SiC/BN层状陶瓷耐损伤性能

采用压痕法在SiC/BN层状陶瓷试样的表面引入不同尺寸的表面裂纹,利用三点弯曲测量含裂纹试样的极限断裂应力,研究了不同尺寸的表面裂纹对层状陶瓷断裂强度的影响;根据压痕载荷-强度实验结果,测定层状陶瓷的阻力曲线,并与单相SiC陶瓷对比。结果表明,层状陶瓷的压痕强度对压痕裂纹深度的变化不敏感,阻力曲线呈上升型;而单相SiC陶瓷的压痕强度随压痕裂纹深度的增加急剧下降,阻力曲线呈平稳型,说明层状陶瓷具有优异的耐损伤性能和升值R-曲线行为。分析认为,裂纹在弱界面处发生偏折是层状陶瓷具有优良耐损伤性能和升值R-曲线行为的主要原因。这为陶瓷材料在含有一定的制备和加工缺陷以及承受冲击、磨损等接触损伤的条件下保持高强度提供了可能。     

基于能量的球压痕硬度的计算方法

采用Oliver-Pharr方法计算压痕硬度容易受到卸载斜率计算偏差的影响。为了提高压痕硬度计算的准确度,减小卸载斜率对压痕硬度的影响,假定最大压入深度下的接触半径与完全卸载后的压痕半径相等,根据Hertz接触理论,基于压痕功与压痕深度的线性关系,建立了压痕硬度与压痕功的显式模型。压痕试验的结果表明,基于能量的压痕硬度计算方法能够有效提高压痕硬度计算的准确度,可以满足工程实际需要。     

平头压痕试验确定铝合金2A12的蠕变剩余寿命研究

基于平头压痕理论和蠕变损伤理论,提出了采用平头压痕试验和拉伸蠕变试验相结合的方法来确定铝合金2A12的蠕变剩余寿命.具体思路为:(1)由拉伸蠕变试验(通过控制试验时间)获得不同损伤的试样;(2)针对受损试样,由压痕加卸载试验确定不同损伤所对应的卸载曲线顶部斜率;(3)通过计算获得拉伸蠕变试样的损伤量;(4)得到卸载曲线顶部斜率与损伤的关系曲线,并以此为标定曲线推断出铝合金拉伸蠕变损伤试样的剩余寿命.     

试样倾斜对球形压头微米划痕测试紫铜摩擦系数的影响

采用球形压头对紫铜进行微米划痕实验,在恒定正压力下研究了试样倾斜对测试摩擦系数的影响。结果表明:实验测得的名义摩擦系数与倾斜角线性相关,且±9°的差别会造成摩擦系数相差约14倍。通过建立球形压头与试样倾斜状态的位置关系模型,发现试样倾斜对粘着摩擦系数无影响,而犁沟摩擦系数随倾斜角线性变化。为了准确计算无倾斜条件下的真实摩擦系数,可通过平面接触力学模型实现名义摩擦系数与真实摩擦系数之间的转换,或者在试样表面的不同位置往复划刻,再取摩擦系数的平均值作为最终结果。     

测量生物组织等效杨氏模量的毫米压痕仪

生物组织的弹性是诊断其是否发生病变的重要依据,杨氏模量是反映组织弹性的重要参数。以Labview为软件平台结合数据采集和运动控制等硬件设备,研制了一套测量弹性模量的毫米压痕弹性仪。以市购冷鲜牛肉、猪肉、猪肝和猪肾为试样,测量了球形压头向试样施加的负载力和对应的试样压痕深度,在对测量数据进行校准的基础上,利用基于赫兹接触力学模型的压头半径R、作用力F、压痕深度δ与试样等效杨氏模量E~*间的解析关系,得到各试样的等效杨氏模量。实验结果表明,测得的组织试样等效杨氏模量数值与文献基本相符,所设计的毫米级压痕法测试装置可用于生物组织等效杨氏模量的检测。     

