基于SHPB的球形压痕实验方法

提出一种基于霍普金森压杆装置的动态球形压痕实验方法,通过将硬质合金小球置于2个试件中间,实现加载过程中2个试件的同时压入,以得到准确的压痕力与位移的关系。利用有限元软件ABAQUS/Explicit对该实验方法进行了数值模拟,从实验结果的可行性、准确性等方面,对新实验方法与传统的动态压痕实验方法进行了比较;采用新方法对铝合金材料进行了实验,并得到了压入力-位移曲线等关系。研究结果表明:采用新实验方法能得到较准确的结果,能较真实地描述压痕过程。     

血红细胞力学性能的纳米压痕实验和有限元模拟

采用纳米压痕技术和有限元方法研究了血红细胞的生物力学性能. 进行了血红细胞的纳米压痕实验, 得到了血红细胞的材料参数和变形形貌; 在实验基础上, 建立了血红细胞的三维有限元模型, 模拟了血红细胞的压痕载荷-位移曲线, 并考虑了参数效应. 数值模拟结果和实验数据符合很好. 通过改变压头与材料之间的摩擦系数和压头曲率半径等参数, 比较了载荷-位移曲线的变化情况. 研究表明摩擦系数对压痕载荷-位移曲线和应力分布影响很小, 而压头曲率对载荷-位移曲线的影响明显.     

球形压痕的残余变形分析

球形压痕技术在材料力学属性,诸如硬度,弹性模量等的测量中得到了广泛的应用.应用Twyman-Green及云纹干涉法并配合相移技术,本文对IN783合金进行了一系列的球形压痕实验研究,并对残余压痕的面内(u, v)及离面(w)变形场进行了定量测量和分析.应用面内变形测量结果,进一步对试件表面的应力-应变分布进行了分析和计算,并在离面变形场的基础上,确立了压痕周围的弹塑性边界,从而进一步应用面内的分析结果,得到材料的屈服强度.应用压痕实验的接触半径和压力并配合Tabor经验公式,本文进一步得到了材料的应力应变曲线.实验结果与已知的IN783合金相吻合.对所涉及的一系列压痕实验,本文也进行了二维有限元分析并得到了比较一致的结果.     

球形压痕的残余变形分析(英文)

球形压痕技术在材料力学属性,诸如硬度,弹性模量等的测量中得到了广泛的应用。应用Twyman-Green及云纹干涉法并配合相移技术,本文对IN783合金进行了一系列的球形压痕实验研究,并对残余压痕的面内(u, v)及离面( w)变形场进行了定量测量和分析。应用面内变形测量结果,进一步对试件表面的应力-应变分布进行了分析和计算,并在离面变形场的基础上,确立了压痕周围的弹塑性边界,从而进一步应用面内的分析结果,得到材料的屈服强度。应用压痕实验的接触半径和压力并配合Tabor经验公式,本文进一步得到了材料的应力应变曲线。实验结果与已知的IN783合金相吻合。对所涉及的一系列压痕实验,本文也进行了二维有限元分析并得到了比较一致的结果。     

球形压痕的残余变形分析

球形压痕技术在材料力学属性，诸如硬度，弹性模量等的测量中得到了广泛的应用。应用Twyman-Green及云纹干涉法并配合相移技术，本文对IN783合金进行了一系列的球形压痕实验研究，并对残余压痕的面内（u,v）及离面（w）变形场进行了定量测量和分析。应用面内变形测量结果，进一步对试件表面的应力一应变分布进行了分析和计算，并在离面变形场的基础上，确立了压痕周围的弹塑性边界，从而进一步应用面内的分析结果，得到材料的屈服强度。应用压痕实验的接触半径和压力并配合Tabor经验公式，本文进一步得到了材料的应力应变曲线。实验结果与已知的IN783合金相吻合。对所涉及的一系列压痕实验，本文也进行了二维有限元分析并得到了比较一致的结果。     

纳米压痕仪接触投影面积标定方法的研究

基于Oliver与Pharr方法的纳米压痕实验以其简单方便获得广泛的应用，但众多因素对压 痕实验结果的影响范围并无明确的结论. 其中压痕接触面积的确定是一个重要环节，该因素 对实验结果，特别是小深度下的实验结果具有重要影响. 仔细分析了Oliver与Pharr 方法并进行了几种材料的纳米压痕实验，针对该方法在接触深度确定、不同深度范围下方法 的适用性进行了说明. 分析结果表明，对所有的材料使用统一的面积公式，只有在大压痕深 度时才是适用的，而在小压痕深度时可能带来较大的误差. 因此，应慎重使用由Oliver与 Pharr方法得到的小压痕深度的硬度数据.     

