Instrumented Indentation Experiments Research Articles

Simulation and dimensional analysis of instrumented dynamic spherical indentation of ductile metals

Finite element (FE) modeling of instrumented dynamic indentation experiments in a miniature Kolsky bar is undertaken. Geometry, including an output bar with machined spherical indenter tip, and velocity history boundary conditions are extracted directly from experimental diagnostics. The test material (i.e., substrate) is polycrystalline aluminum alloy Al 6061-T6. The constitutive model used in simulations accounts for isotropic elasticity and isotropic plasticity with strain hardening, strain-rate hardening, and thermal softening under adiabatic conditions. The FE model, with representative material parameters culled from the literature, accurately reproduces the curvature of the experimental load versus depth data for three different experimental indentation velocity histories. A framework for dimensional analysis of instrumented dynamic spherical indentation is set forth, improving upon prior work. Parametric FE simulations reveal sensitivity, or lack thereof, of the predicted response to variations in the proposed independent dimensionless variables encompassing material properties. For the indenter size, maximum depth, and maximum strain rate imposed experimentally on the order of 103/s, force-depth predictions are nearly unaffected by realistic variations in mass density, melting temperature, and thermal softening parameters when the sample is initially at room temperature. Predictions are affected by elastic constants (the elastic modulus and to a lesser extent, Poisson’s ratio), initial yield strength, two strain hardening parameters, and strain rate sensitivity. Predictions are also notably affected by initial temperature, with thermal softening prominent at high enough initial temperature or much higher loading rates. Based on the dimensional analysis, static indentation and elevated temperature indentation experiments are proposed for extraction of quasi-static and thermal material properties from previously uncharacterized metals, and dynamic indentation is proposed for extraction of rate sensitivity that cannot be obtained from static tests. Rate sensitivity obtained in this way from the novel instrumented dynamic spherical indentation experiments and FE simulations produces a parameterized stress–strain response for Al 6061-T6 reasonably validated by external studies for strain rates up to the order of 103/s.

Synthesis and characterization of metal carbides for nanoindentation tip applications

Instrumented indentation experiments at elevated temperatures require careful attention to a myriad of experimental details. Not the least of these is the choice of the indenter tip material. Traditional room-temperature indenters, e.g., diamond and sapphire, can break down, react, and wear excessively at elevated temperatures. In this work, rf-induction heating float-zone and high-temperature solution single-crystal growth techniques have been used to prepare a suite of bulk refractory carbide specimens (i.e., ZrC, VC0.86, NbC, TiC0.95, WC). These potential indenter tip materials were subsequently characterized using nanoindentation testing techniques to determine their single-crystal elastic modulus, hardness, and fracture toughness in order to evaluate their potential for use as elevated-temperature nanoindentation tips. Additionally, subject carbide crystal characteristics were compared to those of single-crystal sapphire and polycrystalline WC-Co. The cumulative results show that single-crystal WC is a promising candidate for indenter tip material based on a combination of its high elastic modulus, hardness, and resistance to cracking—in addition to being crystallographically favorable for fabrication in the frequently used three-sided pyramidal indenter tip geometries.

Toward Material Property Extraction from Dynamic Spherical Indentation Experiments on Hardening Polycrystalline Metals

Static indentation and dynamic indentation are reviewed, with a focus on extraction of material properties of isotropic strain-hardening polycrystalline metals that may be rate- and temperature-sensitive. Static indentation is reviewed first, followed by dynamic indentation, since the former is regarded as a specialization of the latter with inertia, rate dependence, and adiabatic heating excluded. Extending concepts from the literature review, a treatment of dynamic indentation using dimensional analysis is forwarded, and a general framework for extraction of material property information (i.e., constitutive model parameters) from instrumented dynamic spherical indentation experiments is set forth. In an example application of the methodology, experimental data obtained from instrumented spherical indentation in a miniature Kolsky bar apparatus are evaluated via dimensional analysis. The substrate material is aluminum alloy Al 6061-T6. Several definitions of indentation strain proposed for static indentation are assessed for dynamic indentation, as are indentation strain rates. While the fidelity of the experimental method and inertial effects could inhibit extraction of elastic properties, extraction of certain plastic constitutive properties may be feasible. Current data are insufficient to enable determination of a complete and unique set of all physical properties. Motivated by the present review and analysis, new experiments and simulations are proposed that would identify influences of material properties, facilitating their extraction from data.

