Novel Design of Cruciform Specimens for Planar Biaxial Testing of Soft Materials

Cruciform specimens have long been used in planar biaxial testing of inanimate materials such as metals and composite materials. The efforts to improve the geometric design of cruciform specimens have focused on maximizing the degree of uniformity of stress and strain in the gage section. The standardization of the procedure for the determination of the mean stress in the gage section is lacking, however, because the exact load transferred from the grippers to the gage section during testing is unknown. Here, we introduce a novel split-arm design for cruciform specimens by taking into account three important factors: i) the effectiveness of load transfer from the grippers to the gage section, ii) the uniformity of normal stress (in the loading direction) over the symmetry line, and iii) the compatibility between the nominal stress and the true stress. By ensuring these conditions, one can estimate more accurately the mean stress in the gage section based on the measured force at the grippers and the deformed configuration of a reference length. A genetic algorithm coupled with finite element analysis was utilized to optimize the geometric shape of the novel cruciform design. The identified optimum design provides a load transfer effectiveness of 100 %. The calculated nominal stress deviates from the true stress at the center of the specimen by only ?0.49 %. A numerical experiment was conducted to validate the substantially improved performance of the optimized new design. Experiments were also conducted for natural latex rubber to demonstrate the application of the proposed design.     

Mechanical Behaviors and Bending Effects of Carbon Fiber Reinforced Lattice Materials

To acquire materials of higher specific stiffness and strength, stretching dominated lattice materials reinforced by carbon fibers were designed and manufactured. The mechanical behaviors were predicted and experimented. The imperfections of lattice materials, such as the waviness of the struts, non-circular cross-sections and cantilever ribs, greatly influenced their performance. The bending effects of the imperfections were predicted and compared with the experiment results. Although influenced by the imperfections, carbon reinforced lattice materials are still much stiffer and stronger than foams and honeycombs.     

Mechanical Characterization of Coatings Using Microbeam Bending and Digital Image Correlation Techniques

A new technique for characterizing end-supported microbeams of coating materials is presented. Microbeams are fabricated using micro-EDM machining to isolate the material under investigation from the underlying substrate. Three- and four-point bending is realized by a custom-built microspecimen testing system, and digital image correlation is employed to capture full-field strains and displacements in theses microbeams. These experiments provide the foundation for the use of finite element modeling and inverse methods to determine the mechanical properties (elastic moduli, strength, interfacial toughness) of the coatings. Here, the experimental details of the microbeam bending experiments are explained, discussed and illustrated through application to a multilayered metal/oxide/ceramic thermal barrier coating system commonly used in aero-turbines.     

增材制造三维微点阵材料力学性能表征与细观优化设计研究进展

三维微点阵材料是一种由复杂拓扑胞元周期性排列构成的超轻质结构材料,兼具极低的密度、优越的力学特性和良好的能量吸收等性能,是满足轻量化、抗冲击和多功能集成需求的重要新型战略材料.增材制造技术的快速发展,为三维微点阵材料的制备和优化设计带来了便利的条件,二者的结合为航空航天、轨道交通以及武器装备等领域实现防护结构轻量化和多功能一体化提供了新思路.为阐明增材制造三维微点阵材料的动态力学特性与变形失效机理,进一步开展材料多尺度优化设计,拓展增材制造微点阵材料在冲击防护等领域的应用,对增材制造三维微点阵材料力学行为与设计的研究成果进行了系统的综述和展望.依据增材制造三维微点阵材料的多尺度结构特征,分别评述了不同类型微点阵材料的宏观动力学响应与失效机制、细观性能表征与结构优化设计、微观组织特征与变形机理等方面的研究,展望了未来增材制造三维微点阵材料在冲击防护领域面临的问题和挑战.     

