Mechanical properties and bruise susceptibility of green bananas under repeated impacts

Banana fruits are typically harvested and transported while still green and hard, referred to as green bananas. However, frequent collisions often occur between fruit fingers, working parts, and adjacent fruit fingers during this stage. This study aims to investigate the mechanical properties of fruit fingers at different growth positions. The experimental material was selected as the finger of the banana plant (Musa spp.) on a supporting platform created by 3D printing technology. A complete factorial experiment was performed using a pendulum method. The test conditions included three impact energies (0.27, 0.52, and 0.88 J) and three repeated times (1, 5, and 10) across three growth positions (the inner rows of the first and second nodes, the outer rows of the first and second nodes, and the inner and outer rows of the last nodes) within a green banana bunch. Mechanical parameters were calculated, such as peak acceleration, coefficient of restitution (CoR), bruise area (BA), bruise volume (BV), and bruise susceptibility (BS). Among them, the BS represented the ratio of BV to the total absorbed impact energy. A multi-way analysis of variance was also conducted at a significance level of 5% to determine the statistical significance of the mean values of the parameters. The results demonstrated that there was a strong positive correlation between peak acceleration and bruise sizes, and a strong negative correlation between the CoR and BS. Specifically, the higher peak acceleration resulted in the more severe damage to the fruit fingers, whereas the smaller CoR made the green bananas more susceptible to damage. Moreover, the impact energy, repeated times, their interaction, and the location of banana growth significantly dominated the mechanical parameters of the BA, BV, and BS. The higher impact energy led to the greater peak acceleration, BA, BV, and BS, whereas, the lower CoR was observed. Consequently, the higher impact energy tended to increase the susceptibility of banana fruit to damage. Once the impact energy was below the threshold for the plastic deformation of green bananas, the peak acceleration and CoR were improved non-linearly with each impact up to the first five impacts, after which the increment decreased progressively for each subsequent impact. Similar trends were observed for the BA and BV, while the opposite was true for the BS. Therefore, the susceptibility of banana fruit decreased with the increasing impacts. It is challenging for the reduced number of impacts to less than five during production. Fruit damage can be minimized to reduce the energy required to hit the banana fingers. Once the impact energy exceeded the threshold, an initial increase in the peak acceleration and CoR was followed by a decrease, due to the plastic deformation and ductile fracture of the banana finger from continued impacts. Both BV and the rate of increase were enhanced with the number of repetitions. Furthermore, the BS of banana fingers in the outer row was generally smaller than that in the inner row, indicating that the inner row was more susceptible to impact-induced bruise damage. These findings can provide a strong reference to managing the protective measures and risk factors during banana harvesting and transportation.