bluuebunny/crossref_metadata_2025 huggingface dataset:
The Impact of Sampling Schemes on the Site Frequency Spectrum in Nonequilibrium Subdivided Populations
Authors: Städler Thomas , Haubold Bernhard , Merino Carlos , Stephan Wolfgang , Pfaffelhuber Peter
Published: February 2009

Using coalescent simulations, we study the impact of three different sampling schemes on patterns of neutral diversity in structured populations. Specifically, we are interested in two summary statistics based on the site frequency spectrum as a function of migration rate, demographic history of the entire substructured population (including timing and magnitude of specieswide expansions), and the sampling scheme. Using simulations implementing both finite-island and two-dimensional stepping-stone spatial structure, we demonstrate strong effects of the sampling scheme on Tajima's D (DT) and Fu and Li's D (DFL) statistics, particularly under specieswide (range) expansions. Pooled samples yield average DT and DFL values that are generally intermediate between those of local and scattered samples. Local samples (and to a lesser extent, pooled samples) are influenced by local, rapid coalescence events in the underlying coalescent process. These processes result in lower proportions of external branch lengths and hence lower proportions of singletons, explaining our finding that the sampling scheme affects DFL more than it does DT. Under specieswide expansion scenarios, these effects of spatial sampling may persist up to very high levels of gene flow (Nm > 25), implying that local samples cannot be regarded as being drawn from a panmictic population. Importantly, many data sets on humans, Drosophila, and plants contain signatures of specieswide expansions and effects of sampling scheme that are predicted by our simulation results. This suggests that validating the assumption of panmixia is crucial if robust demographic inferences are to be made from local or pooled samples. However, future studies should consider adopting a framework that explicitly accounts for the genealogical effects of population subdivision and empirical sampling schemes.
Sex-lethalFacilitates the Transition From Germline Stem Cell to Committed Daughter Cell in the Drosophila Ovary
Authors: Chau Johnnie , Kulnane Laura Shapiro , Salz Helen K
Published: February 2009

In Drosophila, the female-specific SEX-LETHAL (SXL) protein is required for oogenesis, but how Sxl interfaces with the genetic circuitry controlling oogenesis remains unknown. Here we use an allele of sans fille (snf) that specifically eliminates SXL protein in germ cells to carry out a detailed genetic and cell biological analysis of the resulting ovarian tumor phenotype. We find that tumor growth requires both Cyclin B and zero population growth, demonstrating that these mutant cells retain at least some of the essential growth-control mechanisms used by wild-type germ cells. Using a series of molecular markers, we establish that while the tumor often contains at least one apparently bona fide germline stem cell, the majority of cells exhibit an intermediate fate between a stem cell and its daughter cell fated to differentiate. In addition, snf tumors misexpress a select group of testis-enriched markers, which, remarkably, are also misexpressed in ovarian tumors that arise from the loss of bag of marbles (bam). Results of genetic epistasis experiments further reveal that bam's differentiation-promoting function depends on Sxl. Together these data demonstrate a novel role for Sxl in the lineage progression from stem cell to committed daughter cell and suggest a model in which Sxl partners with bam to facilitate this transition.
Heparin-Bonded Central Venous Catheters Do Not Reduce Thrombosis in Infants With Congenital Heart Disease: A Blinded Randomized, Controlled Trial
Authors: Anton Natalie , Cox Peter N. , Massicotte M. Patricia , Chait Peter , Yasui Yutaka , Dinyari P. Maria , Marzinotto Velma , Mitchell Lesley G.
Published: February 2009

Background. Infants with congenital heart disease who require central venous lines are at increased risk of thrombosis. Heparin-bonded catheters provide protection from thrombotic events in some children. However, heparin-bonded catheters may not be as effective in infants ≤1 year old because of other potential risk factors (smaller vessel size, longer duration of catheter use). No studies have assessed the benefit of heparin-bonded catheters in such specific high-risk populations. The objective of this study was to assess the efficacy of heparin-bonded catheters for preventing thrombosis in infants aged ≤1 year with congenital heart disease.

Study Design. This study was designed as a randomized, controlled, blinded single-center trial. Infants ≤1 year old with congenital heart disease requiring a central venous line for clinical care were randomly assigned to receive either a heparin-bonded catheter or a standard non–heparin-bonded catheter. Catheters were visually indistinguishable. The primary outcome was incidences of both silent and clinically noticeable thrombosis confirmed by ultrasound. Ultrasounds were reviewed by a blinded central adjudication committee. Interim analysis was performed after enrollment of 97 patients.

Results. Eighty-seven patients were evaluable (41 of the patients were female). Thrombotic events occurred in 17 (42.5%) of 40 patients in the non–heparin-bonded catheter group and in 21 (44.7%) of 47 patients in the heparin-bonded catheters group. The study was stopped when the interim analysis showed convincing evidence for no difference between groups over the alternative hypothesis of 50% risk reduction.