自修复体系中微胶囊壁材力学性能的测试与表征

微胶囊自修复技术成为研发自修复聚合物材料的主要途径。由于尺寸效应的影响和制备工艺的差异,有必要表征和测量自修复体系中微胶囊壁材的力学性能。本文从微胶囊中直接剥离出微胶囊壁试样进行纳米压痕测试,通过数据拟合和和量纲分析的方法,建立载荷-位移(P-h)曲线与微胶囊壁材弹塑性力学参数之间的联系,分析得到了微胶囊壁材的弹性模量和特征应力值(σ_0.033),进而得到微胶囊壁材的应力-应变本构关系。     

材料力学性能检测虚拟实验的设计和开发

针对传统材料实验教学的种种弊端,将虚拟现实技术运用到材料实验教学当中,使教学过程具有交互性好,实验沉浸感强的特点。介绍了基于VRML的材料力学性能检测虚拟实验的设计思想和设计原理,对关键技术进行了阐述和分析。采用Pro/E、3DS MAX建立拉伸机、冲击机,疲劳试验机、压缩机、带表卡尺等实验仪器、工具的三维模型,并将其转换成VRML格式。然后通过VRML的事件路由机制实现了材料力学性能检测实验的全过程,如拉伸实验中,包括试样的测量、加载,拉伸过程的实现和曲线实时生成。作为材料实验教学的辅助工具,使学习过程形象化,充分发挥学生主观能动性,培养创新意识和解决工程问题能力。     

基于气压鼓泡法研究石墨烯力学性能

通常假设二维材料为连续介质薄膜,然后采用连续介质薄膜的研究方法进行二维材料力学性能研究,其中气压鼓泡法是一种主要测试方法．但实验观测发现,悬空石墨烯并非处于气压鼓泡测试分析模型中假设的固支边界条件,而是处于一种粘附边界条件：靠近孔壁边界处,有小部分材料通过范德华吸引粘附在基底柱形孔的侧壁上,而且粘附部分可以在极小载荷作用下剥离．这导致石墨烯悬空部分的实际半径大于基底孔半径,即鼓泡实验中的石墨烯是一种松弛薄膜,而非通常认为的预拉伸薄膜．通过有限元数值模拟研究发现,可基于含有名义松弛应变的鼓泡分析模型获得处于粘附边界条件下的石墨烯弹性模量．     

A Mechanical Testing Capability for Measuring the Microscale Deformation Behavior of Structural Materials

High energy x-ray diffraction can be used to probe the crystal scale mechanical response of polycrystalline alloys. Recently there have been several efforts to create new high energy x-ray experiments. These include the lattice Strain Pole Figure (SPF) technique. By measuring lattice strains in thousands of directions, the lattice strain tensor associated with each orientation can be determined. The focus of this paper is on transforming the SPF technique from a one-off style experiment to a measurement capability. Such a standardization process is of the utmost importance for the field of mechanics of materials and shifts the discovery associated with these experiments from the measurements themselves, to what they reveal about the material. We define a new technique for quantifying how effectively a set of lattice strain measurements (SPFs) probes each crystal orientation. The polycrystal sampling matrix, defined G^*{\boldsymbol\Gamma^*}, represents the mapping between the most likely strain tensor for each orientation and the lattice strain results. The orientation space sampling matrix, defined G (R){\boldsymbol\Gamma ({\bf R})}, represents the set of lattice strain measurements that interrogate each crystal orientation. The rank of G (R){\boldsymbol\Gamma ({\bf R})} can be used to quantitatively compare different experimental configurations and systematically investigate G^*{\boldsymbol\Gamma^*}. The net result is a new tool for selecting experimental conditions to produce optimal sets of SPF data. Results are shown for different experiment configurations and an example of the SPF technique is provided for the Low Solvus High Refractory (LSHR) nickel base superalloy. In addition, we show that for the face centered cubic LSHR, with lattice strains measured for the {111}, {200}, {220}, and the {311} crystallographic families, there are at most 25 lattice strain measurements that interrogate a single orientation.     

On-Chip Electrostatically Actuated Bending Tests for the Mechanical Characterization of Polysilicon at the Micro Scale

The issue of mechanical characterization of polysilicon used in micro electro mechanical systems (MEMS) is discussed in this paper. An innovative approach based on a fully on-chip testing procedure is described; two ad hoc designed electrostatically actuated microsystems are here used in order to determine experimentally the Young’s modulus and the rupture strength of polysilicon. The first device is based on a rotational test structure actuated by a system of comb-finger capacitors which load up to rupture a couple of tapered beams under bending in the plane parallel to the substrate. The second microsystem is based on a large plate with holes. It constitutes with the substrate a parallel plate capacitor moving in the direction orthogonal to the substrate itself. A couple of tapered beams placed at the centre of the plate is loaded up to rupture in bending in the plane orthogonal to the substrate. By means of the two devices, experimental data are obtained: they allow for a careful determination of Young’s modulus and rupture strength. The rupture values are interpreted by means of the Weibull approach; statistical size effects and stress gradient effect are taken into account thus allowing for a direct comparison between the data obtained from the two test structures.     

