316L不锈钢不同取向压痕应变率敏感性研究

本研究探讨了不同取向对激光选区熔化(SLM)成形316L不锈钢力学性能的影响．使用纳米压痕测试系统对SLM成形316L不锈钢扫描平面方向和叠加成形方向两个取向的试样进行测试,并通过相关计算得出了材料在这两个方向的弹性模量、硬度、应变率敏感性指数等力学参数．实验结果表明,扫描平面方向的弹性模量和叠加成形方向的弹性模量基本相同;在相同的压入应变率下,扫描平面方向的硬度大于叠加成形方向的硬度;而对于不同的压入应变率,在高应变率下的压痕硬度相对较大;随着压入深度的增加,硬度逐渐减小并趋于稳定值．此外,本研究分析了不同压入应变率下硬度和应变率敏感性指数m的压痕尺寸效应,并得到与尺寸无关的硬度和应变率敏感性指数m．最后,通过引入扫描平面方向准静态宏观压缩工况下的应变率敏感性指数m加以验证本研究的合理性,揭示了SLM成形316L不锈钢在不同取向上应变率敏感性的差异,进而为研究准静态宏观压缩下叠加成形方向的应变率敏感性提供了支撑．     

泡沫材料的应变率效应

对泡沫材料的应变率敏感性进行了系统深入的讨论,认定这种材料是应变率敏感材料,这种敏感性主要是由于泡孔的变形特性产生的。泡沫材料变形的局部化、微观惯性和致密性导致压垮应力明显提高,基体的应变率效应及泡孔的形状大小并不能对泡沫材料应变率敏感性起主导作用。     

宽应变率范围下2A16-T4铝合金动态力学性能

为了研究2A16-T4铝合金的动态力学性能,利用电子万能试验机、高速液压伺服试验机及霍普金森压杆(SHPB)装置进行常温下准静态、中应变率和高应变率的动态力学性能实验,得到不同应变率下的应力应变曲线,基于修正的Johnson-Cook本构模型对它进行拟合,并分析材料中应变率力学特性对模型应变率敏感参量的影响。结果表明:2A16-T4铝合金在应变率10-4~102 s-1范围内应变率敏感性较弱,而在102~103 s-1范围内应变率敏感性较强,且应变率强化效应随塑性应变的增大而减小;同时,在10-4~103 s-1范围内具有较强的应变硬化效应,且应变硬化效应随应变率的增大而减小;此外,修正Johnson-Cook本构模型的拟合结果与实验结果吻合很好,能够很好表征材料的动态力学行为,且考虑材料中应变率力学特性可提高本构模型参量的准确性。     

SHPB测试中的均匀性问题及恒应变率

利用一维应力波理论对霍普金森压杆(SHPB)测试中的均匀性问题作了较为详尽的讨论,对测试中各种加载波形的优缺点及各参数对均匀性的影响进行了分析与评估。给出了测试脆性材料时实现恒应变率加载的加载条件。对在满足应力均匀性要求下SHPB的可测应变率范围作了讨论并修正了前人不完善的结论。讨论了考虑均匀性时应采用的测试数据处理方法。利用图解的方法对弹塑性材料测试时的均匀性问题及相应加载要求作了定性分析,指出对弹塑性材料,测试中的应变不均匀也应予以考虑。     

不同应变率下泡沫铝的形变和力学性能

对低密度泡沫铝在不同变形率下的形变和力学性能进行了系统的试验研究。结果表明:（1）沿剪切方向骨架首先塌陷,即变形的局部化是低应变率下块体泡沫铝的主要变形特征;（2）在不同应变率下泡沫铝表现出体积应变基本上随工程应变呈线性变化,在低应变率下泊松比随轴向应变呈幂次关系增加,但在高应变率下泊松比随塑性应变增加,从一峰值降低并趋于稳定;（3）低应变率下泡沫铝材料塑性变形均匀,而高应变率下剪切变形较大;（4）泡沫铝材料的强度对应变率不很明显,但随塑性应变增加,它的率敏感性增加。     

微纳米晶金属的应变率敏感性及应变硬化行为分析

基于亚微米、纳米晶粒组织塑性变形过程中多种变形机制(位错机制、扩散机制及晶界滑动机制)共存,建立了理论模型,用于定量研究亚微米、纳米晶粒组织的塑性变形行为.以铜为模型材料,计算分析了晶粒尺度、应变率以及温度对亚微米、纳米晶粒组织塑性变形行为的影响.结果表明:相比粗晶铜,亚微米晶铜表现出明显的应变率敏感性,并且应变率敏感系数随晶粒尺度及变形速率的减小而增大;同时,增大变形速率或降低变形温度都能提高材料的应变硬化能力,延缓颈缩发生,进而提高材料的延性.计算分析结果与实验报道吻合.     

