Finer-Scale Extraction of Viscoelastic Properties from Nanoindentation Characterised by Viscoelastic–Plastic Response

Abstract:  Motivated by recent progress in viscoelastic indentation analysis, the identification of viscoelastic properties from materials exhibiting elastic, viscous and plastic material behaviour by means of nanoindentation is dealt with in this paper. Based on existing solutions for pure viscoelastic material behaviour, two methods allowing us to consider the effect of plastic deformation are presented: (i) the so-called double-indentation technique, with the second indentation characterised by pure viscoelastic material response and (ii) the use of spherical indenter geometries instead of commonly used pyramidal indenters avoiding plastic deformation at all. Both methods are applied to three different polymers, giving access to the model parameters of the fractional dash-pot which is used to describe the viscoelastic behaviour. The results obtained are compared with results from standard (single) indentation tests using a Berkovich indenter. Moreover, the influence of the maximum load, determining the amount of plastic material response, on the identified model parameters is investigated. Finally, the creep-compliance functions identified by nanoindentation are compared with the respective macroscopic creep-compliance functions obtained from bending-beam rheometer tests.     

高聚物粘弹性力学模型计算中容易被忽视的一个基本问题

在应用粘弹性力学模型模拟线形高聚物应力松弛现象时,不能忽视一个基本问题:在维持总形变不变的情况下,总应力是在逐渐衰减的,即应力不是恒定值。本文对高聚物力学松弛及粘弹性的本质进行了讨论。     

试样倾斜对熔融石英硅三棱锥纳米压痕测试的影响

使用三棱锥压头对不同倾斜角下的熔融石英硅进行纳米压痕实验。结果表明,试样倾斜会影响加载曲线的形状。在相同的载荷下,随着试样倾斜角的增加,压痕最大深度、残余深度及接触深度逐渐减小,但卸载曲线不受影响,彼此保持平行,卸载曲线的拟合参数m及接触刚度值保持恒定。另外,试样倾斜将导致测量的压痕接触面积偏小,从而使得测量的硬度和弹性模量偏大。     

应用纳米压痕法测定柱状纳米材料的力学特性（英文）

纳米压痕法在确定纳米结构材料,特别是具有较大高宽比的一维纳米结构/对象的力学特性时,若纳米结构沿压入方向的等效刚度远小于针尖-样品的接触刚度,应用常规数据分析(Oliver-Pharr)模型会导致较大的测量偏差.对常规Oliver-Pharr解析模型进行了推广,以补偿一维纳米材料等效刚度对测量结果的影响,进而提出了适用于此类测量对象的通用纳米压痕分析模型,并应用于分析柱状微纳米结构的准静态压痕测量数据.实验中应用原子力显微镜(AFM)定量测量了湿法刻蚀获得的一维单晶硅柱状结构的几何参数(包括硅纳米柱的直径和长度).实验结果表明,应用常规模型分析对较大高宽比的硅纳米柱(直径386 nm,长500 nm)的压痕数据会导致大于50%的偏差.应用修正模型分析实验数据时,测量结果不受被测对象几何参数的影响,因而可以有效提高应用纳米压痕法对微纳米结构材料,特别是一维材料的测量精度.     

On the Viscoelastic Response of Composite Laminates

The present work deals with the determination of theviscoelastic properties of composite laminates based on the viscoelasticbehaviour of the single lamina. A viscoelastic lamination theory isdeveloped assuming that the lamina stiffness matrix is fully complex andfrequency dependent.For the measurement of the complete setof frequency dependent viscoelastic properties of the single lamina(i.e. complex moduli and Poisson's ratio) a new methodology has beenintroduced, based on the free and forced vibration of unidirectional and[45/–45] _ns composite laminates.Allmeasurements have been performed both in air and in vacuum. Since thevibration amplitude was very small, in order to fulfil linearityrequirements, the results do not show any significant difference due toair damping.The effects of experimental errors on theevaluation of the viscoelastic properties of the single lamina have alsobeen discussed.     

Nanoindentation investigation of asphalt binder and mastic viscoelasticity

An exploratory nanoindentation technique for creep testing of two neat asphalt binders and one mastic at room temperature is developed, tested and verified. This work presents a new approach to obtain viscoelastic properties from low-load spherical (blunt) nanoindentation. Interconverted shear relaxation modulus mastercurves are determined from nanoindentation data. The magnitudes and trends of these mastercurves are found to be in reasonable agreement with Dynamic Shear Rheometer (DSR) results in a stiffness range associated with the range of time and temperature used in nanoindentation testing. Nanoindentation creep data is transformed to develop a mastercurve of dynamic modulus. The portion of this mastercurve corresponding to the frequency and temperature range included in nanoindentation testing demonstrates reasonable agreement with DSR results. These initial results suggest the potential to expand nanoindentation testing to forensic investigations involving testing of preserved asphalt binder and mastic components within field-extracted asphalt concrete composites.     

