Dynamic modeling of soil–tool interaction: An overview from a fluid flow perspective

The study of tillage tool interaction centers on soil failure patterns and development of force prediction models for design optimization. The force-deformation relationships used in models developed to date have been considering soil as a rigid solid or elasto-plastic medium. Most of the models are based on quasi-static soil failure patterns. In recent years, efforts have been made to improve the conventional analytical and experimental models by numerical approaches. This paper aims at reviewing the existing methods of tillage tool modeling and exploring the use of computational fluid dynamics to deal with unresolved aspects of soil dynamics in tillage. The discussion also focuses on soil rheological behaviour for its visco-plastic nature and its mass deformation due to machine interaction which may be analyzed as a Bingham plastic material using a fluid flow approach. Preliminary results on visco-plastic soil deformation patterns and failure front advancement are very encouraging. For a tool operating speed of 5.5 m s⁻¹, the soil failure front was observed to be about 100-mm forward of the tool.     

The cutting of soil by narrow blades

The available models for predicting the forces acting on a narrow soil cutting blade have required separate measurements of the shape of the three-dimensional soil failure pattern ahead of the blade. It is proposed that a three-dimensional model consisting of straight line failure patterns in the soil can be used to predict both the draft forces and the volume of soil disturbed in front of a narrow blade. Limit equilibrium mechanics equations are written for the soil wedges in terms of an unknown angle of the failure zone and the theoretical draft force is minimized with respect to this angle. Force factors are thus found which are of the type to fit Reece's general earthmoving equation, but which vary with the width to depth ratio of the blade as well as with the rake angle of the blade and the friction angle of the soil. In addition the approximate geometry of the three-dimensional failure pattern in the soil is predicted for varying blade shapes and soil strengths. This allows the design of simple tools on the basis of their draft force requirements and their soil cutting efficiency. The draft force predictions and failure geometry calculations are shown to have considerable verification by experimental results.     

A Theoretical Model for Estimating the Peak Cutting Force of Conical Picks

The peak cutting force (PCF) estimation plays an important role in the design of cutting tools for mining excavators. In most of the existing theoretical models for cutting force prediction, the PCF is often modeled as the force on the cutting tool at the moment when the rock fragment is formed. However, according to the theory of fracture mechanics, the PCF is supposed to occur during the crack initiation phase. Consequently, this paper attempts to add to the existing literature by proposing a fracture mechanics-based theoretical model for PCF prediction. The proposed PCF prediction model distinguishes itself from existing models by determining the PCF during initiation of the rock crack. The PCF is determined using the energy and stress criteria of Griffith’s fracture mechanics theory. In this new model, the PCF is positively related to the fracture toughness of the rock and the cutting depth. The experimental results demonstrated the validity of the proposed model. The proposed model performs well in predicting the PCF in terms of reliability and accuracy. Besides, the PCF prediction capability of the proposed model was compared with those of the other rock cutting models existing in the literature.     

Force prediction equation using distorted model as an analog device

The force versus speed response of a distorted model of a soil-engaging tool operating in a soil and at a depth where the performance evaluation of the prototype is required to be determined, is used to compute analog values. A prediction equation relating draft force with tool width, depth of cut, soil specific weight, tool velocity and analog values has been developed. A comparison between the forces predicted by the equation and that measured on flat rectangular blades and angled tools operating at a speed of 2–8.5 km/hr is presented. The closeness in prediction accuracies points that the distorted model itself can be used as a fairly good analog device.     

Developing an excavation torque model for dense cohesionless soils

Torque encountered during the rotary excavation of soils (e.g., when using the DJM method for deep soft ground improvement) poses a serious detrimental effect not only to the excavating machines but also to the viability of a project as a whole. Consequently, this research investigates ways and means of realizing the reduction of torque encountered during the excavation of cohesionless soils. In this paper, the development of a torque model for a rotary excavation of cohesionless soils is proposed. Whereas in most of the soil tillage theories (i) the cutting tool is usually partially exposed at the surface, and (ii) excavation is generally longitudinal, this model is significant because; (i) the excavation process is radial, and (ii) the blade is completely immersed in the excavated medium. Various theories for the prediction of forces acting during the interaction of cutting tools and soils in conjunction with localized modeling of all the other forces, applied and adopted to suit this excavation geometry, have been applied in the development of the torque model. Experimental data was obtained from excavation experiments performed on compacted completely saturated sand samples. Within the experimental and theoretical limitations, the results showed that this model represented the excavation process.     

