Tire inflation and its influence on drawbar characteristics and performance – Energetic indicators of a tractor set

This work deals with the influence of tire inflation on tractive characteristics and performance-energetic parameters of a ploughing set. The test was conducted using two tire sets with different tire pressures under field conditions. Measurements of tractive properties were performed by setting travel speeds to 5, 8, and 10 kph, respectively. The ploughing set was operated at 8 kph, according to the manufacturer’s recommendation. The measurement results were processed graphically and mathematically into the Vehicle Traction Ratio, drawbar power, and slip characteristics. The tire inflation, reduced from 180 to 65 kPa and/or 75 kPa, of tires with wide treads (low-profile) resulted in increase of the front tire footprint by 24.7% and rear tire footprint by 31.1%. This change had a positive impact on the specific tractive fuel consumption that decreased in the range from 3.4% to 16.0%, depending on the travel speed. The results of performed measurements revealed that reducing the tire inflation of appropriate tires can improve the drawbar characteristics and consequently the fuel consumption.     

Theoretical analysis of the forces and torques acting on a tractor during ploughing

This paper presents a physical model of a four-wheeled tractor during ploughing. In this model, the wheels are approximated by ideal non-oscillating harmonic oscillators. The velocity of the tractor is considered to be constant, i.e., the tractor is in an equilibrium state of forces and torques. The equilibrium state is described by Euler’s laws of motion. Finally, an infinite approximation of the wheel stiffness is performed and an exact solution of the forces is presented.     

Dynamics of tractor lateral overturn on slopes under the influence of position disturbances (model development)

In order to investigate the effects of forward speed, ground slope and wheel–ground friction coefficient on lateral stability of tractor at the presence of position disturbances, a tractor dynamic model was developed. In this model two types of instability were considered: instability due to overturn and skid and for each case the stability index was determined. Different geometries and mass specifications of tractor MITSUBISHI-2501D were used to examine the model. According to the results of this model forward speed and ground slope had a reverse effect on all stability indexes. Moreover stability of this tractor was more affected by tractor skidding than overturning. Therefore to improve the overall stability of this tractor, preference should be on increasing the tractor stability index derived from skid dynamics of tractor.     

基于多目标优化NSGA2改进算法的结构动力学模型确认

传统结构动力学模型确认方法通常采用单目标优化,存在精度不足和稳定性差等缺点,难以满足实际工程需求。基于此,提出一种采用神经网络作为代理模型,建立以马氏距离和鲁棒性为不确定性量化指标的多目标优化模型,并将NSGA2多目标进化算法用于求解。针对NSGA2存在无法有效识别伪非支配解、计算效率低和解集质量较差等设计缺陷,提出一种基于支配强度的NSGA2改进算法INSGA2-DS。INSGA2-DS将支配强度引入非支配排序,采用新型拥挤距离公式和自适应精英保留策略,以提高收敛效率和解集质量。GARTEUR飞机算例的仿真结果表明,INSGA2-DS求解复杂工程问题时具有更好的收敛性和分布性,而考虑鲁棒性的结构动力学模型确认方法可以获得同时满足多种目标要求的Pareto解集,提高了模型确认的精度和稳定性。     

鸟体姿态对结构抗鸟撞性能的影响

低于现行标准规定能量的大量鸟撞事故中,航空结构仍然发生实质性破坏的情况,说明只考虑鸟体的质量和速度不足以保证飞机安全。本文中针对弹性平板、雷达罩及机翼前缘等飞机典型结构,开展了不同姿态鸟体的鸟撞分析研究。分析结果发现,鸟体姿态对结构的抗鸟撞性能有比较显著的影响,不同的结构特点反映的响应规律也不同:对吸能结构,姿态角越大,吸收的能量越多,被保护的结构就越安全;而对承力结构,姿态角越大,高应力区域越大,结构就越危险。因此,在结构的抗鸟撞安全性评估中,除了完成特定姿态鸟体的鸟撞实验,针对危险工况还应通过数值分析评估不同鸟体姿态的结构撞击响应,进一步确保结构的抗鸟撞能力。     

