Showing papers on "Indentation published in 2019"

Indentation deformation in oxide glasses: Quantification, structural changes, and relation to cracking

Abstract: Improving the fracture resistance of oxide glasses through adjustment of the chemical composition remains a challenging task, although composition-mechanical property relations have been established for simple model systems. The glass mechanical properties are, among other methods, conventionally tested using instrumented indentation, which is a fast and convenient technique that mimics the real-life damage for certain applications, although interpretation can be challenging due to the complex stress fields that develop under the indenter. Early indentation experiments have shown that oxide glasses exhibit pronounced tendency to densify under compressive load compared to metals and ceramics. After decades of investigations, it is now known that the extent of densification is strongly dependent on the glass' chemical composition and in turn its atomic packing density and Poisson's ratio. Spectroscopic techniques have shed light on the mechanism of densification, which include changes in the bond angle distributions as well as an increase in the coordination number of the network-forming cations. Knowledge of such details is crucial for understanding the link between chemical composition and resistance to cracking in oxide glasses, since densification is an efficient way to dissipate the elastic energy applied to the material during indentation. Here, we review the experimental work on identification and quantification of indentation deformation in glasses, as well as on probing the accompanying structural changes in the glassy network. We also include the conclusions drawn from computer simulation studies, which can provide atomistic details of the indentation deformation mechanisms. Finally, we discuss the link between the mechanism of deformation and the crack resistance.

Anisotropy vs isotropy in living cell indentation with AFM.

Abstract: The measurement of local mechanical properties of living cells by nano/micro indentation relies on the foundational assumption of locally isotropic cellular deformation As a consequence of assumed isotropy, the cell membrane and underlying cytoskeleton are expected to locally deform axisymmetrically when indented by a spherical tip Here, we directly observe the local geometry of deformation of membrane and cytoskeleton of different living adherent cells during nanoindentation with the integrated Atomic Force (AFM) and spinning disk confocal (SDC) microscope We show that the presence of the perinuclear actin cap (apical stress fibers), such as those encountered in cells subject to physiological forces, causes a strongly non-axisymmetric membrane deformation during indentation reflecting local mechanical anisotropy In contrast, axisymmetric membrane deformation reflecting mechanical isotropy was found in cells without actin cap: cancerous cells MDA-MB-231, which naturally lack the actin cap, and NIH 3T3 cells in which the actin cap is disrupted by latrunculin A Careful studies were undertaken to quantify the effect of the live cell fluorescent stains on the measured mechanical properties Using finite element computations and the numerical analysis, we explored the capability of one of the simplest anisotropic models - transverse isotropy model with three local mechanical parameters (longitudinal and transverse modulus and planar shear modulus) - to capture the observed non-axisymmetric deformation These results help identifying which cell types are likely to exhibit non-isotropic properties, how to measure and quantify cellular deformation during AFM indentation using live cell stains and SDC, and suggest modelling guidelines to recover quantitative estimates of the mechanical properties of living cells

Grain boundary effect on nanoindentation: A multiscale discrete dislocation dynamics model

Abstract: Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GBs) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a three-dimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.

Role of indentation depth and contact area on human perception of softness for haptic interfaces.

Abstract: In engineering, the “softness” of an object, as measured by an indenter, manifests as two measurable parameters: (i) indentation depth and (ii) contact area. For humans, softness is not well defined, although it is believed that perception depends on the same two parameters. Decoupling their relative contributions, however, has not been straightforward because most bulk—“off-the-shelf”—materials exhibit the same ratio between the indentation depth and contact area. Here, we decoupled indentation depth and contact area by fabricating elastomeric slabs with precise thicknesses and microstructured surfaces. Human subject experiments using two-alternative forced-choice and magnitude estimation tests showed that the indentation depth and contact area contributed independently to perceived softness. We found an explicit relationship between the perceived softness of an object and its geometric properties. Using this approach, it is possible to design objects for human interaction with a desired level of perceived softness.