A review on traction prediction equations

A variety of methods, ranging from theoretical to empirical, which have been proposed for predicting and measuring soil-vehicle interaction performance are reviewed. A single wheel tyre testing facility at Indian Institute of Technology, Kharagpur, India, was used to check the applicability of the most widely used traction models, for tyres used in Indian soil conditions. Finally, the coefficients of traction prediction equations developed by Brixius [16] were modified to fit traction data obtained from the testing of the tyres in the Indian soil conditions.     

Prediction of soil deformation beneath temporary airfield matting systems based on full-scale testing

This paper presents results from full-scale evaluations of an aluminum structural mat system with regard to carrying heavy aircraft across graded, but unimproved, soil with California Bearing Ratios (CBRs) of 6, 10, 15, 25, and 100. The objective was to determine relationships among soil deformation rate, the mat’s flexural modulus, the number of applied passes, and the underlying soil’s CBR. Current prevailing performance prediction models for aluminum mat systems are based on full-scale tests using historic aircraft loads over soils having a CBR of 4 that were never validated for soils with higher CBR values. Full-scale test results presented herein demonstrated the inability of current models to accurately predict mat permanent deformation. Strong correlations were found between measured and predicted data across the entire spectrum of soil CBRs. These relationships can be used to noticeably improve the accuracy of performance prediction models. An empirical equation was developed to reasonably predict subgrade deformation for any number of passes and soil CBR for the loading and mat system tested.     

LSA model for sinkage predictions

The paper presents a Load-Sinkage Analytical (LSA) model, its validation and comparison with other models used in Terramechanics. The LSA model predicts sinkage and penetration force as functions of soil parameters and plates (or traction devices) shape and dimensions. This model uses invariant soil parameters that can be given or measured before the calculations by routine methods of classical soil mechanics. Soil parameters can also be obtained by recommended empirical equations using four physical soil parameters measured in the field with hand held instruments without time consuming and costly plate tests. The paper includes also an analysis of capabilities and limitations of the observed models.     

Next-generation NATO reference mobility model complex terramechanics – Part 1: Definition and literature review

The US army along with NATO member and partner nations’ militaries need an accurate software tool for predicting ground vehicle mobility (such as speed-made-good and fuel-consumption) on world-wide terrains where military vehicles may be required to operate. Currently, the NATO Reference Mobility Model (NRMM) is the only NATO recognized tool for assessing ground vehicle mobility. NRMM was developed from the 1960s to the 1980s and relies on steady-state empirical formulas which may not be accurate for new military ground vehicles. A NATO research task group (RTG-248) was established from 2016 to 2018 to develop the NG-NRMM (next-generation NRMM) software tool requirements and an NG-NRMM prototype which uses high-fidelity “simple” or “complex” terramechanics models for the terrain/soil along with modern 3D multibody dynamics software tools for modeling the vehicle. NG-NRMM Complex Terramechanics (CT) models are those that utilize full 3D soil models capable of predicting the 3D soil reaction forces on the vehicle surfaces (including tires, tracks, legs, and under body) and the 3D flow and deformation of the soil including both elastic and plastic deformation under any 3D loading condition. In Part 1 of this paper, an overview of the full spectrum of terramechanics models from the highest fidelity to the lowest fidelity is presented along with a literature review of CT ground vehicle mobility models.     

FRP疲劳累积损伤理论研究进展

纤维增强树指复合材料（FRP）已成为高性能结构的先进材料。本文对FRP疲劳累积损伤理论作了详细的回顾,它们可分为剩余寿命模型、剩余刚度模型、耗散能模型、Markov链模型和其它经验公式。还用两个大样本实验数据对这些模型作了分析对比,结果表明:除Yao和Himmel的剩余强度模型略优一些,其它模型则与传统的Miner模型不相上下。      

Comparison of the implementation of three common types of coupled CFD-DEM model for simulating soil surface erosion