二维材料力学行为的压痕测试

准确了解二维材料的力学性能对于推动其应用具有重要意义, 无基底压痕技术是目前最广泛采用的二维材料力学性能测试方法之一, 本文综述了二维材料压痕研究的最新进展以及所面临的问题, 并对将来的研究工作进行了展望.无基底压痕技术是将二维材料转移到带有沟槽或柱形孔的基底上, 制备二维材料"梁"和"鼓"模型, 然后利用原子力显微镜测量其在压针作用下的载荷--位移关系, 最后通过基于连续介质薄膜导出的压痕响应分析模型拟合实验结果, 估算出二维材料的弹性模量和本征强度.由于二维材料的厚度远小于连续介质薄膜, 来自于压头以及基底孔侧壁的范德华力对二维材料的压痕响应具有显著影响, 造成二维材料与传统压痕分析模型中的基本假设不符, 导致不能准确预测二维材料的弹性模量; 另外, 由于传统压痕模型无法准确描述二维材料在大变形下的非线性行为, 以及由缺陷等引起的应力集中, 导致由压痕测试表征的二维材料(特别是多晶二维材料)本征强度具有较大的偏差. 因此, 一方面需要正确了解由压痕技术获得的二维材料力学性能, 另一方面还需对目前的研究方法做进一步的改进和完善.      

聚甲基丙烯酸甲酯材料(PMMA)纳米压/划痕变形热恢复的实验研究

聚甲基丙烯酸甲酯(PMMA)是常见的光伏电池封装材料.本文采用锥形和球形两种压头,利用纳米压痕仪开展了PMMA的纳米压痕和划痕实验.基于表面形貌扫描得到的PMMA材料在不同温度热处理后压痕与划痕变形随时间恢复的演化规律,分析了时间、温度和压头形状等对PMMA材料压/划痕变形恢复过程的影响机理.结果表明,压/划痕变形恢复...     

二维材料力学行为的压痕测试

准确了解二维材料的力学性能对于推动其应用具有重要意义,无基底压痕技术是目前最广泛采用的二维材料力学性能测试方法之一,本文综述了二维材料压痕研究的最新进展以及所面临的问题,并对将来的研究工作进行了展望.无基底压痕技术是将二维材料转移到带有沟槽或柱形孔的基底上,制备二维材料"梁"和"鼓"模型,然后利用原子力显微镜测量其在压针作用下的载荷–位移关系,最后通过基于连续介质薄膜导出的压痕响应分析模型拟合实验结果,估算出二维材料的弹性模量和本征强度.由于二维材料的厚度远小于连续介质薄膜,来自于压头以及基底孔侧壁的范德华力对二维材料的压痕响应具有显著影响,造成二维材料与传统压痕分析模型中的基本假设不符,导致不能准确预测二维材料的弹性模量;另外,由于传统压痕模型无法准确描述二维材料在大变形下的非线性行为,以及由缺陷等引起的应力集中,导致由压痕测试表征的二维材料(特别是多晶二维材料)本征强度具有较大的偏差.因此,一方面需要正确了解由压痕技术获得的二维材料力学性能,另一方面还需对目前的研究方法做进一步的改进和完善.     

Indentation tests investigation of mechanical behavior of two-dimensional materials