描述大应变率范围下材料响应的粘塑性本构模型

以位错动力学理论中的Ｏｒｗａｎ和Ｇｉｌｍａｎ关系为基础建立描述率相关材料非弹性响应的基本方程，选择材料准静态实验的单轴响应作为强化演化的规律，并考虑应变率敏感程度随变形产生变化的特性，建立了适用于大应变率范围内率相关材料的统一型粘塑性本构模型。对铝１１００－０在应变率范围１０－５～１０４ｓ－１内产生的有限塑性应变的单轴响应进行了理论预测，与Ｋｈａｎ和Ｈｕａｎｇ［１］的实验数据及模型预测结果进行了比较，结果表明本文模型具有较高的预测精度，在高应变率和较大应变下不容忽视率敏感参数随变形的变化。     

有限塑性应变与应变率及其在晶体塑性中的表示

首先对变形梯度的塑性乘积分解的唯一性问题进行了分析,结果表明在放松了的或中间构形上所定义的应变对应着唯一的乘积分解,即Ｌｅｅ分解,尔后分析研究了该类型的应变及应变率,建立了客观塑性变率与变形率之间的关系,最后在不同构形中给出了塑性应变在晶体塑性中的表示,建立了塑性滑移率与应变及应变率之间的关系。     

微结构对多孔材料应变率效应影响的机理研究

通过数值模拟讨论了圆环结构对多孔材料应变率效应的影响,计算结果表明,圆环结构与折板结构一样,同样存在结构的屈曲失稳,因此也具有应变率效应。只是其应变率效应不如折板结构敏感,在小应变下的率效应不明显,在后屈曲失稳阶段率效应较为明显。另外,对于壁厚较薄的圆环结构,加载过程中更容易出现结构的屈曲失稳,此时微结构所引起的应变率效应较明显。壁厚较厚的圆环结构,微结构引起的应变率效应则不显著。通过蜂窝结构的模型更清楚地揭示了微结构对多孔材料变形过程的影响。     

应变率历史对记忆合金在高应变率拉伸下力学行为影响的实验研究

利用分离式拉伸霍布金森杆（ＳＨＳＢ）装置考察了高应变率拉伸作用下形状记忆合金的力学行为，并研究了高应变率历史对高应变率拉伸作用下力学行为的影响．研究表明，记忆合金是一种对应变率非常敏感的材料，与准静态载荷作用下应力应变关系相比，高应变率使屈服应力提高，并随着所经历的应变率水平的升高，同一高应变率下屈服强度明显增加．     

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.     

Indentation of a hard film on a soft substrate: Strain gradient hardening effects

The strain gradient work hardening is important in micro-indentation of bulk metals and thin metallic films, though the indentation of thin films may display very different behavior from that of bulk metals. We use the conventional theory of mechanism-based strain gradient plasticity (CMSG) to study the indentation of a hard tungsten film on soft aluminum substrate, and find good agreement with experiments. The effect of friction stress (intrinsic lattice resistance), which is important in body-center-cubic tungsten, is accounted for. We also extend CMSG to a finite deformation theory since the indentation depth in experiments can be as large as the film thickness. Contrary to indentation of bulk metals or soft metallic films on hard substrate, the micro-indentation hardness of a hard tungsten film on soft aluminum substrate decreases monotonically with the increasing depth of indentation, and it never approaches a constant (macroscopic hardness). It is also shown that the strain gradient effect in the soft aluminum substrate is insignificant, but that in the hard tungsten thin film is important in shallow indentation. The strain gradient effect in tungsten, however, disappears rapidly as the indentation depth increases because the intrinsic material length in tungsten is rather small.     

基于微纳米压入法提取无铅焊料合金弹塑性力学参数的研究

本文通过微纳米压入法结合数值模拟研究了无铅焊料合金SnAg3.5 的弹塑性力学性能，分别采用圆柱形压头及两种不同锥角压头对无铅焊料合金进行压入测试：基于圆柱形压头测试过程中接触面积恒定的特点得到了无铅焊料的弹性模量，进一步采用塑性应变梯度理论对两种锥角压头的测试结果予以修正并通过数值模拟反分析得到相应的特征应力值，同时基于压入特征塑性应变与压头锥角的关系式得到两种不同锥角压头下的特征应变值，在此基础上经求解方程组得到焊料合金的初始屈服应力与应变强化因子，进而得到了焊料合金的幂强化弹塑性本构关系．该方法剔除了压入尺度效应的影响并保证了所得本构关系的唯一性，给出了一种通过原位压入测试表征金属材料弹塑性力学性能的有效方法．     

An expanding cavity model incorporating strain-hardening and indentation size effects

An expanding cavity model (ECM) for determining indentation hardness of elastic strain-hardening plastic materials is developed. The derivation is based on a strain gradient plasticity solution for an internally pressurized thick-walled spherical shell of an elastic power-law hardening material. Closed-form formulas are provided for both conical and spherical indentations. The indentation radius enters these formulas with its own dimensional identity, unlike that in classical plasticity based ECMs where indentation geometrical parameters appear only in non-dimensional forms. As a result, the newly developed ECM can capture the indentation size effect. The formulas explicitly show that indentation hardness depends on Young’s modulus, yield stress, strain-hardening exponent and strain gradient coefficient of the indented material as well as on the geometry of the indenter. The new model reduces to existing classical plasticity based ECMs (including Johnson’s ECM for elastic–perfectly plastic materials) when the strain gradient effect is not considered. The numerical results obtained using the newly developed model reveal that the hardness is indeed indentation size dependent when the indentation radius is very small: the smaller the indentation, the larger the hardness. Also, the indentation hardness is seen to increase with the Young’s modulus and strain-hardening level of the indented material for both conical and spherical indentations. The strain-hardening effect on the hardness is observed to be significant for materials having strong strain-hardening characteristics. In addition, it is found that the indentation hardness increases with decreasing cone angle of the conical indenter or decreasing radius of the spherical indenter. These trends agree with existing experimental observations and model predictions.     