Soil erosion is a common process studied by soil science, environmental engineering, geotechnical engineering, coastal engineering, and many other fields. In the areas of hydraulic engineering, the geotechnics of soil erosion remains a high priority topic as the bridge scour is a common cause of bridge failures. Accurate predictions of scour depth and soil erosion rate remain challenging, due to the limitations of existing scaled experimental approach in fulfilling the hydrological and hydrodynamic similarity requirements. Computational model offers a promising alternative to further the microscale understanding of soil erosion which can help to develop engineering tools in practice. Computational model that couples Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) to simulate the behaviors of fluid-solid systems is promising to advance the current tools for soil erosion analyses. Different mathematical forms for laminar fluid flows exist for the coupled CFD-DEM model as documented in published literatures and implemented in commercial and open-source software; each of them is based on certain physics assumptions and corresponding mathematical treatments. There are, however, no direct comparison of the results of CFD-DEM models based on these seemingly different mathematical formulations, which would help researchers to select the proper simulation tool. This study implemented coupled CFD-DEM models based on three most common types of mathematical formats used in the previous modeling work. The results of different CFD-DEM models are firstly validated by comparing the results of simulating the free settling of a particle in fluid. A case study is then designed to compare the models in simulating the surface erosion of cohesionless soil inside a pipe flow using laminar flow equations. Comparison indicates that for a relatively sparse particle-fluid system, the difference of the three models is negligible. For a dense particle-fluid system, simulation with the three different mathematical formats can predict different results (as large as 10% in the fluid velocity and 20% in the particle drag force for the simulation case study analyzed). The results of this case study indicate that the three CFD-DEM models achieved comparable results for simulating soil erosion from an engineering perspective, however, the differences between these models, which originate from their underlying physics assumptions, must be kept in mind in selecting an appropriate simulation model as well as in comparing the results from different models.     

一种考虑虚拟质量力的两相压力波速经验模型

前人建立的两相压力波速经验模型未考虑虚拟质量力,本文考虑虚拟质量力、管壁弹性、管壁粗糙度等因素,通过求解双流体模型的小扰动,提出了一种新的气液两相压力波速经验模型．以一个具体的工程实例为背景,运用数值方法对其求解,得到的计算结果与前人实测的实验数据一致．结果表明,当空隙率较小时(0＜Φ＜15%)时,虚拟质量力对压力波速的影响不大,当空隙率较大时(Φ≥15%),考虑虚拟质量力计算的压力波速远大于不考虑虚拟质量力计算的压力波速．经验公式也可达到准确求解压力波速的目的．     

Experimental evaluation of cutting dynamic models in soil bin facility

Computer Aided Engineering methods in earthmoving machines design and their automation require the development of soil-cutting models. These models both in two or three dimensions, static or dynamic, fitted for frictional or cohesive soils, must be mutually compatible and must function with soil transportation models and with machine locomotion characteristic models. In this work two different methods of soil cutting have been evaluated, both of them based on the classical wedge method, in order to verify their applicability to test conditions in the new soil bin facility of CEMOTER. From experimental results the possibility of using dynamic models of soil cutting in the frequency domain is discussed, to improve earthmoving machinery performance by automation and implementation of open and closed-loop control. After a preliminary analysis of a plane blade under different test conditions in sandy soil, soil cutting theoretical models of a simple implement are compared with respective scale models by tests performed in a soil bin facility at various operating speeds and depths, in order to investigate their applicability and the dynamic behaviour of the soil cutting force.     

Development of a machine integrated strain-based contact force sensor for pad foot soil compactors

An investigation was undertaken to explore the use of measurable pad strains on a non-vibratory pad foot roller to provide real time continuous evidence of compaction and contact force. Individual pads were instrumented with strain gages in a pattern chosen based on pad finite element analysis (FEA). Different pad–soil contact stress distributions were modeled to simulate a range of soil conditions. The FEA revealed that the contact stress distribution has a significant influence on the observed pad strain field, suggesting soil specific interpretation of pad strains in order to determine contact force. Results from uniaxial laboratory testing of pad loading on dry sand verified the FEA, i.e., experimental strains generally matched within 15% of FEA strains. The contact stress distribution was measured using tactile pressure sensors and found to be moderately parabolic. A soil specific empirical calibration factor relating vertical sidewall strains to contact force was determined. Field testing was performed on the dry sand with multiple instrumented pads installed on a Caterpillar CP56 roller. Pad strain magnitudes increased up to 250% during compaction from repeated passes of the roller. Using the empirical calibration factor, the estimated contact force was shown to increase with compaction, represented by the independently-measured soil unit weight.     

Triaxial force measurement at specific positions on a soil-cutting blade

In order to create a mathematical model of a soil-cutting blade, it is necessary to understand thoroughly the behavior of a soil slice and its interaction with the blade surface. The triaxial force transducer was developed to serve as one of the various tools to verify the proposed mathematical model. The prototype model transducer was fabricated, calibrated and tested with a soil slice on a flat cutting blade. The calibration results have indicated high sensitivity and the capability of simultaneous measurement in three directions. As a technological refinement, the detecting part of this triaxial force transducer was tapered to solve the problem of soil clogging in the opening clearance. Furthermore, the effects of the clearance configurations between the bore on the soil-cutting blade and the detecting part which is embedded in this bore were investigated to determine the most desirable configuration. The comparative results indicated that by tapering both the detecting part and the bore, the tangential stress measurement gained the highest value, and provided the most satisfactory condition for three-dimensional stress management.     

