Nanoindentation Creep Tests Research Articles

Characterization of the elastic and viscoelastic properties of dentin by a nanoindentation creep test

Dentin is the main supporting structure of teeth, but its mechanical properties may be adversely affected by pathological demineralization. The purposes of this study were to develop a quantitative approach to characterize the viscoelastic properties of dentin after de- and re-mineralization, and to examine the elastic properties using a nanoindentation creep test. Dentin specimens were prepared to receive both micro- and nano-indentation tests at wet and dry states. These tests were repeatedly performed after demineralization (1% citric acid for 3 days) and remineralization (artificial saliva immersion for 28 days). The nanoindentation test was executed in a creep mode, and the resulting displacement–time responses were disintegrated into primary (transient) and secondary (viscous) creep. The structural changes and mineral densities of dentin were also examined under SEM and microCT, respectively. The results showed that demineralization removed superficial minerals of dentin to the depth of 400μm, and affected its micro- and nano-hardness, especially in the hydrate state. Remineralization only repaired the minerals at the surface layer, and partially recovered the nanohardness. Both the primary the secondary creep increased in the demineralized dentin, while the hydration further enhanced creep deformation of untreated and remineralized dentin. Remineralization reduced the primary creep of dentin, but did not effectively increase the viscosity. In conclusion, water plasticization increases the transient and viscous creep strains of demineralized dentin and reduces load sustainability. The nanoindentation creep test is capable of analyzing the elastic and viscoelastic properties of dentin, and reveals crucial information about creep responses.

A comparative study of indentation induced creep in pure magnesium and AZ61 alloy

Nanoindentation creep tests were performed on pure Mg and Mg–6wt% Al–1wt% Zn (AZ61) alloy at room temperature. Plastic flow and creep were more pronounced in Mg than in AZ61. Pure Mg was found to exhibit two distinct steady state stress regimes, with the values of stress exponent equal to 4.5 and 12. Twinning is most likely to dominate initially, in the high stress regime. Crystallographic re-orientation, caused by twinning renders the grains favorable for 〈c+a〉 slip in low stress regime. Grain boundary creep was also investigated, and appeared to be related to grain boundary sliding (n=2) in low stress regime. Creep response of AZ61 consisted of a single stress exponent regime (n~1) for both grain interior as well as grain boundary. A combination of extension and contraction twinning seems to be the dominant deformation mechanism. Chemical potential gradient induced diffusion is also likely to play an important role in creep deformation.

Nanoindentation study of size effect on shear transformation zone size in a Ni–Nb metallic glass

An apparent reduction in both size and amount of shear bands was observed, as the sample converted from 45μm ribbon to 1.8μm film in a Ni–Nb metallic glass. Such size effect on the macroscopic plastic characteristic could be intrinsically related to the scale variation of the shear transformation zone (STZ). Statistical analysis of the maximum shear stress, which is based on the distribution of first pop-in events in nanoindentation, was employed to estimate the STZ volume and atoms it contains. The result indicates that the STZ size decreases from 83 atoms in the ribbon to 36 atoms in the film. This phenomenon was consistent with the result by another experimental method based on the strain rate sensitivity (SRS). Upon nanoindentation creep test, STZ sizes with 44 atoms and 18 atoms were calculated for the ribbon and film, respectively. The variation trend of the STZ size was discussed in terms of free volume content combined with internal stress which arises during the film deposition and the local atomic structure changes recognized by EXAFS measurement.