Conclusions. Infants with congenital heart disease are at significant risk of both silent and clinically identified thrombosis. There seems to be no advantage in using heparin-bonded catheters in infants ≤1 year of age.
Can Young’s modulus and hardness of wire structural materials be directly measured using nanoindentation?
Authors: Shu S.Q. , Yang Y. , Fu T. , Wen C.S. , Lu J.
Published: February 2009

In recent studies, nanoindentation experiments combined with the Oliver and Pharr method (OP method) are frequently used to measure the mechanical properties of “one-dimensional” structural materials (micro/nanowires and nanobelts) regardless of the corresponding assumptions of the OP method. This article reports the numerical simulation studies of the nanoindentations of wire structural materials on elastic-plastic substrates using dimensional analysis and the finite element method. We find that the measured hardness and Young’s modulus of wire structural materials are significantly influenced by their geometries and indenters as well as the mechanical properties of substrates and wires.
Thermal treatment effects on hard and soft films measured by nanoindentation test
Authors: Tai F.C. , Lee S.C. , Wei C. , Wei P.J. , Lin J.F.
Published: February 2009

Nanoindentation measurements using the Berkovich diamond tip with a load up to 700 and 210 mN were performed on 1-μm hydrogenated diamond-like carbon (DLCH) films and 6-μm polyimide (PI) films under different thermal treatment. The average nanohardness and elastic modulus from nanoindentation measurements are 25.13 and 192.18 GPa for as-deposited DLCH film and 22.93 and 174.22 GPa for as-annealed DLCH film, respectively. The ID/IG ratio of Raman spectra is 1.72 for as-deposited DLCH film and 2.04 for as-annealed DLCH film. The average nanohardness and elastic modulus from nanoindentation measurements are 0.37 and 5.58 GPa for 300 as-cured PI film and 0.42 and 5.19 GPa for 400 as-cured PI film, respectively. The relative cured rate is 92.1% for 300 as-cured PI film and 100% for 400 as-cured PI film. Both the nanohardness and elastic modulus derived from nanoindentation test results can correlate well with the trend of Raman spectra and FTIR spectra for hard DLCH film and soft PI film under different thermal treatment effects.
Mechanism of aging effects on viscoelasticity in ethylene-methacrylic acid ionomer studied by local thermal-mechanical analysis
Authors: Kulkarni Harsha P. , Mogilevsky Gregory , Mullins William M. , Wu Yue
Published: February 2009

A unique atomic force microscope-based local thermal-mechanical analysis (LTA) technique was used to study the influence of room temperature aging on viscoelastic properties of ethylene-methacrylic (E/MAA) acid ionomers. This approach permits easy access to structural relaxation effects on viscoelasticity at a short aging time, for instance, before the occurrence of secondary crystallization differential scanning calorimetry (DSC) melting peak. A Burger model along with finite element method yields quantitative analysis of viscoelastic properties versus the aging time. Creep curves were obtained with LTA after various times of aging at room temperature upon cooling from the melt. Measurements were carried out at both 30 and 70 °C. The results reveal the effects of structural relaxation upon aging in the ion-rich amorphous region, the influence of secondary crystallites on the viscoelastic properties, and shed light on the processes associated with aging in E/MAA ionomers.
Relation between the ratio of elastic work to the total work of indentation and the ratio of hardness to Young's modulus for a perfect conical tip
Authors: Chen J. , Bull S.J.
Published: February 2009

A linear relationship between the ratio of elastic work to the total indentation work and hardness to reduced modulus, i.e., We/Wt = λ H/Er, has been derived analytically and numerically in a number of studies and has been widely accepted. However, the scaling relationship between We/Wt and H/Er has recently been questioned, and it was found that λ is actually not a constant but is related to material properties. In this study, a new relationship between We/Wt and H/Er has been derived, which shows excellent agreement with numerical simulation and experimental results. We also propose a method for obtaining the elastic modulus and hardness of a material without invoking the commonly used Oliver and Pharr method. Furthermore, it is demonstrated that this method is less sensitive to tip imperfections than the Oliver and Pharr approach is.
Modified method for continuous stiffness measurement
Authors: Wei Pal Jen , Lin Jen Fin
Published: February 2009

This study proposes a method developed to simultaneously solve contact hardness and reduced modulus by loading and unloading coefficients together with the inclined angle of an indenter. The ratios of the applied load to the squared slopes of load–depth curves during loading and unloading processes were used to determine loading and unloading coefficients. The values of the contact area estimated by the present method were found to be precise for a variety of materials. Compared to the reduced modulus, errors due to underestimated contact area were found more significant in the evaluation of contact hardness.
Nanoindentation study of slip transfer phenomenon at grain boundaries
Authors: Britton T.B. , Randman D. , Wilkinson A.J.
Published: February 2009

