Effects of reduced inflation pressure and vehicle loading on off-road traction and soil stress and deformation state

Field experiments on off-road vehicle traction and wheel–soil interactions were carried out on sandy and loess soil surfaces. A 14 T, 6 × 6 military truck was used as a test vehicle, equipped with 14.00-20 10 PR tyres, nominally inflated to 390 kPa. Tests were performed at nominal and reduced (down to 200 kPa) inflation pressures and at three vehicle loading levels: empty weight, loaded with 3.6 and 6.0 T mass (8000, 11,600 and 14,000 kg, respectively). Traction was measured with a load cell, attached to the rear of the test vehicle as well as to another, braking vehicle. Soil stress state was determined with the use of an SST (stress state transducer), which consists of six pressure sensors. Soil surface deformation was measured in vertical and horizontal directions, with a videogrammetric system. Effects of reduced inflation pressure as well as wheel loading on traction and wheel–soil interactions were analyzed. It was noticed that reduced inflation pressure had positive effects on traction and increased stress under wheels. Increasing wheel load resulted in increasing drawbar pull. These effects and trends are different for the two soil surfaces investigated. The soil surface deformed in two directions: vertical and longitudinal. Vertical deformations were affected by loading, while longitudinal were affected by inflation pressure.     

基于ALE方法的飞机水面降落过程

为了研究飞机水面降落过程中的动态响应规律,采用arbitrary Lagrangian-Eulerian(ALE)方法开展了飞机水面降落的动力学分析。研究了飞机水面降落过程的速度变化规律,结果表明,降落速度和飞行速度在飞机入水的初始阶段变化较快,随后变化幅度趋于平缓。分析了不同降落速度、飞行速度和降落仰角下的机身结构响应,得到了飞机水面降落时结构响应随时间的变化规律。机身结构应力在入水的初始阶段达到最大值,随后迅速下降,最后保持稳定。飞机结构的最大变形也出现在入水的初始阶段,随后迅速回复到初始状态。对比了降落速度、飞行速度和降落仰角对飞机结构响应的影响程度,结果表明降落速度对结构响应的影响程度最大,降落仰角次之,飞行速度的影响最小。 更多还原     

惯性／DGPS精密进场着陆引导系统

为确保飞机精确直线飞越预定点上空并具备精密进场着陆引导能力 ,研制了机载惯性 /DGPS精密进场着陆引导系统。该系统通过综合技术将惯性导航系统与差分 GPS有效组合 ,不仅定位精度高、工作可靠 ,而且输出信息实时、连续 ;通过着陆引导轨迹及引导画面的精心设计 ,为飞行员提供图形化的、精确安全的进场着陆引导信息 ,引导飞机至决断高处 ,从而解决了飞机全天候尤其是在复杂气象条件的进场着陆引导问题。本文介绍了该系统的方案、组成、系统特点、技术难点以及试飞结果。试飞中飞行员反映该系统精度高 ,引导画面图形直观 ,操作简便 ,能满足复杂气象条件下的进场着陆引导要求     

Effects of a roller and a tracked vehicle on the compaction of a high lifted decomposed granite sandy soil

The aim of this research was to innovate a new compaction machinery by comparing experimentally the effects of a two-axle, two wheel road roller and a tracked vehicle on the compaction of a decomposed granite sandy soil with a high spreading lift. By measuring the amount of sinkage of the terrain surface, the dry density distribution versus depth using a cone penetrometer, the normal earth pressure distribution versus depth using a stress state transducer (SST), the effects of the road roller and the tracked vehicle on the increment of the soil dry density were considered theoretically. It was observed that the tracked vehicle showed a larger amount of sinkage and a larger dry density distribution versus depth than the roller. The ratio of shear stress to normal stress was still large enough at the deep stratum, so that an optimal shear strain was developed on the whole range of the high lifted stratum and it increased the soil compaction density due to the dilatancy effect.     