Design and Simulation of a Novel Multi-Axial Fatigue Test Rig

This research covers the design and simulation of a novel experimental concept for multi-axial fatigue analysis of cylindrical specimens. The resulting design allows a combination of bending and torsional stress to test specimens with critical diameters ranging from 5 to 15 mm at test frequencies up to 50 Hz. Furthermore, the amplitude and frequency of both loadcases can be controlled independently. The test rig will be used to study and validate fatigue criteria for synchronous and asynchronous loading conditions and to analyze the effect of size on the fatigue life of metal and plastic components. The test setup consists of a closed mechanical loop. The primary shaft contains the cylindrical test specimen and is modified to impose rotating bending loads. The secondary shaft is adjusted to introduce fluctuating torque in the transmission loop. Both shafts are connected by means of two double link mechanisms to minimize the clearance and the inertia of the system. The time-varying multi-axial stress state in the cylindrical specimen is analyzed as a function of the amplitudes and frequencies of both bending and torsional loadcases. This is verified by a numerical fatigue analysis in MSC-Patran and MSC-Fatigue. Finally, the dynamical behavior of the test system is studied using a 5 DOF torsional mass-spring representation and the Lagrangian method. A more complex model with 20 DOF is implemented in SimDriveLine and solved via Matlab to analyze the kinematical and dynamical properties more accurately. Both studies take the mechanical properties of steel and plastic test specimens of different sizes into account.     

Design of Inserts for Split-Hopkinson Pressure Bar Testing of Low Strain-to-Failure Materials

In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups—one utilizing conical inserts and the other utilizing a “dogbone” shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insert-specimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress–strain behavior.     

材料高温力学性能理论表征方法研究进展

随着科学技术的迅猛发展,材料在高温领域的应用越来越广泛.然而高温下材料的力学性能和常温相比有很大差异,材料的高温力学性能研究和表征已成为当前的研究热点.论文对材料在高温下力学行为理论表征方法研究的最新进展进行了总结和回顾.着重介绍了近年来高温陶瓷材料的断裂强度、金属材料的屈服强度、弹性模量与本构关系的温度相关性理论表征方法的研究进展.最后,总结已有研究工作的特点和不足之处,对材料高温力学性能理论表征方法的后续研究进行了展望,就进一步研究提供建议.     

A New Testing Machine for the Dynamic Characterization of High Strength Low Damping Fiber Materials

This work describes a novel method for measuring the damping, the elastic modulus and the non-linear behavior of high strength low damping fiber materials such as para-aramids, silicon carbide (SiC) and carbon. The method is based on resonant response characterization of a spring-mass system excited by a sine-wave forcing term which is applied as a vertical force to the suspended mass. The damping is obtained from the measured resonance quality factor Q, the elasticity modulus is calculated from the resonance frequency, and the non-linear coefficient is obtained with the backbone approach from resonance profile variations as a function of the forcing term amplitude. It is argued that the method is very sensitive, to the point that a maximum excitation amplitude of the order of a few percent of resistance is sufficient to obtain an estimate of the non-linear coefficient. This claim is supported by experimental results. A testing machine is also discussed, which provides the necessary sensitivity at such small excitation amplitudes and the capability of evaluating very small damping values, as expected in high strength low damping fiber materials. The sensitivity is guaranteed by an optical position sensor with sub-micron resolution. To evaluate small damping values, particular care has been taken to ensure that energy dispersions in the generator are much smaller than energy dispersions in the fibers themselves. Examples of dynamic characterization are shown for para-aramid, silicon carbide, and carbon fibers.     

