夹芯板结构局部压入力学响应研究

利用材料试验机(MTS)实验研究了复合材料面板、闭孔泡沫铝芯层夹芯板结构在准静态压入时的变形和破坏特征。实验结果表明,夹芯板的破坏主要集中在压头作用的局部区域内;同时,根据最小势能原理建立了泡沫铝夹芯板在半球形压头作用下的压入力学响应理论预测模型。通过引入无量纲参数分析了夹芯板压入载荷-位移响应,并在不同面板厚度、芯层厚度和芯层相对密度情况下,对夹芯板压入响应理论解的有效性和适用性进行了讨论。     

平头压痕试验确定热障涂层弹性模量的方法研究

本文模拟了典型热障涂层(TBC)结构在平头压痕作用下的力学响应,重点研究利用平头压痕法确定TBC涂层弹性模量的方法。研究发现k(单位压入应力下压头的压入位移)不仅与材料的性质有关,还与压头的半径有关。本文提出:通过两个不同尺寸的平压头可以同时确定陶瓷层和粘结层的弹性模量。     

开孔和闭孔泡沫铝的力学与吸能特性研究

研究泡沫铝的力学与缓冲吸能特性有助于其在武器、航天与航空等领域的广泛应用。通过对开孔和闭孔泡沫铝的实验研究发现:开孔和闭孔泡沫铝的应力-应变曲线都明显呈现出线弹性变形、塑性屈服(平台段)和致密化3个阶段;闭孔泡沫铝较开孔泡沫铝的平台段更加明显;闭孔泡沫铝的吸能效率较开孔泡沫铝的高,比开孔泡沫铝更适合作缓冲元件。     

纳米压痕试验中压头尺寸效应的准连续介质法分析

采用准连续介质法模拟了单晶铝纳米压痕试验过程,分析了不同宽度的刚性矩形压头所引起的初始塑性变形特点,获得了载荷-压深、应变能-位移和硬度-压深曲线.从位错理论的角度分析了压头尺寸对纳米压痕测试结果的影响.研究发现:随着压头宽度的不断增大,压头下方位错形核所需要的载荷和压深程度增大,需要的应变能增加,应变能的变化速率递增,纳米硬度值减小,呈现出明显的尺寸效应.同时表明在一定的压入深度下,硬度与压头尺寸之间存在着一定的比例关系,不同尺寸压头获得的硬度值可以相互换算,但当矩形刚性压头宽度大于或等于120时这种尺寸效应消失.研究结果为纳米压痕实验过程中压头尺寸的选择提供了参考依据.     

闭孔泡沫铝的塑性泊松比

在表征闭孔泡沫铝的力学性能中,塑性泊松比是较为重要的参数之一。本文应用Kelvin十四面体模型构建出不同相对密度的闭孔泡沫铝三维细观模型并采用LS-DYNA对所得细观模型进行单轴准静态压缩计算。数值模拟分析发现,随着轴向应变的增加,泡沫铝泊松比-轴向应变曲线呈倒S形,存在峰值和极小值,曲线变化规律与泡沫铝胞孔的变形有密切关系。根据泊松比-轴向应变曲线与胞孔变形之间的关系,给出了平均塑性泊松比的定义。计算结果显示,随着相对密度的提高,闭孔泡沫铝的平均塑性泊松比增大。当闭孔泡沫铝的相对密度低于0.1时,其平均塑性泊松比接近于零,计算中可以忽略;当闭孔泡沫铝相对密 度大于0.1时,其平均塑性泊松比随相对密度的增加而呈线性从0.17增加到0.5     

闭孔泡沫铝低速冲击防护的临界条件与优化设计

建立了高孔隙率闭孔泡沫铝抗低速撞击的分析模型,通过落重冲击试验验证了模型预测的准确性;采用所建立的模型,计算了闭孔泡沫铝作为大质量结构抗低速冲击构件的临界冲击速度,研究了不同冲击条件下结构响应的最小加速度和临界加速度。结果表明,闭孔泡沫铝适合作为大质量结构的低速冲击防护材料：当撞击速度低于临界冲击速度时,泡沫铝的作用应力不会超过其平台压缩应力,具有高孔隙率的泡沫铝甚至可使冲击响应加速度大幅降低,具有优良的防护效果;当撞击速度超过相应条件下的临界速度时,由于泡沫铝压缩密实阶段的应力增强作用,不仅使其作用应力迅速增大（为平台应力的5～15倍）,而且使冲击响应加速度迅速增加甚至超过1000g,从而对结构的安全防护构成威胁。最后,讨论了冲击质量比、泡沫铝孔隙率、泡沫几何尺寸等冲击参数对临界冲击速度和冲击响应加速度的影响。     

