水化硅酸钙纳米压痕分子动力学模拟方法优化

针对水化硅酸钙纳米压痕模型忽视了压头与基底之间相互作用的问题，由尺寸差异引起的金刚石压头难以计算的问题，以及Wittmann模型无法得到实际接触面积的问题，提出了新的模型与计算方法．结合分子动力学方法，采用金刚石压头-Wittmann模型基底的组合方式构建无定形态水化硅酸钙纳米压痕试验模型．在建模阶段，考虑到压头模型与基底模型粒子间尺寸差异，提出了等比例替换模型，通过公式推导并就不同尺寸模拟结果验证了等比例替换模型的可行性．在计算阶段，提出了局部前处理的弛豫方法进行模拟．确定最大荷载位置处的接触面积为546 nm2，进而求出水化硅酸钙模型硬度H为0.84 GPa、折合模量Er为30.52 GPa．并通过纳米压痕试验，验证了模拟结果的准确性，证明了模型的科学性，对今后水化硅酸钙(C-S-H)纳米层面的模拟具有重要借鉴意义．     

多孔氧化铝薄膜横观各向同性弹性性质的研究

电化学阳极氧化生成的氧化铝薄膜含有高度有序的纳米孔阵列,本文首先假设氧化铝薄膜基体(无孔部分)为各向同性,结合其周期性孔结构特点和均匀化理论,可以得到氧化铝基体和薄膜弹性性质之间的关系。然后利用单轴拉伸结合电子散斑干涉(ESPI)的方法得到薄膜面内的杨氏模量为63.4GPa,并根据均匀化方法得到的基体与薄膜弹性性质的关系进一步推出薄膜横观各向同性的其它弹性参数,如基体杨氏模量等。为证明结果的可靠性,利用推出的弹性参数建立三维有限元模型,模拟纳米压痕实验,得到的加卸载曲线与实验曲线相吻合。     

灵芝孢子孢壁的弹性模量和硬度研究

采用纳米压痕技术和数值模拟研究灵芝孢子孢壁的弹性模量和硬度.利用原位纳米力学测试与分析系统,测试灵芝孢子孢壁的弹性模量和硬度.得到了载荷--位移曲线图和硬度、弹性模量随压痕深度变化的值.并用有限元方法模拟压痕过程,利用ANSYS软件,按照灵芝孢子孢壁和Berkovich压头的结构,建立了二维计算模型,得到纳米压痕的等效应力分布以及压痕过程中加载和卸载时的载荷--位移曲线.考察了摩擦、压头尖端半径对模拟结果的影响.结果显示:灵芝孢子孢壁的平均弹性模量为2.0GPa,硬度为0.13GPa.模拟结果在趋势上与实验结果有较好的吻合,与理论分析的载荷--位移关系基本一致.摩擦、压头尖端半径小于100nm时对模拟结果不会造成明显影响.研究结果为分析孢子的破壁机理提供必要参数.     

有机玻璃中球形应力波传播的分析

基于球形发散波实验技术及圆环型电磁粒子速度测试技术,采用0.125 g TNT当量的微型炸药作为爆炸源,对填实爆炸下有机玻璃中球形波的传播规律进行了实验研究,并基于粒子速度波形进行了分析。结果表明:粒子速度峰值及粒子位移峰值符合指数衰减规律,粒子速度、位移峰值的衰减指数分别为1.34和1.28;负向粒子速度峰值随比距离的增加有先增大后减小的趋势;基于强间断假设得到的低压(小于1 GPa)下径向压力峰值-粒子速度峰值关系与一维应变下得到的σ-v Hugoniot曲线吻合较好;采用变模量模型假设,结合粒子速度数据反演的有机玻璃弹性模量E=(6.40±0.64)GPa、体积模量K=(7.12±0.71)GPa、剪切模量G=(2.37±0.24)GPa。     

含能单晶微纳米力学性能试验研究及数值表征

利用微纳米压痕实验测定β-HMX 单晶(010) 晶面和α-RDX 单晶(210) 晶面的力学性能参数和微观破坏特征,并利用数值拟合确定了含能单晶的部分本构参数. 通过微纳米压痕实验连续刚度法(CSM) 得到HMX 单晶和RDX 单晶的弹性模量和硬度,RDX 单晶的硬度和模量都大于HMX 单晶,其硬度值均表现出一定的尺寸效应. 利用原子力显微镜(AFM) 分析了HMX 单晶和RDX 单晶的微观破坏机理,裂纹随着载荷的增大生成并扩展,裂纹面产生方向为晶体的最易解理破坏方向. 利用ABAQUS 有限元软件进行了纳米压痕数值模拟,结合微纳米压痕实验加卸载曲线,选取了合适的含能单晶塑性损伤本构模型的损伤本构参数.      

