【作者】 王芳；

【导师】 王春光；

【作者基本信息】 内蒙古农业大学 ， 农业机械化工程， 2008， 博士

【摘要】 西瓜的采收、贮藏、运输对其破损率均有很大影响。为了确保西瓜贮运、生产、加工、设计的合理性,需对西瓜的承载能力和抗冲击能力进行有效的定量分析,并深入研究西瓜的力学特性和损伤机理。利用力学特性来指导西瓜检测分级,进一步完善西瓜品质评价体系,从而为西瓜的采收、分级、贮运等提供一定的理论指导和数据支持,对减少西瓜腐烂损失,延长鲜食品种供应期有着重要的意义。鉴于目前西瓜贮运损伤引起品质下降的问题,针对国内外西瓜力学损伤研究中存在的不足,本文提出根据西瓜的压缩特性和蠕变特性对其进行研究,通过对自然状态下完整西瓜进行压缩试验、蠕变试验和随贮藏时间变化的蠕变特性与品质关系的试验研究,并在此基础上利用有限元法对西瓜的力学特性进行了仿真。通过试验研究和理论分析,得到如下结论:1.在自然状态下对完整西瓜进行了压缩试验,获得了不同温度、不同硬度、不同速度条件下西瓜不同压缩部位的力—变形曲线并对其曲线进行了分析,从曲线反映出有两个可见特征点:生物屈服点和破裂点,获得了沿瓜梗垂直压缩的破坏极限比水平受压的要大。2.由曲线及对曲线拟合得到了不同条件下压缩特性参数:生物屈服极限、变形能、破坏极限、破坏能和刚度,分别获得了不同温度、硬度和速度条件下西瓜瓜梗水平受压的刚度范围20.6～42.5 N/mm,垂直受压的刚度范围27.9～62.3 N/mm;瓜梗水平受压的破坏极限范围845～1940 N,垂直受压的破坏极限范围1077～2432 N;瓜梗水平受压的破坏能范围8.3～60.4 N·m,垂直受压的破坏能范围16.4～68.7 N·m,对西瓜采收、贮运、收获和加工机械的设计提供理论指导和数据支持。3.研究了不同条件对压缩特性参数的影响并对其进行了方差分析,得到了温度、速度、硬度对压缩特性参数的影响都呈极显著差异(显著性概率Sig.值<0.01)。压缩特性参数之间具有一定的相关性。4.对西瓜的损伤机理进行了分析。西瓜瓜梗水平受压,最大力是在赤道平面的表皮处,方向沿子午线方向,裂纹沿子午线方向扩展。瓜梗垂直受压,瓜皮从瓜梗开始破裂,裂纹沿子午线方向。这说明最大力是造成西瓜在压缩载荷作用下产生破裂的原因,可用最大力来解释西瓜宏观破坏的机理。5.建立了压缩特性参数预测西瓜损伤体积和品质的数学模型,经方差分析所建数学模型有意义,说明用压缩特性参数可以预测西瓜的品质和损伤程度。6.运用有限元法建立西瓜的压缩模型,比较西瓜水平受压的试验值和仿真值,二者最大差异是10%;研究西瓜瓜梗垂直和水平受压的承载特性,验证了仿真数值解的可行性。结果表明:在相同压力作用下,西瓜瓜梗垂直放置时,其受压的应力和应变都小于水平受压的情况。研究结果可为西瓜的贮藏、运输和加工等提供理论依据。7.获得了不同加载力、不同温度和不同硬度的蠕变曲线和相应的蠕变特性参数(γ0、γ1、Trel、η2),建立了西瓜的蠕变模型为四元件Burgers模型和相应的本构方程:分析了不同加载力、温度和硬度对西瓜蠕变曲线、蠕变参数的影响,蠕变参数对蠕变曲线的影响和蠕变参数之间的相关性。并对该本构方程计算的西瓜蠕变结果与本试验结果进行了模拟,误差在5%以内,验证了理论结果的正确性。8.在不同载荷作用下,西瓜的蠕变速率和蠕变变形均有所不同,载荷越大,蠕变速率越快,相应变形也越大,达到极限变形值时西瓜产生损伤,所以可用临界变形来解释西瓜的蠕变损伤机理。9.获得了不同温度、不同硬度和不同载荷下西瓜品质(TSS、PH等)、蠕变及相应的蠕变特性参数随贮藏时间变化的曲线并分析了其变化的原因;建立了蠕变参数随贮藏时间变化的数学模型、蠕变参数预测西瓜品质的数学模型。将其蠕变参数和西瓜品质的相关性进行了分析,用蠕变参数预测西瓜的品质和贮藏时间,从而无损评定西瓜品质,提高产品质量,进一步设法减少西瓜损伤。更多还原

