【作者】 吴杰；

【导师】 郭康权；

【作者基本信息】 西北农林科技大学 ， 农业生物环境与能源工程， 2011， 博士

【摘要】 针对新疆特色的库尔勒香梨碰撞损伤问题,本研究从香梨不同组织的机械特性和果肉动态粘弹力学特性入手,分析香梨果肉的破坏机制及抗冲击损伤的性能,然后应用高速摄像分析的方法,对香梨跌落碰撞中的冲击能损耗与碰压损伤进行深入研究,并采用压力感应胶片测量和有限元数值模拟相结合的方法,对香梨碰撞接触应力分布特性进行进一步研究,确定了香梨碰撞接触应力分布与机械损伤的关系,揭示了香梨碰压损伤的规律,初步实现了香梨碰压损伤的预测和评估,为香梨机械化和自动化作业中的减损结构及工艺优化提供必要的基础数据和科学依据。本研究主要得出如下结论:(1)确定了香梨果肉的弹性模量为2.496 MPa、破坏应力为0.296 MPa、破坏能38.86 N·mm、坚实度为0.022 N/mm2,香梨果核部的各项机械特性参数值明显比果肉部的高;成熟度提高,香梨果肉的弹性模量、破坏应力、破坏能和坚实度都减小,低温储藏期的第2~4月各参数值变化不显著,显著差异发生在香梨由青熟进入黄熟的阶段和香梨过熟阶段;储藏温度升高,香梨果肉各项机械特性参数值呈下降趋势,-2°C低温时的香梨各项机械特性参数较大,但温度在10°C~30°C范围无显著差异。(2)建立了可精确拟合香梨果肉蠕变曲线的六元件广义Kelvin-Voigt模型,相关系数R2≥0.999;香梨果肉在蠕变回复阶段有一定的永久变形,即蠕变损伤,各蠕变参数中延迟弹性柔量对总蠕变柔量的贡献率最大;获得了香梨果肉的动态频谱响应特性,在0.1~10 Hz的频域中,常温下具有常膨压水平下的香梨果肉储能模量G′值范围为0.092~0.177 MPa,损耗模量G″值范围为0.009~0.031 MPa,其储能模量G′远高于损耗模量G″,损耗正切tanδ范围为0.08~0.20,表现出以弹性为主导的粘弹双重特性。(3)随着细胞膨压下降或者温度升高,香梨果肉的动态粘弹模量(G′,G″)逐渐减小,蠕变柔量(J0,J1,J2,1/η0)均逐渐增大,膨压增大或温度降低使香梨果肉趋于硬脆性,反之则使香梨果肉趋于软弹性;香梨的粘弹特性受温度影响较弱,在10°C变温范围内无显著变化,与之相比,膨压调控对香梨果肉粘弹特性有显著影响,是改变梨果肉的质地品质和抗碰撞损伤性能的有效方法;不同膨压水平香梨果肉组织的扫描电镜和光学电镜观察证明,膨压对香梨果肉组织的细胞结构和胞间隙有明显改变,是导致香梨果肉各粘弹参数都发生变化的本质原因,因此通过采摘期梨树适度灌溉或储藏期温湿调控改变香梨果肉的膨压水平,可达到改变香梨抗机械损伤性能的目的。(4)香梨在20~80 cm高度范围跌落,曲率半径较高的胴部比其它部位的损伤严重;在分级、运输和包装作业中应合理摆放香梨的姿态避免胴部碰撞;香梨与钢板、橡胶板、胶合板、瓦楞纸板、EPE板碰撞,在缓冲性能较差的钢板、橡胶板和胶合板上的冲击能损耗率在70%以上,碰撞损伤敏感度为6.32~10.32 N·mm2,香梨冲击能应限制在在0.18 J以下时,香梨才不会发生碰撞损伤;香梨在瓦楞纸板和EPE板这2种缓冲材料上的冲击能损耗率低于70%,碰撞损伤敏感度为2.32~3.74 N·mm2,香梨的冲击能应控制在0.4 J以下,香梨才不会发生碰撞损伤。(5)获得了香梨与不同触件材料碰撞接触应力分布的特征,接触应力峰值为0.5~0.6 MPa,果肉破坏的临界应力为0.2 MPa。与钢板、橡胶板和胶合板碰撞时,接触应力分布特征轮廓接近椭圆形,接触应力为正态分布特征,≤0.2 MPa的接触应力分布在边缘区域且面积很小,0.2~0.3 MPa范围应力的分布面积最大,接触应力分布面积接近香梨损伤面积,应力均值为0.25~0.30 MPa,不随着跌落高度提高而有明显增大趋势;与瓦楞纸板和EPE板碰撞,接触应力分布特征图的轮廓边缘呈现不同程度的放射状,接触应力呈非正态分布,较低冲击水平下,<0.2 MPa应力分布面积较大,接触应力分布面积与香梨损伤面积相差较大,但随着冲击能增大而逐渐减小,当冲击能≥0.55 J时,其分布开始萎缩而趋于边缘化,应力均值为0.19~0.25 MPa,随着跌落高度提高有较明显增大趋势;基于接触应力分布面积和应力均值建立的香梨的损伤面积和损伤体积预测的线性回归模型,可对香梨损伤精确评估。(6)不同储藏温度和成熟度的香梨碰撞接触应力分布面积由大至小顺序依次为:20°C黄熟>20°C青熟>0°C青熟,与之相应,香梨碰撞损伤敏感度由大至小顺序依次为:0°C青熟>20°C青熟> 20°C黄熟,冷藏的青熟香梨出库后,不适宜机械化作业;储藏温度对香梨碰撞损伤敏感度影响不显著,成熟度尽管对香梨碰撞损伤敏感度有显著影响,但远弱于缓冲材料对其影响作用,香梨机械防损应主要依靠接触材料的缓冲性能。(7)以线弹性和各向同性简化考虑,香梨的单层模型,即材料弹性模量取香梨果皮、果肉和果核弹性模量平均值时,可实现香梨在钢板和胶合板上碰撞接触应力分布和损伤的预测,精度可达到80%以上,但在非均质瓦楞纸板上的模拟结果对损伤面积预测的误差还很高,需进一步深入研究。更多还原

【Abstract】 The Korla pear bruising is an important problem in the process of packaging, sorting, storage and transportation. In this work the focus was on dynamic loading due to single impacts as this appeared to be most prevalent. Firstly, the stress-strain property and viscoelasticity of pear tissue were tested to understand the failure mechanism and impact resistance capability of pear. The impact bruise and energy loss of dropped pears were then analyzed using high speed video camera. Moreover, the contact pressure was measured by pressure-sensitive film in order to know the relation between contact pressure distribution and bruise area and volume. Finally the contact pressure of pear against impact surface was dynamic finite element modeled and numerical analytical results were validated with the measurements of film to achieve the bruise predication of pear and provide a design tool for reducing the likelihood of pear bruising occurring. The main contributions