STABILITY ANALYSIS OF REUSABLE LAUNCH VEHICLE LANDING STRUCTURE$^{\bf 1)}$

近年来,包括中国在内的诸多国家相继开展垂直起降重复使用火箭的研究,运载火箭在平台上垂直着陆时的着陆稳定性为实现运载火箭重复使用的关键问题. 由于在运载火箭设计初期结构设计尚未完成,不具有供着陆稳定性分析的详细的动力学模型,难以开展着陆过程动力学仿真,故对运载火箭着陆稳定性评估方法的研究尤为必要. 本文基于广义碰撞定律,对二维运动模式下运载火箭与着陆平台的多点碰撞过程进行了分析,切向采用库伦摩擦模型给出了切向运动学恢复系数的表达式. 本文首先通过机械能约束和接触碰撞中的单边约束给出了一般运动形式下广义运动学恢复系数的值域,再对两种典型运动模式,给出了该两种典型运动模式下广义运动学恢复系数的值域. 然后考虑着陆腿中缓冲器的作用,将运载火箭与平台的碰撞近似为完全非弹性碰撞,得到了其广义运动学恢复系数,并结合运动学分析和能量法提出了一种基于碰撞后速度的着陆稳定性的判别方法. 最后以某型运载火箭着陆样机的参数为例,分析了碰撞前速度、着陆腿跨距、摩擦系数对着陆稳定性的影响,结果表明,本文提出的稳定性判别方法较能量法更为精确,可以考虑触地速度、角速度、摩擦系数等参数间的耦合关系.     

Measurement of compacted soil density in a compaction of thick finishing layer

The compaction of a soil is one of the important construction operations that influences the durability of soil structure. Therefore, the measurement of soil density, used to judge the degree of compaction, has to be performed exactly. Since a compaction of a thick finishing layer could be executed with the enlargement of compaction machinery and the improvement of productivity, new equipment which can measure the soil density in a deep stratum has to be developed. In this paper, we propose a method of accurately estimating compacted soil density based on the three dimensional stresses measured in the ground during compaction by a stress state transducer (SST). A tracked vehicle mounted with a vertical oscillator was used to compact a decomposed granite soil of 45 cm depth. A model experiment was executed at a frequency that was varied from 16 to 25 Hz, setting the load ratio of maximum oscillating force to the vehicle weight (4.9 kN) to be 1.2, 1.6 and 2.0. The three dimensional stresses in the ground were measured by use of the SST. Comparing the dry density converted from cone penetrometer test results and the dry density estimated from Baily’s formula, the compacted soil density at the lowest soil stratum could be estimated by measuring earth pressure using SST.     

重复使用火箭着陆结构稳定性分析

近年来,包括中国在内的诸多国家相继开展垂直起降重复使用火箭的研究,运载火箭在平台上垂直着陆时的着陆稳定性为实现运载火箭重复使用的关键问题.由于在运载火箭设计初期结构设计尚未完成,不具有供着陆稳定性分析的详细的动力学模型,难以开展着陆过程动力学仿真,故对运载火箭着陆稳定性评估方法的研究尤为必要.本文基于广义碰撞定律,对二维运动模式下运载火箭与着陆平台的多点碰撞过程进行了分析,切向采用库伦摩擦模型给出了切向运动学恢复系数的表达式.本文首先通过机械能约束和接触碰撞中的单边约束给出了一般运动形式下广义运动学恢复系数的值域,再对两种典型运动模式,给出了该两种典型运动模式下广义运动学恢复系数的值域.然后考虑着陆腿中缓冲器的作用,将运载火箭与平台的碰撞近似为完全非弹性碰撞,得到了其广义运动学恢复系数,并结合运动学分析和能量法提出了一种基于碰撞后速度的着陆稳定性的判别方法.最后以某型运载火箭着陆样机的参数为例,分析了碰撞前速度、着陆腿跨距、摩擦系数对着陆稳定性的影响,结果表明,本文提出的稳定性判别方法较能量法更为精确,可以考虑触地速度、角速度、摩擦系数等参数间的耦合关系.     

Prediction of wheel performance by analysis of normal and tengential stress distributions under the wheel-soil interface

Prediction of wheel performance by analysis of normal stress distribution under the wheel-soil interface was reported by one of our research members. In this study analysis of both normal and tangential stress distributions are included for the prediction of wheel performance. A visco-elastic soil model based on a three-element Maxwell model is used to evaluate normal stress distribution under a wheel running on soft ground. The values of the parameters characterizing the visco-elastic behavior of the soil can be derived from plate penetration tests. A rigid wheel-soil interface model is used to evaluate the tangential stress distribution under the wheel-soil interface. Shear deformation modulus, cohesion and angle of internal shearing resistance of the soil are derived from shear-displacement tests. Test results indicate that both maximum normal and shear stress occur in front of the wheel axle, and the location of peak normal stress shifts backwards towards the wheel axle while that of tangential stress shifts forwards when slippage is increased from a low value. Increasing slippage also causes a decrease in normal stress and an increase in tangential stress. Coefficients of traction and tractive efficiency are low at low slippage, increase with an increase in slippage, and level off at higher slippage.     

