Instrumented Indentation Measurements Research Articles

Effect of loading/unloading rate on instrumented indentation measurements in alloy Cu-2%Be

The influence of the loading/unloading rate on the indentation strain rate, hardness and elastic modulus in Cu-2wt%Be samples subjected to different aging treatments was analyzed. Homogenized samples were aged for 1 and 10 h at 400 °C to induce the formation of γ′ and γ precipitates, and instrumented indentation tests were performed at different loading rates between 0.15 and 1 mN s−1. The indentation curves of all the samples were influenced by the loading/unloading rate, but the behavior of the samples with precipitates was different from that of the homogenized samples. Strain rate levels during loading were also influenced by the loading rate. The strain rate increased almost linearly with the loading rate in the homogenized sample and the sample with γ′ nanoprecipitates, while it experienced a decelerated increase with the loading rate in the sample with γ phase. The hardness and reduced modulus were estimated from the indentation curves using the Oliver and Pharr method. Hardness remained almost constant at approximately 2.4 GPa as the rate was varied in the homogenized sample. On the other hand, the samples with γ′ and γ phase exhibited higher hardness values at 0.15 mN s−1, while it remained approximately constant for the other rates. The reduced modulus increased with the loading/unloading rate and this increase was more pronounced for the sample with γ precipitates, with a higher sensitivity of the reduced modulus increase.

The Influence of the Third Element on Nano-Mechanical Properties of Iron Borides FeB and Fe2B Formed in Fe-B-X (X = C, Cr, Mn, V, W, Mn + V) Alloys.

In this study, the influence of alloying elements on the mechanical properties of iron borides FeB and Fe2B formed in Fe-B-X (X = C, Cr, Mn, V, W, Mn + V) alloys were evaluated using instrumented indentation measurement. The microstructural characterization of the alloys was performed by means of X-ray diffraction and scanning electron microscope equipped with an energy dispersive X-ray analyzer. The fraction of the phases present in the alloys was determined either by the lever rule or by image analysis. The hardest and stiffest FeB formed in Fe-B-X (X = C, Cr, Mn) alloys was observed in the Fe-B-Cr alloys, where indentation hardness of HIT = 26.9 ± 1.4 GPa and indentation modulus of EIT = 486 ± 22 GPa were determined. The highest hardness of Fe2B was determined in the presence of tungsten as an alloying element, HIT = 20.8 ± 0.9 GPa. The lowest indentation hardness is measured in manganese alloyed FeB and Fe2B. In both FeB and Fe2B, an indentation size effect was observed, showing a decrease of hardness with increasing indentation depth.

Nanomechanical Behaviour of Martensite in Cryogenically Deformed NiTi Alloy

Effect of ageing treatment on work-hardening response of cryogenically deformed martensitic NiTi alloy has been evaluated at the microstructural length scale by conducting nanoindentation or depth-sensing instrumented indentation measurements. Hardening response has been observed in the alloy sample by cyclic indentation. Nanomechanical properties, viz. nanohardness, hardness to elastic modulus (H/E) ratio, and recovery index, were estimated at peak-aged condition.

Review: Models of Instrumented Indentation Measurements of Elastic- Plastic Materials

Review: Models of Instrumented Indentation Measurements of Elastic- Plastic Materials

Atomic-scale simulations of material behaviors and tribology properties for BCC metal film**Project supported by la DirecciónNacional de Investigación of the Universidad Nacional de Colombia, “the Theoretical Study of Physical Properties of Hard Materials for Technological Applications” (Grant No. 20101007903).

This work has two main purposes: (i) introducing the basic concepts of molecular dynamics analysis to material scientists and engineers, and (ii) providing a better understanding of instrumented indentation measurements, presenting an example of nanoindentation and scratch test simulations. To reach these purposes, three-dimensional molecular dynamics (MD) simulations of nanoindentation and scratch test technique were carried out for generic thin films that present BCC crystalline structures. Structures were oriented in the plane (100) and placed on FCC diamond substrates. A pair wise potential was employed to simulate the interaction between atoms of each layer and a repulsive radial potential was used to represent a spherical tip indenting the sample. Mechanical properties of this generic material were obtained by varying the indentation depth and dissociation energy. The load-unload curves and coefficient of friction were found for each test; on the other hand, dissociation energy was varied showing a better mechanical response for films that present grater dissociation energy. Structural change evolution was observed presenting vacancies and slips as the depth was varied.

