Evaluation of litchi impact damage degree and damage susceptibility

Mechanical damage induced by impact during shaking harvesting or transportation debases the quantity and shortens the shelf life of litchi fruit. In this paper, 'Nuomici' and 'Guiwei' litchis were used to study the impact behavior between fruit and a rigid plate with a new measuring method in order to achieve an approach to evaluate the damage degree and the damage resistance. Impact tests were conducted on a pendulum device and the impact force, compressed depth and contact length were recorded with a force sensor and a high-speed camera. Based on these data, a method was introduced to calculate the contact area, average surface pressure and compressed volume at any given moment of the impact. Analysis of the impact process indicated that the end of deformation phase was the most dangerous moment for a fruit during the impact in all cases. For both cultivars, as the drop height increased from 200 mm to 800 mm, the percentage of absorbed energy, the maximum contact area and the damage degree increased, while the contact time decreased. The maximum surface pressure remained stable as the drop height ranged from 400 mm to 800 mm, so that surface pressure was not a main factor affecting the damage degree. The damage degree of 'Guiwei' and 'Nuomici' were 13.34% and 13.27% at drop height of 200 mm and 27.89% and 23.74% at 800 mm. Respectively, 70% and 79% damage degree happened at drop height of 400 mm. The difference of litchi cultivar has low effect on contact area and contact time, but significant effect on damage degree when drop height was 600 mm and 800 mm. Based on elastic theory, power models with high coefficient of determination were successfully used to fitting the relationship between impact energy and compressed depth for both cultivars. Combined with the good linear fittings between compressed volume and damage degree, the damage degree could then be predicted with impact energy. Furthermore, the damage susceptibility (DS) was introduced to describe the susceptibility of impact damage of litchi fruit. The results show that DS decreased with the increase of drop height 'Nuomici' was more resist to impact damage than 'Guiwei' as drop height changed from 600 mm to 800 mm. The new method presented in this study can be used to evaluate the damage of litchi fruit and optimize the design of litchi shakers.