最大挤压载荷应小于53.6 N。最后,用有限元法建立了莲仁的压缩力学模型,分析了莲仁整体受压时的应力分布,仿真结果与莲仁压缩试验结果基本吻合,验证了所测莲仁力学参数的正确性。研究结果可为莲仁加工和输送设备的设计提供参考。

Determination of mechanical characteristic parameters and extrusioncrushing characteristics test for lotus seed kernel

Abstract: In this paper, an innovative compression technology is developed to study and analyze the mechanical properties of lotus seed kernel with regard to elastic modulus, compression strength and ultimate load when the whole lotus seed kernel is under the pressure. The objectives of this paper are to study the damage and crushing characteristics of lotus seed kernel under the condition of squeezing the load and reducing the crushing rate in the processing and transportation. Firstly, the universal testing machine is utilized to implement tests of elastic modulus and compression strength for lotus seed kernel samples. A series of the mechanical properties are measured for the lotus seed kernel with varied moisture content (5.52%, 7.78%, 10.29%, 12.47%, and 15.06%) in terms of vertical elastic modulus, horizontal elastic modulus, longitudinal compression strength and transverse compression strength. The corresponding transverse stress-strain curve and longitudinal stress-strain curve are figured out. The measurement results indicate the vertical and horizontal elastic modulus and compression strength of lotus seed kernel decrease with the increase of moisture content. The longitudinal values are higher than transverse values as to samples with same level of moisture content. Therefore, lotus seed kernel belongs to a kind of anisotropic material. It is much easier to cause lotus seed kernel broken when it suffers horizontal compression. When the moisture content is relatively low (5.52%, 7.78%, 10.29%), there is a linear relationship between stress and strain. Lotus seed kernel shows brittleness. If the moisture content is much higher (12.47%, 15.06%), strain becomes larger, which means lotus seed kernel has great toughness. The regression equation is established for the sake of carrying out regression analysis based on test results of elastic modulus and compression strength. It is obtained that the minimum compression strength and minimum elastic modulus of qualified lotus seed kernel with moisture content smaller than 11% are 5.12 and 37.12 MPa respectively. Furthermore, the crack caused by compression is also discussed on the basis of varied moisture contents and different compression methods. It is measured that the ultimate loads under the flat pressure for 5 experimental samples are 103.92, 89.87, 72.21, 59.60 and 51.73 N, respectively. Similarly, the ultimate loads under the side pressure are 88.19, 72.78, 57.80, 45.58 and 39.16 N, respectively. From the measurement data, the moisture content and compression method of lotus seed kernel have great influence on ultimate loads. The ultimate loads increase with the decrease of moisture content, no matter it suffers side pressure or flat pressure. Furthermore, the ultimate load of side pressure is less than that of flat pressure in terms of same level of moisture content. Therefore, side pressure is much easier to make the lotus seed kernel broken. The first crack positions are the load position, parting surface and area around the drilled hole in flat pressure test. As to the side pressure test, the first crack positions are on the both sides of parting surface. Based on the test results of ultimate loads of lotus seed kernels, a regression model of ultimate loads and moisture content is established. It is calculated that the maximum compression load of the qualified lotus seed kernel (moisture content is less than 11%) should be less than 53.60 N. In practical cases, the compression load should be less than this limit value. Lastly, a dynamic model of lotus seed kernel compression is developed with the finite element method. The stress distribution of pressure states of lotus seed kernel is figured out. The analysis results demonstrate the stress at load position, parting surface and area around the drilled hole is larger in flat and side pressure tests. The parting surface is the weak part of lotus seed kernel. It expresses that the simulation results well fit the crack position and direction in the compression test, which identifies the correctness of measurement parameters. The study results provide a reliable evidence for determining process parameters of processing and conveying equipment in the lotus seed kernel production.