Experimental analysis of vertical soil reaction and soil stress distribution under off-road tires

In this study, the vertical soil reaction acting on a driven wheel was measured by strain gages bonded to the left rear axle of a 2WD tractor driven under steady-state condition on different soil surfaces, tractor operations, and combinations of static wheel load and tire inflation pressure. In addition, the measurements of radial and tangential stresses on the soil–tire interface were made simultaneously at lug’s face and leading side near the centerline of the left rear tire using spot pressure sensors. The experimental results indicate that the proposed method of vertical soil reaction measurement is capable of monitoring the real-time vertical wheel load of a moving vehicle and provides a tool for further studies on vehicle dynamics and dynamic wheel–soil interaction. Furthermore, the measured distributions of soil stresses under tractor tire could provide more real insight into the soil–wheel interactions.     

氟橡胶O型圈低压气体密封黏滞摩擦特性实验

开展了低气体密封压力条件下的氟橡胶O型圈低压黏滞摩擦特性实验研究.对氟橡胶O型圈与不锈钢2Cr13摩擦副的摩擦力的时变曲线进行测量,重点分析了密封压力、O型圈压缩率和滑动速度对O型圈黏滞摩擦力的影响规律.结果表明:随压缩率的增加,氟橡胶O型圈摩擦力增加,并呈现出明显的回弹特征,即滞后摩擦力增加且释放时间增加;随着密封压力的增加,其所受最大静摩擦力、滑动摩擦力与滞后摩擦力均呈现非线性变化,且存在一极大值;减小往复运动速度,摩擦力数值增大,但摩擦力极值对应的密封压力值变小.     

Deformable spherical devices to measure stresses within field soils

The design, calibration and use of two deformable spherical stress transducers are described. They are suitable for detecting principal stresses in deforming media such as soil and have major advantages over many rigid transducers previously used in such situations. One of the transducers is a water-filled rubber ball (WFRB) sensitive to hydrostatic and deviator stresses, the other is a mastic ball which deforms plastically and is sensitive to only deviator stresses. When the two devices are used at similar depths under a surface load, e.g. a wheel, the combined measurements of internal pressure of the WFRB and axial deformation of the mastic ball can be used to derive values for first and third principal stresses (assuming second and third principal stresses are equal). Calibration of the transducers at different temperatures is described.

Field measurements made with the transducer under loaded wheels are compared with predicted values of first principal stresses using equations developed by Söhne. Close correspondence between predicted and measured values was observed, when the existing soil strength conditions were taken into account.

The transducers promise to be useful in the measurement of stresses in field soils.     

An investigation of the use of strain gages to measure welding-induced residual stresses

The measurement of weld-induced residual stress is important in structures that are subjected to cyclic loading during their service life. Depending on their magnitude, stresses can influence the rate of crack growth under cyclic loading and hence affect the life of the structure. Because the level of residual stress may change during service, measurement of these changes is necessary for accurate life prediction of the structures. The measurement of welding-induced residual stress using strain gages poses significant problems, the most important being the potential damage to the gages by high temperatures generated in the welding process. This laboratory study was undertaken to assess the suitability and signal stability of commercially available resistive strain gages for the measurement of postweld residual stresses in a submarine hull structure. Adhesively bonded and weldable-type strain gages were attached to the surface of a 35 mm thick steel plate, which was then subjected to thermal cycles similar to those encountered during welding construction of a submarine pressure hull. This paper describes the strain gage application procedure, changes in the strain gage output at end of each experimental stage and the history of changes in the residual stress.     

