Experimental validation of a differential variational inequality-based approach for handling friction and contact in vehicle/granular-terrain interaction

Daniel Melanz, Paramsothy Jayakumar, Dan Negrut
Journal of Terramechanics, Volume 65, June 2016, Pages 1-13, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.01.004
http://www.sciencedirect.com/science/article/pii/S0022489816000069
Abstract:  

The observation motivating this contribution was a perceived lack of expeditious deformable terrain models that can match in mobility analysis studies the level of fidelity delivered by today’s vehicle models. Typically, the deformable terrain-tire interaction has been modeled using Finite Element Method (FEM), which continues to require prohibitively long analysis times owing to the complexity of soil behavior. Recent attempts to model deformable terrain have resorted to the use of the Discrete Element Method (DEM) to capture the soil’s complex interaction with a wheeled vehicle. We assess herein a DEM approach that employs a complementarity condition to enforce non-penetration between colliding rigid bodies that make up the deformable terrain. To this end, we consider three standard terramechanics experiments: direct shear, pressure-sinkage, and single-wheel tests. We report on the validation of the complementarity form of contact dynamics with friction, assess the potential of the DEM-based exploration of fundamental phenomena in terramechanics, and identify numerical solution challenges associated with solving large-scale, quadratic optimization problems with conic constraints.  
Keywords: Terramechanics; Discrete element method; Friction and contact; Differential variational inequality; Validation; Calibration; Direct shear test; Pressure-sinkage test; Single wheel test; Deformable terrain

Agile tire slippage dynamics for radical enhancement of vehicle mobility

Jeremy P. Gray, Vladimir V. Vantsevich, Jesse Paldan
Journal of Terramechanics, Volume 65, June 2016, Pages 14-37, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.01.002
http://www.sciencedirect.com/science/article/pii/S0022489816000045
Abstract: 
There is a need to radically increase mobility of terrain vehicles through new modalities of vehicle locomotion, i.e., by establishing a new technological paradigm in vehicle dynamics and mobility. The new paradigm greatly applies to military vehicles for the radical improvement of tactical and operational mobility. This article presents a new technological paradigm of agile tire slippage dynamics that is studied as an extremely fast and exact response of the tire–soil couple to (i) the tire dynamic loading, (ii) transient changes of gripping and rolling resistance conditions on uniform stochastic terrains and (iii) rapid transient changes from one uniform terrain to a different uniform terrain. Tire longitudinal relaxation lengths are analyzed to characterize the longitudinal relaxation time constants. A set of agile characteristics is also considered to analyze agile tire slippage dynamics within a time interval that is close to the tire longitudinal relaxation time constants. The presented paradigm of agile tire slippage dynamics lays out a foundation to radically enhance vehicle terrain mobility by controlling the tire slippage in its transient phases to prevent the immobilization of a vehicle. Control development basis and requirements for implementing an agile tire slippage control are also analyzed and considered.  
Keywords: Terrain mobility; Agile tire dynamics; Agility; Mobility enhancement; Tire slippage; Tire longitudinal relaxation length; Tire relaxation time constant

A calculation method of track shoe thrust on soft ground for splayed grouser

Congbin Yang, Ligang Cai, Zhifeng Liu, Yang Tian, Caixia Zhang  Journal of Terramechanics, Volume 65, June 2016, Pages 38-48, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.02.001
http://www.sciencedirect.com/science/article/pii/S0022489816000070
Abstract:  
Thrust of track shoes on soft ground is affected by soil moisture content, shear rate and structure parameters of track shoes. A lack of comprehensive consideration of these factors exists for normal calculation methods. A method to predict thrust for track shoes on soft ground with splayed grouser was established based on experimental results and theoretical derivations. Model track shoe traction experiments were conducted for verification and correction of the thrust formula. It was observed that the thrust for the track shoes decreased with the increase in moisture content of the soil. Increases in shear rate, grouser height, and grouser splayed angle resulted in greater tractions. Effect of grouser thickness and grouser draft angle on tractions was not obvious. A corrected thrust formula allowed accurate prediction of thrust for a single track shoe on soft ground.  
Keywords: Track shoe thrust; Soft ground; Splayed grouser; Shear stress; Soil strength