Room temperature nanoindentation creep of nanograined NiTiW shape memory alloy thin films

In this paper, the creep behaviors of NiTiW thin films at various W contents (2.6–33.6%) were investigated using the nanoindentation creep testing method. With W content ranging from 2.6at.% to 4.5at.%, the films are strengthened and exhibit a much reduced strain rate ϵ˙=6.76×10−4s−1 indicating the highest creep resistance. With further increase in W content beyond 4.5at.% the strain rate increases and therefore the creep resistance of films decreases gradually. The stress exponents were calculated from the loading curves. The results show that stress exponent for NiTi was 8.2 and increased to 20.5 for NiTiW (2.6) and 22.9 for NiTiW (4.5) and decreased rapidly to 9.5 after increasing the W concentration from 9.1 to 33.6at.%. The mechanism for the room temperature creep is discussed in the framework of dislocation dynamics. Grain boundaries play an important role in creep behavior. Studying the deformation behavior of NiTiW thin films has technological importance because of their various applications in micro- and nano-electro-mechanical systems.

Poroelastic response of articular cartilage by nanoindentation creep tests at different characteristic lengths

Nanoindentation is an experimental technique which is attracting increasing interests for the mechanical characterization of articular cartilage. In particular, time dependent mechanical responses due to fluid flow through the porous matrix can be quantitatively investigated by nanoindentation experiments at different penetration depths and/or by using different probe sizes. The aim of this paper is to provide a framework for the quantitative interpretation of the poroelastic response of articular cartilage subjected to creep nanoindentation tests. To this purpose, multiload creep tests using spherical indenters have been carried out on saturated samples of mature bovine articular cartilage achieving two main quantitative results. First, the dependence of indentation modulus in the drained state (at equilibrium) on the tip radius: a value of 500kPa has been found using the large tip (400μm radius) and of 1.7MPa using the smaller one (25μm). Secon, the permeability at microscopic scale was estimated at values ranging from 4.5×10−16m4/Ns to 0.1×10−16m4/Ns, from low to high equivalent deformation. Consistently with a poroelastic behavior, the size-dependent response of the indenter displacement disappears when characteristic size and permeability are accounted for. For comparison purposes, the same protocol was applied to intrinsically viscoelastic homogeneous samples of polydimethylsiloxane (PDMS): both indentation modulus and time response have been found size-independent.

G0310202 押込み試験による予ひずみを考慮したガスケットの長期・高温粘弾性特性評価(材料力学部門一般セッション:変形・ひずみ(2))

Gasket stresses in flanged joints decrease due to creep deformation of gaskets. Prediction of long-term viscoelastic properties of a gasket is important to ensure the tightness of flange joint for a long-term use, especially in elevated temperature applications. Nanoindentation creep test was carried out using TMA with Berkovich indenter to evaluate the viscoelastic characteristics of a PTFE based sheet gasket. The time-temperature superposition principle using W.L.F equation was applied to obtain the master curve of the creep compliance of gasket material at elevated temperature. The elastic-viscoelastic correspondence principle waz also applied to estimate the gasket creep strain in the flanges from the master curve of the creep compliance. It was able to obtain equally prediction equation with results of HPIS Z 105 test by taking into account the pre-strain. In order to obtain a creep strain predictive expression, it is necessary to consider the prestrain. Creep strain predictive expression is useful for estimating the long-term creep modification in a flange with a high temperature.

Nanoindentation creep behavior of enamel biological nanocomposites

Organic matrix and water are essential factors for enamel biological nanocomposite to resist external force as revealed by nanoindentation creep test.

Poroviscoelastic finite element model including continuous fiber distribution for the simulation of nanoindentation tests on articular cartilage

Articular cartilage is a soft hydrated tissue that facilitates proper load transfer in diarthroidal joints. The mechanical properties of articular cartilage derive from its structural and hierarchical organization that, at the micrometric length scale, encompasses three main components: a network of insoluble collagen fibrils, negatively charged macromolecules and a porous extracellular matrix. In this work, a constituent-based constitutive model for the simulation of nanoindentation tests on articular cartilage is presented: it accounts for the multi-constituent, non-linear, porous, and viscous aspects of articular cartilage mechanics. In order to reproduce the articular cartilage response under different loading conditions, the model considers a continuous distribution of collagen fibril orientation, swelling, and depth-dependent mechanical properties. The model's parameters are obtained by fitting published experimental data for the time-dependent response in a stress relaxation unconfined compression test on adult bovine articular cartilage. Then, model validation is obtained by simulating three independent experimental tests: (i) the time-dependent response in a stress relaxation confined compression test, (ii) the drained response of a flat punch indentation test and (iii) the depth-dependence of effective Poisson's ratio in a unconfined compression test.Finally, the validated constitutive model has been used to simulate multiload spherical nanoindentation creep tests. Upon accounting for strain-dependent tissue permeability and intrinsic viscoelastic properties of the collagen network, the model accurately fits the drained and undrained curves and time-dependent creep response. The results show that depth-dependent tissue properties and glycosaminoglycan-induced tissue swelling should be accounted for when simulating indentation experiments.