Nanoindentation was undertaken near grain boundaries to increase understanding of their individual contributions to the material’s macroscopic mechanical properties. Prior work with nanoindentation in body-centered cubic (bcc) materials has shown that some grain boundaries produce a “pop-in” event, an excursion in the load–displacement curve. In the current work, grain boundary associated pop-in events were observed in a Fe–0.01 wt% C polycrystal (bcc), and this is characteristic of high resistance to intergranular slip transfer. Grain boundaries with greater misalignment of slip systems tended to exhibit greater resistance to slip transfer. Grain boundary associated pop-ins were not observed in pure copper (face-centered cubic) or interstitial free steel ~0.002 wt% C (bcc). Additionally, it was found that cold work of the Fe–0.01 wt% C polycrystal immediately prior to indentation completely suppressed grain boundary associated pop-in events. It is concluded that the grain boundary associated pop-in events are directly linked to interstitials pinning dislocations on or near the boundary. This links well with macroscopic Hall–Petch effect observations.
Measuring the constitutive behavior of viscoelastic solids in the time and frequency domain using flat punch nanoindentation
Authors: Herbert E.G. , Oliver W.C. , Lumsdaine A. , Pharr G.M.
Published: February 2009

The purpose of this work is to further develop experimental methodologies using flat punch nanoindentation to measure the constitutive behavior of viscoelastic solids in the frequency and time domain. The reference material used in this investigation is highly plasticized polyvinylchloride (PVC) with a glass transition temperature of −17 °C. The nanoindentation experiments were conducted using a 983-μm-diameter flat punch. For comparative purposes, the storage and loss modulus obtained by nanoindentation with a 103-μm-diameter flat punch and dynamic mechanical analysis are also presented. Over the frequency range of 0.01–50 Hz, the storage and loss modulus measured using nanoindentation and uniaxial compression is shown to be in excellent agreement. The creep compliance function measured using a constant stress test performed in uniaxial compression and flat punch nanoindentation is also shown to correlate well over nearly 4 decades in time. In addition, the creep compliance function predicted from nanoindentation data acquired in the frequency domain is shown to correlate strongly with the creep compliance function measured in the time domain. Time–temperature superposition of nanoindentation data taken at 5, 10, 15, and 22 °C shows the sample is not thermorheologically simple, and thus the technique cannot be used to expand the mechanical characterization of this material. Collectively, these results clearly demonstrate the ability of flat punch nanoindentation to accurately and precisely determine the constitutive behavior of viscoelastic solids in the time and frequency domain.
Nanoindentation of yttria-doped zirconia: Effect of crystallographic structure on deformation mechanisms
Authors: Gaillard Y. , Anglada M. , Jiménez-Piqué E.
Published: February 2009

This article presents a nanoindentation study of polycrystalline and single crystals of yttria-doped zirconia with both tetragonal and cubic phases. Analysis of the deformation mechanisms is performed by both atomic force microscopy (AFM) and micro-Raman spectroscopy. Phase transformation from tetragonal to monoclinic phase is clearly distinguished on tetragonal crystals, whereas in cubic crystals the plastic deformation seems to be controlled by dislocation nucleation and interactions. AFM observations in tetragonal zirconia grains have shown that both grain size and autocatalytic transformation strongly influence the size of the transformed zone. Furthermore, the martensitic phase transformation seems to be also strongly dependent of the indenter shape. Experimental results suggest that a critical contact pressure is necessary to induce the phase transformation.
Comparative investigation of strength and plastic instability in Cu/Au and Cu/Cr multilayers by indentation
Authors: Li Y.P. , Zhu X.F. , Tan J. , Wu B. , Wang W. , Zhang G.P.
Published: February 2009

The mechanical response to indentation (including nano- and microindentation) has been investigated in Cu/Au and Cu/Cr multilayers with respective layer thickness ratios of 1:1 and 2:1, and individual layer thickness ranging from nanometer to submicrometer scale. It was found that the Cu/Cr multilayer has higher strength than the Cu/Au multilayer, although both multilayers have close Hall–Petch slope. Examination of indentation-induced deformation behavior shows that the Cu/Cr multilayer exhibits higher resistance to plastic deformation instability than the Cu/Au multilayer. Theoretical analysis indicates that the significant difference in mechanical response originates from the constituent layer configuration and interface structures, which impose distinguishing confining effect on dislocation activity.
The equivalence of axisymmetric indentation model for three-dimensional indentation hardness
Authors: Qin J. , Huang Y. , Xiao J. , Hwang K.C.
Published: February 2009