Changes in the mechanical, structural and electrical properties of glassy carbon due to strontium and silver co-implantation and annealing

The effect of Sr and Ag co-implantation and recovery annealing on the structural, mechanical and electrical properties of glassy carbon is reported. Glassy carbon was co-implanted with 200 keV Sr and Ag ions to a fluence of 1 × 1016 ions/cm2 at room temperature. Combination of Raman spectroscopy and high resolution transmission electron microscopy (HRTEM) showed that the co-implantation of Sr and Ag resulted in amorphisation of the glassy carbon substrate. Raman spectra and HRTEM micrographs obtained after annealing indicated recovery of the glassy carbon structure, however, the recovered structure appears to be graphite-like. Instrumented indentation experiments on the ion-irradiated glassy carbon substrates revealed the formation of a surface layer with considerably enhanced mechanical performance. When annealing the ion implanted samples, these effects were only partially reversible. The effect of the irradiation-induced amorphisation and graphitisation of glassy carbon on its conductivity was also studied. The sample became less conductive after Sr and Ag co-implantation, whereby annealing resulted in an increase in conductivity.

Multiscale deformation processes during cold sintering of nanovaterite compacts

Cold sintering is a promising route towards the manufacturing of dense ceramics at mild processing conditions, but our poor understanding of the process has prevented the wider spread of this attractive densification approach. Using nanovaterite powders with well-defined multiscale morphology, we perform in-situ X-Ray tomography on compacts subjected to controlled mechanical load and quantify the multiscale deformation processes responsible for the water-assisted cold sintering of this powder with the help of instrumented indentation experiments at the micro- and nano-scale. Our results reveal the crucial effect of water in promoting the macroscopic densification process and highlight the dominant role of the nanoparticle network inside agglomerates in controlling the cold sintering of compacts at high mechanical loads. By providing new insights into the deformation processes responsible for the densification effect, this study can potentially guide the discovery of novel chemical compositions and particle morphologies that can be more easily densified through room-temperature cold sintering with water.

Precipitation Hardenable High Entropy Alloy for Tooling Applications

We present a high entropy alloy (HEA) from the system Al-Co-Cr-Fe-Ni with small additions of W, Mo, Si and C which was designed to allow for precipitation hardening by annealing in the temperature range from 600 to 900 °C. The alloy development was supported by thermodynamic computations using ThermoCalc software and the specimens were produced by arc melting. The microstructure of one selected sample in as-cast and annealed conditions was analysed using SEM/EDS, SEM/EBSD and TEM. The as-cast microstructure consists of spinodally decomposed BCC dendrites enveloped by FCC+Cr23C6 eutectic. Upon annealing at 700 °C for 24 h nanoscale precipitates form within the spinodal BCC as well as from FCC. Precipitation is exquisitely uniform leading to an increase in microhardness from 415 HV0.5 in the as-cast state to 560 HV0.5 after annealing. We investigated coarsening of this microstructure using varying holding time for a constant temperature of 700 °C. The microstructure evolution during coarsening and the corresponding mechanical properties obtained from instrumented indentation experiments are presented in this work.

Ripplocations: A universal deformation mechanism in layered solids

Layered materials and formations are ubiquitous in Nature and span the gamut from individual graphene layers in graphite, to layered composites, to geologic formations. And while the similarities in the deformation of the latter two have been recognized, that the same physics applies at the atomic scale has not. Using atomistic simulations on graphite, and simple instrumented cylindrical indentation experiments on decks of cards and thin steel sheets the authors show that, in all cases, confined buckling leads to the nucleation of multiple ripplocations that rapidly propagate away from under the indenter in a wavelike manner. Upon unloading, they disappear, after dissipating considerable frictional energy. In short, Nature's solution for the deformation of all layered solids, g20 orders of magnitude in scale, is as simple as it is universal: buckling. To be able to shed light on how an earthquake propagates from studying the deformation of graphite and vice versa is quite astonishing and remarkable indeed.