【Abstract】 Watermelon is easily damaged in the process of collecting, storage, transportation. Therefore, it’s necessary to do quantitative analysis on watermelon’s biomechanics and load capacity to ensure the reliability and rationality of processing and design. One hand, it’s favorable to understand watermelon’s mechanics and its laws, to guide watermelon grading with mechanical properties, to improve the evaluation system of watermelon’s quality. On the other hand, since watermelon’s mechanics are more intensively researched and broken mechanism was revealed, which can conduct watermelon’s sorting, collecting and transporting.According to the problems on declining watermelon quality caused by storage and transportation currently, and the shortcomings of study on mechanical damage of watermelon at home and abroad, a new evaluation method was put forward in this paper: the intact watermelon under the natural status was studied by a compression test and creep test, and the relation between creep characteristics and quality with the variation of storage time, as well as mechanical properties of the watermelon were analyzed with Finite element method through test and theoretical analysis. The following conclusions were drawn:1. Deformation-force curves about ellipsesolid body of intact watermelon under quasi -static loading were developed and analyzed under different temperature and hardness, different loading rate. The curve reflects two visible feature points: biological yield point and the rupture point. In addition, the curve shows that rupture force compressed in perpendicular direction is greater than horizontal bearing’s watermelon.2. Compressive parameters had been acquired by the curve and fitness the curve under different conditions, such as biological yield force, deformation energy, rupture force, rupture energy. The scope of stiffness of horizontal and perpendicular bearing’s water melon is 20.6~42.5 N/ mm and 27.9~62.3 N/mm; the scope of rupture force is 845~1940 N and 1077~2432 N; the scope of rupture energy were 8.3~60.4 N?m and 16.4~68.7 N?m.It provide theoretical guidance and data support for watermelon harvest, storage, transportation and design of processing machinery. 3. The effect and variance about temperature, speed and hardness against the compressive parameters was analyzed. It can be concluded that the compressive parameters were all significantly affected by the effective factors (significant probability Sig.Value<0.01). Statistic analysis indicated that there is a significant positive correlation among compression parameters presented.4. The damage principle of watermelon was analyzed. For horizontal bearing’s watermelon, its maximum force position is in the epidermis of equatorial plane, along the direction of meridian direction. Perpendicular bearing’s watermelon stem, its cracks is along the meridian direction. The study results show that the damage of watermelon was caused by the maximum force under the compression load. Therefore the maximum force can be used to explain the mechanism of watermelon macro damage.5. Two Mathematical models were set up: (1) compression parameters forecast watermelon damage volume; (2) compression parameters forecast watermelon quality.The model which was set is meaningful through the analysis of ANOVA. Study result show that parameters of compression can predict injury and quality of watermelon.6. The mechanics model of compressing watermelon was established by the finite element method, and its horizontal bearing’s simulation value of carrying capacity was compared with experimental data, which showed that maximal difference between simulation and experiment is 10.0%. At last, the characteristics of carrying compressing capacity in perpendicularity and horizon of watermelon stem was studied, and the simulation numerical solution was confirmed by comparing with the experiment data. It can be concluded that perpendicularl bearing’s watermelon stem strain and the stress was less than the horizontal bearing under the same pressure. The study will provide the theory basis to watermelon’s storing, transporting and processing and so on.7. The creep curve and the corresponding creep parameters of watermelon (γ0,γ1, Trel,η2) were acquired in different load, temperatures and hardness. At the same time a watermelon creep model was established as the four components Burgers model, and the corresponding constitutive equation: D tFFetFt. It is found that the new model well describes the full creep procedure of watermelon. by a different load, and temperature and hardness of watermelon creep curve, parameters of creep, creep parameters on the creep and creep curve of the correlation between parameters. The effect on temperature, hardness and loading force against the creep curve and parameters, on creep parameters against creep curve, and correlation about creep parameter with itself was analyzed. The parameters of the creep equations are calculated and the forecast curves are drawn. In comparison with the experimental curves, the theory results are proved, within the error of 5 percent. The rheological parameters of the creep equations are calculated and the forecast curves are drawn. In comprison with the experimental curves, the theorial results are proved, within the error of 5 percent.8. The creep rate and deformation of watermelon are different in different load. The greater load, the sooner the creep rate and the greater corresponding deformation is. The reason of creep damage was deformation values in excess of threshhold. It is feasible for explaining the mechanism of injury creep to use critical deformation.9.Watermelon quality(TSS, PH etc.)curve, creep curve and the corresponding creep parameters with different storage time was acquired under different temperature, hardness and different load, and their changeable ressons were analyzed. Two mathematical models were set up: (1) the creep parameters change with the storage time; (2) creep parameter forecast watermelon quality. The relevance of creep parameters and texture of watermelon was analyzed to realize non-destructive assessment watermelon texture and improve product quality, and reduce the watermelon injury.更多还原