基于努氏硬度表征大块金属玻璃的力学性能

采用多种载荷对14种大块金属玻璃进行努氏硬度测试,结果表明努氏硬度随载荷的增加而降低,最后趋于稳定,即材料受到正压痕尺寸效应的影响,并利用Meyer定律、弹塑性变形模型、Hays-Kendall模型和比例试样阻力模型对这种现象进行了解释,因此在压痕表面未产生裂纹时应采用较大载荷下趋于稳定的硬度值对杨氏弹性模量E和屈服强...     

An experimental study on wind loads acting on a high-rise building model induced by microburst-like winds

In the present study, an experimental investigation is conducted to quantify the characteristics of the microburst-induced wind loads (i.e., both static and dynamic wind loads) acting on a high-rise building model, compared to those with the test model placed in conventional atmospheric boundary layer (ABL) winds. The experimental study is performed by using an impinging-jet-based microburst simulator available at Iowa State University. In additional to conducting flow field measurements to quantify the flow characteristics of the microburst-like wind, both mean and dynamic wind loads acting on the test model induced by the microburst-like wind are assessed in detail based on the quantitative measurements of the surface pressure distributions around the test model and the resultant aerodynamic forces. It is found that the microburst-induced wind loads acting on high-rise buildings would be significantly different from their counterparts in conventional ABL winds. Both the static and dynamic wind loads acting on the high-rise building model were found to change significantly depending on the radial locations and the orientation angles of the test model in respect to the oncoming microburst-like wind. The dynamic wind loads acting on the test model were found to be mainly influenced by the periodical shedding of the primary vortices and the high turbulence levels in the microburst-like wind. The findings derived from the present study are believed to be useful to gain further insight into the underlying physics of the flow–structure interactions of high-rise buildings in violent microburst winds for a better understanding of the damage potential of microburst winds to high-rise buildings.     

Non-linear electromechanical response of 1–3 type piezocomposites

Domain switching in piezoelectric materials is caused by external loads such as electric field and stress that leads to non-linear behaviour. A study is carried out to compare the non-linear behaviour of 1–3 piezocomposites with different volume fractions and bulk piezoceramics. Experiments are conducted to measure the electrical displacement and strain on piezocomposites and bulk ceramics under high cyclic electrical loading and constant compressive prestress. A thermodynamically consistent uni-axial framework is developed to predict the nonlinear behaviour by combining the phenomenological and micromechanical techniques. Volume fractions of three distinct uni-axial variants (instead of six variants) are used as internal variables to describe the microscopic state of the material. In this model, the grain boundary effects are taken into account by introducing the back fields (electric field and stress) as non-linear kinematic hardening functions. An analytical model based on equivalent layered approach is used to calculate effective properties such as elastic, piezoelectric, and dielectric constants for different volume fractions of piezocomposites. The predicted effective properties are incorporated in the proposed uni-axial model and the dielectric hysteresis (electrical displacement versus electric field) as well as butterfly curves (strain versus electric field) are simulated. Comparison between the experiments and simulations show that this model can reproduce the characteristics of non-linear response. It is observed that the variation in fiber volume fraction and compressive stress has a significant influence on the response of the 1–3 piezocomposites.     

Establishment of safe axle loads for sugarcane soils under varying moisture content

Studies were conducted for the establishment of safe axle loads for sugarcane hauling vehicles beyond which detrimental soil compaction would be induced. The treatments involved running a loaded test vehicle in field strips previously chosen at random. Safe loads were established by testing the level of significance of the difference in induced soil compaction between treated and non-treated sections. Working under soil moisture contents of 21.4–27.1% (dry basis), safe axle loads for two 18.4 × 30 tires were found to be 55.6 and 60.0 kN for sandy clay loam and sandy loam soils with initial dry bulk density about 1.434 g/cm³. These corresponded to ground contact pressures of 111 and 120 kPa, respectively.     