Measurement of an indentation size effect for Indium as a function of temperature

We present compression and instrumented indentation experiments on high-purity Indium, at temperatures from 22 to −100 °C. A strong indentation size effect is detected at all temperatures, corresponding to a large material length-scale. Therefore the large length-scale is not a consequence of creep mechanisms but an intrinsic property of the material.

A new modified ECM approach on the identification of plastic anisotropic properties by spherical indentation

This paper proposes a new approach on the identification of plastic anisotropic properties (σY, n, m), which is based on new modified expanding cavity model (ECM). The new ECM incorporates Hill's an anisotropic yield criterion in 1948 by equivalent transformation of stress expression form from spherical polar coordinate system to space rectangular coordinate system, and considers piling-up or sinking-in effect by introducing volume correction factor t as a semi-theoretical function. Through the indentation analysis for new ECM, the representative stress-strain method can characterize indentation mean pressure and uniaxial flow properties. Meanwhile, nonlinear functional relations between indentation response and plastic anisotropic properties are established. The effectiveness and reliability of new developed ECM approach are verified by the comparisons between identified properties and input properties in spherical indentation simulations, the comparisons between plastic anisotropic properties identified from instrumented indentation experiments and uniaxial compression experiments, and the comparisons between representative flow stress-strain points and compression experimental curves. The identified results are obtained by iterative calculations using the trust-region techniques based on the nonlinear least-square method. Excellent agreement is achieved. Finally, it is demonstrated that the proposed approach is reliable and useful for identification of plastic anisotropic properties of various anisotropic materials.

On the Measurement of Power Law Creep Parameters from Instrumented Indentation

Recently the measurement of the creep response of materials at small scales has received renewed interest largely because the equipment required to perform high-temperature nanomechanical testing has become available to an increasing number of researchers. Despite that increased access, there remain several significant experimental and modeling challenges in small-scale mechanical testing at elevated temperatures that are as yet unresolved. In this regard, relating the creep response observed with high-temperature instrumented indentation experiments to macroscopic uniaxial creep response is of great practical value. In this review, we present an overview of various methods currently being used to measure creep with instrumented indentation, with a focus on geometrically self-similar indenters, and their relative merits and demerits from an experimental perspective. A comparison of the various methods to use those instrumented indentation results to predict the uniaxial power law creep response of a wide range of materials will be presented to assess their validity.

Indentation size effect of cortical bones submitted to different soft tissue removals

Properties of elasticity, hardness and viscosity are determined for the study of the visco-elastoplastic behavior of bones. The mechanical properties are compared in two upright sections of the bone due to their anisotropy. Besides, influence of hydration treatments leading to structural modifications of collagen and ground substance contents of bones on the mechanical properties is studied on a femoral cortical bovine bone. The treatments applied to the bone are used by forensic anthropologists to remove the soft tissue and modifying the hydration degree coupled to the collagen content. From instrumented indentation experiments, the hardness is characterized by the macrohardness and a hardness length-scale factor stating the hardness-load dependence. The elastic modulus results from the application of the methodology of Oliver and Pharr (1992). The coefficient of viscosity is deduced from a rheological model representing the indenter time–displacement observed under the application of a constant load. As a result, all the mechanical properties are found to be lower in the transverse section in an extent depending on the hydration treatment, i.e. the different values are located between 5% and 25% for the hardness around 0.5GPa, between 25% and 40% for the elastic modulus around 20GPa and between 2% and 35% for the coefficient of viscosity around 60GPa.s. Unexpectedly, the elastic modulus to coefficient of viscosity ratio is found to be independent on the hydration treatment.