Modeling of macroscopic response of phase transforming materials under quasi-static loading

Based on a continuum model of solid-solid phase transformations, the macroscopic response of a bar of a thermoelastic phase transforming material loaded quasistatically is investigated. A critical loading rate is identified for the evolution of a single phase boundary in the bar during an isothermal process. It is shown that, when the loading rate is larger than this critical loading rate, nucleation occurs either continuously or at multiple sites; when the loading rate is lower than this critical loading rate, the size of the hysteresis loop increases with increasing loading rate, and decreases with an increase in the mobility of the phase boundary. The heat conduction due to the heat generated by the latent heat of the phase transformation is considered for a special case.     

Effect of variations in front wheels driving lead on performance of a farm tractor with mechanical front-wheel-drive

Most previous researches indicate that about 20–55% of available tractor power is lost in the process of interaction between tires and soil surface. Vertical wheel loads and tire performance are parameters that play a significant role in controlling slip and fuel consumption of a tractor. Tractor’s slip is adjusted by attaching additional weights and/or reducing tire pressures, and this may have an impact on driving lead of front wheels. Mechanical Front-Wheel-Drive (MFWD) tractors work efficiently when driving lead of front wheels is 3–4% in soft soil and 1–2% in hard soil. This research was aimed to experimentally determine such tire pressures that allow adjusting tractor’s slip without deviating from set value of driving lead of front wheels. The research was also aimed to determine the effect of driving lead of front wheels on MFWD tractor’s slip and fuel consumption. Experimental results showed that front/rear tire pressure combinations that generate a well-targeted driving lead of front wheels have no effect on slip on hard soil; however, it significantly affect fuel consumption. Results show that when air pressures in front/rear tires varied within 80–220 kPa, driving lead of front wheels varied in the range from +7.25% to −0.5%.     

Viscoplastic dynamics of isotropic plates of variable thickness under explosive loading

A problem of viscoplastic dynamic bending of isotropic plates of variable thickness is formulated. A method for integrating the initial-boundary problem is developed. Numerical results are compared with a known analytical solution obtained within a rigid-plastic model; good agreement is demonstrated. The efficiency of the method developed is verified by numerical computations. It is shown that the final flexure of plates can be reduced severalfold by applying rational design. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 123–134, March–April, 2007.     

柔性多体系统动力学研究现状与展望

对柔性多体系统动力学的研究现状进行了概括和总结,主要从柔性体建模方法、刚柔耦合动力学、接触碰撞问题、多物理场耦合、微分代数方程求解技术、控制方法、设计优化及软件开发和实验研究等几个研究方向进行总结,并对未来的研究方向做了展望.     

Implementation and validation of a polycrystalline model for a bi-phased steel under non-proportional loading paths

The aim of the present paper is to propose a polycrystalline approach in order to model the elastic–plastic behavior of an austenitic–ferritic stainless steel. In order to take into account the specific character of the steel, the multi-scale polycrystalline approach proposed by Cailletaud–Pilvin [Pilvin, P., 1990. Approches multiéchelles pour la prévision du comportement anélastique des métaux, PhD Thesis, Université Pierre et Marie Curie; Cailletaud, G., 1992. A micromechanical approach to inelastic behavior of metals. International Journal of Plasticity 8, 55–73; Cailletaud, G., Pilvin, P., 1994. Utilisation des modéles polycristallins pour lecalcul par éléments finis. Revue Européenne des Eléments Finis 3 (4), 515–541] is extended to bi-phased material. In particular, two interaction laws and two local behaviors, based on the crystallographic slip and the dislocation density evolution, are simultaneously considered. After an identification of the model parameters on simple tests (monotonous tension, tension-compression), we propose an evaluation of the predictive capabilities of the multi-scale approach for non-proportional loading paths. A good agreement is observed between simulation and experimental data.     

