First results on the assessment of impact damage of vegetables by means of the FEM approach

Mechanical harvest and post-harvest handling induce numerous mechanical impacts on vegetables. These impacts may cause damage such as black-spot bruise, resulting in severe economic losses. Impact forces and accelerations arising from collisions, are among the main indices taken into account when studying the damage of fruit and vegetables during post-harvest activities. A miniaturised Acceleration Measuring Unit (AMU) has been recently developed at the Institut für Agrartechnik, Potsdam-Bornim (ATB): when implanted into a real product like a potato tuber, it is able to measure the accelerations at the centre of the fruit deriving from a impact. This PhD Thesis represents a first contribution on the study of mechanical impacts of vegetables (potato tubers), arising from mechanical harvest and post-harvest handling, by means of simulations based on the Finite Element Method (FEM) approach. Simulations were developed by using the Linux distribution CAELinux2011, that contains several technical-engineering software, among which stand out Salome-Meca and Code-Aster. Salome-Meca was used for modelling, meshing and post-processing activities, while Code-Aster was used for processing models. The work has been developed in collaboration with the Institut für Agrartechnik, Potsdam-Bornim (ATB), Germany, where they were conducted laboratory tests (drop tests to measure impact forces and texture analyses to measure modulus of Young) with two spherical artificial fruits. After the development of several preliminary simple models to gain familiarity with the computational software Salome-Meca and Code-Aster and to acquire an acceptable agreement between simulated and experimental tests, it was carried out an extensive set of drop test simulations with a spherical artificial fruit aimed at evaluating the effects of drop height, size of the fruit, density and modulus of Young of the material, on the impact indices (maximum impact force and maximum acceleration at the centre of the fruit). Simulated material parameters were chosen to approach potato tubers properties. All the factors examined (drop height, sphere diameter, modulus of Young and density of the material, mass of the fruit) affected the maximum impact force and the maximum acceleration at the centre of the sphere. Their increase always caused an increase in the maximum impact force, whereas the maximum acceleration at the centre of the sphere decreased vs sphere diameter, material density and mass, and increased vs drop height and modulus of Young. The decreasing trends are due to the cushioning effect produced by the sphere material itself. Moreover, the maximum impact forces reported in the experimental results by Geyer et al. (2009), referring to drop tests of potato tubers onto steel plates, are in good agreement with the values of impact forces provided by the simulations. Instead, simulations provided acceleration values about twice as many those measured in the experimental results with the AMU device. This difference could be due to the implantation system of the AMU inside the tuber. In fact, comparing the measured impact force and the force computed by means the second law of Newton (F = m · a), Geyer et al. in the cited work report that the computed force was approximately half the measured one, meaning an under-estimation of the acceleration provided by the AMU. Ultimately, the concordance between measured and simulated impact forces confirmed the validity of FEM approach, although the limitations owing to the simplicity of the model developed in this work.