Numerical Analysis of Effect of Friction on Mechanical Properties of Ti-6Al-4VAlloy Through Nanoindentation

Nanoindentation is a non-destructive technique used for evaluating mechanical properties from a very small volume of material by deforming it with a indenter. In the present investigation, FE modelling has been carried out to study the influence of friction on Ti-6Al-4V alloy widely used for aerospace applications. The emphasis is placed on providing a comprehensive evaluation of the influence of friction coefficient on load Vs displacement characteristics, calculated hardness and young's modulus in conical and spherical indentations. The development of pile-up/sink-in during indentation process and its dependence on contact friction was also analyzed. Results indicates that, during indentation process, the amount of pile-up cannot be related solely to the strain-hardening exponent, rather, the contact friction also affects the pile-up geometry, Therefore, the material parameters which are related to projected area will lead to large error due to friction contradicting with the statement of researchers i the published literature i.e, the instrumented indentation experiments are not significantly affected by friction. The error in hardness and Young's modulus were found to be as high as 14.59% and 6.78% respectively.

Nanochemomechanical assessment of shale: a coupled WDS-indentation analysis

Establishing the links between the composition, microstructure and mechanics of shale continues to be a formidable challenge for the geomechanics community. In this study, a robust methodology is implemented to access the in situ chemomechanics of this sedimentary rock at micrometer length scales. Massive grids of coupled wave dispersive spectroscopy (WDS) and instrumented indentation experiments were performed over representative material surfaces to accommodate the highly heterogeneous composition and microstructure of shale. The extensive datasets of compositional and mechanical properties were analyzed using multi-variate clustering statistics to determine the attributes of active phases present in shale at microscales. Our chemomechanical analysis confirmed that the porous clay (PC) mechanical phase inferred by statistical indentation corresponds to the clay mineral phase defined strictly on chemical grounds. The characteristic stiffness and hardness behaviors of the PC are realized spatially in regions removed from silt inclusions of quartz and feldspar. At the microscale shared by indentation and WDS experiments, a consistent chemomechanical signature for shale emerges in which the heterogeneities of the PC are captured by the standard deviations of indentation properties and concentrations of chemical elements. However, these local behaviors are of second order compared to the global trend observed for mean mechanical properties and the clay packing density, which synthesizes the relative volumes of clay and nanoporosity in the material. The coupled statistical indentation and WDS technique represents a viable approach to characterize the chemomechanics of shale and other natural porous composites at a consistent scale below the macroscopic level.

Experimental Researches on Relation between Martens Hardness and Traditional Mechanical Properties

For finding the relation between Martens hardness and Vickers hardness, Rockwell hardness, tensile strength, yield strength, research are done on carbon alloy steel by experiment, linear regression analysis and derivation of equation .Martens hardness, Vickers hardness and Rockwell hardness of 30 samples which contains 3 material and 10 hardness levels are tested by ZHU 2.5 hardness tester. On the basis of 180 groups of data obtained and analyzed from instrumented indentation experiments, mathematic model between Martens hardness and Vickers hardness is established, by linear regression analysis. And mathematic model between Martens hardness and Rockwell hardness, tensile strength, yield strength is established on that basis. By means of comparing the value calculated by the mathematic model and obtained from tensile test using Electronic universal testing machine, the accuracy of the mathematic model is verified. The result show that the relative error of the four mathematics models which established in this test is below 5%, and the four mathematics models can be applied to the practical engineering application.

Study of mechanical deformation of Zr55Cu30Al10Ni5 bulk metallic glass through instrumented indentation

Abstract Instrumented sharp indentation experiments using both conical and Vickers diamond pyramidal indenters were carried out to study deformation characteristics of Zr55Cu30Al10Ni5 bulk metallic glass. Finite element simulations of instrumented indentation were also performed to formulate an overall constitutive response. Comparing the experimentally obtained results with the finite element predictions, it can be stated that mechanical deformation of the bulk metallic glass can be described well by both Mohr–Coulomb and Drucker–Prager constitutive criteria. Using these criteria, the extent of material pile-up observed around the indenter was also estimated very well.

