Indentation of transversely isotropic power-law hardening materials: computational modelling of the forward and reverse problems

Using a combination of dimensional analysis and large deformation finite element simulations of triple indentations of 120 materials, a framework for capturing the indentation response of transversely isotropic materials is developed. By considering 4800 combinations of material properties within the bounds of the original set of 120 materials, forward algorithms that predict the indentation response of materials and reverse algorithms that predict the materials’ elastic and plastic properties from experimentally measured indentation responses are formulated for both longitudinal and transverse indentations. Issues of accuracy, reversibility, uniqueness and sensitivity within the context of the indentation of transversely isotropic materials are addressed carefully. Using 1400 combinations of material properties, it is demonstrated that there is perfect reversibility between the material properties and their indentation responses as predicted by the forward and reverse algorithms. On average, the differences between the results of the finite element analysis and those predicted by the forward algorithms for longitudinal or transverse indentations are less than 1%, thus demonstrating the high accuracy and uniqueness of the forward analysis. For longitudinal and transverse indentations, the reverse algorithms provide accurate results in most cases with an average error of 3 and 6%, respectively. A sensitivity analysis with a ±2% variation in the material properties in the forward algorithm and ±2% variation in the indentation responses in the reverse algorithms demonstrated the robustness of the algorithms developed in the present study, with the longitudinal indentations providing relatively less sensitivity to variability in indentation responses as compared to the transverse indentations.     

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.     

Rate sensitivity and deformation mechanism of semi-crystalline polymers during instrumented indentation

Instrumented indentation tests using both constant loading rate (CLR) and continuous stiffness measurement (CSM) operation modes were performed to investigate the deformation mechanism and their sensitivity to the deformation rate in semi-crystalline polymers through the quantitative analysis of load-depth loading and unloading curves. The strain rate was constant during the CSM tests, while the strain rate decreased with the increasing of loading time in CLR tests. The mechanical response mechanism of the semi-crystalline polymers to these tests was very complicated because of the combined effects of strain-hardening in the crystal phase and strain-softening in the amorphous phase. Results show that the loading index m reflects the strain-hardening or strain-softening response during indentation. When m > 2, the mechanical response was due to the strain-hardening, and when m < 2, the response was due to strain-softening. A method based on the measured contact hardness was proposed to obtain the unloading stiffness, and the other mechanical parameters could then be determined according to the unloading stiffness.     

Study on the formation defect of nano imprinted optical waveguide devices and nano‐indentation detection

In this study, we try to discuss the formation defects found in the application of air‐assisted soft mold UV‐cured nano imprint lithography technology in the manufacture of optical waveguide devices, and find a solution. Meanwhile, we try to utilize the nano‐indentation technology in the material quality detection for optical waveguide devices. The results tell us that there is a corresponding relationship between the indentation hardness and procedure parameters under nano‐meter level depth. For example, the indentation tends to be harder when it is lowly loaded and shallow in depth. Closer it gets to the edge of waveguide's turning, lower the indentation hardness will be. At the same time, different exposal process results in different structural intensity. Therefore, the high structural intensity without forming defects of optical waveguide with less optical loss and better optical transmission. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanical characterization of nanoclay-filled PDMS thin films

Polydimethylsiloxane (PDMS) reinforced by nanoclay platelets exhibits excellent mechanical properties for microfluidic systems. We tested PDMS-clay nanocomposite thin films under flat-ended cylindrical indentation and measured the elastic modulus and shear strength. A simple formulation based on a shear-lag model, fed by numerical simulations, was introduced to estimate the interfacial shear strength of thin films. Increase in the nanoclay content improved the elasticity of PDMS-clay thin films but reduced their interfacial shear strength. Shear thinning behavior of nanoclay platelets probably reduced the strength of PDMS nanocomposites. The proposed approach can be used for characterization of any polymeric thin films.     

