Prediction of internal mechanical damage in pineapple compression using finite element method based on Hooke?s and Hertz?s laws

Fruits are susceptible to damage from external loads during harvesting, transport and storage. Fruit damage is determined by its tissue mechanical properties. The aim of this study was to evaluate two theories (Hooke's and Hertz's Laws) on the apparent elastic modulus of pineapple and predict the internal mechanical damage of pineapple under compression. Two multi-scale finite element models (FEMs) were developed to predict the pineapple internal damage. One model, based on Hooke's Law contained three layers, including peel, pulp, and core. Another model, based on Hertz's Law, was a two layers model, including peel and pulp (containing core). The difference of these two FEMs was evaluated in terms of different compression displacements. The results showed that, when the compression displacement <= 10 mm, the Hooke three-layer model was closer to reality. But when the compression displacement >10 mm, the Hertz two-layer model was better in reflecting real pineapple compression. Simulated data confirmed the experimental results and predicted the internal mechanical damage of pineapple. Finite element results indicated that, when a force was applied to the fruit, pulp tissue suffered mechanical damage before peel and core tissue. When the compression level was <= 5% (5.5 mm; 86 N), there was no damage to pulp, core, and peel. But, if compression level >5%, the fruit was damaged. These results indicated that pineapple placed horizontally with an allowable numbers of stackable pineapples must be less than eight. Otherwise, the maximum allowable force of stacked fruit on the lowest one will reach the pulp failure force.