摩擦因素对不同形状压头微压痕测试的影响

本文基于低阶应变梯度塑性有限元,分析了摩擦因素在不同形状压头的微压痕测试中的影响.结果表明对于目前普遍采用的Berkovich 压头(简化为半后顶角为70.3°并具有一定球头半径的圆锥),摩擦的影响并不重要,可以简化为无摩擦情况.而对于Cube-corner压头(简化为圆锥半后顶角为 42.3°),摩擦不可以忽略.同样压深情况下,相对于较钝压头,尖锐压头下材料变形更加激烈,几何必需位错密度更大,同时受摩擦因素影响更加明显.模拟结果还显示,摩擦对卸载曲线的影响可以忽略不计.     

数字图像相关方法在闭孔泡沫铝压缩试验中的应用

为了了解相对密度与胞孔结构对闭孔泡沫铝力学性能的影响,本文采用放大成像及数字图像相关技术对两种不同密度的泡沫纯铝试样进行了实验研究.利用数字图像相关方法对泡沫纯铝变形前后的图像进行相关计算,获得了弹性范围内静态压缩情况下闭孔泡沫铝材料表面的全场变形及局部孔结构的变形,同时根据试验结果计算了试件的名义弹性模量.实验结果表明泡沫铝整体孔结构的变形与泡沫金属材料相对密度有关,而单个孔结构的变形主要与孔壁面光滑程度和皱褶有关.实验结果还表明图像相关方法能够有效地应用于闭孔泡沫金属的力学性测量和评估的研究.     

泡沫铝铜合金静态压缩力学行为和吸能性能的实验研究

通过静态压缩实验研究一种新型闭孔泡沫铝材料泡沫铝铜合金的静态压缩力学行为和吸能性能。对实验得到的一定密度范围内的材料的弹性模量E和平台应力pl关于相对密度进行曲线拟合,发现将Gibson等给出的开孔泡沫理论公式中的幂次常数修正为由实验确定的常数,可以给出一定密度范围的闭孔泡沫铝铜合金材料的弹性模量和平台应力与相对密度之间关系的一个较好的估计。采用比能-应力或比能-应变曲线,可以对不同密度的泡沫铝铜合金材料的吸能情况进行较为直观的比较和分析。该曲线对工程设计具有较好的指导意义。     

泡沫铝填充薄壁圆管的三点弯曲实验的数值模拟

泡沫金属填充薄壁结构的应用日趋广泛,建立合理的数值计算模型对结构设计和工程应用非常重要.该文通过对泡沫铝填充薄壁铝合金圆管的三点弯曲实验的数值模拟,研究了它的力学行为.采用ABAQUS软件,建立了空管和泡沫铝全填充管的有限元模型,并对这两种结构在三点弯曲下的力学行为进行了数值模拟,所得结果与实验结果符合得较好.通过数值模拟分析了结构的承载机理和不同压头直径对结构承载能力的影响.此外,还研究了泡沫铝部分填充圆管的三点弯曲行为,分析了不同填充长度对结构承载能力的影响.     

泡沫金属压痕试验的数值模拟及其反演

在理论研究的基础上，将泡 沫金属压痕试验的有限元数值模拟结果与用无量纲分析法构造出的一系列无量纲函数相结 合，建立了泡沫金属压痕试验中载荷-压痕深度关系曲线与泡沫金属的弹塑性材料参数之 间的联系. 利用这种联系，就可以实现用压痕试验通过反演分析来确定泡沫金属的材料参数. 研究结果表明，泡沫金属材料的杨氏模量，屈服强度及塑性可压缩因子等参数均可由其压痕 试验唯一的确定，但其塑性平台区终点应变的确定还需进一步的研究.     