Evolution of shear bands in bulk metallic glasses under dynamic loading

Shear band spacing in Zr-based bulk metallic glasses (BMGs) under dynamic loads is found to vary with position and local strain rate in the indented region. To investigate the dependence of shear band evolution characteristics on local strain rate and normal stress, a micromechanical model based on momentum diffusion is proposed. The thermo-mechanical model takes into account the normal stress dependence of yield stress, the free volume theory and the associated viscosity change within the shear band region. Temperature rise is obtained from the balance between the heat diffusion to the adjacent regions from a shear band and the heat generation due to the accumulated plastic work in a shear band. The parametric study has revealed that thermal effects play a minor role when the critical shear displacement is below 10 nm (as in nanoindentation) but become significant when the shear displacement accumulated in a shear band is of the order of hundreds of nanometers (as in uniaxial compression and in dynamic indentations). Finally, it is found that the normal stress plays a crucial role in the deformation behavior of BMGs by not only decreasing the time for shear band formation but also increasing the temperature rise significantly.     

A numerical investigation of side‐loads resulting from rigid body motions of an overexpanded engine nozzle

A numerical model for fluid–structure interactions is presented. Its purpose, within the context of 2D overexpanded engine nozzles, is to improve understanding of interactions between side‐loads and rigid body rotations, and more generally of the underlying physics between a shock in motion and nozzle movements. The model is based on three different solvers, for fluid, structure and mesh deformation respectively, which are linked to a coupling scheme in a parallel environment. In particular it is shown that the nozzle has a natural torsional frequency for which the measured side‐loads are the greatest. This phenomenon is associated with a transversal wave in the flow between the two internal walls of the nozzle. For free coupling cases, our calculations go some way to explain how the mechanical energy is dissipated with a transfer of energy to the shock that encounters the largest motions to dissipate it. It has also been observed that the compression shock may adopt a quasi‐steady state response with regard to nozzle rotations at low frequencies, whereas this will no longer be the case at higher frequencies, where a phase shift may occur between side‐loads and rotational position. This study is aimed at enhancing the only current aeroelastic stability model for overexpanded nozzles (AIAA, 29th Joint Propulsion Conference and Exhibit, Monterey, CA, 28–30 June 1993). Copyright © 2010 John Wiley & Sons, Ltd.     

An experimental investigation on the characteristics of fluid–structure interactions of a wind turbine model sited in microburst-like winds

An experimental investigation is performed to assess the characteristics of the fluid–structure interactions and microburst-induced wind loads acting on a wind turbine model sited in microburst-liked winds. The experiment study was conducted with a scaled wind turbine model placed in microburst-like winds generated by using an impinging-jet-typed microburst simulator. In addition to quantifying complex flow features of microburst-like winds, the resultant wind loads acting on the turbine model were measured by using a high-sensitive force–moment sensor as the turbine model was mounted at different radial locations and with different orientation angles with respect to the oncoming microburst-like winds. The measurement results reveal clearly that, the microburst-induced wind loads acting on the turbine model were distinctly different from those in a conventional atmospheric boundary layer (ABL) wind. With the scales of the wind turbine model and the microburst-like wind used in the present study, the dynamic wind loadings acting on the turbine model were found to be significantly higher (i.e., up to 4 times higher for the mean loads, and up to 10 times higher for the fluctuation amplitudes) than those with the same turbine model sited in ABL winds. Both the mean values and fluctuation amplitudes of the microburst-induced wind loads were found to vary significantly with the changes of the mounted site of the turbine model, the operating status (i.e., with the turbine blades stationary or freely rotating), and the orientation angle of the turbine model with respect to the oncoming microburst-like wind. The dynamic wind load measurements were correlated to the flow characteristics of the microburst-like winds to elucidate underlying physics. The findings of the present study are helpful to gain further insight into the potential damage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of the wind turbines in thunderstorm-prone areas.     

Numerical study on wave loads and motions of two ships advancing in waves by using three-dimensional translating-pulsating source

A frequency domain analysis method based on the three-dimensional translating-pulsating (3DTP) source Green function is developed to investigate wave loads and free motions of two ships advancing on parallel course in waves. Two experiments are carried out respectively to measure the wave loads and the free motions for a pair of side-byside arranged ship models advancing with an identical speed in head regular waves. For comparison, each model is also tested alone. Predictions obtained by the present solution are found in favorable agreement with the model tests and are more accurate than the traditional method based on the three dimensional pulsating (3DP) source Green function. Numerical resonances and peak shift can be found in the 3DP predictions, which result from the wave energy trapped in the gap between two ships and the extremely inhomogeneous wave load distribution on each hull. However, they can be eliminated by 3DTP, in which the speed affects the free surface and most of the wave energy can be escaped from the gap. Both the experiment and the present prediction show that hydrodynamic interaction effects on wave loads and free motions are significant. The present solver may serve as a validated tool to predict wave loads and motions of two vessels under replenishment at sea, and may help to evaluate the hydrodynamic interaction effects on the ships safety in replenishment operation.     