Mechanical behaviour of sintered silver nanoparticles reinforced by SiC microparticles

Abstract: SiC microparticles with various weight ratios (0.0, 0.5, 1.0 and 1.5 wt%) are incorporated into sintered silver nanoparticles (AgNP) as one of the promising packaging materials for high-power electronic devices. Mechanical properties and constitutive behaviour of sintered AgNP reinforced by SiC microparticles are investigated based on nanoindentation experiment and analytical approach. Nanoindentations were performed in the manner of continuous stiffness measurement for a maximum penetration depth of 2000 nm at a strain rate of 0.05 s−1. Particularly, a Berkovich indenter is utilized to evaluate the values of Young's modulus and hardness, and a spherical indenter is utilized to describe the constitutive behaviour. For sintered AgNP with 0.5 wt% SiC, the morphology exhibits uniformly compact microstructures to enable optimizing the heat conductivity, the yield strength and hardening capacity of sintered AgNP material is enhanced. To describe the constitutive behaviour, an analytical approach is proposed to simulate the indentation behaviour. The parameters in the modified power-law model are determined by fitting the average indentation responses. The developed correlation between microstructure and macroscopic properties facilitates the design of AgNP paste morphology and improves the mechanical properties of sintered AgNP in electronics packaging.

Assessment of Nano-Indentation Method in Mechanical Characterization of Heterogeneous Nanocomposite Materials Using Experimental and Computational Approaches.

Abstract: This study investigates the capacity of the nano-indentation method in the mechanical characterization of a heterogeneous dental restorative nanocomposite using experimental and computational approaches. In this respect, Filtek Z350 XT was selected as a nano-particle reinforced polymer nanocomposite with a specific range of the particle size (50 nm to 4 µm), within the range of indenter contact area of the nano-indentation experiment. A Sufficient number of nano-indentation tests were performed in various locations of the nanocomposite to extract the hardness and elastic modulus properties. A hybrid computational-experimental approach was developed to examine the extracted properties by linking the internal behaviour and the global response of the nanocomposite. In the computational part, several representative models of the nanocomposite were created in a finite element environment to simulate the mechanism of elastic-plastic deformation of the nanocomposite under Berkovich indenter. Dispersed values of hardness and elastic modulus were obtained through the experiment with 26.8 and 48.5 percent average errors, respectively, in comparison to the nanocomposite properties, respectively. A disordered shape was predicted for plastic deformation of the equilateral indentation mark, representing the interaction of the particles and matrix, which caused the experiment results reflect the local behaviour of the nanocomposite instead of the real material properties.

On the study of mystical materials identified by indentation on power law and Voce hardening solids

Abstract: We conduct an incisive investigation of the existence of a one-to-one correspondence between a material's elastoplastic properties and its indentation responses, with particular emphasis on the residual imprint. We first unravel a so-called "mystical material" pair reported by Chen et al. (2007) by examining the specimens' post-indentation morphologies, despite using a single self-similar indenter. Next, using Metric Multidimensional Scaling (MDS), we mitigate the mystical material issue for materials hardening according to the popular Hollomon's power law equation. In contrast, the same exact protocol reveals the absence of a one-to-one correspondence between the single indentation response and Voce hardening parameters. To alleviate this, we propose a multi-depth indentation strategy.

Nanoindentation studies of the mechanical behaviours of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites

Abstract: In this study, the influence of multiwall carbon nanotubes (MWCNT) additions on the mechanical properties of sintered Ti6Al4V-based nanocomposites was investigated. The nanocomposites were fabricated with varying weight fractions of MWCNT (0.5, 1.0 & 1.5 wt%) using the spark plasma sintering (SPS) technique. Investigations were carried out using nanoindentation of varying indentation loads (50 mN, 75 mN and 100 mN) to assess the nanohardness (H) and reduced elastic modulus (Er) of the alloy and nanocomposites. Further analysis was done to evaluate the elastic recovery index ( W e W t ) , plasticity index ( W p W t ) , elastic strain resistance ( H E r ) and yield pressure ( H 3 E r 2 ) at the maximum load. Microstructural analysis revealed the presence of the MWCNT dispersed across the alpha and beta phases of the Ti6Al4V matrix. The nanoindentation studies showed that the nanohardness, elastic modulus, elastic recovery index, elastic strain resistance and anti-wear properties improved with the MWCNT addition and continually increased with increase in nanotubes content. Also, it was observed that the nanohardness and reduced elastic modulus of the fabricated nanocomposites are in the range of 4677–9276 MPa and 29.3–60.9 GPa respectively which declined with increase in indentation load. The sintered Ti6Al4V displayed the least resistance to plastic deformation.