非稳态载荷下轮轨滚动接触及其钢轨波磨研究

利用有限元法,考虑材料反复滚压条件下棘轮效应和局部滑动的影响,研究了非稳态机车和车辆车轮载荷作用下轮轨滚动接触的弹塑性应力、应变和变形,进而分析了塑性流动型钢轨波浪形磨损的形成和发展过程以及波谷和波峰处材料的力学行为.结果表明:在非稳态载荷作用下,钢轨接触表面产生不均匀塑性变形引起的波磨,波磨发展速率呈衰减趋势,最终趋于稳定状态;在相同载荷下,与车辆车轮相比,机车车轮对钢轨波磨影响较大;波谷处的残余应力、应变和变形大于波峰处.     

Design and development of a new transformable wheel used in amphibious all-terrain vehicles (A-ATV)

Conventional ground-wheeled vehicles usually have poor trafficability, low efficiency, a large amount of energy consumption and possible failure when driving on soft terrain. To solve this problem, this paper presents a new design of transformable wheels for use in an amphibious all-terrain vehicle. The wheel has two extreme working statuses: unfolded walking-wheel and folded rigid wheel. Furthermore, the kinematic characteristics of the transformable wheel were studied using a kinematic method. When the wheel is unfolded at walking-wheel status, the displacement, velocity and acceleration of the wheel with different slip rates were analyzed. The stress condition is studied by using a classic soil mechanics method when the transformable wheel is driven on soft terrain. The relationship among wheel traction, wheel parameters and soil deformation under the stress were obtained. The results show that both the wheel traction and trafficability can be improved by using the proposed transformable wheel. Finally, a finite element model is established based on the vehicle terramechanics, and the interaction result between the transformable wheel and elastic–plastic soil is simulated when the transformable wheel is driven at different unfold angles. The simulation results are consistent with the theoretical analysis, which verifies the applicability and effectiveness of the transformable wheel developed in this paper.     

一种轿车车轮应力状态测量与分析方法

基于台架应变测试试验方法,研究轿车车轮在疲劳试验中的应力状态。结果表明:车轮的径向疲劳试验时,轮胎会对车轮的载荷产生较大的影响,轮胎与转鼓的挤压变形以及正反转动都会对车轮的应力状态产生影响;车轮径向疲劳试验时,最大应变出现在轮辋与轮胎接触位置沿车轮圆周方向,而在靠近轮心位置的应变较小;车轮弯曲疲劳试验时,最大应变出现在轮辐靠近轮心的位置,最大应变出现在轮辐的长度方向;不同的载荷对车轮应变的变化规律并没有影响,但是会对最大和最小峰值产生影响。     

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth.     

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

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.     

Prediction of wheel-soil interaction and performance using the finite element method

In this experimental-analytical study of wheel-soil interaction, a technique based on the finite element method is used for predicting continuous wheel performance and subsoil response behaviour. The evaluation of wheel-soil interaction performance at any degree of slip is performed using energy principles. The analytical technique utilizes experimentally determined wheel-soil particle path as displacement input for load simulation to predict the soil response beneath the wheel.

An incremental loading approach is adopted to satisfy as closely as possible the soil loading path. The solution requires initial conditions which establish the soil at zero energy level (no stress history) and proceeds to stationary wheel positions with wheel-soil penetration equal to its dynamic sinkage. The method of analysis then proceeds to the steady-state wheel travel mode. The predicted drawbar pulls and subsoil behaviour results are presented and shown to compare well with the experimentally measured values.     

Mobility performance of a rigid wheel in low gravity environments

To investigate influences of gravity on mobility of wheeled rovers for future lunar/planetary exploration missions, model experiments of a soil-wheel system were performed on an aircraft during variable gravity maneuvers. The experimental set-up consists of a single rigid wheel and a soil bed with two kinds of dry sands: lunar soil simulant and Toyoura sand. The experimental results revealed that a lower gravity environment yields higher wheel slippage in variable gravity conditions. In addition to the partial gravity experiments, the same experiments with variable wheel load levels were also performed on ground (1 g conditions). The on-ground experiments produced opposite results to those obtained in the partial gravity experiments, where a lower wheel load yields lower slippage in a constant gravity environment. In low gravity environments, fluidity (flowability) of soil increases due to the confining stress reduction in the soil, while the effect of the wheel load on sinkage decreases. As a result, both of these effects are canceled out, and gravity seemingly has no effect on the wheel sinkage. In the meantime, in addition to the effect of wheel load reduction, the increase of the soil flowability lessens the shear resistance to the wheel rotation, as a result of which the wheel is unable to hold sufficient traction in low gravity environments. This suggests that the mobility of the wheel is governed concurrently by two mechanisms: the bearing characteristics to the wheel load, and the shearing characteristics to the wheel rotation. It appears that, in low gravity, the wheel mobility deteriorates due to the relative decrease in the driving force while the wheel sinkage remains constant. Thus, it can be concluded that the lunar and/or Mars’ gravity environments will be unfavorable in terms of the mobility performance of wheels as compared to the earth’s gravity condition.     