含有序孔洞的多孔氧化铝薄膜的力学性能测量和数值模拟

高纯铝箔在特定的溶液下经过电化学阳极氧化腐蚀,可在其表面生成一层多孔的非晶氧化铝层,其孔径分布非常均匀,孔大致呈六方密排布。由于此类薄膜具有规则的纳米级孔径,大的比表面积,良好的自组织排列性,所以其日益受到人们的关注。然而,到目前为止,对于此类薄膜力学性能的研究还很少,所以在一定程度上限制其功能的开发和应用。为了获得此类多孔膜的弹性常数,本文首先由实验出发,通过光力学检测(双光束散斑干涉)的方法得到薄膜拉伸时的整体表观弹性模量。然后对薄膜建立二维有限元模型,运用均匀化理论反推出其基体(无孔结构)的弹性模量,同时考察了不同的基体泊松比对模型整体表观模量的影响,并且用一般有限元方法验证了沿特定方向拉伸时均匀化模型计算的有效性。     

杆式冲击装置测定材料动态J积分

本文介绍了一种研究材料动态断裂韧度的实验技术。该方法采用冲击杆对常规三点弯曲试件进行动态加载。由于长杆条件下存在荷载、位移及冲击速度之间的定量关系,试验中只需对荷载的时间历程进行测定,就能间接地计算出位移和J积分值。与目前广泛使用的示波摆锤(落锤)冲击法相比,不仅测试过程明显简化,而且力学分析的基础也更为可靠。     

Correction to: Material and Mechanical Characterization of Multi-Functional Carbon Nanotube Reinforced Hybrid Composite Materials

Experimental Techniques - In the original article, the first author A. Paul Praveen’s name was mentioned incorrectly in the author list. The details given in this correction are correct.     

Experimental and Numerical Simulation of Elastomeric Outsole Bending

In this paper, we deal with the mechanical behaviour of elastomeric sole shoes subjected to bending. First, a methodology for material parameter identification is described for a visco-hyperelastic constitutive law with discontinuous damage, modelling the Mullins effect. This constitutive law was implemented in the finite element software CODE-ASTER (Abbas and Baranger 2010). Then, numerical results of the bending behaviour of a simple geometric shoe sole model are compared to experimental measurements obtained with a stereoscopic vision system. Finally, an industrial application of dynamic bending sole shoe is presented and experimental results and simulated load history are compared.     

A Device for Biaxial Testing in Uniaxial Machines. Design,Manufacturing and Experimental Results Using Cruciform Specimens of Composite Materials

The present study deals with the design and manufacturing of a mechanical device to perform biaxial testing in universal (uniaxial) testing machines. A review of previous definitions of similar devices is carried out and a new device is conceived and developed. The main improvement with the present device is that it allows different types of biaxial loadings (tension-compression) to be performed with few manipulations. The device allows variable displacement ratio to be used in each loading direction, giving then rise to variable loading ratios. Biaxial tension-tension tests on cruciform specimens made of composite material were carried out using very brittle samples in which the fibre direction was perpendicular to the loading plane. Strain gages were used to monitor the percentage bending parameter so that the correct alignment of the loading could be checked. Values below 5% for the bending parameter were achieved at the moment of failure.     

Mechanical Behavior Characterization of a Stainless Steel Dissimilar Metal Weld Interface : In-situ Micro-Tensile Testing on Carburized Martensite and Austenite

Experimental Mechanics - Stainless Steel Dissimilar Metal Welds (SS DMW) between low-alloy steel 18MND5 and austenitic 316L stainless steel are critical junctions in the currently operating...     

The Application of a Digital Computer to Mechanical Testing

A digital computer is being used to record raw data and to provide automatic data calculations for mechanical tests. The computer which is connected through an analog-digital converter to a universal testing machine and to a torsion tester, stores the raw loadstrain data, converts it to stress-strain data, and calculates any of the mechanical properties desired from the test. The computer can also provice a printout and a paper tape, both of which contain the stress-strain data points and the calculated data. In addition, the loadstrain data that are normally obtained from the test machine are still available. An oscilloscope display is obtained as the data are being recorded. Tests which are presently being performed using the computer are: tensile, compression, pin bearing, double shear and torsion. Data which are being recorded include: ultimate strength, offset yield strength, elastic and shear moduli, energy under the stress-strain curve, and a mathematical expression for the stress-strain curve. In addition to the mechanical load-strain curve provided by the test machine, the computer can provide a stressstrain curve and a listing of stress-strain data points.