Original papersCalibration of discrete element parameters of crop residues and their interfaces with soil
Introduction
There is a constant need to improve the productivity of agriculture in order to cope up with the increasing demand for food. One way of ensuring improved productivity is to design and develop high-efficiency agricultural machinery for tillage and seeding. Soil-engaging tools are among the main constituents of these agricultural machines and they directly interact with the materials present in the fields (Zeng, 2019). In developing effective soil-engaging tools, comprehensive knowledge of the mechanical behavior and properties of the materials these tools interact with is necessary. Shear properties, which include internal friction and cohesion, are among the mechanical material properties that play a significant role in optimizing the design of a tool (Sadek et al., 2011a, Sadek et al., 2011b). Previously, only the interaction of soil and tools has been the major focus of the study to develop agricultural equipment. But with the rise in popularity of conservation tillage, crop residues present in the field have to be considered as a critical factor as well (Chen et al., 2016, Zeng et al., 2021). Understanding the interaction between soil and crop residues, and how both interact with soil-engaging tools, became integral for the design of agricultural equipment for conservation tillage (Zeng and Chen, 2019). However, the mechanical properties of crop residue and its interaction with soil have not been well documented.
There are many approaches that can be considered to study the particulate behavior of soil and crop residues; among them, discrete element method (DEM) has been effectively utilized due to its capability to consider the complex and heterogeneous nature of both materials (Sadek and Chen, 2015, Wang et al., 2020, Zeng et al., 2020, Zeng and Chen, 2019). DEM simulates the mechanical behavior of a particulate system by using a collection of distinct particles, like discs and spheres, that interact with each other (Cundall and Strack, 1979). This makes it effective in simulating material interactions in tillage as the behavior of materials in this process involves excessive displacements, separation, mixing, and the flow of particles (Itzhak Shmulevich et al., 2009).
Just like any other numerical models, the reliability of a DEM model depends on the input particle contact parameters (microparameters) of the materials involved (Coetzee, 2016). Hence, microparameters of crop residues and soils, and at their interfaces need to be identified. The identification of material microparameters is one of the main difficulties in using DEM (Sadek et al., 2021). There are not a lot of common laboratory experiments that can measure the microparameters that describe the particulate behavior of materials. Also, these microparameters cannot be directly correlated to the macroscopic/continuum-based parameters (macroparameters) that most laboratory experiments measure (Ahlinhan et al., 2018). The standard approach in identifying microparameters is to back-calculate them using the Bulk Calibration Approach (Coetzee, 2017). In this approach, an actual laboratory test is performed to measure a particular material macroparameter. Subsequently, a DEM model of the laboratory test is developed, and sensitivity analysis is performed to see how the microparameters influence the macroscopic behavior of the model. Based on the results of the sensitivity analysis, a calibration approach is then designed to systematically adjust the microparameter values and match the simulated material macroparameter to the ones measured from the actual laboratory test. Some of the laboratory tests simulated for the calibration of microparameters of different materials are ring shear test (Simons et al., 2015), direct shear test (Coetzee and Els, 2009), and triaxial test (Belheine et al., 2009, Yang et al., 2011). Sadek et al., 2011a, Sadek et al., 2011b specifically used this approach and developed a direct shear test model to calibrate microparameters of an agricultural material like hemp fiber. This approach has also been utilized by Nandanwar and Chen (2018) for the calibration of the microparameters of agricultural soil through simulations of a triaxial test.
In summary, to improve the reliability of discrete element models simulating soil and crop residue interactions in tillage, the microparameters of both the materials involved in the process need to be calibrated. This research focused on the calibration of the microparameters of different crop residues and their interfaces with soil. The objectives of this research were to (1) measure the shear properties of crop residues using a ring shear test; (2) measure the shear properties at the interface of crop residues and soil using a direct shear test; (3) develop a discrete element model of both tests in Particle Flow Code in Three-Dimensions (PFC3D); and (4) calibrate the microparameters of the crop residues (residue-residue) and soil-residue interface by matching the measured and simulated test results.
Section snippets
Material description
Crop residues of canola, corn, flax, oats, and wheat (Fig. 1) were collected at the research farm of Agriculture and Agri-Food Canada in Portage La Prairie, located in the west of Winnipeg at the Central Plains region of the province of Manitoba, Canada. Samples were air-dried to mitigate decomposition. The moisture content of the samples was measured (ASAE Standard, 2012a, ASTM Standard, 2012b), and it was observed to range between 14 and 15% (dry basis).
The agricultural soil sample to be
Stress-displacement relationships
Examples of the stress-displacement relationships measured from one set of ring shear and direct shear tests are shown in Fig. 8. These examples were obtained from tests using oats, and only one kind of crop residue was shown as similar trends of stress-displacement relationships governed while using the other crop residues. In Fig. 8a, it is apparent from the stress-displacement relationships of the ring shear test that as the residue is subjected to increasing normal pressure, its resulting
Conclusion
Ring shear tests were performed to identify the internal friction angle of five different crop residues: canola, corn, flax, oats, and wheat. The measured internal friction angles ranged from 16.4 to 26°. Direct shear tests were performed to quantify the interface friction angle of the crop residues against soil. The soil-residue interface friction angles measured ranged from 41.3 to 47.4°. The PFC3D models of both ring shear and direct shear tests developed in this research simulated well the
Declaration of Competing Interest
The authors declare that they have no competing interests.
Acknowledgment
This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).
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