Cosserat连续体模型中本构参数对应变局部化模拟结果影响的数值分析

基于所发展的压力相关弹塑性Cosserat连续体模型及相应的数值方法,以一维剪切层及二维平板压缩问题为例,数值分析了Cosserat连续体模型中的本构参数Cosserat剪模、软化模量及内部长度参数对应变局部化数值模拟结果的影响.结果表明在一定取值范围内,Cosserat剪模对数值模拟结果几乎没有影响,并给出了具体数值计算时的取值范围;软化模量绝对值越大,后破坏段的荷载-位移曲线越陡,计算得到的剪切带宽度越窄;内部长度参数越大,后破坏段的荷载-位移曲线越平缓,计算得到的剪切带越宽.     

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。     

纳米压痕过程的三维有限元数值试验研究

采用有限元方法模拟了纳米压痕仪的加、卸载过程，三维有限元模型考虑了纳米压痕仪的标准Berkovich压头．介绍了有限元模型的几何参数、边界条件、材料特性与加载方式，讨论了摩擦、滑动机制、试件模型的大小对计算结果的影响，进行了计算结果与标准试样实验结果的比较，证实了模拟的可靠性．在此基础上，重点研究了压头尖端曲率半径对纳米压痕实验数据的影响．对比分析了尖端曲率半径r＝0与r＝100nm两种压头的材料压痕载荷—位移曲线．结果表明，当压头尖端曲率半径r≠0时，基于经典的均匀连续介质力学本构理论、传统的实验手段与数据处理方法，压痕硬度值会随着压痕深度的减小而升高．     

考虑压头曲率半径和应变梯度的微压痕分析

在压头尖端曲率半径取100nm的前提下，采用Chen和Wang的应变梯度理论，对微压痕实验进行了系统的数值分析. 首先通过拟合载荷-位移实验曲线的后半段来确定材料的屈服应力和幂硬化指数值,然后用有限元方法数值模拟压痕实验，并将计算得到的整段载荷-位移曲线及硬度-位移曲线和实验结果进行了比较. 结果表明应变梯度理论所预测的计算结果和实验结果很好地符合,包括压痕深度在亚微米和微米范围内的整段曲线.     

仪器柔度标定误差对两种压入测试方法的精度影响

针对目前最具代表性的两种压入测试方法:Oliver-Pharr方法和Ma方法,通过有限元数值模拟分析了仪器柔度标定误差对测试精度的影响.结果表明仪器柔度的标定精度直接影响压入测试结果的准确度,尤其是当材料较软且压入深度较大时更为显著;对同一材料,压入载荷越大,由仪器柔度标定误差引入的压入测试结果误差越大;在同一压入深度下,针对不同材料,由仪器柔度标定误差引入的压入测试结果误差差别不大;就测试方法而言,Ma方法具有比Oliver-Pharr方法更高的精度和更低的仪器柔度敏感性.     

基于粗粒化分子动力学的自由水与水化硅酸钙孔隙水冻结模拟

水化硅酸钙是水泥基材料的主要水化产物,其孔隙内的水分是影响水泥基材料抗冻性的主要因素。本文基于粗粒化分子动力学方法研究水化硅酸钙孔隙水的冻结机制,针对水的粗粒化P₄粒子和水化硅酸钙胶体颗粒,建立了水化硅酸钙孔隙水的冻结模型。根据此模型计算了不同孔径孔隙水冰点,分析了水泥基材料孔径孔隙在冻融破坏中的危害程度;模拟得到了水化硅酸钙孔隙内水的冻结分布特征和密度分布特征。研究工作表明,本文建立的模型有效提高了分子动力学模拟水化硅酸钙孔隙水冻结问题的规模,为后续进行水泥基材料的冻融破坏分析提供了研究基础。     

Evaluation of the stress–strain curve of metallic materials by spherical indentation

