Study On Damage Mechanism Of Big Rice Seedling Stalks By Mechanical Transplantation Based On Rigid-flexible-fluid Coupling Dynamics

The transplanting of big rice seedlings can effectively alleviate the contradiction of stubble in the multi cropping rice planting area in the middle and lower reaches of the Yangtze River,and achieve high,stable and high-quality yield.However,the existing transplanting machine has problems such as high damage rate,long re-greening period and low live planting rate.Stalk damage seriously affects the material transport and physiological metabolism of rice seedlings after planting,which is a problem that needs to be overcome urgently for transplanting big rice seedlings.However,the research on mechanical transplanting damage of rice seedlings is weak,lacking systematic and in-depth exploratory research,data support,theoretical basis and analytical methods.As a result,the mechanism damage of rice seedling stalk by mechanical transplanting is still unclear,which greatly restricts the performance optimization of big rice seedling transplanting mechanism.A combination of theoretical analysis,finite element simulation and experimental study was used to investigate the research method.An in-depth study on the damage mechanism of rice large seedling stalk by mechanical transplanting under the actual operating conditions was carried out.The main research work and conclusions are as follows:1)Dynamic biomechanical properties of rice seedling stalks.Shear,tensile,compression and bending mechanical experiments were carried out based on the loading form of the stalk in the process of transplanting.The load-strain curves and mechanical behaviors of rice seedling stalks during loading were analyzed to determine the dynamic changes of yield strain and yield load with seedling age under different loading forms.As well as the damage behaviors and failure forms of rice seedling stalks under different loading forms.The correlation between the structural characteristics,physical properties and chemical components of rice seedling stalks and their mechanical properties was analyzed.And the covariance among the influencing factors was examined.The partial least squares regression analysis(PLSR)method was used to construct a prediction model for the dynamic biomechanical properties of rice seedling stalks.Which is helpful to quantify the contribution of the influencing factors to the mechanical properties of rice seedling stalks,and providing a theoretical basis for the analysis of the mechanical properties of rice seedling stalks.The results showed that the mechanical behavior of rice seedling stalks under the same load form varied in a consistent pattern.Stem diameter,air cavity ratio,cellulose and lignin are the main factors affecting the shear strength and tensile strength of the rice seedling stem.The contribution of cellulose to the bending strength of rice seedling stem was significantly higher than other factors.There was no significant difference in the contribution rate of each factor to the compression strength of rice seedling stem.Rice seedling stalks have strong tensile and shear resistance and weak resistance to bending and compressive loads.2)Biomechanical model construction and damage characteristics of rice seedling stalks.In view of the complexity of microstructure geometry,the hierarchical nature of tissue structure,the diversity of material properties and the inhomogeneity of material spatial distribution of rice seedling stalks,a three-dimensional geometric model of rice seedling stalks was constructed using Micro-CT technology to obtain the high-resolution multi-scale spatial structure of rice seedling stalks.The nanoindentation test method was used to obtain the material property parameters of rice seedling stalk tissues and construct the intrinsic equations of rice seedling stalk tissues.The mapping relationships between grayscale-density-elastic modulus of rice seedling stalk tissue’s CT images were established.And a 3D finite element model of rice seedling stalk with spatial material property distribution and microstructure characteristics was constructed.The model was verified to be reliable and accurate in terms of macroscopic mechanical response,microscopic mechanical behavior and actual load effect through mechanical tests.Meanwhile,the simulation results were analyzed to observe the deformation behavior,stress transfer and distribution,and damage failure process of rice seedling stalks under different load forms.And the mechanical damage characteristics of rice seedlings stalk has been revealed.The results show that the proposed biomechanical model of rice seedling stalk can truly reflect the mechanical functions of different tissues and structures,which provides a powerful tool for accurate quantification of rice seedling stalk damage analysis and breaks through the limitations of traditional damage measurement methods for biological materials.3)Construction and validation of the coupled dynamics model of transplanting mechanism-rice seedling-paddy soil.Taking the mechanical transplanting operation system of rice seedlings as the research object,the fluid-structure coupling theory,three-phase field theory and multi-body dynamics theory are adopted to integrate the transplanting mechanism,rice seedlings and paddy soil in a rigid-flexible-fluid interaction operation system.A coupled kinetic model of splitting mechanism-rice seedling-paddy soil is constructed,and the model is validated and modified with bench tests to improve the prediction accuracy of the model.From the simulation results,it is clear that the impact load is the main reason for the bending and deformation of the rice seedling stalks,the base is the vulnerable part of the stalk.The proposed model can provide technical support for the analysis and extraction of difficult-to-measure physical quantities such as kinetic characteristics,mechanical behavior and system interactions parameters of rice seedlings during transplanting,and realize the dynamic visual analysis of rice seedling transplanting process.Seedling needle cause a high level of collision and extrusion damage to rice seedling stalks,which seriously affects the growth and development of rice seedlings after planting,and can even cause seedling death.4)Study on the mechanical damage mechanism of big rice seedling.Based on the coupled kinetic model of rice seedling-rice seedling-paddy soil,we analyze the kinetic parameters and deformation behaviors of seedling movement such as velocity,attitude,stress and viscous resistance during transplanting.The mechanical analysis and theoretical derivation of the collision and extrusion process in the transplanting process were carried out.The mechanical behavior changes and damage evolution process of the rice seedling stalk under the collision and extrusion of the seedling needle in the major and minor axes were analyzed based on the multi-scale parameter fusion of seedling stalk biomechanical model.And the test platform was used for test verification.The results showed that a single bending or twisting deformation caused less damage to rice seedling stalks,and the composite load would aggravate the degree of damage to seedling stalks.The resistance to impact damage of the short axis of rice seedlings is lower than that of the long axis.Therefore,it is best to ensure that the vertical plane where the long axis of the seedling stalk is located should be parallel to the plane of movement of the seedling needle during transplanting.Which means that enough offset space is reserved between the seedling needle and the seedling stalk,which reduces the value of eccentricity,reduces the probability of damage and the degree of damage of rice seedling stalk,and also ensures the vertical state of rice seedlings to the maximum extent,which is beneficial to the transplanting quality.In summary,this study combined the biomechanical model of rice seedling stalk and the coupled kinetic model of transplanting mechanism-rice seedling-soil in the paddy field based on the biomechanical characteristics of rice seedling stalk.The kinetic properties of rice seedling stalks during transplanting and the mechanical behavior and damage evolution of rice seedling stalks under different loading forms were investigated.The damage mechanism of rice seedling stalk during transplanting was revealed.The research results can provide theoretical basis and technical support for the optimization analysis of low damage performance of rice seedling transplanting mechanism.