On the use of the stratified momentum balance for the deduction of shear stress in horizontal gas–liquid pipe flow

The use of the stratified flow momentum balance for the deduction of interfacial and liquid wall shear stresses from experimental measurements is examined. A systematic error analysis is applied to the governing equations using the principle of maximum uncertainty. A series of air–water experiments were conducted in 50 and 80 mm diameter pipes, in which gas pressure drop, liquid height and gas wall shear stress were measured. A framework for the correlation of the deduced shear stresses is proposed from the experimental measurements. The uncertainty analysis is used to show that the definition of mean liquid height does not significantly influence the overall results. The development of empirical equations based on such methods may lead to total uncertainties of up to 40%, irrespective of accuracy of the experimental data or the appropriateness of the correlating technique. Comparisons with state-of-the-art correlations for the liquid wall and interfacial friction factor data showed even larger discrepancies between measurement and prediction.     

On the use of the stratified momentum balance for the deduction of shear stress in horizontal gas–liquid pipe flow

The use of the stratified flow momentum balance for the deduction of interfacial and liquid wall shear stresses from experimental measurements is examined. A systematic error analysis is applied to the governing equations using the principle of maximum uncertainty. A series of air–water experiments were conducted in 50 and 80 mm diameter pipes, in which gas pressure drop, liquid height and gas wall shear stress were measured. A framework for the correlation of the deduced shear stresses is proposed from the experimental measurements. The uncertainty analysis is used to show that the definition of mean liquid height does not significantly influence the overall results. The development of empirical equations based on such methods may lead to total uncertainties of up to 40%, irrespective of accuracy of the experimental data or the appropriateness of the correlating technique. Comparisons with state-of-the-art correlations for the liquid wall and interfacial friction factor data showed even larger discrepancies between measurement and prediction.     

A bending-to-stretching analysis of the blister test in the presence of tensile residual stress

The adhesion of films and coatings to rigid substrates is often measured using blister geometries, which are loaded either by an applied pressure or a central shaft. The measurement will be affected if there are residual stresses that make a contribution to the energy release rate. This effect is investigated using analytical solutions based on the principle of virtual displacements. A geometrically nonlinear finite element analysis is conducted for comparison. Furthermore, the relationships among strain energy release rate, load, deflection, and fracture radius are discussed in detail. Both analytical solutions and numerical results reveal that uniform tensile residual stresses reduce a specimen’s deflection if it experiences plate behavior under small loads. However, this effect becomes negligible when membrane stresses induced by the loading become dominant.     

Soil compaction and tires for harvesting and transporting sugarcane

Four tire types (A, block-shape tread; B, rib-shape tread; C, low-lug tread; D, high-lug tread) used to harvest and transport sugarcane were compared regarding the compaction induced to the soil. Tires were tested at three inflation pressures (207, 276, 345 kPa) and six loads ranging from 20 to 60 kN/tire. Track impressions were traced, and 576 areas were measured to find equations relating inflation pressure, load, contact surface and pressure. Contact surface increased with increasing load and decreasing inflation pressure; however, the contact pressure presented no defined pattern of variation, with tire types A and B generating lower contact pressure. The vertical stresses under the tires were measured and simulated with sensors and software developed at the Colombian Sugarcane Research Center (Cenicaña). Sensors were placed at 10, 30, 50 and 70 cm depth. Tire types A and B registered vertical stresses below 250 kPa at the surface. These two tires were better options to reduce soil compaction. The equations characterizing the tires were introduced into a program to simulate the vertical stress. Simulated and measured stresses were adjusted in an 87–92% range. Results indicate a good correlation between the tire equations, the vertical stress simulation and the vertical stress measurement.     

复合材料圆筒自由边效应的有限元分析

本文分析复合材料圆筒在内压作用下的自由边效应，讨论圆筒的叠层顺序对自由边应力的影响，预报最易发生边界脱层的界面．有限元分析结果表明：层间剪切破坏是该结构发生边界失效的主要原因，不同的叠层顺序可较大地影响自由边层间剪切应力，而大变形情况下线性有限元分析结果与非线性有限元数据相差较大，几何非线性的影响不容忽视．     