Nix and Gao [J. Mech. Phys. Solids46, 411 (1998)] established an important relation between the microindentation hardness and indentation depth for axisymmetric indenters. We use the conventional theory of mechanism-based strain gradient plasticity [Huang et al., Int. J. Plast.20, 753 (2004)] established from the Taylor dislocation model [Taylor, Proc. R. Soc. London A145, 362 (1934); Taylor, J. Inst. Met.62, 307 (1938)] to study the Berkovich and other triangular pyramid indenters. The three-dimensional finite element analysis shows that the widely used equivalence of equal base area leads to significant errors, particularly in microindentation. A new equivalence of equal angle is proposed for triangular pyramid indenters, and it has been validated for a large range of indenter angles and indentation depths.
A quantitative analysis for the stress field around an elastoplastic indentation/contact
Authors: Feng Gang , Qu Shaoxing , Huang Yonggang , Nix William D.
Published: February 2009

In our previous paper [Feng et al., Acta Mater.55, 2929 (2007)], an analytical model is proposed to estimate the stress field around an elastoplastic indentation/contact, matching nicely with the finite element analysis. The model is related to an embedded center of dilatation (ECD) in a half-space. In this paper, we focus on determining the ECD strength B* and the ECD depth ξ. By matching an expanding cavity model and the ECD model, we find that B* ≈ Yc3/6 and ξ ≈ 0.4c, where Y is the yield strength and c is the plastic zone radius. We provide a method to predict Y, c, and thereby B* as well as ξ through nanoindentation data, and we also demonstrate that pileup is the physical reason for the existence of the upper limit for the ratio of hardness to Y. Thus, our ECD model is completed by combining our previous paper (the analytical expression) and this paper (the essential parameters).
Molecular dynamics study of crystal plasticity during nanoindentation in Ni nanowires
Authors: Dupont V. , Sansoz F.
Published: February 2009

Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on <111>-oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of 1 m·s−1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation.
Measurements of diffusion thickness at polymer interfaces by nanoindentation: A numerically calibrated experimental approach
Authors: Yang Chunyu , Lo Chieh-Tsung , Bastawros Ashraf F. , Narasimhan Balaji
Published: February 2009

The interfacial fracture toughness and the adhesion strength of two dissimilar materials are governed by the diffusion interfacial thickness and its mechanical characteristics. A new testing methodology is implemented here to estimate the actual interfacial thickness from a series of nanoindentations across the interface, under the same applied load, with tip radius and indentation depth many times larger than the interface thickness. The bimaterial system used is a semicrystalline polymer interface of isotactic polypropylene and linear low-density polyethylene. The laminate is prepared under a range of diffusion temperature to yield diffusion interfaces of 0 to 50 nm. A numerical relationship is developed using two-dimensional (2D) finite element simulation to correlate the true interfacial thickness, measured by transmission electron microscopy, with the experimentally estimated apparent interfacial thickness, derived from the transition domain of a series of indents across the interface. A range of material-pairs property combinations are examined for Young’s modulus ratio E1/E2 = 1 to 3, yield strength ratio σY1/σY2 = 1 to 2.5, and interfacial thickness of 0 to 100 nm. The proposed methodology and the numerically calibrated relationship are in good agreement with the true interfacial thickness.
Indentation creep of a Ti-based metallic glass
Authors: Huang Y.J. , Chiu Y.L. , Shen J. , Chen J.J.J. , Sun J.F.
Published: February 2009

In this work, the time-dependent plastic deformation behavior of Ti40Zr25Ni3Cu12Be20bulk and ribbon metallic glass alloys was investigated using a nanoindentation technique at room temperature with the applied load ranging from 5 to 100 mN. The stress exponentn, defined as, has been derived as a measure of the creep resistance. It was found that the measured stress exponent increases rapidly with increasing indentation size, exhibiting a positive size effect. The size effect on the stress exponentnobtained from the bulk sample is more pronounced than that obtained from the ribbon sample. The deformation mechanism involved will be discussed.
Determination of plastic properties by instrumented spherical indentation: Expanding cavity model and similarity solution approach
Authors: Jiang Peng , Zhang Taihua , Feng Yihui , Yang Rong , Liang Naigang
Published: February 2009

The present paper aims to develop a robust spherical indentation-based method to extract material plastic properties. For this purpose, a new consideration of piling-up effect is incorporated into the expanding cavity model; an extensive numerical study on the similarity solution has also been performed. As a consequence, two semi-theoretical relations between the indentation response and material plastic properties are derived, with which plastic properties of materials can be identified from a single instrumented spherical indentation curve, the advantage being that this approach no longer needs estimations of contact radius with given elastic modulus. Moreover, the inconvenience in using multiple indenters with different tip angles can be avoided. Comprehensive sensitivity analyses show that the present algorithm is reliable. Also, by experimental verification performed on three typical materials, good agreement of the material properties between those obtained from the reverse algorithm and experimental data is obtained.
Response of power-law-viscoelastic and time-dependent materials to rate jumps
Authors: Ngan A.H.W. , Tang B.
Published: February 2009