A comparison of ground vehicle mobility analysis based on soil moisture time series datasets from WindSat, LIS, and in situ sensors

Maria T. Stevens, George B. McKinley, Farshid Vahedifard  Journal of Terramechanics, Volume 65, June 2016, Pages 49-59, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.02.002
http://www.sciencedirect.com/science/article/pii/S0022489816000203
Abstract:  
Soil moisture is a key terrain variable in ground vehicle off-road mobility. Historically, models of the land water balance have been used to estimate soil moisture. Recently, satellites have provided another source of soil moisture estimates that can be used to estimate soil-limited vehicle mobility. In this study, we compared the off-road vehicle mobility estimates based on three soil moisture sources: WindSat (a satellite source), LIS (a computer model source), and in situ ground sensors (to represent ground truth). Mobility of six vehicles, each with different ranges of sensitivity to soil moisture, was examined in three test sites. The results demonstrated that the effect of the soil moisture error on mobility predictions is complex and may produce very significant errors in off-road mobility analysis for certain combinations of vehicles, seasons, and climates. This is because soil moisture biases vary in both direction and magnitude with season and location. Furthermore, vehicles are sensitive to different ranges of soil moistures. Modeled vehicle speeds in the dry time periods were limited by the interaction between soil traction and the vehicles’ powertrain characteristics. In the wet season, differences in soil strength resulted in more significant differences in mobility predictions.  
Keywords: Soil moisture; Off-road mobility; Soft soil trafficability; Remote sensing

Design of manned lunar rover wheels and improvement in soil mechanics formulas for elastic wheels in consideration of deformation

Wanshen Xiao, Yan Zhang
Journal of Terramechanics, Volume 65, June 2016, Pages 61-71, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.03.004
http://www.sciencedirect.com/science/article/pii/S0022489816300027
Abstract: 
Most of the current lunar rover vehicle wheels are inconvenient for changing broken wheels and have poor shock absorbing in driving, so they cannot be used to carry people on the moon. To meet the demands for manned lunar transportation, a new wheel possessing a woven metal wire mesh tire and using hub-rim combination slide mechanism is designed in this article. The characteristics of the new wheel is analyzed by comparing with the same-size conventional rover wheels after demonstrating the validity of FEM simulation. The new wheel possesses lighter structure and superior shock absorbing. It also provides stronger traction because the deformation of the designed wheel increases the contact area between the tire and lunar terrain. In order to establish an on-line soil parameter estimation algorithm for low cohesion soil, the stress distribution along a driven deformable wheel on off-road terrain is simplified. The basic mechanics equations of the interaction between the wheel and the lunar soil can be used for analytical analysis. Simulation results show that the soil estimation algorithm can accurately and efficiently identify key soil parameters for loose sand. 
Keywords: Manned lunar rover; Woven metal wire mesh tire; Wheel deformation; Soil parameter

Development of a dynamometric vehicle to assess the drawbar performance of high-powered agricultural tractors

M. Cutini, C. Bisaglia
Journal of Terramechanics, Volume 65, June 2016, Pages 73-84, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.03.005
http://www.sciencedirect.com/science/article/pii/S0022489816300039
Abstract:  
Agricultural tractors are machines originally designed to mechanize agricultural tasks, especially tillage and pulling. A large part of research activities have been interested in optimizing tractor efficiency, in particular in terms of emissions and energy. In this frame, the OECD Tractor Code 2 sets out a drawbar test in specific controlled conditions with the aim of evaluating the power of the tractor available at the drawbar. The principal measurement chain relies on dynamometric vehicles (DV) that are instrumented vehicles specifically engineered to develop horizontal force at the drawbar of agricultural tractors. The CREA Laboratory of Treviglio, Italy, engineered a new dynamometric vehicle to test tractors with up to 200 kW at the drawbar (245 kW at the engine flywheel) and a maximum of 118 kN drawbar force. The chosen basis is a FIAT 6605 N truck (TM 69 6 × 6) which has been transformed into a hydrostatic vehicle driven by a hydraulic system and an auxiliary gearbox. The maximum drawbar force was verified up to 122 kN. The drawbar power verification (200 kW) was successfully carried. The final verification confirmed that the project is valid for the investigation and optimization of the parameters regarding the traction efficiency of agricultural tractors.
Keywords: Drawbar power; Driveline; Tractor efficiency; Agricultural tyres