Indentation deformation and cracking in oxide glass –toward understanding of crack nucleation

Abstract: Cracking is one of the most serious and unfavorable phenomena in glass. To enhance the damage resistance of glass, methods for controlling factors related to cracking have been discussed for many years. In this brief review, various works on indentation-induced fracture in oxide glass are introduced and relevant issues regarding indentation techniques are discussed. In particular, various aspects of the indentation fracture (IF) method, which is a well-known empirical technique for evaluating the apparent fracture toughness of brittle materials, are discussed and it is emphasized that indentation techniques are powerful tools for comparing crack resistances between glass compositions. Finally, some novel approaches to analyzing indentation deformation and fracture in oxide glass are introduced to aid in understanding the brittle behaviors of glass and improving the mechanical properties of glass.

Hardness and Indentation Fracture Toughness of Slip Cast Alumina and Alumina-Zirconia Ceramics

Abstract: Alumina (Al2O3) and zirconia (ZrO2) have good overall properties and thus are widely used oxide technical ceramics. The biggest drawback of Al2O3 is its low fracture toughness. In contrast, ZrO2 is relatively tough, but is also much more expensive. In order to improve the alumina toughness, composite ceramics are being developed. Slip casting technology has economic advantages over the conventional hot isostatic pressure technology, but problems may arise when preparing stable highly-concentrated suspensions (slip) for filling the mold. The purpose of this study is to prepare aqueous suspensions using 70 wt. % α-Al2O3, with 0, 1, 5 and 10 wt. % of added t-ZrO2. Suspensions were electrosterically stabilized using the ammonium salt of polymethylacrylic acid, an alkali-free anionic polyelectrolyte dispersant. Also, magnesium oxide in form of magnesium aluminate spinel (MgAl2O4) was used to inhibit the abnormal alumina grain growth during the sintering process. Minimum viscosities were used as stability estimators, where an increase in ZrO2 content required adding more dispersant. After sintering, the Vickers indentation test was used to determine the hardness and the indentation fracture toughness from the measurement of the crack length. Also, the brittleness index (Bi, μm-1/2) was calculated from values of Vickers hardness and the Vickers indentation fracture toughness. It was found that with increasing ZrO2 content the fracture toughness increased, while the hardness as well as the brittleness index decreased. Zirconia loading reduces the crystallite sizes of alumina, as confirmed by the X-ray diffraction analysis. SEM/EDS analysis showed that ZrO2 grains are distributed in the Al2O3 matrix, forming some agglomerates of ZrO2 and some pores, with ZrO2 having a smaller grain size than Al2O3.

Molecular dynamics and experimental studies of nanoindentation on nanoporous silica aerogels

Abstract: The nanomechanics during the indentation test on low-density nanoporous silica aerogels remains one of the least understood and explored areas of mechanics. In the present work, we performed nanoindentation using a spherical indenter on silica aerogels to investigate the mechanical properties, such as elastic modulus and hardness, and also, the deformation behaviour. Using all-atom simulations on large samples, the elastic modulus is computed from the elastic part of force–depth curves that can be fitted to the Hertz law, which shows that it increases with density. We proposed a novel approach to calculate the projected true contact area in nanoindentation and to estimate an accurate hardness of silica aerogel, which has a highly complex and randomly arranged network of atoms structure. The experimental studies of nanoindentation are performed on silica aerogel, which reveals that the measured elastic modulus is in good agreement with the simulations. However, the measured hardness values are nearly close to the projected contact area method. It suggests that in all-atom simulations the computed high hardness values using the proposed true area method are the actual local contact pressure. This new understanding may help to expand the use of computer simulations to explore the nanoindentation processes at the molecular level and to advance the macroscopic hardness calculation.

Discrete dislocation dynamics simulations of nanoindentation with pre-stress: Hardness and statistics of abrupt plastic events

Abstract: Nanoindentation of crystalline materials has been thought as a primarily surface-driven technique that is not able to probe bulk mechanical properties directly, such as material yield strength or bulk plastic flow rates. We elucidate this question through extensive discrete dislocation plasticity simulations of nanoindentation on a single crystal. We consider the competition between nanoindentation and tensile loading (pre-stress) towards crystal plasticity. For this purpose, we study a two-dimensional discrete dislocation model where indentation is performed by using cylindrical indentation with varying radius under both displacement and load control. We focus on the behavior of the hardness and pop-in event statistics during nanoindentation under various pre-stress levels and we correlate them to the spatially correlated dislocation microstructure behavior. At small indentation depths (relative to other microstructural or tip length scales), we find that the hardness is inversely dependent on the plastic strain/dislocation density induced by the applied tensile pre-stress; consequently, we argue that small-depth indentation may be useful for identifying bulk plastic yielding behavior prior to indentation. In contrast, for larger indentation depths, pre-stress has no effect on hardness. However, effect of pre-stress can be revealed through plastic events statistics, both in load and displacement controlled protocols. Moreover, post-indentation surface morphology clearly shows the effect of the pre-stress.