Modeling instrumented indentation testing to evaluate the behavior of PA11 and CaCO3 composites for offshore applications

PA/PCC micro-composites were prepared with different PCC contents. Compatibility tests were conducted with crude oil at aging times up to 28 days. The experimental phases of the instrumented indentation test (loading, creep and unloading) were modeled. Moreover, the heuristic method of differential evolution was used to fit the parameters. Loading curves showed higher scattering than the subsequent unloading, regardless of the aging time. The two unloading parameters exhibited quasi-linear dependency, but their dispersion was quite different. The mechanical and viscoelastic properties revealed that the PCC filler acted as a reinforcing agent. However, a higher PCC content did not lead to an increase in modulus, probably due to the poor interaction between particles and polymer. The hardness results showed that this property is not so sensitive to the material's morphology as are modulus data. All systems still had predominantly plastic behavior even after aging.     

Flat‐punch indentation of viscoelastic material

The indentation of standard viscoelastic solids, that is, the three‐element viscoelastic material, by an axisymmetric, flat‐ended indenter has been investigated theoretically. Under the boundary conditions of flat‐punch indentation of a viscoelastic half‐space, the solutions of the equations of viscoelastic deformation are derived for the standard viscoelastic material. Their generality resides in their inclusion of compressible as well as incompressible solids. They cover the two transient situations: flat‐punch creep test and load‐relaxation test. In experimental tests of their applicability, nanoindentation and microindentation probes under creep and relaxation conditions yielded a modulus from 0.1 to 1.1 GPa and viscosity from 1 to 37 Gpa · s for a crosslinked glassy polyurethane coatings. For bulk polystyrene, the values vary from 1 to 2 GPa and from 20 to 40 Gpa · s, respectively. The analysis here provides a fundamental basis for probing elastic and viscous properties of coatings with nanoindentation or microindentation tests. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 10–22, 2000     

Oliver–Pharr indentation method in determining elastic moduli of shape memory alloys—A phase transformable material

Instrumented indentation test has been extensively applied to study the mechanical properties such as elastic modulus of different materials. The Oliver–Pharr method to measure the elastic modulus from an indentation test was originally developed for single phase materials. During a spherical indentation test on shape memory alloys (SMAs), both austenite and martensite phases exist and evolve in the specimen due to stress-induced phase transformation. The question, “What is the measured indentation modulus by using the Oliver–Pharr method from a spherical indentation test on SMAs?” is answered in this paper. The finite element method, combined with dimensional analysis, was applied to simulate a series of spherical indentation tests on SMAs. Our numerical results indicate that the measured indentation modulus strongly depends on the elastic moduli of the two phases, the indentation depth, the forward transformation stress, the transformation hardening coefficient and the maximum transformation strain. Furthermore, a method based on theoretical analysis and numerical simulation was established to determine the elastic moduli of austenite and martensite by using the spherical indentation test and the Oliver–Pharr method. Our numerical experiments confirmed that the proposed method can be applied in practice with satisfactory accuracy. The research approach and findings can also be applied to the indentation of other types of phase transformable materials.     

Analyse thermomécanique des problèmes de fissure fixe sous chargement dynamique

The purpose of this work is the study of thermomechanical coupling in the mechanisms of dynamic rupture in a fixed crack under dynamic loading.     

基于剪切修正GTN模型的金属弹性退化机理研究

许多金属材料在进行显微或宏观压入测试时,测得的弹性模量会随着压入深度的增大而不断降低.考虑到在压痕测试过程中金属材料并不产生明显的裂纹,这种性能退化应是由材料的损伤引起.然而,经典损伤理论认为以受压和受剪为主的压入变形不会引起材料软化和损伤.本文结合剪切变形下材料损伤的萌生和演化机理,对经典的GTN(Gurson-Tvergaard-Needleman)模型进行了修正,对压入测试进行了有限元模拟.模拟结果显示,材料在压入过程中的损伤是由现有空洞的扭曲变形和次级空洞的萌生共同引起,如果只考虑现有空洞变形则会低估材料在压入变形过程中的损伤演变.