Simulation and experiment on the mechanical properties of Coptis chinensis root-soil composites based on image reconstruction

Chinese goldthread (Coptis chinensis) is one of the most important herbal medicines in eastern Asia. However, the manual harvesting cannot fully meet the large-scale production in recent years. Taking the Coptis chinensis at harvest period as the research object, this study aims to explore the mechanical properties of root-soil composites using image reconstruction. The incremental structure from motion (ISFM) was used to feature match the unordered images of Coptis chinensis roots, and then the geometric correction and triangulation were performed to restore the sparse point cloud structure. The relative pose was re-estimated using existing point clouds after local and global optimization. All camera poses and sparse 3D point clouds were output for the latter use. Then, the multi-view stereo (MVS) was used to estimate the depth map. Dense reconstruction of point clouds and dense 3D point clouds were obtained to contain the information, such as image color texture and a strong sense of reality. And then the images were converted into the polygonal data for optimization processing. A high-fidelity root morphology model was established using multi-view image reconstruction. The approach was applied into the mechanical research of root-soil composites under different conditions. The ring knife sampling was conducted on the rootless soil and in situ root-soil composites at different depths through field stratified sampling. The root profile was dug to measure the distribution range of the root system. It was found that the root system of Coptis chinensis was distributed mainly in the soil layer with a depth of 0-90 mm. The root quality first increased and then decreased, as the depth increased. The average shear modulus and Poisson's ratio of rhizomes were 4.08 MPa and 0.45, respectively, after tensile and shear mechanical testing. Direct shear tests were conducted on the soil and root-soil composites at depths of 0-30, >30-60, and >60-90 mm under loads of 50, 100, 150, and 200 kPa, respectively. The physical and mechanical parameters were obtained for the soil and root-soil composites at different depths, such as the average shear strength. The design expert software was used for the experimental design. Significant parameters with a high impact rate were identified through Plackett-Burman experiments among numerous contact parameters. The steepest climbing test was conducted to narrow the parameter range. Then, the response value was taken as the average shear strength of soil and root-soil composites with a depth of 0-30 mm from direct shear tests. The Central Composite test was conducted to calibrate the optimal combination of simulation parameter. After that, numerical simulations of direct shear tests were conducted on soil and root-soil composites in EDEM software. The results showed that the average errors of the cohesion and internal friction angle of the discrete element model of the Coptis chinensis root-soil composites during the harvest period were 3.48% and 5.21%, respectively, with the standard deviations of 3.44% and 1.63%, under the average percentage changed in the vertical mass distribution of the root system at different depths, when the soil moisture content was between 49.58% and 62.96%, and the root moisture content was within the range of 80% to 230%. The discrete element model of soil and root-soil composites was verified to reconstruct the root system model using images data and simulation. The mechanical behavior of root-soil composites was validated for the subsequent research on root soil interaction and separation mechanisms under different conditions. The finding can provide the theoretical design basis for the key working components and the optimal working parameters during mechanized harvesting of Coptis chinensis.