Effect of multiple passes of tractor with varying normal load on subsoil compaction

A field experiment was conducted on alluvial soil with sandy loam texture, in a complete randomized design, to determine the compaction of sub-soil layers due to different passes of a test tractor with varying normal loads. The selected normal loads were 4.40, 6.40 and 8.40 kN and the number of passes 1, 6, 11 and 16. The bulk density and cone penetration resistance were measured to determine the compaction at 10 equal intervals of 5 cm down the surface. The observations were used to validate a simulation model on sub-soil compaction due to multiple passes of tractor in controlled conditions. The bulk density and penetration resistance in 0–15 cm depth zone continuously increased up to 16 passes of the test tractor, and more at higher normal loads. The compaction was less in different sub-soil layers at lower levels of loads. The impact of higher loads and larger number of passes on compaction was more effective in the soil depth less than 30 cm; for example the normal load of 8.40 kN caused the maximum bulk density of 1.53 Mg/m³ after 16 passes. In 30–45 cm depth layer also, the penetration resistance increased with the increase in loads and number of passes but to a lesser extent which further decreased in the subsoil layers below 45 cm. Overall, the study variables viz. normal load on tractor and number of passes influenced the bulk density and soil penetration resistance in soil depth in the range of 0–45 cm at 1% level of significance. However, beyond 45 cm soil depth, the influence was not significant. The R² calculated from observed and predicted values with respect to regression equations for bulk density and penetration resistance were 0.7038 and 0.76, respectively.     

Simulations of nanowire bend tests for extracting mechanical properties

Mechanical properties of nickel nanowires are characterized based on the numerical simulations of bend tests performed with a customized atomic force microscope (AFM) and scanning electron microscope (SEM). Nickel nanowire specimens are subjected to bending loads by the tip of the AFM cantilever. The experimental force versus bending displacement curves are compared against simulations from finite element analysis and peridynamic theory, and the mechanical properties are extracted based on their best correlations. Similarly, SEM images of fractured nanowires are compared against peridynamic failure simulations. The results of this study reveal that nickel nanowires have significantly higher strengths than their bulk counterparts, although their elastic modulus values are comparable to bulk nickel modulus values.     

A second strain gradient elasticity theory with second velocity gradient inertia – Part I: Constitutive equations and quasi-static behavior

A multi-cell homogenization procedure with four geometrically different groups of cell elements (respectively for the bulk, the boundary surface, the edge lines and the corner points of a body) is envisioned, which is able not only to extract the effective constitutive properties of a material, but also to assess the “surface effects” produced by the boundary surface on the near bulk material. Applied to an unbounded material in combination with the thermodynamics energy balance principles, this procedure leads to an equivalent continuum constitutively characterized by (ordinary, double and triple) generalized stresses and momenta. Also, applying this procedure to a (finite) body suitably modelled as a simple material cell system, in association with the principle of the virtual power (PVP) for quasi-static actions, an equivalent structural system is derived, featured by a (macro-scale) PVP having the typical format as for a second strain gradient material model. Due to the surface effects, the latter model does work as a combination of two subsystems, i.e. the bulk material behaving as a Cauchy continuum, and the boundary surface operating as a membrane-like boundary layer, each subsystem being in (local and global) equilibrium by its own. Further, the applied (ordinary) boundary traction splits into two (response-dependent) parts, i.e. the “Cauchy traction” transmitted to the bulk material and the “Gurtin–Murdoch traction” acting, together with all other boundary tractions, upon the boundary layer. The role of the boundary layer as a two-dimensional manifold enclosing a Cauchy continuum is elucidated, also with the aid of a discrete model. A strain gradient elasticity theory is proposed which includes a minimum total potential energy principle featuring the relevant boundary-value problem for quasi-static loads and its (unique) solution. A simple application is presented. Two appendices are included, one reports the proof of the global equilibrium of the boundary layer, the other is concerned with double and triple stresses. The paper is complemented by a companion Part II one on dynamics. Previous findings by the author [Polizzotto, C., 2012. A gradient elasticity theory for second-grade materials and higher order inertia. Int. J. Solids Struct. 49, 2121–2137] are improved and extended.     