Nonlinear viscoelastic problems are in general not analytically solvable. However, it is shown here that, for any viscoelastic materials describable by a constitutive law with linear elastic and (in general) nonlinear viscous elements arranged in any network fashion, such as the Maxwell or standard linear solid arrangements, it is always possible to eliminate the viscous terms by replacing the displacement, strain, and stress fields of the problem by the jumps in rates of these fields. After the viscous terms are eliminated, the problem is reduced to a linear elastic problem defined on the same spatial domain and with the same elastic constant as in the original viscoelastic problem. Such a reduced elastic problem is analytically solvable in many practical cases, and the solution yields a relation between jumps in the load rate and the displacement rate, pertinent to the boundary conditions in the original problem. Such a relation can often be used as the basis for an experimental scheme to measure the elastic constants of materials. The material can be time- or strain-dependent, and the value of the elastic constant measured corresponds to the time instant or the strain value when the jump in load or displacement rate is implemented.
Indentation strength of a piezoelectric ceramic: Experiments and simulations
Authors: Kamble S.N. , Kubair D.V. , Ramamurty U.
Published: February 2009

The spherical indentation strength of a lead zirconate titanate (PZT) piezoelectric ceramic was investigated under poled and unpoled conditions and with different electrical boundary conditions (arising through the use of insulating or conducting indenters). Experimental results show that the indentation strength of the poled PZT is higher than that of the unpoled PZT. The strength of a poled PZT under a conducting indenter is higher than that under an insulating indenter. Poling direction (with respect to the direction of indentation loading) did not significantly affect the strength of material. Complementary finite element analysis (FEA) of spherical indentation of an elastic, linearly coupled piezoelectric half-space is conducted for rationalizing the experimental observations. Simulations show marked dependency of the contact stress on the boundary conditions. In particular, contact stress redistribution in the coupled problem leads to a change in the fracture initiation, from Hertzian cracking in the unpoled material to subsurface damage initiation in poled PZT. These observations help explain the experimental ranking of strength the PZT in different material conditions or under different boundary conditions.
Influence of specimen preparation, microstructure anisotropy, and residual stresses on stress–strain curves of rolled Al2024 T351 as derived from spherical indentation tests
Authors: Heerens J. , Mubarok F. , Huber N.
Published: February 2009

In the present work, a previously developed neural network approach for analyzing spherical indentation experiments is applied to prestressed specimens to determine the effect of residual stresses on the identified stress–strain curves. Within this scope, a comparison to other measurement errors has been made, which are caused by surface preparation and anisotropy of the material. To validate the experimental and analysis approach, the effect of compressive and tensile prestresses was also simulated using a three-dimensional finite element model. The material investigated is a rolled 2024 T351, which is widely used for manufacturing airplanes. It is shown that the existing neural network approach is able to determine the stress–strain behavior in agreement with that obtained from tensile tests. The method is robust against most error sources, such as surface roughness, coarse grain structure, and anisotropy, if a sufficient number of experiments are available. The most important influencing factor can be the residual stress causing errors up to 20% in the identified stress–strain curves.
Determination of plastic properties of metals by instrumented indentation using a stochastic optimization algorithm
Authors: Peyrot I. , Bouchard P-O. , Ghisleni R. , Michler J.
Published: February 2009

A novel optimization approach, capable of extracting the mechanical properties of an elasto-plastic material from indentation data, is proposed. Theoretical verification is performed on two simulated configurations. The first is based on the analysis of the load–displacement data and the topography of the residual imprint of a single conical indenter. The second is based on the load–displacement data obtained from two conical indenters with different semi-angles. In both cases, a semi-analytical approach [e.g., Dao et al., Acta Mater.49, 3899 (2001) and Bucaille et al., Acta Mater.51, 1663 (2003)] is used to estimate Young’s modulus, yield stress, and strain hardening coefficient from the load–displacement data. An inverse finite element model, based on a commercial solver and a newly developed optimization algorithm based on a robust stochastic methodology, uses these approximate values as starting values to identify parameters with high accuracy. Both configurations use multiple data sets to extract the elastic-plastic material properties; this allows the mechanical properties of materials to be determined in a robust way.
The correlation between the internal material length scale and the microstructure in nanoindentation experiments and simulations using the conventional mechanism-based strain gradient plasticity theory
Authors: Backes B. , Huang Y.Y. , Göken M. , Durst K.
Published: February 2009

In the present work a new equation to determine the internal material length scale for strain gradient plasticity theories from two independent experiments (uniaxial and nanoindentation tests) is introduced. The applicability of the presented equation is verified for conventional grained as well as for ultrafine-grained copper and brass with different amounts of prestraining. A significant decrease of the experimentally determined internal material length scale is found for increasing dislocation densities due to prestraining. Conventional mechanism strain gradient plasticity theory is used for simulating the indentation response, using experimentally determined material input data as the yield stress, the work-hardening behavior and the internal material length scale. The work-hardening behavior and the yield stress were taken from the uniaxial experiments, whereas the internal material length scale was calculated using the yield stress from the uniaxial experiment, the macroscopic hardness H0 and the length scale parameter h* following from the nanoindentation experiment. A good agreement between the measured and calculated load–displacement curve and the hardness is found independent of the material and the microstructure.
Analysis of the effects of conical indentation variables on the indentation response of elastic–plastic materials through factorial design of experiment
Authors: Rodríguez Pulecio Sara Aida , Moré Farias María Cristina , Souza Roberto Martins
Published: February 2009

