沥青胶浆拉伸全曲线的试验研究

细观尺度上, 沥青混合料是由沥青胶浆、粗骨料和孔隙等构成的多相复合材料, 而沥青胶浆的力学参数为沥青混合料进行细观尺度研究的重要部分, 对抗拉强度有着决定性的影响. 本文通过设计制作加载夹具针对不同温度和不同加载速率的条件, 对9 组试件进行拉伸试验, 得到沥青胶浆拉伸全曲线. 通过对沥青胶浆拉伸全曲线的研究和计算, 确定了沥青胶浆的最大破坏应力σmax、刚度Knn 和破坏位移U, 这些参数为采用粘聚带本构模型模拟沥青混合料的开裂行为提供了力学参数.     

Murray lecture tensile testing at the micrometer scale: Opportunities in experimental mechanics

The measurement of mechanical properties using specimens whose minimum dimensions are of the order of micrometers is an important new area of experimental solid mechanics. One obvious application is in the area of micro-electromechanical systems (MEMS) where the final product is on the millimeter or micrometer scale. This paper describes techniques developed at Johns Hopkins University for tensile testing of materials used in MEMS. Polycrystalline silicon is currently the most widely used material; its modulus has been measured as 158±10 GPa, and its Poisson's ratio as 0.22±0.01, with fracture strengths ranging from 1.2 to 3.0 GPa depending upon the manufacturer. The properties of silicon nitride, silicon carbide, and electroplated nickel have also been measured and are presented. In addition to the quasi-static tensile tests, new techniques and procedures for measuring strengths at stress concentrations in brittle thin-film materials, fatique testing, and high-temperature testing are described.     

A review of MEMS-based microscale and nanoscale tensile and bending testing

Thin films at the micrometer and submicrometer scales exhibit mechanical properties that are different than those of bulk polycrystals. Industrial application of these materials requires accurate mechanical characterization. Also, a fundamental understanding of the deformation processes at smaller length scales is required to exploit the size and interface effects to develop new and technologically attractive materials. Specimen fabrication, small-scale force and displacement generation, and high resolution in the measurements are generic challenges in microscale and nanoscale mechanical testing. In this paper, we review small-scale materials testing techniques with special focus on the application of microelectromechanical systems (MEMS). Small size and high force and displacement resolution make MEMS suitable for small-scale mechanical testing. We discuss the development of tensile and bending testing techniques using MEMS, along with the experimental results on nanoscale aluminum specimens.     

Coupled Hydraulic System for Tensile Testing in Compression-only Machines

Fracture theory for normal strength concrete has thoroughly been studied over the past decades. Through indirect and direct tensile testing techniques, the post-peak softening response of conventional concrete has been established and utilized in analysis and design. However, for more recently developed concrete materials (e.g. fiber reinforced, high performance) under complex loading conditions, the required fracture properties to predict response are extremely limited. Considering this lack of knowledge, the objective of this research was to develop a uni-axial tensile testing technique to attain the post-peak softening response for ultra-high performance concrete for ultimate use in conjunction with an applied confining pressure system. Specifically, this research was conducted for implementation into an existing, large compression-only machine at the US Army Engineer Research and Development Center (ERDC). The new methodology enabled the existing testing frame to apply a stiffening force, while an external hydraulic plunger cylinder performed the tensile test. The scheme enables tensile testing under confining pressures in the compression-only machine.     

The Potential of MEMS for Advancing Experiments and Modeling in Cell Mechanics

Response to mechanical stimuli largely dictates cellular form and function. A host of extraordinary yet unexplained responses have been identified within the hierarchical cell structure. As experimental and model-based investigations in cell mechanics advance, the underlying structure-function mechanisms dictating these responses emerge. Here we explore the potential of microelectromechanical systems (MEMS) for advancing understanding of cell mechanics. To motivate the discussion, existing experimental techniques are summarized. Interrelated model-based approaches, which aim to interpret or predict observed results, are also outlined. We then focus on a representative set of MEMS-based devices designed for investigations in cell mechanics and point to the fact that, while these devices have yet to maximize their functionality through higher levels of sensor/actuator integration, they are highly complementary to existing techniques. In closing, novel MEMS sensor and actuator schemes that have yet to materialize in this field are discussed to motivate the next generation of MEMS for investigations in cell mechanics.     