are as follows:(1) The module of elasticity, failure stress, failure energy and toughness of Korla pear flesh was 2.496 MPa, 0.296 MPa, 38.86 N·mm and 0.022 N/mm2, respectively. The values of mechanical parameters of pear core were significantly higher than those of pear flesh. There was no difference between the modulus of skins in transverse and longitudinal directions. The results showed that all mechanical properties decrease with the increasing storage time. The higher effect of storage was seen at first stage and final stage of ripeness. The mechanical property did not changed significantly from second to fourth month of storage period of pear. Storage temperature had weaker influence on all mechanical properties, although the values of -2°C pear were relatively higher among pears at different storage temperatures.(2) The creep compliance responses have been best characterized by a generalized Kelvin-Voigt model with six elements, with a correlation coefficient≥0.999. All pear tissues remained considerable plastic (unrecovered) strain in the creep recovery test. The relative contribution of retarded compliance to the overall compliance was highest among all types of compliance. The dynamic spectra of pear tissue have been obtained. The values of storage moduli (G′) and loss moduli (G″) of the Korla pear tissue with normal turgor level at 20°C from 0.1 to 10 Hz were 0.092~0.177 MPa and 0.009~0.031 MPa, respectively. The loss tangent tanδ(G″/G′) was 0.08-0.20 over the entire frequency range. Therefore, the Korla pear tissue behaved an elastic solid with storage moduli (G′) much higher than loss moduli (G″). (3) The instantaneous (J0), retarded compliance (J1 and J2) and steady-state fluidity (1/η0) increased while the storage and loss moduli (G′, G″) decreased as turgor was reduced or temperature was raised. The results indicated that the tissue was more brittle with increasing turgor of decreasing temperature. Conversely, the tissue was more ductile. The values of rheological parameters differed insignificantly when temperature interval was 10°C or so. The changes caused by temperature were much less greater than the changes caused by turgor adjustment. Many rheological parameters (G″, G′, J0, J1, J2 and 1/η0) showed the greater changes due to turgor levels, which was related with the changes of cells and intercellular spaces due to the turgor manipulation, as observed in light and scanning electron microscopy. Therefore, the turgor manipulation for pear tissue through storage temperature and humidity control and inrrigration schedule is considered be more effective measure than temperature control to improve the mechanical bruise susceptibility of Korla pear.(4) It was found that cheek impacts will give larger bruise than stem of calix shoulder impacts as this region tends to have a high radius of curvature when Korla pears dropped at the heights from 20 cm to 80 cm. The cheek of pear should be avoided being impacted through its proper position during the processes of package, transport and grading. The impact energy loss rate was above 70% and the bruise susceptibility ranged from 6.32-10.32 N·mm2 when Korla pear dropped onto steel, rubber and plywood at impact energies over 0.18 J. In comparison with these, the impact energy loss rate was below 70% and the bruise susceptibility was 2.32-3.74 N·mm2 for pear impacts against with expanded polyethylene (EPE) and corrugated board at same impact levels. Moreover, the bruise of pear occurred only impact energy was more than 0.4 J.