基于Visco-SCRAM模型的侵彻装药点火研究

针对弹体侵彻过程中装药的安全性,基于黏弹性统计裂纹力学(visco-statistical crack mechanics, Visco-SCRAM)模型计算装药整体温升、装药裂纹摩擦生热以及弹体装药与壳体摩擦生热,考察这3种机制对装药温升的贡献以及侵彻装药的点火机制,得到了装药点火对应的弹体侵彻临界初始速度。结果表明:(1)装药与弹体内壁摩擦生热对装药温升有一定贡献,随着弹体初始撞击速度的提高,摩擦生热对温升的贡献逐渐增大;(2)黏性、损伤和绝热体积变化导致的装药整体温升对装药点火的作用有限; (3)裂纹摩擦形成热点是侵彻装药点火的物理机制;(4)采用Visco-SCRAM模型可预测低强度、长脉冲载荷作用下的装药点火响应。     

Analytical study of the dynamic response of an embedded railway track to a moving load

Summary In this paper, the steady-state dynamic response of an embedded railway track to a moving train is investigated theoretically. The model for the track consists of a flexible plate performing vertical vibrations, two beams that are connected to the plate by continuous visco-elastic elements and an elastic foundation that supports the plate. Two harmonic loads that move uniformly along the beams describe the train load. The plate, the beams and the elastic foundation are employed to model a concrete slab of an embedded track, the rails and the ground reaction, respectively. The problem is studied by employing the Fourier integral transforms in the following way. Firstly, the dispersion analysis of waves that may propagate along the system is accomplished in the frequency-wavenumber domain. On the basis of this analysis, critical velocities of the loads are found both for the in-phase and anti-phase vibrations of the loads. Secondly, the vertical displacement of the rails and the slab, along with the stresses in the slab, are investigated as functions of the velocity and frequency of the loads. Finally, the response of the two-dimensional model is compared to that of a simplified one-dimensional model.     

Prediction of soil compaction under pneumatic tires a using fuzzy logic approach

Artificial intelligence systems are widely accepted as a technology offering an alternative way to tackle complex and ill-defined problems. They can learn from examples, are fault tolerant in the sense that they are able to handle noisy and incomplete data, are able to deal with non-linear problems, and once trained can perform prediction and generalization at high speed. Compared with traditional approaches, fuzzy logic is more efficient in linking the multiple inputs to a single output in a non-linear domain. The purpose of this study was to investigate the relationship between tire working parameters and soil compaction characteristics, and to illustrate how Fuzzy expert system might play an important role in prediction of soil. All experimental values were collected from soil bin. The trials were conducted in different tire types, vertical loads, inflation pressures and forvard velocities. In this paper, a sophisticated intelligent model, based on Mamdani approach fuzzy modeling principles, was developed to predict the changes in penetration resistance, final pressure and bulk density of soil due to wheel traffic. The verification of the proposed model is achieved via various numerical error criteria. For all parameters, the relative error of predicted values was found to be less than the acceptable limits (10%).     

A fractal analysis of subcooled flow boiling heat transfer

A fractal model for the subcooled flow boiling heat transfer is proposed in this paper. The analytical expressions for the subcooled flow boiling heat transfer are derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for the subcooled flow boiling heat transfer is found to be a function of wall superheat, liquid subcooling, bulk velocity of fluid (or Reynolds number), fractal dimension, the minimum and maximum active cavity size, the contact angle and physical properties of fluid. No additional/new empirical constant is introduced, and the proposed model contains less empirical constants than the conventional models. The proposed model takes into account all the possible mechanisms for subcooled flow boiling heat transfer. The model predictions are compared with the existing experimental data, and fair agreement between the model predictions and experimental data is found for different bulk flow rates.     