Universal hierarchical symmetry for turbulence and general multi-scale fluctuation systems

Scaling is an important measure of multi-scale fluctuation systems. Turbulence as the most remarkable multi-scale system possesses scaling over a wide range of scales. She-Leveque （SL） hierarchical symmetry, since its publication in 1994, has received wide attention. A number of experimental, numerical and theoretical work have been devoted to its verification, extension, and modification. Application to the understanding of magnetohydrodynamic turbulence, motions of cosmic baryon fluids, cosmological supersonic turbulence, natural image, spiral turbulent patterns, DNA anomalous composition, human heart variability are just a few among the most successful examples. A number of modified scaling laws have been derived in the framework of the hierarchical symmetry, and the SL model parameters are found to reveal both the organizational order of the whole system and the properties of the most significant fluctuation structures. A partial set of work related to these studies are reviewed. Particular emphasis is placed on the nature of the hierarchical symmetry. It is suggested that the SL hierarchical symmetry is a new form of the self-organization principle for multi-scale fluctuation systems, and can be employed as a standard analysis tool in the general multi-scale methodology. It is further suggested that the SL hierarchical symmetry implies the existence of a turbulence ensemble. It is speculated that the search for defining the turbulence ensemble might open a new way for deriving statistical closure equations for turbulence and other multi-scale fluctuation systems.     

混凝土高温动态劈拉行为细观数值分析

为研究高温作用下混凝土的动态劈裂拉伸破坏行为,考虑了力学性能的高温退化与应变率增强效应的联合作用,结合混凝土材料内部非均质性,建立了细观尺度数值分析模型与方法。将该数值方法分为两个步骤：首先对混凝土进行热传导行为模拟,进而将输出结果作为初始条件对混凝土动态劈裂拉伸行为进行细观模拟。在模拟结果与已有试验现象良好吻合的基础上,分析了高温下混凝土动态劈裂拉伸行为及其细观破坏机制,对比了不同应变率及加热温度下混凝土的劈裂拉伸应力-应变关系,揭示了混凝土应变率效应与温度退化效应的相互影响规律。研究结果表明：(1) 高温作用后,试件损伤区域较常温下更集中;(2) 名义应变率较大时,破坏过程急促,常温下骨料发生破坏,而经历高温后骨料基本没有破坏;(3) 由于混凝土试件细观结构的非均质性,其内部应力呈枣核状不连续分布;(4) 相比于应变率效应,混凝土劈裂拉伸强度受温度退化作用的影响更显著。

     

Development of testing systems for dynamic assessment of sheet metal specimens

The work presented herein regards the analysis of an experimental technique for the execution of dynamic tensile tests on structural material sheet specimens. Dynamic tensile testing of sheet is becoming more important due to the need for more optimized vehicle crashworthiness analysis in the automotive industry. Positive strain-rate sensitivity, i.e. the strength increases with strain-rate, offers a potential for improved energy absorption during a crash event. Tests have been carried out in the Reliability and Safety Laboratory of the 2^nd Engineering Faculty of the Politecnico di Torino. Different types of testing techniques have been used to generate data under dynamic conditions. However, no guidelines are available for the testing method, specimen dimensions, measurement devices, and other important issues which are critical for the quality of the results. Accurate signal processing and curve smoothing are often necessary to make the testing data usable.     

多尺度复合材料力学研究进展

多尺度复合材料力学是运用多尺度分析思想研究空间分布非均匀材料力学性能的学科.近年来,多组分、多层级先进材料的蓬勃发展和微纳米实验观测手段的不断进步,有力地推动了该学科的研究.论文围绕非均匀材料力学性能的多尺度分析,首先从微纳米尺度到宏观尺度综述了常用的理论分析方法;接着分别针对非均匀连续介质和离散体系介绍了常用的多尺度计算模拟方法;然后结合本课题组在纳米复合材料、抗冲击吸能材料、随机网络材料和多层级自相似材料等方面的研究工作,举例说明了如何综合运用多种方法对各种复杂材料系统进行多尺度分析;最后,展望了该领域还需进一步发展和完善的若干方向.     