(5) The measurements of contact pressure distribution of Korla pear for impacts against different contact materials have been obtained. The peak contact pressure was 0.5-0.6 MPa and the critical pressure of pear flesh failure was 0.2 MPa for all impacts. For pears against steel, rubber and plywood surfaces, the outlines of pressure distribution region were elliptic and the pressures tended to a normal distribution with relative small pressure area which was approximated bruising area. Additional, the 0.2~0.3 MPa pressure covered largest area and the average pressure was 0.25-0.30 MPa, which increased insignificantly with the increasing drop height. In the case of pear dropping onto expanded polyethylene (EPE) and corrugated board at low impact level, the contact pressure distributed in radiate. The pressures did not conformed to normal distribution and the pressure area was much larger than bruise area of pear. Also, it was founded that the pressure below 0.2 MPa in larger area and the average pressure was 0.19-0.25 MPa for pear contacts with these cushion materials, which tended to increase with the increasing drop height. The linear regress models fitted by the production of pressure area and average pressure can precision predicate and assess pear bruise area. (6) The increasing pressured areas of Korla pear were in order of 0°C green pear, 20°C green pear and 20°C yellow pear. Accordingly, the bruising of Korla pear was in order of increasing impact susceptibility as 0°C green pear > 20°C green pear > 20°C yellow pea, indicating that the green pear from cold storage room can not be desirable for mechanical treatments. The impact bruise susceptibility of pear differed insignificantly with the changes of temperature. Although the impact bruise susceptibility was affected weakly by the changes of pear ripeness, but it was sharply decreased when pear dropped onto the cushion materials. To conclude, the impact bruising reduction of Korla pear should depend on the use of cushion materials, not on control of storage temperature and ripeness.(7) The charactering pressure and bruising for pear impacts has been achieved using finite element analysis. The linear elastic model of pear was developed and its material was assumed be isotropic. When the average value of Young’s module of skin, flesh and core was used, the differences between the numerical bruise predications for steel and plywood and experimental results were relatively small. It proved that the analytical modelling of pear impacts can be applied to optimal design for mechanized equipment to reduce pear bruising. However, the predication error is larger for soft counterfaces such as corrugated board due to its inhomogeneous material property.更多还原