Comparison between Berkovich,Vickers and conical indentation tests: A three-dimensional numerical simulation study

Three-dimensional numerical simulations of Berkovich, Vickers and conical indenter hardness tests were carried out to investigate the influence of indenter geometry on indentation test results of bulk and composite film/substrate materials. The strain distributions obtained from the three indenters tested were studied, in order to clarify the differences in the load–indentation depth curves and hardness values of both types of materials. For bulk materials, the differentiation between the results obtained with the three indenters is material sensitive. The indenter geometry shape factor, β, for evaluating Young’s modulus for each indenter, was also estimated. Depending on the indenter geometry, distinct mechanical behaviours are observed for composite materials, which are related to the size of the indentation region in the film. The indentation depth at which the substrate starts to deform plastically is sensitive to indenter geometry.     

基于微纳米压入法的高熵合金柏氏矢量研究

采用微纳米压入法对CoCrFeNiMn高熵合金进行多种应变率下的压入测试研究,实验获得了材料硬度与压深之间的关系并通过计算分析得到了其不同工况下的柏氏矢量值,探究了压入深度和应变率对柏氏矢量的影响．实验结果表明,所测材料柏氏矢量值在一定范围内呈现出一定的波动性,随着压深的增大,柏氏矢量表现出尺寸效应,即柏氏矢量随压深呈增加趋势;并且在同等压深下,柏氏矢量存在率效应,随着应变率的增加,柏氏矢量值先减小后增加,柏氏矢量从滑移主导向原子失配主导的转变是其率效应转变的主要原因．     

Influence of Specimen Geometry on Split Hopkinson Tensile Bar Tests on Sheet Materials

In recent years numerous studies on the high strain rate behaviour of sheet materials using split Hopkinson tensile bar set-ups have been reported in literature. For these experiments mostly dogbone-shaped specimens are used. However, widely divergent specimen dimensions can be found. In the present study the influence of this specimen geometry on the test results is investigated experimentally. An extensive series of Hopkinson tests on a steel sheet material using different specimen geometries is performed. An advanced optical technique is used to obtain the true distribution of the deformation along the length of the specimen. Important issues such as the contribution of the deformation of the transition zones to the total deformation and the (non-)homogeneity of the strain in the specimen are thus determined. From the experiments it is clear that the influence of the specimen geometry on the observed behaviour cannot be neglected. It is shown that inconsistencies between the assumed and real specimen behaviour account for these differences. For the TRIP steel considered in the study, accurate deformation values are only guaranteed if the length to width ratio of the central zone is larger than 1.25 and if the radius of the transition zone is sufficiently small.     

Wedge indentation of thin films modelled by strain gradient plasticity

A plane strain study of wedge indentation of a thin film on a substrate is performed. The film is modelled with the strain gradient plasticity theory by Gudmundson [Gudmundson, P., 2004. A unified treatment of strain gradient plasticity. Journal of the Mechanics and Physics of Solids 52, 1379–1406] and analysed using finite element simulations. Several trends that have been experimentally observed elsewhere are captured in the predictions of the mechanical behaviour of the thin film. Such trends include increased hardness at shallow depths due to gradient effects as well as increased hardness at larger depths due to the influence of the substrate. In between, a plateau is found which is observed to scale linearly with the material length scale parameter. It is shown that the degree of hardening of the material has a strong influence on the substrate effect, where a high hardening modulus gives a larger impact on this effect. Furthermore, pile-up deformation dominated by plasticity at small values of the internal length scale parameter is turned into sink-in deformation where plasticity is suppressed for larger values of the length scale parameter. Finally, it is demonstrated that the effect of substrate compliance has a significant effect on the hardness predictions if the effective stiffness of the substrate is of the same order as the stiffness of the film.     

高速冲击表面处理对金属材料力学性能和组织结构的影响

高速冲击表面处理过程中的应变率对金属材料的宏观力学性能和微观组织结构都具有重要影响。根据当前应变率效应的研究成果,从宏观与微观相结合的角度出发,综述了高速冲击表面处理过程中应变率对金属材料强度和塑性的影响规律,并重点阐述了不同应变率下金属材料内部微观组织结构的演变规律,主要包括晶粒结构、绝热剪切带、相变、位错组态和析出相以及变形孪晶等。此外,还分析了组织结构随应变率的演化和微观变形机制的转变对材料力学性能的强化和弱化机理。最后,对高速冲击表面处理梯度组织的变形特点进行了总结。提出了不同组织结构对材料性能影响的综合效应模型,以期为应变率效应的深入研究奠定基础。