Determination and quantitative evaluation of early postharvest hidden damage in fresh strawberry fruit based on coupling of dynamic finite element method and response surface methodology

Collision damage is the most common type of damage during the mechanized harvesting, stacking and transportation of strawberries. Aiming at the problem that hidden bruises on fruits caused by collision behaviors are difficult to detect and accurately quantify in the early stage of damage, this paper carried out simulation and experimental research on the bruise susceptibility of strawberry fruits at the moment of collision based on dynamic finite element method and response surface method. By measuring the physical characteristics parameters of three different varieties of strawberry fruits, the three-dimensional solid model of the fruit including cortex, central pith, and achene was established. The multi-scale finite element model of the fruit was further established based on the mechanical property parameters obtained based on the quasi-static compression experiment. A total of 240 different experimental scenarios were set up in this paper, and the cloud diagrams of fruit’s equivalent stress and the changing law of system energy under different conditions were obtained. The experimental results showed that the equivalent stress and contact force inside the fruit vary due to differences in contact material, drop height and impact angle. In order to further obtain the comprehensive effects of contact material, drop height and impact angle on bruise susceptibility, four empirical models for predicting bruise susceptibility were established by using response surface methodology. By comparing the experimental results with the predicted results of the model, it was found that under the conditions where the contact material was steel and the drop height was 1 m, the relative error between the measured value and the predicted value was the smallest (1.38 %) when the impact angle was −19°; the relative error between the measured value and the predicted value was the largest (6.43 %) when the impact angle was −31°. The results of this study showed that the predicting models of strawberry fruit’s bruise susceptibility based on response surface methodology were reasonable and correct. These models can be used to determine the potential mechanical damage of strawberry fruits during mechanized harvesting, stacking and transportation, and can provide a basis for the development of end-effectors/manipulators in strawberry or other fruit and vegetable picking robots, the formulation of picking/harvesting strategies, and the design of packaging container structures.