鼓泡流化床中流动特性的多尺度数值模拟

鼓泡流化床因其较高的传热特性以及较好的相间接触已经被广泛应用于工业生产中,而对鼓泡流态化气固流动特性的充分认知是鼓泡流化床设计的关键.在鼓泡流化床中,气泡相和乳化相的同时存在使得床中呈现非均匀流动结构,而这种非均匀结构给鼓泡流化床的数值模拟造成了很大的误差.基于此,以气泡作为介尺度结构,建立了多尺度曳力消耗能量最小的稳定性条件,构建了适用于鼓泡流化床的多尺度气固相间曳力模型.结合双流体模型,对A类和B类颗粒的鼓泡流化床中气固流动特性进行了模拟研究,分析了气泡速度、气泡直径等参数的变化规律.研究表明,与传统的曳力模型相比,考虑气泡影响的多尺度气固相间曳力模型给出的曳力系数与颗粒浓度的关系是一条分布带,建立了控制体内曳力系数与局部结构参数之间的关系.通过模拟得到的颗粒浓度和速度与实验的比较可以发现,考虑气泡影响的多尺度曳力模型可以更好地再现实验结果.通过A类和B类颗粒的鼓泡床模拟研究发现,A类颗粒的鼓泡床模拟受多尺度曳力模型的影响更为显著.      

Large-scale rotating bending fatigue tests for offshore pipe connections

A testing apparatus featuring rotating bending fatigue techniques was designed and constructed to investigate the fatigue behavior of full-sized threaded connections used to couple offshore piping and structures. Rotating bending tests were performed where flexural loads are applied to the pipe with a hydraulic loading system while simultaneously rotating the pipe by belt drive attached to an electric motor. While rotating bending fatigue testing is not a new concept, the relatively large scale of the tests presented special problems that make the apparatus distinctive. This rotating bending apparatus allows more rapid testing than conventional closed-loop systems. Additionally, conventional closed-loop axial fatigue tests would have required a testing frame with fatigue load capacity exceeding 4455 kN (1,000,000 lbs). Several specially manufactured devices are featured that require large service loads combined with exacting machine tolerances. Currently, a fatigue frequency of 2.0 Hz has been achieved with an equivalent deformation range of 25.4 mm (1.0 in.). Faster speeds are believed possible.     

蒙脱土/尼龙6纳米复合材料刚度预测

本文采用交错模型对蒙脱土／尼龙6纳米复合材料的弹性模量进行预测。模型划分为四个区域：以拉伸变形为主的A（蒙脱土片）、B和D区,和同时承受拉伸和剪切两种变形的C区。通过对各区之间合力平衡和变形协调的简单分析,本文得到了封闭形式的蒙脱土／尼龙6纳米复合材料模量预测公式。引用文献[1]中的蒙脱土材料的几何参数和弹性常数,以及纯尼龙6的弹性常数,对纳米复合材料的模量进行预测,预测结果与实验结果基本吻合。有限元法对交错模型的分析证实了理论模型中变形区划分的合理性,同时也发现FEM的预测结果与实验更为接近。     

Experimental techniques for mechanical characterization of hydrogels at the microscale

Hydrogel actuators in microfluidic devices must endure the forces of aqueous flow, the constraint of device walls, and the restoring force of elastic membranes. In order to assess the capabilities of hydrogels, three experimental techniques for determining the uniaxial tensile properties and functional swelling properties of microscale hydrogel structures have been developed. Tensile tests were conducted to determine Young's modulus and Poisson's ratio at varying degrees of swelling equilibrium. Force response tests were performed to determine the force exerted by cylindrical hydrogel structures on compression platens held at fixed displacement. Particle image velocimetry, a method originally developed to measure velocity fields in fluid flows, was adapted to investigate the deformation rates at various times within hydrogel structures during volumetric swelling. The techniques and sample fabrication methods outlined are applicable to a variety of hydrogel chemistries.     