A critical reassessment of elastic unloading in sharp instrumented indentation experiments and the extraction of mechanical properties

This work examines the extraction of mechanical properties from instrumented indentation P–h s curves via extensive three-dimensional finite element analyses for pyramidal tips in a wide range of solids under frictional and frictionless contact conditions. Since the topography of the imprint changes with the level of pile-up or sink-in, a relationship is identified between correction factor β in the elastic equation for the unloading indentation stage and the amount of surface deformation effects. It is shown that the presumption of a constant β significantly affects mechanical property extractions. Consequently, a new best-fit function is found for the correlation between penetration depth ratios h e /h max, h r /h max and n, circumventing the need for the assumption of a constant value for β, made in our prior investigation [Acta Mater. 53 (2005) pp. 3545–3561]. Simulations under frictional contact conditions provide sensible boundaries for the influence of friction on both h e /h max and h r /h max. Friction is essentially found to induce an overestimation in the inferred n. Instrumented indentation experiments are also performed in three archetypal metallic materials exhibiting distinctly different contact responses. Mechanical property extractions are finally demonstrated in each of these materials.

Theoretical and experimental analysis of contact radius for spherical indentation

The study of analytical models for the determination of the contact radius between a spherical indenter and an elastic–plastic material is presented in this paper. Experimental results obtained for different spheroidized steels are compared to those obtained numerically and with various analytical models. The comparison between experimental and numerical indentation curves demonstrates that the indentation response of the studied steels can be correctly obtained starting from the work hardening behaviour determined by tensile test with the Bridgman method. The experimental and numerical comparison also shows that an increase in friction coefficient leads to a non-negligible decrease in contact radius at large penetration depths. In consequence, an analytical model which is proposed with the assumption of friction contact allows a better contact radius to be determined. The comparison between the experimental and theoretical contact radius shows that the theoretical models, dependent only on the strain hardening exponent over-estimate the dimensionless contact radius, especially for low dimensionless contact radii, a/ R. The models of Matthews (1980), Hill et al. (1989), Taljat et al. (1998) and Alcala et al. (2000) lead to acceptable results only for dimensionless contact radii, a/ R, higher than 0.3. Below this value, only the models of Lee et al. (2005), Hernot et al. (2006) and Kim et al. (2006) produce results that are close to those obtained experimentally. The present study shows that these last models enable an accurate assessment of contact area from measurements of penetration depth which is central to the analysis of instrumented indentation experiments.

Influence of friction in material characterization in microindentation measurement

A comprehensive computational study is undertaken to identify the influence of friction in material characterization by indentation measurement based on elasto- plastic solids. The impacts of friction on load versus indentation depth curve, and the values of calculated hardness and Young’s modulus in conical and spherical indentations are shown in this paper. The results clearly demonstrate that, for some elasto-plastic materials, the curves of load versus indentation depth obtained either by spherical or conical indenters with different friction coefficients, cannot be distinguished. However, if utilizing the parameter β (see text for details), to quantify the deformation of piling-up or sinking-in, it is easy to find that the influence of friction on piling-up or sinking-in in indentation is significant. Therefore, the material parameters which are related to the projected area will also have a large error caused by the influence of friction. The maximum differences on hardness and Young’s modulus can reach 14.59% and 6.78%, respectively, for some elastic materials shown in this paper. These results do not agree with those from researchers who stated that the instrumented indentation experiments are not significantly affected by friction.

Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This effect is in contrast to uniaxial compressive yield strength, σy, which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/σy, of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discussed.

Materials characterization by instrumented indentation using two different approaches

Experimental applications of two different approaches for interpretation of instrumented indentation experiments are proposed in this paper. The first approach is the application of methodology developed for spherical indentation based on models. The second approach is the application of inverse algorithm based on the minimization between experiments and simulated data. It was shown that inverse approach lead to a good prediction of elastic modulus for metals with a face-centred cubic crystal structure. However, for metals with a body-centred cubic crystal structure identified elastic modulus was 10% under values obtained by ultra-sound method. It was then shown that models developed for spherical indentation give more accurate results than inverse approach. Moreover, a sensitivity study of mesh density in inverse identification shows that the more the mesh density the more accurate the results.