Mathematical models for soil displacement under a rigid wheel

Soil–wheel interaction especially soil deformation caused by the wheel motion was investigated experimentally using a sophisticated soil bin test apparatus and an on-line measurement system for soil displacement. Based on these test results, characteristics of soil deformation were summarized focusing on the behavior and distribution of displacement increment vectors. Mathematical models were examined in order to describe the displacements of soil particles. Properties of the displacement loci are described. The magnitude of the displacement increment vector, its horizontal and vertical components are discussed, and characteristics of these distributions with respect to the relative horizontal distance from the vertical centerline of the wheel to the target point are clarified. Shapes of these distribution curves were closely similar to those of the derivatives of a Gaussian function. A distribution curve of the horizontal displacement increment had two peaks and that of the vertical one had three peaks. Based on the results, mathematical models for those displacement increments were proposed by employing a Gaussian function through multiplication of a linear function and a quadratic function. Predicted distributions and displacement loci of the models agreed with high accuracy to the measured values. The mathematical models were extended taking into consideration the wheel slip. The predicted distributions according to test conditions agreed very well to the measured results.     

Analysis of rheological deformation of soil by means of finite element method

A linear time-dependent viscoelastic behavior of soil is analyzed by the finite element method (FEM), which has great advantage in obtaining an approximate numerical solution of deformation or stress of a continuous body under complex boundary conditions, as known nowadays worldwide. A rheological three-element model, which is easily handled and represents rationally the actual behavior of soil, is suggested to obtain the rheological constants and the constitutive equation of soil. As actual examples of soil behavior, a stress relaxation of a soil block and a time-dependent sinkage of a rigid wheel are calculated by FEM and are also compared with test results and theoretical values.     

考虑海底接触的钢悬链式立管触地点动力响应以及疲劳损伤分析

基于考虑接触的钢悬链式立管SCR(Steel Catenary Riser)触地点处的结构特性,分别采用了考虑管土分离的线性截断模型以及包含土体吸力效应的帽盖模型来描述P-y曲线。通过改变上端浮体的垂荡运动幅度、土体吸力系数以及海床刚度,对SCR触地点处的动力响应以及疲劳损伤特性进行了分析。分析结果表明,SCR触地点的垂向位移、弯矩、等效应力以及疲劳损伤均随着浮体垂荡运动幅度的增加而呈上升趋势。SCR触地点的垂向位移随着土体吸力系数的增大由高幅低频响应转变为低幅高频响应。SCR触地点的疲劳损伤随着海床刚度的增加呈现先稳定再增加再稳定的趋势。     

Prediction of soil vertical stress under off-road tire using smoothed-particle hydrodynamics

Evaluation of vertical stress distribution in clay-loam soil using Smoothed-Particle Hydrodynamics-Finite Element Analysis (SPH-FEA) technique is presented in this research. The moist soil is modelled using the hydrodynamic elastic–plastic material and Murnaghan equation of state, while the tire is modelled using FEA in Visual Environment’s Pam-Crash software. Soil-tire interaction is performed using the node symmetric node to segment with edge treatment method. A single-wheel tester in a soil bin environment was utilized to provide experimental data. The objectives of the experimental test were to (1) calculate maximum subsoil stresses in the subsoil at 1 to 15 passes of a wheel with loads of 1, 2, 4, and 5 kN on soil with moisture levels of 0, 10, 17, and 24%.; (2) calibrate soil with different levels of moisture (3) compare predicted soil stresses with experiments. The maximum stress at 20 cm depth increased with increasing soil moisture and also with high levels of tire load. In contrast, successive traffic showed a decreasing effect on soil stress. The coefficient of determination 0.97 shows the predictions agreed very well with experiments. The moist soil-tire interaction model will be further used to analyze the soil stress in different soil depths and different forward velocity.