Showing papers on "Indentation published in 2017"

Indentation Hardness Measurements at Macro-, Micro-, and Nanoscale: A Critical Overview

Abstract: The Brinell, Vickers, Meyer, Rockwell, Shore, IHRD, Knoop, Buchholz, and nanoindentation methods used to measure the indentation hardness of materials at different scales are compared, and main issues and misconceptions in the understanding of these methods are comprehensively reviewed and discussed. Basic equations and parameters employed to calculate hardness are clearly explained, and the different international standards for each method are summarized. The limits for each scale are explored, and the different forms to calculate hardness in each method are compared and established. The influence of elasticity and plasticity of the material in each measurement method is reviewed, and the impact of the surface deformation around the indenter on hardness values is examined. The difficulties for practical conversions of hardness values measured by different methods are explained. Finally, main issues in the hardness interpretation at different scales are carefully discussed, like the influence of grain size in polycrystalline materials, indentation size effects at micro- and nanoscale, and the effect of the substrate when calculating thin films hardness. The paper improves the understanding of what hardness means and what hardness measurements imply at different scales.

Extraordinary Indentation Strain Stiffening Produces Superhard Tungsten Nitrides

Abstract: Transition-metal light-element compounds are a class of designer materials tailored to be a new generation of superhard solids, but indentation strain softening has hitherto limited their intrinsic load-invariant hardness to well below the 40 GPa threshold commonly set for superhard materials. Here we report findings from first-principles calculations that two tungsten nitrides, hP4-WN and hP6-WN_{2}, exhibit extraordinary strain stiffening that produces remarkably enhanced indentation strengths exceeding 40 GPa, raising exciting prospects of realizing the long-sought nontraditional superhard solids. Calculations show that hP4-WN is metallic both at equilibrium and under indentation, marking it as the first known intrinsic superhard metal. An x-ray diffraction pattern analysis indicates the presence of hP4-WN in a recently synthesized specimen. We elucidate the intricate bonding and stress response mechanisms for the identified structural strengthening, and the insights may help advance rational design and discovery of additional novel superhard materials.

Anisotropic Mechanical Behavior of AlSi10Mg Parts Produced by Selective Laser Melting

Abstract: AlSi10Mg cylinders produced by laser powder-bed fusion have somewhat different yield behavior for cylinders with XY orientation and Z orientation. Earlier yielding for Z-oriented samples is likely related to micro-residual stress, resulting from the difference in thermal expansion of the aluminum matrix and cellular silicon. Smaller tensile reduction in area of Z-oriented samples is related to tearing along the softer region at the boundaries of melt pools, where the silicon cell spacing is larger. Indentation measurements confirmed the lower hardness at the edges of melt pools.

On the determination of elastic moduli of cells by AFM based indentation

Abstract: The atomic force microscopy (AFM) has been widely used to measure the mechanical properties of biological cells through indentations. In most of existing studies, the cell is supposed to be linear elastic within the small strain regime when analyzing the AFM indentation data. However, in experimental situations, the roles of large deformation and surface tension of cells should be taken into consideration. Here, we use the neo-Hookean model to describe the hyperelastic behavior of cells and investigate the influence of surface tension through finite element simulations. At large deformation, a correction factor, depending on the geometric ratio of indenter radius to cell radius, is introduced to modify the force-indent depth relation of classical Hertzian model. Moreover, when the indent depth is comparable with an intrinsic length defined as the ratio of surface tension to elastic modulus, the surface tension evidently affects the indentation response, indicating an overestimation of elastic modulus by the Hertzian model. The dimensionless-analysis-based theoretical predictions, which include both large deformation and surface tension, are in good agreement with our finite element simulation data. This study provides a novel method to more accurately measure the mechanical properties of biological cells and soft materials in AFM indentation experiments.

Characterizing white matter tissue in large strain via asymmetric indentation and inverse finite element modeling.