Predicting USCS soil classification from soil property variables using Random Forest

Daniel R. Gambill, Wade A. Wall, Andrew J. Fulton, Heidi R. Howard
Journal of Terramechanics, Volume 65, June 2016, Pages 85-92, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.03.006.
http://www.sciencedirect.com/science/article/pii/S0022489816300040
Abstract:
Soil classification systems are widely used for quickly and easily summarizing soil properties and provide a shorthand method of communication between scientists, engineers, and end-users. Two of the most widely used soil classification systems are the United States Department of Agriculture (USDA) textural soil classification system and the Unified Soil Classification System (USCS). Unfortunately, not all soil map units are classified according to the USDA or USCS systems, and previous attempts to provide a crosswalk table have been inconsistent. Random Forest machine learning model was used to create a USCS prediction model using USDA soil property variables. Important variables for predicting USCS code from available soil properties were USDA soil textures, percent organic material, and available water storage. Prediction error rates less than 2% were achieved compared to error rates of approximately 40% using crosswalk methods.
Keywords: USDA; USCS; Random Forest model; Crosswalk table

Calibration of a simple cone-penetration model for snow

Jonah H. Lee 
Journal of Terramechanics, Volume 64, April 2016, Pages 36-45, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2016.01.001.
http://www.sciencedirect.com/science/article/pii/S0022489816000021
Abstract:
Numerical studies using the Material Point Method (MPM) have been conducted recently to model snow penetration tests for fine-grained and coarse-grained snows using small cones with diameters ranging from 2.5 mm to 4 mm, and cone half-angles between 15° and 45°. Although numerical studies have gained physical insight of these tests, due to the lengthy computation time needed for the MPM simulations, it is not feasible to use these simulations to develop a stochastic model to assess the large variations of the mechanical properties of snow typically shown in tests. In this paper, we present a simple and efficient physics-based analytical model based on equilibrium and a cavity expansion solution upon which a stochastic model is built to obtain calibrated material parameters for a Drucker–Prager (DP) model such that prediction of the model can be made. Sensitivity analysis of the analytical model indicates that cohesion and interfacial shear (friction) factor contribute significantly to the penetration hardness whereas the friction angle has little contribution. The calibrated material parameters are similar to those estimated via the MPM simulations. The quality of the stochastic model, when compared with test data, was assessed using four interval-based validation metrics with good results.
Keywords: Cone penetration; Snow; Drucker–Prager; Validation; Calibration; Sensitivity; Metrics; Hardness; Cavity expansion; Gaussian process

Effect of gravity on the mechanical properties of lunar regolith tested using a low gravity simulation device

Meng Zou, Shichao Fan, Ruiyang Shi, Yanjing Yang, Jianqiao Li
Journal of Terramechanics, Volume 60, August 2015, Pages 11-22, ISSN 0022-4898, http://dx.doi.org/10.1016/j.jterra.2015.04.003
http://www.sciencedirect.com/science/article/pii/S0022489815000476
Abstract:
Simulations of the bearing capacity and shear strength of regolith under Earth’s gravity produce different results from those under low gravity. A low-gravity simulation device was developed in this study, and an internal stress model of regolith simulant was established to correct the errors. The model revealed additional force on both shear plane in the shear test and the press plate area in the pressure–sinkage test. The sinkage and shear test results showed that low gravity decreased the deformable index n, frictional modulus kφ and cohesion c, whereas there were no obvious changes to the cohesive modulus kc and internal friction angle φ. The sinkage generally increased as the gravity decreased under a consistent normal load larger than 50 N, but when the wheel load was lower than 50 N, the sinkage of the TYII-1 simulant was larger under 1 G than 1/6 G. Gravity had little effect on the shear strength of the regolith. However, the tractive thrust of the TYII-1 simulant was lower under 1/6 G than 1 G. The smaller difference was due to differences in the way the soils responded to changes in the gravity level for the TYII-2 simulant.
Keywords: Regolith; Low gravity; Shear strength; Bearing capacity