树脂基三维编织复合材料粘弹性能的数值预报

建立了基于三细胞模型数值预报三维编织复合材料粘弹性能的方法。首先构造了三维编织复合材料的三细胞模型并施加周期性边界条件,随后利用标准线性固体模型模拟树脂基体的粘弹性能,导出基体的松弛模量,再通过有限元计算及Prony级数拟合,得到三种胞元的粘弹性参数。然后根据三种胞元的体积分数和粘弹性参数,利用三个标准线性固体模型并联,模拟得到三维编织复合材料沿编织方向的粘弹性参数和蠕变本构关系。最后,分析了编织角和纤维体积含量对粘弹性能的影响。     

Mechanical Properties Measurement of PECVD Silicon Nitride after Rapid Thermal Annealing Using Nanoindentation Technique

Abstract:  This paper presents the results of mechanical characterisation of residual stress, elastic modulus, hardness and fracture toughness of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films subjected to rapid thermal annealing (RTA), processed between 200 and 800 °C. Additional tensile residual stresses were generated during the RTA period and the stress reached peak values after a 400 °C RTA process. On the other hand, nanoindentation testing revealed that both the modulus and hardness varied significantly with different RTA temperatures. Finally, the fracture toughness of the nitride was estimated to be 1.33 MPa √m based on a series of Vickers micro-indentation tests and it can be enhanced by the RTA process. These results should be useful for microelectromechanical systems (MEMS) or integrated circuit (IC) structure fabrication as regards maintaining the structural integrity and improving fabrication performance.     

Simultaneous Measurements of Geometric and Viscoelastic Properties of Hydrogel Microbeads Using Continuous‐Flow Microfluidics with Embedded Electrodes

Geometric and mechanical characterizations of hydrogel materials at the microscale are attracting increasing attention due to their importance in tissue engineering, regenerative medicine, and drug delivery applications. Contemporary approaches for measuring the these properties of hydrogel microbeads suffer from low‐throughput, complex system configuration, and measurement inaccuracy. In this work, a continuous‐flow device is developed to measure geometric and viscoelastic properties of hydrogel microbeads by flowing the microbeads through a tapered microchannel with an array of interdigitated microelectrodes patterned underneath the channel. The viscoelastic properties are derived from the trajectories of microbeads using a quasi‐linear viscoelastic model. The measurement is independent of the applied volumetric flow rate. The results show that the geometric and viscoelastic properties of Ca‐alginate hydrogel microbeads can be determined independently and simultaneously. The bulky high‐speed optical systems are eliminated, simplifying the system configuration and making it a truly miniaturized device. A throughput of up to 394 microbeads min^?1 is achieved. This study may provide a powerful tool for mechanical profiling of hydrogel microbeads to support their wide applications.     

用纳米压痕法研究玻璃表面复合薄膜的力学性能

采用溶胶凝胶法在玻璃基体上制备了CeO2-TiO2单层膜、TiO2-SiO2单层膜和CeO2-TiO2/TiO2-SiO2双层膜,采用纳米压痕和划痕法对薄膜的机械性能(纳米硬度、弹性模量、临界载荷、摩擦系数)进行了分析.实验结果表明,双层膜与玻璃基体的附着力以及弹性模量、硬度等指标均大于单层膜,由TiO2-SiO2组成的内层,对强化附着力起到了关键作用.     

The Role of Dilatation in the Nonlinearly Viscoelastic Behavior of PMMA under Multiaxial Stress States

In order to better understand phenomena related to yield-like behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22 and 110°C. In contrast to the mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong influence of the first stress or strain invariant on shear creep at shear strains in excess of 0.5%. While shear stresses alone elicit nonlinear response in creep (rates) as intrinsically nonlinear shear response, the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in qualitative agreement with free volume arguments when comparison is effected via maximum shear. In addition, an isochronal representation of the intrinsically nonlinear shear response demonstrates that the nonlinear behavior becomes more pronounced the closer one approaches the glass transition temperature from below.     