Dynamics modeling and robotic-assist,leader-follower control of tractor convoys

This paper proposes a generalized dynamics model and a leader-follower control architecture for skid-steered tracked vehicles towing polar sleds. The model couples existing formulations in the literature for the powertrain components with the vehicle-terrain interaction to capture the salient features of terrain trafficability and predict the vehicles response. This coupling is essential for making realistic predictions of the vehicles traversing capabilities due to the power-load relationship at the engine output. The objective of the model is to capture adequate fidelity of the powertrain and off-road vehicle dynamics while minimizing the computational cost for model based design of leader-follower control algorithms. The leader-follower control architecture presented proposes maintaining a flexible formation by using a look-ahead technique along with a way point following strategy. Results simulate one leader-follower tractor pair where the leader is forced to take an abrupt turn and experiences large oscillations of its drawbar arm indicating potential payload instability. However, the follower tractor maintains the flexible formation but keeps its payload stable. This highlights the robustness of the proposed approach where the follower vehicle can reject errors in human leader driving.     

Size and temperature effects on the fracture mechanisms of silicon nanowires: Molecular dynamics simulations

We present molecular dynamics simulations of [1 1 0]-oriented Si nanowires (NWs) under a constant strain rate in tension until failure, using the modified embedded-atom-method (MEAM) potential. The fracture behavior of the NWs depends on both temperature and NW diameter. For NWs of diameter larger than 4 nm, cleavage fracture on the transverse (1 1 0) plane are predominantly observed at temperatures below 1000 K. At higher temperatures, the same NWs shear extensively on inclined {1 1 1} planes prior to fracture, analogous to the brittle-to-ductile transition (BDT) in bulk Si. Surprisingly, NWs with diameter less than 4 nm fail by shear regardless of temperature. Detailed analysis reveals that cleavage fracture is initiated by the nucleation of a crack, while shear failure is initiated by the nucleation of a dislocation, both from the surface. While dislocation mobility is believed to be the controlling factor of BDT in bulk Si, our analysis showed that the change of failure mechanism in Si NWs with decreasing diameters is nucleation controlled. Our results are compared with a recent in situ tensile experiment of Si NWs showing ductile failure at room temperature.     

Modeling the deformation of rock with rough surfaces of block contact under quasistatic and dynamic loading conditions

The effect of layers consisting of small particles or asperities on contact surfaces on the deformation of a block medium as a whole and the failure of the blocks constituting the medium was studied experimentally. The samples were subjected to quasistatic uniaxial compression perpendicular to the contact surfaces. Numerical modeling was performed of wave propagation during pulse loading of a pair of blocks having rough surfaces of contact and made of a material with elastic characteristics close to the characteristics of marble and limestone. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 173–178, May–June, 2007.     

人体上肢运动学动力学建模与仿真技术的研究

根据多体动力学原理,以人体解剖学为基础,对人体上肢进行建模,推导了其动力学和运动学方程,建立了人体上肢四刚体四自由度动力学模型,运用多系统动力学软件ADAMS,结合UG建模功能,对人体上肢动力学和运动学特性进行了分析计算,对人体上肢收臂翻掌过程的运动进行了仿真,并将计算结果与实测数据进行了对比,验证了模型的正确性和有效性。     

Fundamental validation of the finite volume particle method for 3D sloshing dynamics

The use of the finite volume particle (FVP) method is validated for three‐dimensional sloshing dynamics with a free surface by comparing with results from experiments. In the first part, two typical sloshing experiments for a single liquid phase are simulated, and slosh characteristics that include the free surface behavior and hydrodynamic pressure are reported. Moreover, the influences of the circular wall geometry and spatial resolution in the simulation are studied in a sensitivity analysis. In the second part, two sloshing problems with solid bodies are simulated to preliminarily verify the applicability of the FVP method to three‐dimensional solid bodies' motion in liquid flow. Good agreement between simulations and corresponding experiments indicates that the present FVP method well reproduces three‐dimensional sloshing behavior. Copyright © 2010 John Wiley & Sons, Ltd.     

