| 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.
|
PDF Full Text Request