A criterion to identify sinking-in and piling-up in indentation of materials

The instrumented indentation test is usually used to determine the mechanical properties of materials. Depending on the nature of the material, the way the matter flows under the indenter by piling-up or sinking-in affects the calculation of these mechanical properties. Consequently, corrections proposed by Oliver and Pharr and Loubet et al. should be done according to these two behaviors in addition to other corrections associated with the indenter tip defect as well as the compliance of the instrument. In this work we tested different materials having supposedly piling-up or sinking-in behavior: low-carbon steel, aluminum, brass, copper, beta tricalcium phosphate (β-TCP) bioceramic, rolled or sintered stainless steel and ceramic composite TiB2–60% B4C by using two types of indenter, i.e. Vickers and Berkovich ones. From the corrected load–indenter displacement curve, we showed that a criterion, defined as the ratio between the residual indentation depth and the maximum indentation depth reached at the maximum load, is able to identify the predominant deformation mode. For materials for which this ratio is higher than 0.83 piling-up prevails while it is sinking-in when it is lower than 0.83. When the ratio equals 0.83, the two modes of deformation should coexist since the calculations made using either correction of Oliver and Pharr or Loubet et al. give the same results. This novel way of considering the instrumented indentation measurements renders more accurate the determination of the hardness and the elastic modulus since the observation of the indent is then not required for identifying the deformation mode which affects the contact area calculation.

Instrumented indentation measurements of residual stresses around a crack tip under single tensile overloads

Abstract In this study, the residual-stress fields around crack tip of M(T) specimens under three single tensile overload ratios ( R ol ) during fatigue crack growth of aluminum alloy 2524-T3 were investigated using the instrumented indentation tests (IIT). The results showed that both the magnitude and region of the compressive residual stress around the crack tip increase with the increase of R ol . Finite element method was also used to simulate the residual stress distributions near the crack tip at the same loading conditions as experiments, which confirmed that the estimation of the crack-tip residual stress using Suresh et al.'s model based on IIT was feasible. The profiles of the crack tip under constant-amplitude loading and three overload ratios were examined using optical microscopy. It was found that the geometry of crack tip changes significantly from the sharp crack to the blunt crack with increasing R ol after applying a tensile overload. The probable mechanisms resulting in the change of the crack-growth behavior due to tensile overloads were also discussed. It was suggested that the combined effects of the residual stress, crack tip profile and the crack tip hardening may be responsible for the observed overload retardation effect of the present M(T) specimens during fatigue crack growth.

Use of instrumented indentation testing to study local mechanical properties of 304L SS welded joints subjected to low-cycle fatigue loadings

In this paper, a series of experiments including room-temperature low-cycle fatigue tests, instrumented indentation measurements, optical microscopy examinations, etc. were carried out to study the local mechanical properties and microstructures in particular zones of 304L SS welded joints in both the as-welded and cyclic straining conditions. An analytical algorithm proposed by Dao et al. [12] was utilized to estimate the basic mechanical properties (E, σy and n) from the indentation load-depth data. It is shown that in the as-welded condition, the weld metal has a higher strength and strain hardening capacity but a lower stiffness than does the base metal, while the material within HAZ exhibits a significant gradient of the mechanical properties. A strain amplitude-dependent variation in the local mechanical properties in particular joint regions was also obtained. Optical microscopy examinations showed that the microstructure in particular zones of 304L SS welded joints was not only decided by the metallurgical condition resulting from the welding process, but also determined by the strain amplitude applied during cyclic straining. In this study, an empirical criterion for judging the fracture locations of a weldment based on the yield stress mismatch ratio was also introduced to assess the integrity of the present 304L SS welded joints subjected to low-cycle fatigue loadings. The research work performed in this paper would further expand the applications of instrumented indentation technique in fatigue strength analysis of welded components.

Understanding the Shape-Memory Behavior in Ti-(~49 At. Pct) Ni Alloy by Nanoindentation Measurement

The influence of aging treatment on the work-hardening behavior of near-equiatomic NiTi alloy has been studied at the microstructural scale by conducting the instrumented indentation measurement. The maximum shape recovery is achieved at the peak aged condition. The improvement in shape recovery has been attributed to the delayed onset of plasticity. A comparison has been made between the recoverable strain obtained from the tensile experiments and the recovery index parameter determined from the nanoindentation measurements.

The reduced modulus in the analysis of sharp instrumented indentation tests

Abstract

Measurement of ceramic powders using instrumented indentation and correlation with their dynamic response

This article discusses an experimental approach for measuring the properties of individual powder particles sized between 25 μm and 100 μm of four common very hard ceramic materials: tungsten carbide, alumina, boron carbide, and silicon carbide. Fused silica is also investigated as the reference material. Experiments on the individual powder particles were performed to a depth of 100 nm and critically compared against consolidated compacts of the same materials. To draw a correlation between the instrumented indentation measurement and their dynamic properties, this investigation exploits the low modulus to hardness ratio of these materials, whether tested as powder or in consolidated form. The indentation experiments, as they were performed in this study, lie within a regime that allows the use of the expanding cavity model to provide a more rigorous mechanical analysis to the results. Through the use of the expanding cavity model framework, this article concludes by discussing the plausibility of a relationship between the hardness of the individual powder particles, static and dynamic strength, and the high rate deformation behavior of compacts fabricated from these hard ceramic powders.