Response of skin panels to combined self- and boundary layer-induced fluctuating pressure

Fluctuating pressures are a critical consideration in the life-prediction of thin-gauge hot-structures operating in high-speed flow. Sources include both boundary layer turbulence and self-induced components, where the latter arises from panel vibrations. While a considerable body of research is available for the structural response of thin-gauge panels to self-induced pressure fluctuations, the response to boundary layer turbulence is not well-understood due to the complexity in modeling the loads. Important open issues are the degree of coupling between the boundary layer induced fluctuating loads and the thermo-structural response, and also the potential for interactions between a turbulent boundary layer and structural response to result in structural instabilities. This study seeks to address these issues by incorporating a phenomenological model for turbulent boundary layer loads into an aerothermoelastic framework. The enhanced aerothermoelastic model is then used to study the combined effect of self- and boundary layer-induced fluctuating pressures on responses of simple panels, and to characterize features in the turbulent boundary layer loads that can lead to large amplitude structural vibrations. The developed phenomenological model predicts that the magnitude of the boundary layer induced fluctuating pressure increases with increasing panel inclination, and decreases with increasing temperature. Furthermore, it is found that both RMS magnitude and phase angle of the boundary layer induced pressure loads play key roles in panel response. Certain combinations of these features, coupled with the self-induced pressure fluctuations, are found to cause onset of fluid–structural instabilities earlier than observed when pressure fluctuations from the turbulent boundary layer are either neglected or decoupled from the panel response.     

复杂工况下的大型风力机气动性能和尾迹研究Investigation of aerodynamic performance and wake of the large scale wind turbine in complicated conditions

在复杂工况下,大型风力机非定常特性会更严重,导致风力机气动性能变化和尾迹预测更加复杂。本文主要针对稳态偏航、动态偏航、风剪切和随机风速场等复杂工况,基于自由涡尾迹方法,嵌入复杂工况的模块,加入了动态失速模型和三维旋转效应模型修正,实现了复杂工况数值模拟计算,比较了不同复杂工况的气动载荷和尾迹形状。最后,得出了风力机在复杂工况下的气动性能、载荷和尾迹叶尖涡线特性,并计算出风力机在复杂工况下的气动载荷超调量和迟滞时间。对推进自由涡尾迹方法应用于风力机工程的大批工况载荷计算,提高大型风力机的载荷计算精度和设计水平等具有重要意义。     

Compaction characteristics for the towed and driven conditions of a wheel operating in an agricultural soil

High axle loads, duration of strain as well as strain rate due to applied stresses, and field moisture condition have been found to contribute to compaction in the field. Numerous previous investigations on agricultural soil compaction were carried out with relatively dry soil. The aim of this study was to investigate the interrelationships between compaction, applied load, vehicle speed and a certain practical range of soil moisture content through a soil bin investigation of the compaction which results from the passage of a towed and a driven wheel. Soil pressure and the corresponding bulk density were analysed using a model proposed by Bailey et al. (J. agric Engng Res. 33, 257–262 (1986)) and ANOVA techniques. The results showed that compaction was higher at the higher moisture content level for both towed and driven conditions of the wheel, and that it was applied load that had the greatest contributory effect. Also, compaction was higher in the case of the driven wheel as compared to the towed wheel due to the phenomenon of slip sinkage. Bailey's model, it appears, can be utilized in the field for a practical estimation of compaction resulting from the passage of a towed wheel.     

基于等效静态载荷法的区间参数结构可靠性拓扑优化

研究了用等效静态载荷法,解决动态响应约束下的区间参数结构可靠性拓扑优化问题。对等效静态载荷赋予了新的含义:由等效静态载荷产生的区间静态响应与由动态载荷产生的区间动态响应,其对应的中值与离差均相等。利用泰勒展开计算出区间参数结构动态响应所有可能值组成的集合,再根据集合映射获得包含结构所有不确定信息的等效静态载荷集合,继而建立静态可靠性拓扑优化数学模型。通过集合映射和区间自然扩展,获得静态位移响应区间。基于区间非概率可靠性指标的定义,给出区间非概率可靠性约束的伴随法灵敏度分析算法。采用移动渐近线法完成此优化问题的求解。数值算例验证了模型的正确性和算法的有效性。