Abstract: Characterizing the mechanical properties of white matter is important to understand and model brain development and injury. With embedded aligned axonal fibers, white matter is typically modeled as a transversely isotropic material. However, most studies characterize the white matter tissue using models with a single anisotropic invariant or in a small-strain regime. In this study, we combined a single experimental procedure - asymmetric indentation - with inverse finite element (FE) modeling to estimate the nearly incompressible transversely isotropic material parameters of white matter. A minimal form comprising three parameters was employed to simulate indentation responses in the large-strain regime. The parameters were estimated using a global optimization procedure based on a genetic algorithm (GA). Experimental data from two indentation configurations of porcine white matter, parallel and perpendicular to the axonal fiber direction, were utilized to estimate model parameters. Results in this study confirmed a strong mechanical anisotropy of white matter in large strain. Further, our results suggested that both indentation configurations are needed to estimate the parameters with sufficient accuracy, and that the indenter-sample friction is important. Finally, we also showed that the estimated parameters were consistent with those previously obtained via a trial-and-error forward FE method in the small-strain regime. These findings are useful in modeling and parameterization of white matter, especially under large deformation, and demonstrate the potential of the proposed asymmetric indentation technique to characterize other soft biological tissues with transversely isotropic properties.

Structural origin of high crack resistance in sodium aluminoborate glasses

Abstract: Sodium aluminoborate glasses are found to exhibit favorable mechanical properties, especially high crack resistance, due to their relatively low resistance to network compaction during sharp-contact loading. We here reveal the origin of the high crack resistance by investigating changes in structure and mechanical properties in compositions ranging from peralkaline to peraluminous and by varying the pressure history through an isostatic N 2 -mediated pressure treatment at elevated temperature. This approach allows us to study the composition dependence of the ease of the glassy network compaction and the accompanying changes in structure and properties, which shed light on the processes occurring during indentation. Through solid state NMR measurements, we show that the network densification involves an increase in the average coordination number of both boron and aluminum and a shortening of the sodium-oxygen bond length. These changes in the short-range order of the glassy networks manifest themselves as an increase in, e.g., density and indentation hardness. We also demonstrate that the glasses most prone to network compaction exhibit the highest damage resistance, but surprisingly the crack resistance scales better with the relative density increase achieved by the hot compression treatment rather than with the extent of densification induced by indentation. This suggests that tuning the network structure may lead to the development of more damage resistant glasses, thus addressing the main drawback of this class of materials.

Characterization of indentation damage resistance of hybrid composite laminates using acoustic emission monitoring

Abstract: This paper focuses on the experimental investigation of impact damage resistance in hybrid composite laminates. In this case, the low velocity impact behaviour of quasi-isotropic glass/epoxy, glass/basalt/epoxy (G/B/G, B/G/B) and glass/carbon/epoxy (G/C/G, C/G/C) composite laminates was simulated by carrying out quasi-static indentation (QSI) tests with online acoustic emission (AE) monitoring. The dent depth, back surface crack size and load-deflection behaviors were examined and there is no distinct differences could be seen between low velocity impact tests and quasi-static indentation tests. The QSI tests were performed on specimens with rectangular section, of size 150 mm x 100 mm, which were loaded at the centre by a hemispherical steel indenter with 12.7 mm diameter. The indentation response was evaluated by measuring peak force, absorbed energy and linear stiffness. The residual strength of the laminates following indentation was measured by testing them under compression load in a 100 kN universal testing machine, once again with AE monitoring. AE parameters, such as amplitude, rise time, cumulative counts and cumulative energy were considered for monitoring damage progression during quasi-static indentation loading. Also other parameters linked to AE monitoring, such as the rise angle (RA) and Felicity ratio (FR) were measured for evaluating the damage resistance in each cycle of indentation. In addition, sentry function was also computed based on the combination of mechanical strain energy accumulated in the materials and of the acoustic energy propagates by fracture events made it possible to evaluate the amount of induced damage. These results showed that the combination of glass and carbon fibres in glass/carbon/epoxy (C/G/C) laminates improved their interlaminar shear strength at a level well above the other configurations tested.

Mechanical characterization of carbon nanotubes based polymer composites using indentation tests

Abstract: In this paper, instrumented indentation testing was used to determine the elastic mechanical properties of multiwall carbon nanotubes (CNTs) reinforced polymer composites. A finite element analysis model for simulating micro-indentation test was developed using ABAQUS software and confronted to experimental tests. It seems that CNTs reinforced polymer composites shows an improved of mechanical properties. The microstructure of indentation mark has been evaluated using scanning electronic microscopes and compared with the microstructure of numerical models. It results that adding small amount of CNTs improves the fracture interfacial rigidity and can stop micro-cracks evolution.