In this work, the effects of conical indentation variables on the load–depth indentation curves were analyzed using finite element modeling and dimensional analysis. A factorial design 26 was used with the aim of quantifying the effects of the mechanical properties of the indented material and of the indenter geometry. Analysis was based on the input variables Y/E, R/hmax, n, θ, E, and hmax. The dimensional variables E and hmax were used such that each value of dimensionless Y/E was obtained with two different values of E and each value of dimensionless R/hmax was obtained with two different hmax values. A set of dimensionless functions was defined to analyze the effect of the input variables: Π1 = Pl/Eh2, Π2 = hc/h, Π3 = H/Y, Π4= S/Ehmax, Π6 = hmax/hf, and Π7 = Wp/WT. These six functions were found to depend only on the dimensionless variables studied (Y/E, R/hmax, n, θ). Another dimensionless function, Π5 = β, was not well defined for most of the dimensionless variables and the only variable that provided a significant effect on β was θ. However, β showed a strong dependence on the fraction of the data selected to fit the unloading curve, which means that β is especially susceptible to the error in the calculation of the initial unloading slope.
On the usage of the effectively shaped indenter concept for analysis of yield strength
Authors: Herrmann M. , Richter F.
Published: February 2009

Using the extended Hertzian approach (EHA), the “effectively shaped indenter” corresponding to Pharr's concept is described in terms of a parameter set {d0,d2,d4,d6}, which can be determined by a fitting procedure from the unloading curve of an indentation experiment. Owing to the limited accuracy of measurement, a given experimental curve may in principle correspond to more than one such parameter set. Based on indentation experiments with a Berkovich indenter into fused silica, we have investigated the influence of the fitting procedure itself on the results. We suggest a certain manual fitting procedure, which delivered a yield strength Y = (7.1 ± 0.1) GPa independent of the maximum load. Manual fitting always includes some degree of subjectivity, however, both Y and the elastic field as a whole proved to be relatively robust against modifications of the parameter set. We also suggest a preliminary objective procedure, which delivered Y = 6.8 − 7.1 GPa. In addition, we have performed finite element method (FEM) simulations of elastic–plastic indentations of a conical indenter into a von Mises solid with a yield strength of Y = 7.0 GPa. The simulated unloading curve was analyzed using the EHA in the same manner as the experimental curves, and yield strength of 6.95 GPa was obtained being very close to the input value of the FEM.
A simple method for evaluating elastic modulus of thin films by nanoindentation
Authors: Wei Zhongxin , Zhang Guoping , Chen Hao , Luo Jian , Liu Ranran , Guo Shengmin
Published: February 2009

A simple empirical method that extracts the elastic moduli of both thin films and the underlying substrates is proposed and validated by both new nanoindentation experiments and published data. Deconvolution of thin film’s elastic properties from the substrate is achieved by statistical estimation, where a simple function relating the elastic moduli of the thin film and substrate to the film-substrate composite modulus is used to fit the experimental data plotted against the logarithmic indentation depth normalized by film thickness. Experimental data from a wide range of soft and hard films on substrate were used to demonstrate the deconvolution and validate the method. The estimated elastic moduli of thin films and substrates agree well with their corresponding standard values or values obtained by other methods. The advantages of this method are discussed, and recommendations are made on how to design experiments to obtain reliable data for this method.
Indentation fracture of silicone gels
Authors: Lin Wei-Chun , Otim Kathryn J. , Lenhart Joseph L. , Cole Phillip J. , Shull Kenneth R.
Published: February 2009

Indentation tests were performed, using a flat punch probe, on silicone gels to induce failure under compression. The silicone gels were formed from networks of vinyl-terminated polydimethylsiloxane (PDMS) with molecular weights of 800 and 28,000 g/mol and a sol fraction of trimethylsiloxy-terminated PDMS with molecular weights ranging from 1250 to 139,000 g/mol. Cone cracks were observed in samples that fractured from defects at the sample surface, but failure more commonly originated from the corners of the indenter. Ring cracks were observed for the most highly compliant samples that fractured at indentation depths approaching the overall thickness of the sample. In these cases we generally observed a delayed fracture response, with a time delay that increased with increasing sol fraction and decreased with increasing indentation load.
Measuring substrate-independent modulus of dielectric films by instrumented indentation
Authors: Hay Jennifer
Published: February 2009