A method for deducing the stress–strain uniaxial properties of metallic materials from instrumented spherical indentation is presented along with an experimental verification.An extensive finite element parametric analysis of the spherical indentation was performed in order to generate a database of load vs. depth of penetration curves for classes of materials selected in order to represent the metals commonly employed in structural applications. The stress–strain curves of the materials were represented with three parameters: the Young modulus for the elastic regime, the stress of proportionality limit and the strain-hardening coefficient for the elastic–plastic regime.The indentation curves simulated by the finite element analyses were fitted in order to obtain a continuous function which can produce accurate load vs. depth curves for any combination of the constitutive elastic–plastic parameters. On the basis of this continuous function, an optimization algorithm was then employed to deduce the material elastic–plastic parameters and the related stress–strain curve when the measured load vs. depth curve is available by an instrumented spherical indentation test.The proposed method was verified by comparing the predicted stress–strain curves with those directly measured for several metallic alloys having different mechanical properties.This result confirms the possibility to deduce the complete stress–strain curve of a metal alloy with good accuracy by a properly conducted instrumented spherical indentation test and a suitable interpretation technique of the measured quantities.     

Balancing strength and toughness of calcium-silicate-hydrate via random nanovoids and particle inclusions: Atomistic modeling and statistical analysis

As the most widely used manufactured material on Earth, concrete poses serious societal and environmental concerns which call for innovative strategies to develop greener concrete with improved strength and toughness, properties that are exclusive in man-made materials. Herein, we focus on calcium silicate hydrate (C-S-H), the major binding phase of all Portland cement concretes, and study how engineering its nanovoids and portlandite particle inclusions can impart a balance of strength, toughness and stiffness. By performing an extensive +600 molecular dynamics simulations coupled with statistical analysis tools, our results provide new evidence of ductile fracture mechanisms in C-S-H – reminiscent of crystalline alloys and ductile metals – decoding the interplay between the crack growth, nanovoid/particle inclusions, and stoichiometry, which dictates the crystalline versus amorphous nature of the underlying matrix. We found that introduction of voids and portlandite particles can significantly increase toughness and ductility, specially in C-S-H with more amorphous matrices, mainly owing to competing mechanisms of crack deflection, voids coalescence, internal necking, accommodation, and geometry alteration of individual voids/particles, which together regulate toughness versus strength. Furthermore, utilizing a comprehensive global sensitivity analysis on random configuration-property relations, we show that the mean diameter of voids/particles is the most critical statistical parameter influencing the mechanical properties of C-S-H, irrespective of stoichiometry or crystalline or amorphous nature of the matrix. This study provides new fundamental insights, design guidelines, and de novo strategies to turn the brittle C-S-H into a ductile material, impacting modern engineering of strong and tough concrete infrastructures and potentially other complex brittle materials.     

A study on robust indentation techniques to evaluate elastic–plastic properties of metals

Spherical indentation is studied based on numerical analysis and experiment, to develop robust testing techniques to evaluate isotropic elastic–plastic material properties of metals. The representative stress and plastic strain concept is critically investigated via finite element analysis, and some conditions for the representative values are suggested. The representative values should also be a function of material properties, not only indenter angle for sharp indenter and indentation depth for spherical indenter. The pros and cons of shallow and deep spherical indentation techniques are also discussed. For an indentation depth of 20% of an indenter diameter, the relationships between normalized indentation parameters and load–depth data are characterized, and then numerical algorithm to estimate material elastic–plastic curve is presented. From the indentation load–depth curve, the new approach provides stress–strain curve and the values of elastic modulus, yield strength, and strain-hardening exponent with an average error of less than 5%. The method is confirmed to be valid for various elastic properties of indenter. Experimental validation of the approach then is performed by using developed micro-indentation system. For the material severely disobeying power law hardening, a modified method to reduce errors of predicted material properties is contrived. It is found that our method is robust enough to get ideal power law properties, and applicable to input of more complex physics.     