准确了解二维材料的力学性能对于推动其应用具有重要意义, 无基底压痕技术是目前最广泛采用的二维材料力学性能测试方法之一, 本文综述了二维材料压痕研究的最新进展以及所面临的问题, 并对将来的研究工作进行了展望.无基底压痕技术是将二维材料转移到带有沟槽或柱形孔的基底上, 制备二维材料"梁"和"鼓"模型, 然后利用原子力显微镜测量其在压针作用下的载荷--位移关系, 最后通过基于连续介质薄膜导出的压痕响应分析模型拟合实验结果, 估算出二维材料的弹性模量和本征强度.由于二维材料的厚度远小于连续介质薄膜, 来自于压头以及基底孔侧壁的范德华力对二维材料的压痕响应具有显著影响, 造成二维材料与传统压痕分析模型中的基本假设不符, 导致不能准确预测二维材料的弹性模量; 另外, 由于传统压痕模型无法准确描述二维材料在大变形下的非线性行为, 以及由缺陷等引起的应力集中, 导致由压痕测试表征的二维材料(特别是多晶二维材料)本征强度具有较大的偏差. 因此, 一方面需要正确了解由压痕技术获得的二维材料力学性能, 另一方面还需对目前的研究方法做进一步的改进和完善.     

The Nano-Jackhammer effect in probing near-surface mechanical properties

Because of its ease of implementation and insensitivity to indenter drift, dynamic indentation techniques have been frequently used to measure mechanical properties of bulk and thin film materials as a function of indenter displacement. However, the actual effect of the oscillating tip on the material response has not been examined. Recently, it has been shown that the oscillation used with dynamic indentation techniques alters the measured hardness value of ductile metallic materials, especially at depths less than 200 nm. The alteration in the hardness is due to the added energy associated with the oscillation which assists dislocation nucleation. Atomistic simulations on nickel thin films agree with experiments that more dislocations are nucleated during dynamic indents than with quasi-static indents. Through the analysis of quasi-static and dynamic indents made into nickel single crystals and thin films, a theory to describe this phenomenon is presented. This is coined the Nano-Jackhammer effect, a combination of dislocation nucleation and strain rate sensitivity caused by indentation with a superimposed dynamic oscillation.     

准脆性单晶材料动态压痕试验研究

本研究设计了由钛合金压杆与金刚石压头组成的动态压痕试验装置,研究准脆性单晶材料(冰糖和RDX含能晶体)等的高应变率响应。试验装置引入了动量缓冲块,可以避免压杆造成的多次压痕加载,对样品施加产生80μs脉冲宽度的动态压痕载荷。对比了多次与单次加载压痕加载的冰糖单晶压痕坑,其动态硬度值约为5.18MPa。RDX单晶在压痕载荷作用下非常容易发生断裂破碎,测得动态硬度值为1.304MPa。相比RDX单晶,冰糖单晶的塑性变形能力更好。用光学显微镜观察到了压痕坑的径向裂纹和侧边裂纹,试验研究给出了准脆性晶体在动态压痕载荷下的裂纹扩展模式。     

固体力学中侵入问题的若干新进展

本文主要介绍了固体力学中侵入问题近10余年来的发展情况。重点综述了侵入断裂实验及理论研究的新成果;讨论了动静态侵入断裂的关系;简述了侵入方法在测定材料断裂韧性方面的应用。      

Spherical indentation of incompressible rubber-like materials

In recent years, indentation tests have been proven very useful in probing mechanical properties of small volumes of materials. However, a class of materials that very little has been done in this direction is rubber-like materials (elastomers). The present work investigates the spherical indentation of incompressible rubber-like materials. The analysis is performed in the context of second-order hyperelasticity and is accompanied by finite element computations and an extensive experimental program with spherical indentors of different radii. Uniaxial tensile tests were also performed and it was found that the initial elastic modulus correlates well with the indentation response. The experiments suggest stiffer indentation response than that predicted by linear elasticity, which is somehow counter-intuitive, if the uniaxial material response is to be considered. Regarding the uniqueness of the inverse problem, that is to establish material properties from spherical indentation tests, the answer is disappointing. We prove that the inverse problem does not give unique answer regarding the constitutive relation, except for the initial stiffness.     

Measurement of the Viscoelastic Properties of Bitumen Using Instrumented Spherical Indentation

Indentation testing as a tool for determination of the viscoelastic mechanical properties of bitumen is examined in some detail using theoretical, numerical as well as experimental methods. In particular Brinell indentation is analysed and simple but rigorous formulae for a complete characterization of linear viscoelastic materials are presented. Numerical methods (finite element methods) are used in order to verify and substantiate these relations for an experimental situation. Indentation experiments are then performed on bitumen and special efforts are made in order to avoid size effects, i. e. anomalous results due to the fact that the indented specimens are too small and as a result, far field boundary conditions will influence the interpretation of the experimental output. The mechanical properties determined experimentally by indentation are compared with corresponding results from standard mechanical tests, and the results are encouraging considering the fact that non-linear effects are also influencing the outcome of the experiments.     