Breaking the Limit of Micro‐Ductility in Oxide Glasses

Abstract: Oxide glasses are one of the most important engineering and functional material families owing to their unique features, such as tailorable physical properties. However, at the same time intrinsic brittleness has been their main drawback, which severely restricts many applications. Despite much progress, a breakthrough in developing ultra-damage-resistant and ductile oxide glasses still needs to be made. Here, a critical advancement toward such oxide glasses is presented. In detail, a bulk oxide glass with a record-high crack resistance is obtained by subjecting a caesium aluminoborate glass to surface aging under humid conditions, enabling it to sustain sharp contact deformations under loads of ≈500 N without forming any strength-limiting cracks. This ultra-high crack resistance exceeds that of the annealed oxide glasses by more than one order of magnitude, making this glass micro-ductile. In addition, a remarkable indentation behavior, i.e., a time-dependent shrinkage of the indent cavity, is demonstrated. Based on structural analyses, a molecular-scale deformation model to account for both the ultra-high crack resistance and the time-dependent shrinkage in the studied glass is proposed.

Dome-Shape Auxetic Cellular Metamaterials: Manufacturing, Modeling, and Testing

Abstract: We present in this work the manufacturing, modelling and testing of dome-shaped cellular structures with auxetic (negative Poisson's ratio) behaviour. The auxetic configurations allow the creation of structures with synclastic (i.e. dome-shaped) curvatures, and this feature is used to evaluate the performance of cellular metamaterials under quasi-static indentation conditions. We consider here different cellular geometries (re-entrant, arrow-head, tri-chiral, hexagonal) and the implications of their manufacturing using 3D printing techniques with PLA material. The dome-shaped configurations are modelled using full-scale non-linear explicit FE models that represent both the geometry and approximate constitutive models of the PLA filament material derived from tensile tests on dogbone specimens. The cellular metamaterials samples are subjected to indentation tests, with maps of strains obtained through DIC measurements. The correlation between experimental and numerical simulations is good, and shows the peculiar indentation behaviour of these cellular structures. We also perform a comparative analysis by simulation of the force/displacement, strain and fracture history during quasi-static loading, and discuss the performance of the different cellular topologies for these dome-shape metamaterial designs.

New insights into the indentation size effect in silicate glasses

Abstract: When glasses resist permanent deformations, for applications such as a windshield or the cover of a touch-screen device, it is important to understand how they respond to inelastic energy dissipation processes. Many glasses exhibit the so-called Indentation Size Effect (ISE), where the indentation hardness is dependent on the maximum force exerted on the probe. In this study, we perform microindentation on silica and soda lime silicate glasses over a wide range of maximum forces and extract the Vickers hardness by the Oliver and Pharr method. The inelastic volume responsible for dissipating the inelastic energy is decomposed into densification and plastic flow, using surface topography and annealing. We show that the ISE is intimately linked to these mechanisms. Finally, we hypothesize the cause of the ISE is an increase in plasticity in a zone of material under the indenter probe experiencing reduced viscosity due to high strain rates and shear thinning.

Processing, microstructure and mechanical properties of HfB2-ZrB2-SiC composites: Effect of B4C and carbon nanotube reinforcements

Abstract: The fully dense HfB2-ZrB2-SiC composites were processed using spark plasma sintering (SPS) at 1850 °C. The effect of reinforcements (B4C and CNT) on the densification as well as mechanical properties were investigated and compared (with monolithic) in the present study. The study showed that the addition of B4C and CNT were not only beneficial for the densification but also towards enhancing the mechanical properties (hardness, elastic modulus, and fracture toughness) of HfB2-ZrB2-SiC composites. The augmentation in the mechanical properties establish the synergy between solid solution formation (with the equimolar composition of HfB2/ZrB2) and the reinforcements (SiC, B4C, and CNT). The highest increase in the indentation fracture toughness with the reinforcements of B4C as well as CNT is >3 times (~13.8 MPam0.5 when it is 3–4 MPam0.5 for monolithic ZrB2/HfB2) on HfB2-ZrB2-SiC composites, which is attributed to the crack deflection and pull-out mechanisms. An increase in the analytically quantified interfacial compressive residual stresses in the composites during SPS processing with the synergistic addition of reinforcements (SiC, B4C, and CNT) and its effect on the indentation fracture toughness has also been addressed.