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。利用Meyer定律、弹塑性变形模型、Hays-Kendall模型和变形阻力模型对材料的压痕尺寸效应进行分析,结果表明实验材料受到正压痕尺寸效应的影响,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进行杨氏弹性模量E和屈服强度σy的分析。对于大多数实验材料,未修正的Marshall模型和Conway模型计算的杨氏弹性模量值偏大。Marshall模型中的参数α随着压痕对角线比值b′/d的增大而线性减小,Conway模型中的修正系数β随着压痕短对角线比值的平方(b′/b)2线性增加;对两模型进行修正时α和β可以分别用b′/d与(b′/b)2线性表示;同时发现金属玻璃的努氏硬度和杨氏弹性模量成正比,比例系数为0.0445。分别利用Tabor、Lockett、Yu、Marsh、Johnson和Vandeperre模型计算屈服强度时,除Johnson模型的计算值接近实际值外其它模型计算结果偏低,为了不同的模型能够得到正确的屈服强度,需要修正努氏硬度HK与名义硬度H之间的比例关系。当大块金属玻璃的努氏硬度小于6 GPa、玻璃态转变温度小于800 K时,缺口韧性KQ分别随着努氏硬度HK和玻璃态转变温度Tg线性增加。     

An indentation technique for estimating the energy density as fracture toughness with Berkovich indenter for ductile bulk materials

A technique is proposed to estimate the energy density as fracture toughness for ductile bulk materials with an indentation system equipped with a Berkovich indenter based on the theory of plastic deformation energy transforming into the indentation energy of fracture. With progressive increase of penetration loads, the material damage is exhibited on the effective elastic modulus. A quadratic polynomial relationship between the plastic penetration depth and penetration load, and an approximate linear relationship between logarithmic plastic penetration depth and logarithmic effective elastic modulus are exhibited by indentation investigation with Berkovich indenter. The parameter of damage variable is proposed to determine the critical effective elastic modulus at the fracture point. And the strain energy density factor is calculated according to the equations of penetration load, plastic penetration depth and effective elastic modulus. The fracture toughness of aluminum alloy and stainless steel are evaluated by both indentation tests and K_IC fracture toughness tests. The predicted S_cr values of indentation tests are in good agreement with experimental results of CT tests.     

Effect of Varying Test Parameters on Elastic–plastic Properties Extracted by Nanoindentation Tests

A systematic experiment was performed in an effort to investigate how the levels of certain test parameters affect the values of elastic modulus, hardness, yield stress, and strain hardening constant obtained using nanoindentation test. Maximum applied load, loading (unloading) rate, and hold time at maximum load were varied at three levels. The effects of these testing parameters were investigated through a three-level, full factorial design of experiment. The experiments were conducted on ultrafine Al-Mg specimens that were mechanically extruded. Both longitudinal and transverse extrusion directions were examined to investigate effects of anisotropy on mechanical properties and evaluate the persistence of observed variations due to test parameters on different materials orientations. An indentation size effect (ISE) was observed demonstrating that maximum load—and thereby maximum indentation depth—can have a significant effect on values of hardness and elastic modulus. Hardness values decreased with higher loading rates, and higher rates of unloading resulted in higher values of elastic modulus (5–10 GPa increases). Strain-hardening exponent showed a decreasing trend with increasing loading rate while yield stress exhibited a consistent correlation to hardness across all studied parameters. The material exhibited very little creep during the hold period, and values of the calculated properties were not significantly altered by varying the length of the hold time. Anisotropy effect was observed, particularly in the values of yield strength. This is attributed to the preferred grain orientation due to extrusion.     

Analysis of elastic and plastic deformation associated with indentation testing of thin films on substrates

A study has been made of the elastic and plastic deformation associated with submicrometer indentation of thin films on substrates using the finite element method. The effects of the elastic and plastic properties of both the film and substrate on the hardness of the film/substrate composite are studied by determining the average pressure under the indenter as a function of the indentation depth. Calculations have been made for film/substrate combinations for which the substrate is either harder or softer than the film and for combinations for which the substrate is either stiffer or more compliant than the film. It is found, as expected, that the hardness increases with indentation depth when either the yield strength or the elastic modulus of the substrate is higher than that of the film. Correspondingly, the hardness decreases with indentation depth when the yield strength or elastic modulus of the substrate is lower than that of the film. Functional equations have been developed to predict the hardness variation with depth under these different conditions. Finite element simulation of the unloading portion of the load displacement curve permits a determination of the elastic compliance of the film/substrate composite as a function of indentation depth. The elastic properties of the film can be separated from those of the substrate using this information. The results are in good agreement with King's analytical treatment of this problem.