Substrate influence is a common problem when using instrumented indentation (also known as nanoindentation) to evaluate the mechanical properties of thin films. In this work, finite element analysis was used to develop an ad hoc model that predicts the substrate influence when testing thin dielectric films on silicon. The model was evaluated experimentally using three sets of films that were nominally the same except for thickness. Using the model significantly reduced the measurement error for the thinnest films (<250 nm) by accurately accounting for the influence of the substrate. The model also significantly reduced the measurement uncertainty, because properties were evaluated using larger indents that would normally be unduly affected by the substrate. The process for developing this model may be useful in developing other ad hoc models for analyzing film-substrate systems.
Finite element analysis and experimental investigation of the Hertzian assumption on the characterization of initial plastic yield
Authors: Ma Li , Morris Dylan J. , Jennerjohn Stefhanni L. , Bahr David F. , Levine Lyle
Published: February 2009

Sudden displacement excursions during load-controlled nanoindentation of relatively dislocation-free surfaces of metals are frequently associated with dislocation nucleation, multiplication, and propagation. Insight into the nanomechanical origins of plasticity in metallic crystals may be gained through estimation of the stresses that nucleate dislocations. An assessment of the potential errors in the experimental measurement of nucleation stresses, especially in materials that exhibit the elastic–plastic transition at small indentation depths, is critical. In this work, the near-apex shape of a Berkovich probe was measured by scanning probe microscopy. This shape was then used as a “virtual” indentation probe in a 3-dimensional finite element analysis (FEA) of indentation on 〈100〉-oriented single-crystal tungsten. Simultaneously, experiments were carried out with the real indenter, also on 〈100〉-oriented single-crystal tungsten. There is good agreement between the FEA and experimental load–displacement curves. The Hertzian estimate of the radius of curvature was significantly larger than that directly measured from the scanning probe experiments. This effect was replicated in FEA simulation of indentation by a sphere. These results suggest that Hertzian estimates of the maximum shear stresses in the target material at the point of dislocation nucleation are a conservative lower bound. Stress estimates obtained from the experimental data using the Hertzian approximation were over 30% smaller than those determined from FEA.
Nanoindentation investigations to study solid solution hardening in Ni-based diffusion couples
Authors: Franke Oliver , Durst Karsten , Göken Mathias
Published: February 2009

In this work the hardening effect of Ta and Mo in Ni-base alloys was investigated using a combinatorial approach with diffusion couples. Furthermore, the Ni-Fe system was used as a reference system taking advantage of the full miscibility at high temperatures. Ta was chosen, as aside from having a technical relevance in the Ni-base superalloys, it also has a high miscibility in Ni. The main focus of this paper will be solid solution hardening. It will be shown that even though the determination of hardness is subject to varying indentation size effects (ISE) [Durst et al., Acta Mater.55(20), 6825 (2007)], only a few modifications are necessary to describe solid solution strengthening measured by nanoindentations using the Labusch theory [Labusch, Acta Metall.20(7), 917 (1972)]. Moreover, after a careful evaluation of the results, the data can be used to investigate solid solution hardening effects quickly and efficiently with small amounts of material.
Nanoindentation of thin films: Simulations and experiments
Authors: Nair Arun K. , Cordill M.J. , Farkas Diana , Gerberich W.W.
Published: February 2009

Atomistic simulations of nanoindentation of a 20-nm-thick Ni thin film oriented in the [111] direction were carried out to study the effects of indenter velocity and radii, interatomic potentials, and the boundary conditions used to represent the substrate. The simulation results were compared directly with experimental results of Ni thin film of the same thickness and orientation. It was found that the high indenter velocity does not affect the hardness value significantly. Different radii used for indentation also have negligible effects on the hardness value. Two different interatomic potentials were tested, giving significantly different hardness values but both within 20% of the experimental result. Different boundary conditions used to represent the substrate have a significant effect for relatively deep indentation simulations.
Importance of surface preparation on the nano-indentation stress-strain curves measured in metals
Authors: Pathak Siddhartha , Stojakovic Dejan , Doherty Roger , Kalidindi Surya R.
Published: February 2009

In this work, we investigated experimentally the various factors influencing the extraction of indentation stress-strain curves from spherical nanoindentation on metal samples using two different tip radii. In particular, we focused on the effects of (i) the surface preparation techniques used, (ii) the presence of a surface oxide layer, and (iii) the occurrence of pop-ins at the elastic-plastic transition on our newly developed data analysis methods for extracting reliable indentation stress-strain curves. Rough mechanical polishing was shown to introduce a large scatter in the measured indentation yield strengths, whereas electropolishing or vibropolishing produced consistent results reflective of the pristine sample. The data analysis techniques used were able to discard the portions of the raw data affected by a thin oxide layer, present on most metal surfaces, and yield reasonable indentation stress-strain curves. Experiments with different indenter tip radii on annealed and cold-worked samples indicated that pop-ins are caused by delayed nucleation of dislocations in the sample under the indenter.
Experimental measurement and numerical simulation of the plastic strain during indentation and scratch tests on polymeric surfaces
Authors: Pelletier Hervé , Gauthier Christian , Schirrer Robert
Published: February 2009