影响布氏硬度计准确测量的因素及排除方法

计布氏硬度计的工作原理是把一定直径的钢球,在一定试验力作用下,以一定的速度压入试样表面,经规定的试验力保持时间后卸除试验力,以试样压痕球形表面积上的平均压力来表示金属的布氏硬度值。凡影响压痕产生及测量的因素,均影响布氏硬度值。影响测量准确度的因素主要由四部分组成。一、试样试样厚度及表面质量能明显影响布氏硬度测量结果。(1)尽管布氏方法受粗糙表面的影响小,对表面研磨要求不高,但为使压痕边缘清晰,保证测量结果的准确性,该方法对表面粗糙度也有一定的限制。(2)表面清洁度对布氏硬度测量值和不均匀性也是比较关键的,油脂、…     

摩擦力和样品厚度对压痕法测量生物试样弹性的影响

生物试样的弹性测量可为生物体疾病的早期诊断和治疗提供依据。利用压痕法对生物试样的弹性进行了测量,并用有限元软件对压痕过程进行了模拟。研究发现,试样厚度对弹性测量存在影响,试样厚度越大,测量结果越接近试样真实的杨氏模量。当试样厚度为压痕深度的75倍时,测量误差仅为0.74%。又研究了压头速度对弹性测量结果的影响。研究发现,当压头速度较大时,由于摩擦力的作用,测量结果与试样弹性的真实值之间存在一定的差异。在模拟过程中添加摩擦力可准确反演试样的弹性,误差在5%以下。     

一种钠钙硅酸盐玻璃的纳米压痕测试分析

采用纳米压痕测试技术对一种钠钙硅酸盐玻璃进行微观力学性能的测试分析.测得加载-卸载过程载荷与压入深度曲线,发现被测玻璃的最大压深、残余深度和弹性回复量随最大加载力的增加而增大,但其相对弹性回复率系数基本稳定,平均值为58.2%.通过电子显微镜观察了不同最大载荷下的压痕形貌,发现压痕区域出现了边界沉陷现象.当最大加载力为1 000 mN左右时,三棱锥工具头测试的压痕区域出现了较明显的微裂纹;采用四棱锥工具头时出现微裂纹的最大加载力要小于该值,且裂纹取向均与金刚石工具头的棱角取向一致.利用非线性有限元软件MSC.Marc对纳米压痕过程进行了仿真分析,得到载荷与压入深度的仿真曲线,该曲线与试验结果基本相符;分析了载荷作用下材料内部的应力分布.利用Oliver-Pharr模型得到不同压入深度下被测玻璃的接触刚度值,该值随压入深度的增加而增大.     

多孔氧化铝膜的纳米力学性能研究

采用二次铝阳极氧化技术, 制备了高度有序的铝阳极氧化膜(AAO膜), 孔径为50nm. 利用纳米压缩仪结合纳米压痕仪对不同压缩位移条件下的AAO膜进行了纳米力学测试和卸载后的压痕原位扫描实验. 通过测量加载-卸载曲线并用Oliver-Pharr模型分析计算得到, 在压缩位移为0、3.3、6.6、9.9μm时, AAO膜的纳米硬度和弹性模量分别为1.49、1.79、1.69、1.55GPa和11.79、12.32、12.82、13.19GPa. 由卸载曲线和原位扫描的压痕形貌可以看出, 压缩位移为6.6和9.9μm时的压痕在压缩力的作用下发生了明显的塑性变形, 因而AAO膜的纳米硬度减小, 而弹性模量一直增大.     

布氏硬度试样的最小直径

为了保证布氏硬度测量结果的准确性,避免压痕之间以及试样边缘对试样变形的影响,GB231-84“金属布氏硬度试验方法”中规定:压痕中心距试样边缘距离不应小于压痕平均直径的2.5倍,两相邻压痕中心距离不应小于压痕平均直径的4倍。当布氏硬度值小于35时,上述距离应分别为压痕平均直径的3倍和6倍。虽然一般情况下,都能满足上述要求,但是,多大试样才能满足测量要求,即在布氏硬度值HB及F/D~2等一定的条件下,试样的最小直径是多少,是实验人员制作试样时关心的数据。本文按GB231—84的规定,计算出圆试样的最小直径。     