Effect of residual stresses on nanoindentation creep behavior of Zr-based bulk metallic glasses

The effect of residual stresses on the time-dependent deformation of a bulk metallic glass is investigated by the nanoindentation technique. In order to induce residual stresses, a beam sample was elastically bent and constrained in a steel ring. The upper side of the beam experiences the tensile residual stress, the lower side the compressive residual stress, and the central line nearly nil stress. Afterward, nanoindentation creep tests were performed on this stressed sample at room temperature. The creep rate is apparently higher on the tensile side, and remains lower and nearly fixed on the compressive side. The data were analyzed using the viscoelasticity iso-strain Kevin model. Individual strain contributions from anelasticity and creep were evaluated. The behavior can be explained by the joint influence of the residual stress and indention loading.

An improved long-term nanoindentation creep testing approach for studying the local deformation processes in nanocrystalline metals at room and elevated temperatures

Abstract

A study on time dependent properties of aluminum alloy by nanoindentation technique

PurposeLightweight alloys are of major concern, due to their applicability, in transport and industry applications. The purpose of this paper is to perform a comprehensive analysis of time dependent properties of aluminum alloy by nanoindentation technique, through investigation of creep behavior. Additionally, possible explanations on the time dependent behavior and the influence of the hold period at maximum load and the loading rate on the elastic modulus and hardness results are also analyzed and discussed.Design/methodology/approachIn this work, a comprehensive analysis of time dependent properties of aluminum alloy by nanoindentation technique was performed, by varying the loading rate, the maximum applied load and the loading time. The stress exponent values are derived from the displacement‐holding time curves. The present experimental setup includes three different approaches: variation of loading rate, maximum applied load and loading time. The creep deformation mechanisms of the alloy, which are dependent on experiment setup, are discussed and the characteristic “elbow” behavior in the unloading part of the curves is also reported.FindingsThe authors found that the stress exponent values obtained are dependent on the applied peak loads and indentation loading rates. Nanoindentation creep testing of aluminum AA6082‐T6 revealed significant creep displacements, where the strain rate reached a steady state after a certain time and the stress decreased with time as the displacement increased during the creep process. The slopes of strain rate versus stress curves (exponent of power‐law creep) for different maximum loads and various holding times, were investigated.Originality/valueThe stress exponent of the constant‐load indentation creep, in all three types of experiments, was found to reduce at low load region. In case of different holding load and time, the stress exponent increased almost linearly and increased very rapidly as the indent size increased, exhibiting an intense size effect.

Nanoindentation creep of ultrafine-grained Al2O3 particle reinforced copper composites

In this paper, the creep behaviors of two Al2O3 particle reinforced copper composites were investigated using nanoindentation creep testing method. It has been shown that stress exponent determined by nanoindentation creep is much larger than that determined by traditional uniaxial tensile/compressive creep. A low externally applied loading results in a large scattering of the stress exponent due to limited grains joining the creep deformation and thus the different strain rates resulted from different grain orientations to the applied stress. The stress exponent of the composites increases with the externally applied loading for both composites, and the composite with higher reinforcement concentration shows larger stress exponents.