For elastic–plastic contacts, we propose a complete description of the plastic strain field beneath the indenter during indentation and scratch with a spherical indenter (with R, the tip radius), as a function of the testing conditions, defined by the geometrical strain, noted a/R (with a, the contact radius), and the local friction coefficient μloc. The main parameter of the description is the level of the plastic deformation imposed during test into amorphous polymeric surfaces, related in first approximation to the ratio a/R. An equivalent average plastic strain, noted (εp)av, is calculated over a representative plastically deformed volume, both for indentation and scratch tests. The equivalent average plastic strain (εp)av, is observed to increase with the ratio a/R, as predicted by the empirical Tabor's rule, but also with the local friction coefficient μloc for a given ratio a/R, especially during scratching. The plastic zone dimensions and the plastic strain gradient developed beneath the moving tip are shown to depend both on the geometrical strain a/R and also on the friction coefficient μloc.
The role of crystalline anisotropy in mechanical property extractions through Berkovich indentation
Authors: Alcalá J. , Esqué-de los Ojos D. , Rodríguez S.A.
Published: February 2009

This work uses crystal plasticity finite element simulations to elucidate the role of elastoplastic anisotropy in instrumented indentation P–hs curve measurements in face-centered cubic (fcc) crystals. It is shown that although the experimental fluctuations in the loading stage of the P–hs curves can be attributed to anisotropy, the variability in the unloading stage of the experiments is much greater than that resulting from anisotropy alone. Moreover, it is found that the conventional procedure used to evaluate the contact variables ruling the unloading P–hs curve introduces an uncertainty that approximates to the more fundamental influence of anisotropy. In view of these results, a robust procedure is proposed that uses contact area measurements in addition to the P–hs curves to extract homogenized J2-plasticity-equivalent mechanical properties from single crystals.
Computational mechanical property determination of viscoelastic/plastic materials from nanoindentation creep test data
Authors: Wang Jianjun , Ovaert Timothy C.
Published: February 2009

Nanoindentation is a widely accepted test method for materials characterization. On account of the complexity of contact deformation behavior, design of parametric constitutive models and determination of the unknown parameters is challenging. To address the need for identification of mechanical properties of viscoelastic/plastic materials from nanoindentation data, a combined numerical finite element/optimization-based indentation modeling tool was developed, fully self-contained, and capable of running on a PC as a stand-alone executable program. The approach uses inverse engineering and formulates the material characterization task as an optimization problem. The model development consists of finite element formulation, viscoelastic/plastic material models, heuristic estimation to obtain initial solution boundaries, and a gradient-based optimization algorithm for fast convergence to extract mechanical properties from the test data. A four-parameter viscoelastic/plastic model is presented, then a simplified three-parameter model with more rapid convergence. The end result is a versatile tool for indentation simulation and mechanical property analysis.
Numerical analysis of plastic deformation evolution into metallic materials during spherical indentation process
Authors: Beghini M. , Bertini L. , Fontanari V. , Monelli B.D.
Published: February 2009

The present paper deals with the plastic deformation process into metallic materials occurring in the subindenter region during the loading cycle of spherical indentation test. Load–indentation-depth curve and plastic strains field evolution in the region beneath the indenter are examined using finite element analysis (FEA). The FE model was set up and validated by comparison with experimental spherical indentations carried out on two different materials (Al6082-T6, AISI H13) under four different friction conditions, corresponding to friction coefficients equal to 0.0, 0.1, 0.3, and 0.5. It is confirmed that friction effects on load–indentation-depth curves are negligible for the investigated penetration depths, whereas the plastic deformation process is affected by the contact conditions. The investigation shows that, although the L–h curve is not affected by the contact conditions up to medium values of the penetration depth, remarkable effects are produced in the overall plastic core under the indenter. A strong correlation between plastic strains field and friction coefficient is especially observed at low values of this parameter, whereas a saturation of the phenomena is found for medium-high values of the friction coefficient.
Spherical indentation of lungs: Experiments, modeling and sub-surface imaging
Authors: Silva Maricris R. , Yuan Zhijia , Kim Jae Hun , Wang Zhenguo , Hoyos Melissa , Pan Yingtian , Gouldstone Andrew
Published: February 2009

Knowledge of mechanics in atelectasis (alveolar collapse) and reinflation would be useful during anesthesia and critical care. Here an investigation is presented in which atelectasis is induced in a controlled manner on excised inflated lungs using spherical indentation, and noninvasive imaging of the deformed subsurface region is performed using optical coherence tomography (OCT). Indentation loads are physiologic, and spatial dimensions are far larger than alveolar size to allow continuum discussions. Experimental observations of atelectasis are compared with finite element model calculations of maximum stresses. Finally, atelectasis is compared during inflation of lungs with different gases (e.g., air, oxygen/anesthesia mixture).