A Two-Parameter Function for Nanoscale Indentation Measurement of Near Surface Properties

A two-parameter function for estimation of projected area in instrumented indentation measurement is obtained to account for indenter tip imperfection. Imperfection near indenter tip is modeled using a spherical function and combined with a linear function describing the edge boundary of the indenter. Through an analytical fusion technique, the spherical and linear functions are combined into a single function with two unknown geometric parameters of tip radius of curvature and edge slope. Data from indentation measurement of force and displacement, using a Berkovich tip and single crystal alumina and silica samples, are implemented in the proposed area function yielding estimated values of Young’s modulus. Results were compared with that obtained from Oliver and Pharr technique for deep as well as shallow indentation regimes. The estimates for Young’s modulus were found to agree quite favorably. More importantly, in contrast to the Oliver–Pharr technique, the use of the two-parameter function resulted in a significantly more accurate estimation of Young’s modulus for shallow indentation depth of 0–50nm. The error in estimation of Young’s modulus was found to be within 10% for indentation depths of 25–50nm and within 20% for indentation depths of 0–25nm.

Assessment and Verification of a Novel Method for Near Surface Measurement of Mechanical Properties

A novel two-parameter area function for determination of near surface properties of Young’s modulus of elasticity and hardness has shown promise for compensating for the imperfection of the tip-end in an instrumented indentation measurement. This paper provides a comprehensive study involving a Berkovitch tip. The tip is utilized in an MTS nanoindentation measurement machine and is used to establish load indentation information for fused silica samples. The geometry of the tip is then characterized independently using a highly accurate atomic force microscope. Using the indentation data along with the two-parameter area function methodology, the tip-end radius of curvature is found to provide the most consistent value of modulus of elasticity. Independently, the data from the scanning electron microscope measurement of the same tip is used to obtain the least-squares estimation of the tip curvature. The two approaches yield favorable agreement in the estimation of tip-end radius of curvature. Therefore, the validity of the two-parameter area function method is proved. The method is shown to provide a robust, reliable, and accurate measurement of modulus of elasticity and hardness in the nanoscale proximity of a surface.

Determining the Instantaneous Modulus of Viscoelastic Solids Using Instrumented Indentation Measurements

Instrumented indentation is often used in the study of small-scale mechanical behavior of “soft” matters that exhibit viscoelastic behavior. A number of techniques have recently been proposed to obtain the viscoelastic properties from indentation load–displacement curves. In this study, we examine the relationships between initial unloading slope, contact depth, and the instantaneous elastic modulus for instrumented indentation in linear viscoelastic solids using either conical or spherical indenters. In particular, we study the effects of “hold-at-the-peak-load” and “hold-at-the-maximum-displacement” on initial unloading slopes and contact depths. We then discuss the applicability of the Oliver–Pharr method (Refs. 29, 30) for determining contact depth that was originally proposed for indentation in elastic and elastic-plastic solids and recently modified by Ngan et al. (Refs. 20–23) for viscoelastic solids. The results of this study should help facilitate the analysis of instrumented indentation measurements in linear viscoelastic solids.

Scaling, dimensional analysis, and indentation measurements

We provide an overview of the basic concepts of scaling and dimensional analysis, followed by a review of some of the recent work on applying these concepts to modeling instrumented indentation measurements. Specifically, we examine conical and pyramidal indentation in elastic–plastic solids with power-law work-hardening, in power-law creep solids, and in linear viscoelastic materials. We show that the scaling approach to indentation modeling provides new insights into several basic questions in instrumented indentation, including, what information is contained in the indentation load–displacement curves? How does hardness depend on the mechanical properties and indenter geometry? What are the factors determining piling-up and sinking-in of surface profiles around indents? Can stress–strain relationships be obtained from indentation load–displacement curves? How to measure time dependent mechanical properties from indentation? How to detect or confirm indentation size effects? The scaling approach also helps organize knowledge and provides a framework for bridging micro- and macro-scales. We hope that this review will accomplish two purposes: (1) introducing the basic concepts of scaling and dimensional analysis to materials scientists and engineers, and (2) providing a better understanding of instrumented indentation measurements.