Rheological properties of fresh and cryopreserved human arteries tested in vitro

Cryopreservation is widely used to preserve blood vessels for a while but is controversially suspected to affect the mechanical behavior of these allografts. The aim of this study was to determine whether differences in the three-dimensional mechanical behavior exist or not between fresh and cryopreserved arteries. Using a previously developed experimental system, in vitro inflation tests were performed on twenty segments of human fresh and cryopreserved arteries, in static conditions. Opening angles were also measured from images of rings in zero-stress state. The initial reference state was chosen as the unloaded state and tests were performed on specimens stretched at natural “in vivo” length. Mechanical measures calculated are “natural” (Hencky) strains (finite deformations), “true” (Cauchy) stresses in radial, circumferential, and longitudinal directions as well as strain energy per unit volume. Tangent moduli are derived from radial and circumferential stress-strain characteristics using non-linear curve fitting. Values of incremental and pressure-strain elastic parameters, wall stiffness, and compliance per unit length are also calculated. Results are presented in terms of characteristics of stresses and strains in the three directions, axial force, tangent moduli vs strains or stresses, and energy per unit volume, for both types of artery, with reference to transmural pressure. Detailed numerical results are given at mean transmural pressure or in the physiological range. Significant differences are indicated by statistic Student T-tests. Results obtained show that significant differences exist between rheological properties of fresh and cryopreserved segments of human artery. Strains, stresses, axial force, strain energy, and wall stiffness values highlight those differences whereas elastic parameters, compliance, and opening angle do not. The usefulness of some parameters to compare the mechanical behavior existing between fresh and cryopreserved arteries is therefore underlined. Received: 3 January 2000 Revision received: 12 April 2000 Accepted: 8 May 2000     

Constitutive function,residual stress,and state of uniform stress in arteries

Fung (1983) has conjectured that the residual stress in an artery distributes itself in such a way to assure that the circumferential stress is uniform across the artery wall under physiological conditions. In this work, we identify the entire class of constitutive functions for which the circumferential stress is uniform across the artery wall when it is subjected to an internal pressure. It is found that these constitutive functions do not necessarily endow residual stresses. Furthermore, a subset of the class of the constitutive functions is identified for which the circumferential stresses are uniform for the entire range of internal pressures. The constitutive functions in this sub-class are found to have zero residual stress. It is the inhomogeneity, rather than residual stress, that assures uniform circumferential stresses. We also examine the possibility of utilizing these constitutive functions in the design and fabrication of an engineered blood vessel with optimal mechanical properties.     

Interstitial fluid flow: simulation of mechanical environment of cells in the interosseous membrane

In vitro experiments have shown that subtle fluid flow environment plays a significant role in living biological tissues,while there is no in vivo practical dynamical measurement of the interstitial fluid flow velocity.On the basis of a new finding that capillaries and collagen fibrils in the interosseous membrane form a parallel array,we set up a porous media model simulating the flow field with FLUENT software,studied the shear stress on interstitial cells’ surface due to the interstitial fluid flow,and analyzed the effect of flow on protein space distribution around the cells.The numerical simulation results show that the parallel nature of capillaries could lead to directional interstitial fluid flow in the direction of capillaries.Interstitial fluid flow would induce shear stress on the membrane of interstitial cells,up to 30 Pa or so,which reaches or exceeds the threshold values of cells’ biological response observed in vitro.Interstitial fluid flow would induce nonuniform spacial distribution of secretion protein of mast cells.Shear tress on cells could be affected by capillary parameters such as the distance between the adjacent capillaries,blood pressure and the permeability coefficient of capillary’s wall.The interstitial pressure and the interstitial porosity could also affect the shear stress on cells.In conclusion,numerical simulation provides an effective way for in vivo dynamic interstitial velocity research,helps to set up the vivid subtle interstitial flow environment of cells,and is beneficial to understanding the physiological functions of interstitial fluid flow.     