Material structure and size effects on the nonlinear dynamics of electrostatically-actuated nano-beams

An accurate nonlinear model for electrostatically actuated beams made of nanocrystalline materials is proposed accounting for the beam material structure and the beam size effects. Two sets of measures are incorporated in the context of the proposed model to account for the inherent properties (the material structure related properties) and the acquired properties (the size dependent properties) of the beam. The inherent properties of the beam are modeled via a micromechanical model while the acquired properties are modeled via a non-classical continuum beam theory. The micromechanical model for nanocrystalline materials is proposed where the necessary measures to account for the effects of the grain size, the voids percent and size, and the interface (grain boundary) are incorporated. All the measures presented in the micromechanical model are related to the material structure to correctly model the structure of nanocrystalline materials. According to the classical couple stress and Gurtin-Murdoch surface elasticity theories, a size-dependent Euler-Bernoulli beam model is developed to model the mechanics of electrostatically actuated nano-beams. For the first time, the impacts of the beam material structure along with the beam size on the nonlinear dynamics and pull-in instability behaviors of electrostatically actuated nano-beams are intensively studied. The performed analyses through the present effort reflect the great impacts of the beam material structure and the beam size on the static pull-in, the natural frequencies, the dynamic pull-in, and nonlinear dynamics of electrostatically actuated nano-beams.     

Process of selective laser sintering of polymer powders: Modeling,simulation, and validation

Selective laser sintering (SLS) of polymer powders involves multiphysical transient phenomena. A numerical tool for simulating such a process is developed on the basis of the reliable modeling of the corresponding thermo-physical transient phenomena and appropriate numerical methods. The present paper addresses modeling, simulation, and validation aspects that are indispensable for studying and optimizing SLS process. The coupled multiphysical models are detailed, and the numerical tool based on the finite volume method is presented, with validations in terms of numerical and physical accuracy, by considering the shrinkage involved in the process and the successive layers deposition. A parametric analysis is finally proposed in order to test the reliability of the model in terms of representing real physical phenomena and thermal history experienced by the material during the process.     

Modeling of system dynamics of a slewing flexible beam with moving payload pendulum

When a tower crane is handling payload via rotation and moving the carriage simultaneously the jib structure and the payload can be modeled as a system consisting of a slewing flexible clamed-free beam with the spherical payload pendulum that moves along the beam. The present work completes the dynamic modeling of the system mentioned above. The clamed-free beam attached to a rotating hub is modeled by Euler–Bernoulli beam theory. The payload is modeled as a sphere pendulum of point mass attached to via massless inextensible cable the carriage moving on the rotating beam. Non-linear coupled equations of motion of the in- and out-of-plane of the beam and the payload pendulum are derived by means of the Hamilton principle. Some remarks are made on the equations of motion.     

Finite element modeling of contact conditions in multibody system dynamics

In this paper a new method is developed for the dynamic analysis of contact conditions in flexible multibody systems undergoing a rolling type of motion. The relative motion between the two contacting bodies is treated as a constraint condition describing their kinematic and geometric relations. Equations of motion of the system are presented in a matrix form making use of Kane's equations and finite element method. The method developed has been implemented in a general purpose program called DARS and applied to the simulation and analysis of a rotating wheel on a track. Both the bodies are assumed flexible and discretized using a three dimensional 8-noded isoparametric elements. The time variant constraint conditions are imposed on the nodal points located at the peripheral surfaces of the bodies under consideration. The simulation is carried out under two different boundary conditions describing the support of the track. The subsequent constraint forces associated with the generalized coordinates of the system are computed and plotted. The effects of friction are also discussed.