Measurement of contact pressure of Korla pear under compression and bruising predication using finite element analysis

Abstract: Bruise of Korla pears due to mechanical damage caused by static loading is considered as a significant problem during orchard harvest and transportation. In practice, there is a greater interest in damage area for visible bruising assessed by commercial operators and consumers. In this respect, bruise area is essentially related to contact pressure. Experimentally, the contact pressures are difficult to measure as fruit exhibit both anisotropic properties and viscoelastic behavior. It is worth noting that a non-invasive pressure-sensitive film is now used to study the contact area and pressure of fruit due to its ease of application. Additionally, an alternative approach is to estimate stresses using finite element analysis (FEA), which provides a user-friendly format in interpreting bruise formation of fruit. However, the output from FEA is very difficult to interpret and compare. Therefore, the objectives of this work were to study static contacts for Korla pear compressed at different loads using a novel pressure sensitive film technique. The film measurements were then used to validate the analytical results of FEA aiming to accurately predicate the bruise area of Korla pear under compression. In this study, the pear samples were compressed at loads of 50, 100, 150, 200 and 250 N by two parallel plates. The Prescale(r) film (Fuji Film Corporation, Japan) for ultra super low pressure was placed between the sample and the upper plate to measure contact pressure and contact area. When the force was loaded on the film, microcapsules were broken to release their color-forming material at a density that corresponds to the specific level of applied pressure. A prescale pressure graphic system (FPD-8010E), composed of a scanner, was used to evaluate multicolor of results and obtain the data of contact. Meanwhile, the model geometry of a real Korla pear was developed using the ''Spline" option and imported into ANSYS software to create a mesh with hexahedral elements. Assuming that a pear was a linear elastic material, the FEA simulation was run to obtain equivalent stress that followed the Von Mises stress criteria for failure. The film measurements showed that the maximum pressure in the pear contacts remained at a level of 05-0.6 MPa for all loads. Although the film presented the contact pressure area when the load was 50 N, this load did not cause any visible discoloration. The contact area for load ranged from 100 to 200 N was larger than actual bruise area. Only the area occupied by the pressure greater than 0.2 MPa was well correlated to the bruising area (R2=0.83), so this data was applied to determine an appropriate stress area of pear model for estimation of bruise area. As a result, the relative error rate of bruise predications with FEA in pear compressions was around 6% for 100N and 150 N while it was 13% for 200 N. This shows that the use of FEA can comparatively precisely predicate the bruise area of Korla pear. Thus, this approach will help for design of harvesting and sorting equipment and packaging media in reducing likelihood of pear bruising resulting from static loads.