Indentation into polymeric foams

The effects of the nose shape of rigid indenters on the indentation behaviour of polymethacrylimide (PMI) and polyetherimide (PEI) foams with different densities are investigated. Experimental results show that indentation resistance depends on the geometry of the indenter and the density of the foam. Analytical models based on the deformation mechanisms observed in experiments are developed to predict the indentation resistance. It shows that the analytical predictions are in good agreement with experimental measurements for a range of polymeric foams. This study presents a complete and systematic experimental data on the indentation behaviours of a range of polymeric foams and demonstrates the capability of the analytical model to predict the indentation behaviours of PMI and PEI foams.     

Determination of Asphalt Binder Creep Compliance Using Depth-Sensing Indentation

Asphalt binders are common construction materials, however due to time- and temperature- dependence, their mechanical properties are often difficult to characterize. Several standard tests methods exist to describe their complex behavior. This paper presents an exploratory feasibility study of a flat-tip indentation testing to analyze the linear viscoelastic properties of asphalt binders. Depth-sensing indentation testing has been extensively used to characterize the properties of many engineering materials, however the applications to asphalt binders are very limited. This paper presents a simple solution for the creep compliance in tension derived for flat-tipped indenter. This solution was verified with the Finite Element Analysis and then applied to the experimental results from the indentation testing performed on one typical unmodified asphalt binder. The testing was conducted at three different low temperatures and under three different creep load levels to verity the linearity of the response, and to evaluate the robustness and applicability of the indentation method. Furthermore, the creep compliance functions determined from the indentation testing were compared with a more traditional 3-point bending experiments. The results show that there is a non-uniform discrepancy between the two testing methods, most likely due to nonlinear behavior of the asphalt binder at higher temperatures and micro-damage of the binder samples at lower temperatures. Other possible sources of error between indentation and 3-point bending are problems determining the initial tip-specimen contact surface and possible tip-specimen adhesion. It is concluded that flat-tipped indentation at low temperatures should be performed at lower load levels to avoid excessive stress concentrations that leads to micro-damage and nonlinear response of asphalt binders. Alternatively, asphalt binders at low temperatures could be evaluated using different indenter geometries, such as spherical or pyramidal, using corresponding parameter interpretation procedures.     

ANALYSIS OF MECHANICAL PROPERTIES OF CEMENT CLINKER UNDER DIFFERENT TEMPERATURES BASED ON HERTZIAN INDENTATION METHOD1)

本文通过高温球压法得到各种脆性和准脆性材料的表面压痕应力与应变之间的关系曲线。通过压痕应力-应变曲线的分析既可比较方便地确定出材料的压痕弹性模量、剪切模量和布氏硬度,又可比较不同温度下水泥熟料的变形性能。在不同温度(25 ${^\circ}\!$C$\sim $1400 ${^\circ}\!$C)处理下,球压应力松弛试验载荷松弛,在载荷峰值为100 N时,随着温度的升高,水泥熟料载荷松弛更明显。随着温度从500 ${^\circ}\!$C升高到1400 ${^\circ}\!$C,载荷松弛非常明显,尤其温度高于1200 ${^\circ}\!$C,水泥熟料样品内部的硅酸三钙(Ca$_3$SiO$_5$, 简称C3S)分解以及有部分液相的出现引起的应力松弛现象最为明显,在1275 ${^\circ}\!$C时熟料基本上已经软化,载荷急速松弛,所以认为1275 ${^\circ}\!$C为熟料的脆延转化温度。通过水泥熟料高温球压松弛试验可以确定水泥熟料在二次加热过程中的脆-延转化温度,测定熟料弹性模量和抗压强度急剧变化的温度范围。研究水泥熟料在不同温度下的力学行为和力学特性,探索提高粉磨效率的新途径,实现高温下的低能耗粉碎。     

A four-part boundary value problem in elasticity — indentation by a discontinuously concave punch

A solution is given for the frictionless indentation of an elastic half-space by a flat-ended cylindrical punch with a central circular recess, when the load is large enough to establish a circular region of contact in the recess. The problem is reduced to two simultaneous Fredholm equations using the method of complex potentials due to Green and Collins. Results are presented for the relationship between load, contact radius and penetration for various punch geometries.     

Dislocation model of localized plastic deformation initiated with a flat punch

We consider a classical contact mechanics problem, namely, the indentation of a ductile half-plane by a rigid flat punch (in plane strain), and revisit it using the dislocation mechanics approach. The dislocation nucleation and dislocation interaction beneath the indenter are examined. The threshold load for dislocation nucleation and the dislocation emission angle are obtained in analytical form. Moreover, based on the consideration of dislocation interaction, we explore the mechanism of contact load evolution (hardening). A triangular “dead zone” beneath the indenter, which could not be thus far accurately explained by traditional continuum models, is predicted in good agreement with the results of careful experiments that are reported in the literature. The proposed model is likely to be useful for the analysis of contacts at both the micro- and macro-scales.     