Ramp loading in Russian doll poroelasticity

Aporoelastic model for porous materials with a nested pore space structure is developed to represent the interstitial fluid flow in bone tissue. The nested porosity model is applied to the problem of determining the exchange of pore fluid between the vascular porosity (PV) and the lacunar-canalicular porosity (PLC) in bone tissue in a ramp loading in the case where the fluid and solid constituents are assumed to be compressible. The compressibility assumption is appropriate for hard tissues while the incompressibility assumption is appropriate for soft tissues. The influence of blood pressure in the PV is included in the analysis. A formula for the fluid that moves between the two porosities is developed. The analysis showed the coupling of the two porosities and their influence on each other and concluded that the PV pore pressure has an influence less than 3% on the PLC pore pressure while the absence of the PV pore pressure will affect the fluid exchange between the PV and PLC by less than 6% (the blood pressure range is 40-60 mmHg). Also the analysis has shown that the draining time of the PLC is inversely proportional to its permeability. The significance of the result is basic to the understanding of interstitial flow in bone tissue that, in turn, is basic to understanding of nutrient transport from the vasculature to the bone cells buried in the bone tissue and to the process of mechanotransduction by these cells.     

残余应力测定的围箍压痕法模拟

本文从同工量测方法得到启发，提出了一种新型的残余应力测试方法－围箍压痕法。采用轴对称弹塑性有限元计算模拟了围箍压痕法测量结构残余应力的力学过程，得到了围箍压痕时不同残余应力程度下材料的变形，塑性区和接触应力分布。     

15万立米气柜残余应力及工作应力测试分析

报导了１５万立米大型煤气气柜挂圈走台的残余应力及气柜升降时工作应力的测试结果。该气柜服役十余年后发生煤气泄漏，为分析其原因，进行了残余应力及气柜修复前后的工作应力测试，结果表明：修复前残余应力及工作应力均达到很高水平，气柜升起时伴有严重侧偏现象，修复后应力状态有明显好转。     

The application and extension of Sachs method

In this paper, we present the simplification of Sachs formulas for the measurement and calculation of the residual stresses of the cylinder only with the plane stresses. Furthermore, we present the method for the measurement and calculation of the residual stresses of the cylinder not only with the finite length but with the longitudinal stress. These can be applied to the investigation on the residual stresses of the autofretted gun tube.Communicated by Loo Wen-da     

生物组织中的残余应变

本文综述了生物组织中残余应力和残余应变的研究历史和现状,着重分析了心血管系统的残余应变和零应力状态,讨论了残余应变对血管和心脏等器官的力学分析和组织改造的重要意义,并提出了一些有待研究的问题．      

A new apparatus for studying mechanical behavior under pressure

Using a servohydraulic intensifier, a servohydraulic universal test machine and a novel test chamber, pressures of 750 MPa and axial stresses of almost any magnitude are simultaneously generated and applied to the gage section of a solid, cylindrical specimen. Under combined axial-stress/external-pressure loadings, a solid specimen experiences a truly three-dimensional, homogeneous stress state in which the axial stress equals σ₁ and the radial and circumferential stresses equal the negative of the pressure, —P. Through independent computer control of the pressure and axial stress, amterial behavior under monotonically and cyclically applied multiaxial stresses are investigated. This paper describes the experimental details of the test chamber design, high-pressure fluid production and sealing, load and strain measurement under high pressure, computer control and data acquisition. Experimental results obtained from triaxial monotonic tests on low alloy steel and triaxial cyclic tests on a Ni-Ti alloy using this unique apparatus are presented.     

受移动简谐力作用的多孔弹性半平面问题

研究了匀速移动的振动荷载作用下半无限多孔饱和固体中产生的应力和孔隙水压力．应用Fourier变换求解该问题的控制偏微分方程,考虑了荷载的移动速度及振动频率对多孔饱和固体中应力与孔隙水压力的影响,并与相应的弹性介质的解答进行了比较．结果显示多孔饱和半平面中应力和孔隙水压力随荷载的移动速度与振动频率的增加而增大,多孔饱和固体在移动荷载下的动力响应与相应的单相弹性固体的动力响应有较大的差别。