| Sweet cherry is an important economic crop.The import and consumption of sweet cherry in China ranks first in the world.Since sweet cherries are exposed to a wide range of environmental temperatures(about 0 ～ 40℃)during post-processing such as picking,grading,packaging,transportation and sales,it is impossible to predict the damage of sweet cherries during mechanical processing at a specific environmental temperature in a certain postprocessing stage without research on the relationship between mechanical damage and temperature.It is difficult to provide theoretical guidance for the design and working environment of cherry picking,grading and packaging equipment.In order to solve this problem,this research took sweet cherry as the main object of study.On the basis of experiments,finite element analysis software Abaqus was used to construct fruit-to-rigid surface and fruit-to-fruit collision models of sweet cherry,and then solved them.The impact factors such as collision type,initial velocity and ambient temperature are studied.The main research contents and results are as follows:(1)Mechanical properties of sweet cherry in different environmental temperaturesA constant temperature and humidity box was used to create storage conditions with fixed ambient temperature to store sweet cherry,and texture analyzer was used to measure mechanical parameters of sweet cherry fruit,exocarp and mesocarp.The mathematical model of failure stress,failure strain,elastic modulus and failure energy with environmental temperature was established.With the increase of environmental temperature from 5℃ to40℃,the elastic modulus range of exocarp of Black pearl,Summit and Tieton were 2.28 ～1.72 MPa,2.57 ～ 1.47 MPa and 1.98 ～ 1.24 MPa,respectively.The range of mesocarp were0.33 ～ 0.21 MPa,0.27 ～ 0.19 MPa and 0.43 ～ 0.23 MPa,respectively.The exponential function between environmental temperature and mechanical parameters provided systematic mechanical properties parameters for sweet cherry fruit and its tissue,and provided systematic basic parameters for numerical simulation method to predict mechanical damage of sweet cherry.(2)Development of horizontal collision test device and collision testA horizontal collision test device which can real-time monitoring impact force during sweet cherry collison process has been designed.Combining the experimental device,impact velocity and buffer material as test factors in sweet cherry horizontal impact test.The maximum impact force and damage area of sweet cherry under different conditions were calculate to explore the influencing factors of sweet cherry damage caused by horizontal collision.The results showed that the higher the initial velocity of sweet cherry before impact,the greater the impact force and the more serious the damage.Compared with non-cushion material and foam cushion material,sponge cushion material has better cushioning effect.(3)Prediction and prevention strategy of horizontal collision damage of sweet cherryBased on the geometric and mechanical parameters of the fruit,the finite element model of horizontal collision was established based on Abaqus software.The process of horizontal collision between fruit-to-rigid surface and fruit-to-fruit was simulated with the impact type,environmental temperature and initial impact velocity as dependent variables.The results showed that the fruit model based on average elastic modulus of tissue and failure stress could reproduce the maximum impact force and contact time in the compression stage,with relative errors of 1.58% and 1.87%,respectively.In the process of fruit-to-rigid surface and fruit-tofruit collision,the damage amount of fruit and the contact time of fruit are nonlinear increasing.In the fruit-to-rigid surface collision model,the maximum impact force and contact time are mainly sensitive to the elastic modulus and failure stress,the initial fruit velocity and the curvature radius of the fruit surface contact point,but not to the plastic strain of the fruit surface.The most important factors affecting horizontal impact damage sensitivity of sweet cherry were impact type,followed by initial fruit speed and environmental temperature.The three multiple linear regression models can be used as a quantitative evaluation tool for the internal damage degree of sweet cherry in the post-harvest process.The study provides an effective numerical simulation method for objectively predicting the sensitivity of horizontal impact damage of sweet cherry and other fresh fruits.The research provided a theoretical basis for the post-harvest mechanical treatment of sweet cherry,especially the packaging and transportation process.According to the research model,avoiding frequent braking,fixing the packaging box of sweet cherry and correctly regulating the temperature inside the transport vehicle can play a role for mechanical damage preventation. |