The mechanics of soil cutting by a rotating wire

The cutting of soil by a rotating wire analogous to the tip of a rotary tiller blade while cutting a two-dimensional soil slice over a range of ‘fetch-ratios’ (bite length/depth-ratios) in a quasi-static condition is presented. A theoretical models based on Mohr-Coloumb soil mechanics has been proposed to predict forces on the wire (tip). The model is dependent upon observed passive general shear failure of the soil slice towards the curved free surface of a previous cut and the lateral local shear failure towards the undeformed soil. The predicted forces in a frictional soil and in a pure cohesive medium (artificial clay) agreed well with experimental results.     

A method for designing multiload component dynamometers incorporating octagonal strain rings

To optimize the design of force dynamometers incorporating octagonal ring elements it is important to be able to predict the dynamometer sensitivities. Previous methods relying on thin ring theory have been inadequate because octagonal rings often have a thickness which cannot be considered thin and, further, the thickness is not uniform. In this paper, empirical equations that describe the deflections, strains and von Mises stresses of individual octagonal rings due to radial, tangential and axial forces are developed using finite-element models. These models are loaded and constrained to simulate the most common uses of octagonal rings in force dynamometers. A nonlinear regression routine is used to develop the above equations from the data given by the finite-element analysis. The performance of these equations is evaluated and presented in tabular form. A procedure is also outlined to describe the use of these equations in the design of six-load-component dynamometers.     

Force and pressure distribution under vibratory tillage tool

Experiments were conducted to study the force requirement and pressure distribution under vibratory tillage tools in a soil bin with a sandy loam soil. The tool was oscillated sinusoidally in the direction of soil bin travel. An octagonal ring transducer and pressure sensors were used to measure the forces and soil pressure on the blade. The tool was operated at oscillating frequency of 4.5–15.6 Hz and amplitude of 11–26 mm. The soil bin travel speed was varied from 0.05 to 0.224 m/s. The test results obtained showed both the horizontal force and the vertical force decreased with increase in oscillating frequency. The normal pressure on the blade surface varied considerably. The peak normal pressure was found to increase with increase in oscillating frequency, oscillating amplitude and soil bin travel speed. The change in average normal pressure with change in oscillating frequency and amplitude was also investigated.     

Traction performance of a pushed/pulled drive wheel

A comprehensive method for prediction of off-road driven wheel performance is presented, assuming a parabolic wheel–soil contact surface. The traction performance of a driven wheel is predicted for both driving and braking modes. Simulations show significant non-symmetry of the traction performance of the driving and braking wheels. The braking force is significantly greater than the traction force reached in the driving mode. In order to apply the suggested model for prediction of the traction performance of a 4WD vehicle, the load transfer effect was considered. Simulated traction performances of front and rear driven wheels differ significantly, due to the load transfer. In the driving mode, the rear driven wheel develops a net traction force greater than that of the front wheel. On the other hand, in the braking mode the front driven wheel develops a braking force significantly greater than that of the rear driven wheel due to a pushed/pulled force affected by the load transfer. The suggested model was successfully verified by the data reported in literature and by full-scale field experiments with a special wheel-testing device. The developed approach may improve the prediction of off-road multi-drive vehicle traction performance.     

Reverse-rotational rotary tiller for reduced power requirement in deep tillage

In this paper details of rotary tillage regarding the movement of tilled soil are presented. A noticeable reduction of tillage power requirement was achieved during rotary tillage. The soil movement depended upon the direction of rotation and the ratio of tilling depth (H) to blade radius (R). With the differences in the soil movement, four kinds of rotary tilling patterns were determined. Increase in operating power generally resulted when a large amount of tilled soil was re-tilled in the zone of blade rotation. Improvement of backward throwing of the soil was required for power reduction, especially in deep tillage. A backward throwing model of soil by the blade was developed on the basis of trochoidal motion of the blade and sliding motion of the soil over a scoop-surface on the horizontal portion of the blade. The throwing model estimated the conditions for avoiding re-tillage, such as direction of rotation and shape of scoop-surface. The throwing model was applied to the design of the shape of the scoop-surface which enabled maximum backward throwing of the soil sufficient to avoid re-tilling. At tilling depths greater than 300 mm, reverse rotation with the new shaped blades brought about a tillage power reduction by about a half compared to forward or reverse rotation with conventional blades.