Evaluation of the Material Properties of an OFHC Copper Film at High Strain Rates Using a Micro-Testing Machine

The material properties of an oxygen-free high thermal conductivity (OFHC) film with a thickness of 0.1 mm were evaluated at strain rates ranging from 10⁻³/s to 10³/s using a high-speed material micro-testing machine (HSMMTM). The high strain-rate material properties of thin films are important especially for an evaluation of the structural reliability of micro-formed parts and MEMS products. The high strain-rate material testing methods of thin films, however, have yet to be established to the point that the testing methods of larger specimens for electronics, auto-body, train, ship, and ocean structures are. For evaluation, a new type of HSMMTM was developed to conduct high-speed tensile tests of thin films. This machine is capable of testing at a sufficiently high tensile speed with an electromagnetic actuator, a novel gripping mechanism, and an accurate load measurement system. The OFHC copper film shows high strain-rate sensitivity in terms of the flow stress, fracture elongation, and strain hardening. These measures increase as the tensile strain rate increases. The rate-dependent material properties of an OFHC copper film are also compared with those of a bulk OFHC copper sheet with a thickness of 1 mm. The flow stress of an OFHC copper film is relatively low compared to that of a bulk OFHC copper sheet in the entire range of strain rates, while the fracture elongation of an OFHC copper film is much larger than that of a bulk OFHC copper sheet. A quantitative comparison would provide material data at high strain rates for the design and analysis of micro-appliances and different types of micro-equipment.     

Formulation for actuators' number enumeration for main planar structures in robotics

Mechanisms and Machines Theory (MMT) contributed greatly to planar and spatial mechanism synthesis with different degrees of freedom. Some of the current industrial robots with planar chains have a main structure created by the kinematic graphs of MMT. This paper deals with a formulation allowing computing the number of possibilities for actuators' attachment for planar pin-jointed main driving mechanisms in robotics. It is based on the sub-chains symmetries of structures taking into account the position of the actuator of the robot. Pairs of groups of mutually symmetrical mechanisms are detected. Thus, eliminating the symmetrical ones confines the number of possible configurations. Different cases of symmetries have been studied. Expressions for calculation of the number of actuator position of a mechanism are presented. They enable the reduction of the number of structures by avoiding those that are isomorphic. Following this, examples for applications for various planar pin-jointed kinematic structures are presented, enabling the field of research to be restricted to the possible solutions.     

Tensile failure of micro-concrete: from mechanical tests to FE meso-model with the help of X-ray tomography

In this work, concrete is studied at meso-scale (aggregates, macro-pores and mortar matrix), where the local failure mechanisms are known to drive the macroscopic behaviour of the material. In order to highlight the impact of the mechanical and morphological properties of each phase (along with their interfaces), micro-concrete specimens are prepared with rather small dimensions compared to the size of the heterogeneities. X-ray tomography is used to reliably obtain the morphology of the heterogeneous meso-structure, which is then given as an input to a 3D FE meso-model with enhanced discontinuities. A uniaxial tensile numerical simulation is performed as a first application. To validate the numerical model, a uniaxial tensile test of the same micro-concrete specimen is performed inside the X-ray scanner and the in-situ evolution of the micro-structure is followed. Thus, both a direct validation of the model and a valuable insight of the 3D fracture mechanisms while the load progresses are obtained. After identification of the numerical parameters, comparison of experimental and numerical results reveals the capability of the meso-model to reproduce the actual material response (in terms of macroscopic strength, Young’s modulus and fracture patterns), with the explicit representation of the meso-scale heterogeneities being its key feature. To further challenge the meso-model, a new morphology coming from an X-ray scan of another characteristic micro-concrete specimen is introduced and its macroscopic behaviour is computed without a priori numerical identification. Starting from an X-ray scan in meso-scale, it is shown that the 3D meso-model is capable to predict the macroscopic behaviour and the failure patterns of the material.