Advanced Nanoindentation Testing for Studying Strain-Rate Sensitivity and Activation Volume.

Abstract: Nanoindentation became a versatile tool for testing local mechanical properties beyond hardness and modulus. By adapting standard nanoindentation test methods, simple protocols capable of probing thermally activated deformation processes can be accomplished. Abrupt strain-rate changes within one indentation allow determining the strain-rate dependency of hardness at various indentation depths. For probing lower strain-rates and excluding thermal drift influences, long-term creep experiments can be performed by using the dynamic contact stiffness for determining the true contact area. From both procedures hardness and strain-rate, and consequently strain-rate sensitivity and activation volume can be reliably deducted within one indentation, permitting information on the locally acting thermally activated deformation mechanism. This review will first discuss various testing protocols including possible challenges and improvements. Second, it will focus on different examples showing the direct influence of crystal structure and/or microstructure on the underlying deformation behavior in pure and highly alloyed material systems.

Vickers indentation fracture toughness of Y-TZP dental ceramics

Abstract: The purpose of this study was to investigate and analyse the fracture toughness of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) dental ceramics by means of the Vickers indentation technique. Indentation tests are commonly used for the evaluation of the fracture toughness of brittle materials, particularly glass and ceramics, because this technique requires a small sample from which a large number of data points can be generated rapidly. However, a wide variety of equations for the evaluation of the fracture toughness by means of Vickers indentation are available. Therefore, it is important to choose the most appropriate model for the description of this ceramics' fracture behavior. In the present work, nine different fracture toughness equations are considered and the results are compared with the literature value obtained by a conventional method, single edge V-notched beam (SEVNB) test. The influence of the applied indentation load on fracture toughness was also observed. Tests were conducted under four different loading conditions (29.42 N, 49.03 N, 196.13 N, and 294.20 N). Obtained results indicate that the fracture toughness of Y-TZP dental ceramics depends on the indentation load, morphology of indentation cracking as well as on the applied model of fracture resistance.

From elasticity to capillarity in soft materials indentation

Abstract: For soft materials with Young's moduli below 100 kPa, quantifying mechanical and interfacial properties by small scale indentation is challenging because in addition to adhesion and elasticity, surface tension plays a critical role. Until now, microscale contact of very soft materials has only been studied by static experiments under zero external loading. Here we introduce a combination of the colloidal probe technique and confocal microscopy to characterize the force-indentation and force-contact radius relationships during microindentation of soft silicones. We confirm that the widespread Johnson-Kendall-Roberts theory must be extended to predict the mechanical contact for soft materials. Typically a liquid component is found within very soft materials. With a simple analytical model, we illustrate that accounting for this liquid surface tension can capture the contact behavior. Our results highlight the importance of considering liquid that is often associated with soft materials during small scale contact.

Ultrasonic damage investigation on woven jute/poly (lactic acid) composites subjected to low velocity impact

Abstract: In the present contribution, particular attention is devoted to the fabrication and characterization of fully bio laminated structures based on a commercial poly (lactic acid) (PLA) including a jute fabric as reinforcements, even because of the limited availability of the reference literature. An ultrasonic technique is used to investigate the delamination caused by low velocity impact tests, on plates obtained by typical film stacking and compression molding techniques. The square specimens, 100*100 mm, were impacted by a falling dart machine at five increasing impact energy values of 2 J, 5 J, 10 J, 12 J, and 15 J. The delamination is investigated by the very commonly used Ultra Sound technique employing a linear phased array probe. The delaminated area is correlated with both the impact energy and the measured indentation depth. The results showed a threshold energy for the beginning of the internal damage. Moreover, a linear relationship between the delaminated area, the energy and the indentation depth was found.

Instrumented indentation of fused silica by Berkovich indenter

Abstract: Scaling relationships among indentation variables such as the maximum indentation displacement, contact depth, the ratio of load over contact stiffness, and the ratio of permanent displacement over the maximum displacement were investigated via Berkovich indentation of fused silica. Load-displacement curves were characterized by power-law functions for both loading and unloading. The power exponents can be assumed to be constant for loading and unloading. Hardness, elastic modulus, and yield stress were measured to be 9.8 GPa, 73.3 GPa, and 5.5 GPa, respectively, for fused silica that can be assumed to be elastic-perfectly plastic. The empirical algorithm for estimating yield stress was verified for fused silica. The experimental results accorded well with theoretical prediction and dimensional analysis. The expressions obtained can be used to check indentation data, and general implications of the relations are discussed.

Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics

Abstract: Using embedded atom method potential, extensive large-scale molecular dynamics (MD) simulations of nanoindentation/nanoscratching of nanocrystalline (nc) iron have been carried out to explore grain size dependence of wear response. MD results show no clear dependence of the frictional and normal forces on the grain size, and the single-crystal (sc) iron has higher frictional and normal force compared to nc-samples. For all samples, the dislocation-mediated mechanism is the primary cause of plastic deformation in both nanoindentation/nanoscratch. However, secondary cooperative mechanisms are varied significantly according to grain size. Pileup formation was observed in the front of and sideways of the tool, and they exhibit strong dependence on grain orientation rather than grain size. Tip size has significant impact on nanoscratch characteristics; both frictional and normal forces monotonically increase as tip radii increase, while the friction coefficient value drops by about 38%. Additionally, the increase in scratch depth leads to an increase in frictional and normal forces as well as friction coefficient. To elucidate the relevance of indentation/scratch results with mechanical properties, uniaxial tensile test was performed for nc-samples, and the result indicates the existence of both the regular and inverse Hall–Petch relations at critical grain size of 110.9 A. The present results suggest that indentation/scratch hardness has no apparent correlation with the mechanical properties of the substrate, whereas the plastic deformation has.

Atomistic Studies of Nanoindentation—A Review of Recent Advances

Abstract: This review covers areas where our understanding of the mechanisms underlying nanoindentation has been increased by atomistic studies of the nanoindentation process While such studies have been performed now for more than 20 years, recent investigations have demonstrated that the peculiar features of nanoplasticity generated during indentation can be analyzed in considerable detail by this technique Topics covered include: nucleation of dislocations in ideal crystals, effect of surface orientation, effect of crystallography (fcc, bcc, hcp), effect of surface and bulk damage on plasticity, nanocrystalline samples, and multiple (sequential) indentation In addition we discuss related features, such as the influence of tip geometry on the indentation and the role of adhesive forces, and how pre-existing plasticity affects nanoindentation

Determination of equi-biaxial residual stress and plastic properties in structural steel using instrumented indentation

Abstract: This study describes a method that allows estimation of the equi-biaxial residual stress (σR), yield strength (σy), the strain hardening exponent (n), and the ratio (α) between strain at starting-point of strain hardening (est) and yield strain (ey) of structural steel from the load-depth curve of one sharp indentation test. In this method, the relationships between the structural steel properties, residual stress, and the characteristics of the indentation loading-unloading curves were established in the form of dimensionless functions via extensive finite element (FE) analyses. Based on the FE analysis results of the indentation process with a large number of different material properties combinations, the effects of residual stress and plastic parameters on the indentation response were investigated, and a reverse algorithm for estimating four unknown quantities (σR, σy, n, and α) of steel from an indentation test was proposed. A stress-applying apparatus that allows the introduction of a compressive or tensile stress to the sample by applying a compressive or tensile force and maintaining the stress after releasing the force was also developed in this study. Indentation and tensile tests of structural steels (SS400 and SM490) were carried out and the proposed algorithm was validated through both numerical analyses and experimental results.

Indentation metrology of clamped, ultra-thin elastic sheets

Abstract: We study the indentation of ultrathin elastic sheets clamped to the edge of a circular hole. This classical setup has received considerable attention lately, being used by various experimental groups as a probe to measure the surface properties and stretching modulus of thin solid films. Despite the apparent simplicity of this method, the geometric nonlinearity inherent in the mechanical response of thin solid objects renders the analysis of the resulting data a nontrivial task. Importantly, the essence of this difficulty is in the geometric coupling between in-plane stress and out-of-plane deformations, and hence is present in the behaviour of Hookean solids even when the slope of the deformed membrane remains small. Here we take a systematic approach to address this problem, using the membrane limit of the Foppl–von-Karman equations. This approach highlights some of the dangers in the use of approximate formulae in the metrology of solid films, which can introduce large errors; we suggest how such errors may be avoided in performing experiments and analyzing the resulting data.