Evaluation of mechanical properties using spherical ball indentation and coupled finite element–element-free galerkin approach

In the present work, the mechanical properties of pressure tube material (Zr-Nb2.5) are evaluated using the coupled finite element–element-free Galerkin approach. Penalty approach is used to impose contact constraints and non-penetration condition at the interface. An efficient node-to-segment algorithm is employed to model the contact behavior. An updated Lagrangian approach is used to model the large deformation. Loading and unloading response of the indentation process is analyzed using von-Mises and Gurson-Tvergaard-Needleman (GTN) plasticity models. In multiple indentations, the indentation depth is progressively increased up to a maximum specified limit with partial unloading. Load-indentation depth curves are used to extract the flow properties of the material.     

Effect of residual stress on the nanoindentation response of aerospace aluminium alloys

Experimental measurements and finite element simulations have been used to study the effect of residual stresses on the nanoindentation response of an aerospace-grade aluminium alloy. Tensile and compressive residual stresses lead to changes in the nanoindentation load–displacement curves. Loading and unloading curves were studied to observe the effect of residual stresses. The maximum load of indentation, curvature of the loading curve, elastically recovered depth, work of indentation, pile-up and contact area were measured and found to have a linear relationship with residual stress. To calculate residual stress from the load–displacement curve, it was concluded that pile-up should be measured carefully. The paper presents a methodology of calculation of area of contact based on the work of indentation which can be extracted from the nanoindentation load–displacement data. This allows extraction of the residual stresses from experimental nanoindentation data for aerospace aluminium alloys which generate pile-up and for which the true calculation of contact area without imaging is very difficult. Methods previously published in the literature have been assessed against the current approach.     

A New Model Developed to Evaluate the Contact Area Arising During Nano-indentation Tests With Pileup Behavior of Metal Materials

A new mechanical model is developed in this paper for metal materials to investigate the behavior arising during the loading/unloading process of an indentation test. Two governing differential equations are derived for the depth solution of the indenter tip and the depth solution formed at the separation point, expressed in a power form. All spring/block and damping coefficients shown in these governing differential equations associated with the elastic/plastic and viscous behaviors are determined by the real-coded genetic algorithm. With the aid of experimental results of the depth at the indenter tip shown at large and small indentation depths, aluminum and steel were used as two examples of soft materials in this paper. The pileup behavior is implicitly included in the evaluation of the contact projected area (A) in the present model. A significant difference in is caused if the pileup is not considered in the model. Under a constant maximum load, the contact area is slightly increased by decreasing the loading/unloading rate.     

Experimental validation of atomic force microscopy-based cell elasticity measurements

Atomic force microscopy (AFM) is widely used for measuring the elasticity of living cells yielding values ranging from 100 Pa to 100 kPa, much larger than those obtained using bead-tracking microrheology or micropipette aspiration (100-500 Pa). AFM elasticity measurements appear dependent on tip geometry with pyramidal tips yielding elasticities 2-3 fold larger than spherical tips, an effect generally attributed to the larger contact area of spherical tips. In AFM elasticity measurements, experimental force-indentation curves are analyzed using contact mechanics models that infer the tip-cell contact area from the tip geometry and indentation depth. The validity of these assumptions has never been verified. Here we utilize combined AFM-confocal microscopy of epithelial cells expressing a GFP-tagged membrane marker to directly characterize the indentation geometry and measure the indentation depth. Comparison with data derived from AFM force-indentation curves showed that the experimentally measured contact area for spherical tips agrees well with predicted values, whereas for pyramidal tips, the contact area can be grossly underestimated at forces larger than ～0.2 nN leading to a greater than two-fold overestimation of elasticity. These data suggest that a re-examination of absolute cellular elasticities reported in the literature may be necessary and we suggest guidelines for avoiding elasticity measurement artefacts introduced by extraneous cantilever-cell contact.     

