基于霍普金森压杆系统的动态压痕实验

设计了基于分离式Hopkinson压杆系统(SHPB)的动态压痕实验装置,使其能够获得动态压痕实验中试样的压痕位移和所受冲击载荷的实验数据,从而得到一种新的材料动态性能测试技术.对3种不同的材料进行了动态压痕实验,得到了材料的动态硬度和率敏感性等动态性能,与采用其他实验技术所测得的性能数据具有很好的一致性,并实现了利用...     

On testing of charpy specimens using the one-point bend impact technique

This paper reports our methodology and results for the assessment of the dynamic fracture energy of notched Charpy A508 steel specimens. The fracture tests consist of one-point bend impact applied to the specimen in contact with an instrumented bar. Fracture is caused by the inertia of the unsupported specimen only. The fracture energy is determined from the incident, reflected and single wire fracture gage signals. High-speed photographic recordings show that for all the specimens investigated in the “lower shelf” temperature regime, fracture occurs relatively early and prior to “taking off” of the bar by rigid body motion. It also confirms that the fracture gage readings indeed coincide with the formation of a crack from the notch tip. The present methodology is relatively easy to implement, and it allows the investigation of the fracture properties of materials at loading rates (and velocities) that are substantially higher than those achieved in a conventional Charpy test. Moreover, this test is attractive for modeling purposes since its boundary conditions are simple and well defined.     

Effect of gelatin molecular charge heterogeneity on formation of intermolecular complexes and coacervation transition

We investigate the molecular charge heterogeneity of gelatin molecules at three different pHs: isoelectric pH = 5 (pI), and at pHs = pI ± 1 by measuring the zeta potential distributions. Its effect on the formation of soluble intermolecular complexes induced by the presence of a nonsolvent, ethanol, was explored. The charge distributions were found to be symmetric at pH = 5, where the onset of binding, and formation of soluble complexes were observed to be facilitated by the presence of a small net charge (close to zero) with the molecules exhibiting polyampholyte (PA) behavior. These distributions turned asymmetric at pH = 6, yet complex formation and coacervation occurred. On the other hand, for pH = 4 samples, these distributions were found to be strongly asymmetric with the molecules possessing very high net positive charge, such a system did not yield coacervation. The PA to polyelectrolyte transition and its repercussion on coacervation has been discussed in the light of the experimental results obtained from electrophoresis, turbidimetry, atomic force microscopy, and nanoindentation studies. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1511–1520, 2007     

Contact stiffness of initially stressed neo‐Hookean solids

This work, using the solution given by Dhaliwal and Singh, presents analytical expressions of the incremental stress and displacement fields for the axisymmetrical indentation of initially stressed, incompressible neo‐Hookean solids. A simple relation for the contact stiffness, contact area, elastic constants, and finite stretch can be obtained for the indentation by any rigid axisymmetric indenter, which can be described as a smooth function. The contact stiffness increases with the initial finite stretching; the finite stretching makes materials harder to deform. The results provide a basis for evaluating the effects of residual stresses on the nanoindentation of materials from the viewpoint of finite deformation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2513–2521, 2004     

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al₃Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.     

Size-dependent sharp indentation-II: a reverse algorithm to identify plastic properties of metallic materials

In part I of this paper (Cao and Lu, J. Mech. Phys. Solids, in press), a closed-form expression of the size-dependent sharp indentation loading curve has been proposed. In this second part, which concerns the direct application of the analytical model, a reverse algorithm has first been established to extract the plastic properties of metallic materials on a small scale where the size effect caused by geometrically necessary dislocations is significant. Second, from the viewpoint of the mathematical theory of inverse problems, the properties of the present inverse problem i.e. the existence, uniqueness and stability of the solution, have been investigated systematically. The results have identified the extent to which the plastic properties of ductile materials can be determined effectively using the present method. Third, experimental verifications of the reverse algorithm using a standard Berkovich indenter have been carried out for 316 stainless steel and pure titanium, respectively. The results show that, by taking a maximum indentation depth of 1.5 and , respectively for the two materials, a good engineering estimation of the representative stresses, σ_0.033, in the absence of a strain gradient can be made using the present method, which can be used in conjunction with the representative stress corresponding to another indenter with a different tip apex angle to determine the plastic properties of metallic materials, i.e. the yield strength σ_y and the strain hardening exponent n. The material length scale can also be identified by using the present algorithm. Experimental results show that it has the correct order of magnitude, but is more sensitive to data errors than the identified representative stress.     

Indentation failure in composite sandwich structures

A combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken. Two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor; and a sandwich beam with symmetrical facing and core materials as in the sandwich panels. The load-deflection behavior of the loaded facing was monitored during the test. Strains were also measured near the load on both surfaces of the facing using embedded strain gages. A full-field analysis of the in-plane displacements in the foam was conducted using the moiré method. The problem was modeled as an elastic beam resting on an elastic-plastic foundation. Initiation of indentation failure occurs when the foundation yields, while catastrophic failure takes place when the compression facing fractures. The experimental results are in good agreement with the results of the analytical modeling based on the Winkler foundation.     

A MECHANICAL MODEL OF CELL DURING INDENTATION BASED ON MEMBRANE THEORY

基于膜理论和细胞压痕实验,本文提出了一种可用于分析细胞变形的新的力学模型,通过对乳胶球和聚氯乙烯(PVC)球进行宏观压痕试验、理论分析以及与不同细胞压痕试验曲线的比较,验证了细胞力学模型的适用性。同时,利用新模型对单个软骨细胞进行了具体的力学分析,得到了细胞的力学参数、细胞膜表面的应力分布等,为单细胞力学特性的研究提供了一种新思路。     

Spherical indentation of incompressible rubber-like materials

In recent years, indentation tests have been proven very useful in probing mechanical properties of small volumes of materials. However, a class of materials that very little has been done in this direction is rubber-like materials (elastomers). The present work investigates the spherical indentation of incompressible rubber-like materials. The analysis is performed in the context of second-order hyperelasticity and is accompanied by finite element computations and an extensive experimental program with spherical indentors of different radii. Uniaxial tensile tests were also performed and it was found that the initial elastic modulus correlates well with the indentation response. The experiments suggest stiffer indentation response than that predicted by linear elasticity, which is somehow counter-intuitive, if the uniaxial material response is to be considered. Regarding the uniqueness of the inverse problem, that is to establish material properties from spherical indentation tests, the answer is disappointing. We prove that the inverse problem does not give unique answer regarding the constitutive relation, except for the initial stiffness.     

蠕变压痕实验的计算机模拟

采用有限元的方法对双相材料的蠕变压痕实验进行了数值模拟，在有限元数值解的基础上，定义了相应于传统单轴蠕变实验的“等效应力”和“等效应变”，建立了蠕变压痕实验同传统单轴蠕变实验之间的关系，给出了确定薄膜蠕变应力指数和蠕变常数的方法；同时数值解的结果表明，实验中通过控制压痕深度不超过薄膜厚度的 ５％～１０％，忽略基体的硬化指数对确定薄膜性能的影响存在一定的误差，但基体的弹性模量对确定薄膜的蠕变性能影响不大。