Indentation responses of piezoelectric films ideally bonded to an elastic substrate

The influences of elastic substrate on the indentation force, contact radius, electric potential and electric charge responses of piezoelectric film/substrate systems are investigated by the integral transform method. The film is assumed to be ideally bonded to the substrate and the contact interaction between the indenter and the film is assumed to be frictionless, with three kinds of axisymmetric insulating and conducting indenters (i.e., punch, cone and sphere) considered. Obtained results show that when the ratio of the contact radius to the film thickness is close to zero, the influences of the elastic substrate disappear and the indentation behaviors converge to the piezoelectric half space solutions while the indentation responses approach the corresponding ones of elastic half space as the ratio gets to infinity. The transition between the piezoelectric and the elastic half space indentation solutions for the film/substrate system is quantified in terms of the film thickness and the elasticity of the substrate. Finite element analysis on an insulating sphere indentation is conducted to verify the numerical calculations and good agreement is observed. The obtained results are believed to be useful for developing experimental techniques to extract the material properties of piezoelectric film/substrate systems.     

Indentation responses of piezoelectric films

Frictionless indentation responses of transversely isotropic piezoelectric film/rigid substrate systems under circular cylindrical indenter (i.e., punch), conical indenter (i.e., cone), and spherical indenter (i.e., sphere) are investigated. Both insulating and conducting indenters are considered. The technique of Hankel transformation is employed to derive the corresponding dual integral equations for the mixed boundary value indentation problems. For the two limiting cases of infinitely thick and infinitely thin piezoelectric films, closed-form solutions are obtained. For piezoelectric films of finite thickness, a numerical method is constructed to solve the dual integral equations and semi-empirical models having only two unknown parameters are proposed for the responses of indentation force, electric charge and electric potential, and contact radius. With the two parameters inferred from the numerical results, the semi-empirical formulae are found to provide good estimates of the indentation responses for the two limiting cases of infinitely thick and thin piezoelectric films, as well as those in between. The inferred parameters in the proposed semi-empirical formulae for normalized indentation force and electric charge are checked against four different piezoelectric materials and are found to be insensitive to the selection of piezoelectric materials. It is believed that the proposed semi-empirical indentation formulae are useful in developing experimental indentation techniques to extract the material properties of piezoelectric films.     

Effects of Geometry and Dimension of Specimen and Rig on Small Punch Creep Property

The purpose of this study was to examine the effects of the geometries and dimensions of the rig and specimen of a small punch (SP) test on the determined creep property. The SP creep property of a specimen was evaluated using finite element analysis by varying the specimen thickness and diameter, diameter of the spherical and hemispherical indentation punches, inner diameter of the lower die, and dimensions of the chamfer of the lower die. We observed that the rupture time decreased with decreasing specimen thickness and punch diameter and with increasing chamfer size and inner diameter of the lower die. Under similar analytical conditions of static average equivalent stress in steady state, the SP creep curves reasonably agreed even in the case where the specimens or rigs have different geometries, which implied the possibility of direct comparison of the test results obtained from specimens and rigs with different dimensions and geometries.     

Orientation effect on the Boussinesq indentation of a transversely isotropic piezoelectric material

Three-dimensional finite element analysis was used to study the effect of the angle between the loading direction and the axisymmetric direction on the indentation behavior of a transversely isotropic piezoelectric half-space by a cylindrical indenter of flat end. Two cases were considered in the analysis, which included (a) the indentation by an insulating indenter, and (b) the indentation by a conducting indenter. Both the indentation load and the indentation-induced potential were found to be proportional to the indentation depth. Using the simulation results and the analytical relationship for the indentation by a rigid, insulating indenter, semi-analytical relationships were developed between the indentation load and the indentation depth and between the indentation-induced potential on the indenter and the indentation depth for the conducting indenter, respectively. The proportionality between the indentation-induced potential and the indentation depth is only a function of the angle between the loading direction and the poling direction, independent of the type of indenters, which may be used to measure the relative direction of the loading axis to the axisymmetric axis of transversely piezoelectric materials from the indentation test.