线性粘弹性本构方程及其动力学应用研究综述

综述了近年来线粘弹性材料本构方程的研究成果,介绍和讨论了复常数模量模型、标准流变学模型、分数阶导数模型、分数指数模型、微振子模型等典型和常用的五种模型,同时简单介绍了这些模型的动力学应用。     

Calculation of Residual Stress Development in Injection Moulding Using a Nonlinear Viscoelastic Model

The residual stress development in amorphous polymers iscalculated for rapid cooling under pressure during injection moulding.This is done primarily to predict the warpage and shrinkage behaviour ofcomponents. A nonlinear viscoelastic model based on the free-volumeconcept is used as a constitutive equation to describe polymerbehaviour. The cooling process in injection moulding can be divided intotwo stages: until ejection, the part is clamped into the mould, whilethe remaining cooling down to room temperature happens free ofconstraints. Both steps are calculated viscoelastically in thesimulation procedure. First, the stress profiles are determined in onematerial point of a plate-like component. In these calculations, theinfluence of different processing conditions, such as ejection time,packing pressure, or different wall temperatures, is investigated. Next,to be able to take more complex geometries into account, the nonlinearconstitutive equation is used in a commercially available finite-elementprogram. This is done by programming user-defined subroutines. Thesolution of the coupled thermal-mechanical problem allows the predictionof the residual stresses as well as the warpage and shrinkage behaviourfor components with arbitrary geometry. Finally, the calculated warpageis compared with the behaviour of an experimental component.     

Ni针尖/Au基体纳米压痕的分子动力学模拟

采用原子镶嵌势函数（EAM）模拟Ni针尖（约1．5mm）／Au基体纳米压痕过程．研究结果表明,当Ni针尖与Au基体间距离达到一定值时（约0．23 mm）,机械的不稳定性使得针尖与基体间发生跳跃接触,产生纳米压痕和黏附现象（Au原子包裹在Ni针尖周围）．当压头离开基体表面,Ni针尖被拔起,随后在针尖与基体间形成连续的由Au组成的细颈．同时计算得到整个系统在针尖接近基体、跳跃接触、压痕、黏附、形成缩颈和一系列分离过程中的势能变化．     

高压环境中聚合物玻璃化转变温度的测量方法

对高压环境中聚合物玻璃化转变温度(Tg)测量方法的现状与进展进行了综述。在常压测量法基础上发展起来的高压测量方法主要有:高压差示扫描量热法(HP-DSC)、蠕变柔量法等;原位光学法则是新近发展起来的一种非接触式测量方法。随着超临界流体技术与应用的发展及仪器科学自身的不断进步与完善,必将涌现出更多先进的测量技术与方法。     

Molecular Dynamics Simulations of the Nanoindentation for Aluminum and Copper

Atomistic simulations were performed to study the nanoindentation for two kinds of FCC metals, aluminum and copper. Due to the higher stacking faults in aluminum than in copper, two different deformation mechanisms were observed in our simulation under exactly the same simulation condition. Aluminum and copper also showed different mechanical properties in the unloading stage. The influence of stacking sequence along the loading direction on deformation mechanism was also investigated in this paper.     

Atomistic Understanding of the Competition between Dislocation and Twinning in Silver under Nanoindentation

In this study, the competition mechanisms between dislocation slip and twinning in silver with a low stacking fault energy using molecular dynamics (MD) simulation from an atomistic point of view are reported. Herein, three crystallographic surface orientations of $(001)$, $(011)$, and $(111)$ are considered and compared. The indentation stress–strain curves are successfully obtained from the load–displacement curves of nanoindentation. The stress of $(001)$, $(011)$, and $(111)$ orientations drops at the strains of 0.140, 0.133, and 0.136, which corresponds to the yield stress of 3.83, 4.33, and 4.99 GPa, respectively. Dislocation slip and twinning simultaneously form in silver as indicated by the total potential energy of the system. Furthermore, the typical four-, two-, and sixfold symmetries of the out-of-plane displacement as in copper are not observed for $(001)$, $(011)$, and $(111)$ orientations in silver. Hence, this observation can be supported by the simultaneous occurrence of dislocation slip and twinning in silver.     

Nanoindentation response of interphase precipitation in a Nb microalloyed steel

ABSTRACT

In this work, dedicated isothermal transformation studies were performed in a Fe–C–Mn–Nb alloy to obtain two types of precipitate morphology embedded in a similar ferritic matrix. Nano-hardness of individual ferrite grain was measured by using an in situ nano-mechanical tester. The peak of nano-hardness distribution from interphase and dispersed precipitate strengthened ferrite corresponds to the nano-hardness range of 2.6–2.8 and 2.5–2.6?GPa, respectively. A wider spread of nano-hardness from interphase precipitate strengthened ferrite was observed and rationalised by considering the variation in inter-particle spacing on slip planes. The ‘pop-ins’ on the load-depth curve was not able to be correlated with the morphology of a precipitate.     

粘弹性介质中球形空腔嵌入层的共振特性研究

潜艇吸声覆盖层在低频对抗主动声纳的探测是目前面临的一个重要技术难题，利用空腔共振时的吸收是解决低频吸声问题的一种有效途径，文中在假设入射球形纵波完全被腔壁反射的情形下，对球形空腔嵌入层的共振特性进行了理论分析，结果表明，在粘弹性介质中嵌入适当厚度的低剪切波速包覆层能有效地降低空腔共振的频率。