A New Application of Dynamic Indentation: Indentation Machining Technology

While the indentation method is an excellent way to evaluate the mechanical properties of various sizes of materials, from the nano-scale to the macro-scale, its applications have been limited to measuring mechanical properties. In this study we propose a new application of the dynamic indentation method, in an indentation machining technology for mass-production. The core idea is that the array of residual indentations generated by dynamic indentation testing can be used to fabricate a lens array suitable for thinner and brighter displays. We developed an advanced system from a dynamic indentation system, whose maximum speed and maximum specimen size were about 10Hz and 250 mm*250 mm, respectively. Using dual actuating heads this system was used to produce arrays of lenses having depths of 1 μm to 6 mm. Pile-up is a critical reason why indentation machining technology had been not widely used in display industries. Since lower pile-up is observed in more ductile copper-based metals, we increased the annealing time of the metal molds to reduce the amount of pile-up. Then, following a quantitative analysis of the annealing heat treatment and resulting amount of pile-up, a lens array was successfully machined on a metal mold fabricated by the developed system. The machined metal mold was used to manufacture optical plates for a lens array. The results verified that the indentation machining technology proposed in this study, based on the dynamic indentation method, can be applied for the manufacturing of optical components for better displays.     

On the uniqueness and sensitivity of indentation testing of isotropic materials

Instrumented indentation is a popular technique to extract the material properties of small scale structures. The uniqueness and sensitivity to experimental errors determine the practical usefulness of such experiments. Here, a method to identify test techniques that minimizes sensitivity to experimental erros is in indentation experiments developed. The methods are based on considering “shape functions,” which are sets of functions that describe the force–displacement relationship obtained during the indentation test. The concept of condition number is used to investigate the relative reliability of various possible dual indentation techniques. Interestingly, it was found that many dual indentation techniques can be as unreliable as single indentation techniques. Sensitivity analyses were employed for further understanding of the uniqueness and sensitivity to experimental errors of indentation techniques. The advantage of the Monte Carlo approach over other procedures is established. Practical guidelines regarding the selection of shape functions of force–displacement relationship and geometric parameters, while carrying out indentation analysis are provided. The results suggest that indentation experiments need to be very accurate to extract reliable material properties.     

Spherical indentation of freestanding circular thin films in the membrane regime

We present theoretical and experimental results to describe the mechanics of indentation of a clamped circular membrane with a frictionless spherical indenter. Analytical expressions and numerical simulations are presented for the relationships between contact radius, finite indentation strains (and stresses), pre-stretch, loads and deflection. These closed-form solutions are contrasted with point-load models that neglect the contact size (i.e. classical Schwerin-type solutions), and lead to important differences in the indentation strain and load-deflection response. The accuracy of these closed form expressions is illustrated by comparisons with detailed numerical results and experiments on thin elastomer films. We show that the closed-form solutions can be used to extract mechanical properties from indentation testing of freestanding films, with important implications for developing new tests on nanoscale films and/or compliant materials such as polymers and biological substances.     

Sensitivity analysis,calibration and validation of a snow indentation model

Quantification of the mechanical behavior of snow in response to loading is of importance in vehicle-terrain interaction studies. Snow, like other engineering materials, may be studied using indentation tests. However, unlike engineered materials with targeted and repeatable material properties, snow is a naturally-occurring, heterogeneous material whose mechanical properties display a statistical distribution. This study accounts for the statistical nature of snow behavior that is calculated from the pressure-sinkage curves from indentation tests. Recent developments in the field of statistics were used in conjunction with experimental results to calibrate, validate, and study the sensitivity of the plasticity-based snow indentation model. It was found that for material properties, in the semi-infinite zone of indentation, the cohesion has the largest influence on indentation pressure, followed by one of the the hardening coefficients. In the finite depth zone, the friction angle has the largest influence on the indentation pressure. A Bayesian metamodel was developed, and model parameters were calibrated by maximizing a Gaussian likelihood function. The calibrated model was validated using three local and global confidence-interval based metrics with good results.