     

Multistable switching series in chaotic nets

Examples of conscious and interpretable responses that have two or more forms alternating to the same stimuli have been known for centuries, and methods of describing how such situations arise have evolved in biological science. When switches between transient, perceptual or cognitive responses can occur and are mixed serially within time series exhibiting local terminal stability, then patterns arise where psychological data series are too brief to analyse empirically, and neurophysiological data and mathematical simulation are necessary. Modelling such conditions can be approached by using one modified Markov matrix, which we illustrate if we allow some singularities to exist in the dynamics. As soon as networks cease to be homogeneous and have a number of attractors present and operate with different local structures, then one or more response patterns may potentially exist at the same time. The patterns may be addressed within the behavioural dynamics by incorporating in turn very short transients that can be voluntary or involuntary, in sensory and cognitive data. Related software work for modelling, employing hierarchical Dirichlet structures projected into hidden Markov matrices is noted.     

钢筋锈胀引发混凝土保护层开裂破坏的细观数值研究

钢筋锈蚀膨胀引起保护层混凝土开裂是影响钢筋混凝土结构耐久性和服役寿命的重要因素。考虑到混凝土细观结构组成对保护层破坏模式的影响,从细观角度出发,将混凝土看作由骨料、砂浆基质及两者间界面过渡区组成的三相复合材料,建立了描述钢筋锈胀力学行为的混凝土随机骨料模型。采用塑性损伤本构关系模型来表征砂浆基质和过渡区界面的力学行为,假定钢筋均匀锈蚀,对钢筋锈胀引起的混凝土保护层开裂破坏过程进行了细观数值研究。对比了宏观均匀模型与细观非均质模型下获得的保护层破坏模式,探讨了径厚比(c/d)、钢筋位置(中部和角区)及混凝土拉伸强度对保护层破坏模式及保护层胀裂时钢筋锈蚀水平的影响,得到了一些有益结论。     

考虑扭转耦联效应的附属结构最优位置分析

实际建筑物大多为偏心结构,扭转耦联效应使同层楼板上不同位置处附属结构的动力响应也不相同,通常存在一个平面最优位置.本文建立了主附结构体系的扭转耦联模型,利用复模态理论和模式搜索方法研究了影响附属结构最优位置的几个重要因素,包括地震输入方向、场地类别、主体结构偏心、附属结构质量、频率及阻尼比等,通过数值分析得出了一些有益的结论.     

A simple meso-macro model based on SQP for the non-linear analysis of masonry double curvature structures

A 3D model for the evaluation of the non-linear behavior of masonry double curvature structures is presented. In the model, the heterogeneous assemblage of blocks is substituted with a macroscopically equivalent homogeneous non-linear material. At the meso-scale, a curved running bond representative element of volume (REV) constituted by a central block interconnected with its six neighbors is discretized through of a few six-noded rigid wedge elements and rectangular interfaces. Non linearity is concentrated exclusively on joints reduced to interface, exhibiting a frictional behavior with limited tensile and compressive strength with softening. The macroscopic homogenous masonry behavior is then evaluated on the REV imposing separately increasing internal actions (in-plane membrane actions, meridian and parallel bending, torsion and out-of-plane shear). This simplified approach allows to estimate heuristically the macroscopic stress–strain behavior of masonry at the meso-scale. The non-linear behavior so obtained is then implemented at a structural level in a novel FE non-linear code, relying on an assemblage of rigid infinitely resistant six-noded wedge elements and non-linear interfaces, exhibiting deterioration of the mechanical properties.Several numerical examples are analyzed, consisting of two different typologies of masonry arches (a parabolic vault and an arch in a so-called “skew” disposition), a ribbed cross vault, a hemispherical dome and a cloister vault. To fully assess numerical results, additional non-linear FE analyses are presented. In particular, a simplified model is proposed, which relies in performing at a structural level a preliminary limit analysis – which allows to identify the failure mechanism – and subsequently in modeling masonry through elastic elements and non-linear interfaces placed only in correspondence or near the failure mechanism provided by limit analysis. Simulations performed through an equivalent macroscopic material with orthotropic behavior and possible softening are also presented, along with existing experimental evidences (where available), in order to have a full insight into the capabilities and limitations of the approach proposed.