Step by step building of a model for the Berkovich indentation cycle

The indentation cycle obtained from hardness testing of material has two singular points. The first one is the right end of the definition interval of loading/unloading curves; it corresponds to the cycle tip and poses no difficulties for mathematical analysis. The second one is the latest contact point between the material and the indenter tip in phase of withdrawal, it is located inside the interval of definition, more towards the left- or the right-hand side depending on the elasticity degree of the material. This second point is impractical for the analytical modelling of the cycle as the unloading curve loses there all mathematical properties of derivability and differentiability.This difficulty induced a tendency for empirical models built from experimental results that are, like any empirical laws, affected by some lack of precision. It also led to an intense focusing on the unloading curve to which main nanomechanical and structural properties have been connected, forgetting sometimes the loading curve and the valuable information it can provides.In this article, we work out an analytical model to represent the two curves of the indentation cycle as accurately as possible. In this step by step modeling we use functional analysis and force the modelling curves to fit the interval of definition, the concavity direction and the materials energetic properties.     

Determination of the hardness and elastic modulus from continuous vickers indentation testing

Continuous Vickers (H _v) indentation tests were performed on different materials (ion crystals, metals, ceramics, silica glass and plastic). Load-indentation depth curves were taken during the loading as well as during the unloading period by a computer controlled hydraulic mechanical testing machine (MTS 810). The indentation work measured both the loading and the unloading periods, and these were used for the evaluation of parameters characterizing the materials. It was found empirically that there were linear connections between the maximum load to the power 3/2 and the indentation work. These connections were used to relate the conventional hardness number, H _v, and Young's modulus, E, with the work performed during loading and unloading. This work can be determined with great accuracy from the measurements. The values of the Young's modulus and the Vickers hardness determined this way agree well with those obtained by conventional methods. On the basis of continuous indentation tests, materials can be easily classified into the isomechanical groups introduced by Ashby. For this classification the H _v/E ratio is generally used. As a substitute for H _v/E another parameter is recommended which can be determined easily from a single measurement.     

确定纳米压入面积函数的改进方法

针对薄膜等微小结构材料力学性能测试面积函数测量问题,提出了一种改善微小压入深度下压头面积函数准确性的方法.该方法将压头与被测样品之间的投影接触面积与压头针尖曲率半径和角度相关联,解决常用的面积函数确定方法在50 nm以下浅压痕测量中不可靠的问题.经实验验证,该面积函数确定方法可以提高微小接触深度下压入硬度和折合模量的测...     

基于非接触位移测量的层压板低速冲击性能分析

为通过冲击点位移获得复合材料层压板的低速冲击性能,首先,在层压板落重冲击试验中,使用高速激光位移传感器测得冲击点背部的位移和速度;然后,基于弹簧-质量冲击模型,以冲击点位移计算了冲击接触力,并通过无损冲击试验标定了弹簧-质量模型参数;接着,模拟了有损伤冲击过程的接触力,利用实测冲击点速度响应判定了分层产生的时刻,并根据模拟结果得出了分层阈值力;最后,根据无损检测结果进行了层压板抗弯刚度的折减,使用弹簧-质量模型计算了产生冲击损伤后的接触力及能量吸收曲线,使用冲头和冲击点背部位移计算凹坑的实时深度。结果显示:结构受冲击发生分层时,由于层压板刚度的突降,冲击点速度出现剧烈震荡,该现象可以作为结构出现分层损伤的识别特征;产生冲击损伤后的接触力及能量吸收曲线的计算结果与试验测试结果吻合。计算所得凹坑的实时深度随冲击能量的变化趋势与冲击后测试的凹坑深度变化趋势一致。这些结果表明提出的基于非接触测量技术的方法可用于无法直接测量接触力情况下的层压板冲击性能分析。      

Mechanical properties of multilayered films using different nanoindenters

The effects of interface, contact hardness, deformation, and adhesion of Al/Ni multilayered films under nanoindentation were investigated using molecular dynamics (MD) simulations. The results show that the indentation force of the sphere indenter is the largest among nanoindentations using sphere, cone, Vickers, or Berkovich type indenters at the same penetration depth. Force increasing, relaxation and adhesion took place during loading, holding depth and unloading, respectively. The interface occurred along the {111} (110) slip systems and the maximum width of the glide bands was about 1 nm. The reaction force and plastic energy of the indented films are also discussed.