High-speed creep process mediated by rapid dislocation absorption in nanocrystalline Cu

A high-speed creep process mediated by rapid dislocation absorption was found in the nanoindentation creep test on nanocrystalline Cu. The creep strain and creep strain rate depend strongly on the loading strain rate and are far higher than those predicted by the models of Coble creep and thermally activated grain boundary sliding. Our analysis revealed that grain boundary dislocation sources can be activated and emitted dislocations from grain boundaries can be stored effectively at a high loading strain rate, but cannot at a low loading strain rate. The observed high-speed creep process is mediated mainly by the rapid absorptions of the stored dislocations and the dislocations newly nucleated during the holding period. An implication of our experimental finding is that dislocation structure in nanocrystalline metals is highly unstable and dislocation activity can proceed after loading and lead to a significant post-loading plasticity.

Effect of grain boundary structures on grain boundary sliding in magnesium

The effect of grain boundary structures on the deformation behavior at the grain boundaries in magnesium was examined by the nanoindentation creep test. The results of the nanoindentation creep test showed that the dominant deformation mechanism around the grain boundary was grain boundary sliding; however, the occurrence of grain boundary sliding was closely related to the grain boundary energy. The grain boundary with high energy showed high strain rate sensitivity, which was the same tendency as that of the other metallic materials. Furthermore, the addition of aluminum atoms into magnesium tended to prevent the grain boundary sliding due to the decrease in grain boundary energy.

Determination of the Creep Properties of Pb-free Solders for Harsh Environments Using Meso-scale Testing

Solder joints in electronic packages are prone to failure due to the evolution of thermal expansion mismatch strains during thermal cycling. The comparatively wide operating temperature range and long lifetimes of aerospace electronics require high reliability solder joints. Since 2006, high reliability industries (aerospace and military amongst others) that are exempt from lead-free RoHS regulation on account of concerns over the reliability of Pb-free solders have found it increasingly difficult and expensive to continue using traditional Sn-Pb-based solders. Hence there is a pressing need to find a suitable alternative that can match the manufacturing and reliability performance of Sn-Pb. There remains a dearth of data for the constitutive behaviour of Pb-free solders under harsh environment scenarios. Unfortunately, conventional test approaches, particularly in the case of creep behaviour which is critical to solder lifetimes, are expensive and time-consuming. High temperature nanoindentation has been recently developed as a quick method for the determination of creep properties of solder alloys. This paper compares and contrasts nanoindentation creep results for bulk Pb-Sn and lead-free solders. However, there are limits to nanoindentation creep, in particular the load-dependence of the technique. A new meso-scale test approach that lies between nanoindentation and bulk creep testing has been developed. Real ball grid arrays using Pb-free solders have been creep tested in the temperature and stress ranges of operating solder joints. High temperature creep constitutive data has been obtained. The technique offers promising time and materials savings in obtaining important mechanical property data for subsequent use in life-prediction models.

Room temperature nanoindentation creep of nanocrystalline Cu and Cu alloys

Nanoindentation creep tests were conducted on nanocrystalline Cu, Cu–2.3Al and Cu–7.2Al (at.%) alloys at room temperature. The stress exponents were calculated from the loading curves. Results show that the stress exponents are much higher than those in the diffusional creep process, ranging from 5 to 50. The stress exponents increase with the indentation load increasing. However, for Cu–Al alloys, the stress exponents are very high even at low loads. The mechanism for the room temperature creep is discussed in the framework of the dislocation dynamics. Grain boundaries may play an important role in the creep behavior. However, there is no clear correlation among the stress exponent, the alloy composition and the grain size. The dislocation histories and solute atoms may affect the creep behaviors, i.e. both the generation and the recovery of dislocations.

Nanoindentation creep behavior of grain boundary in pure magnesium

Nanoindentation creep tests were performed at the grain boundary and grain interior in pure magnesium. The grain boundary showed a high strain rate sensitivity exponent and was dominated by grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior of the area at a distance of 2 µm into the grain interior. On the other hand, the grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was determined to be dislocation slip.

Nanocharacterization of creep behavior of multiwall carbon nanotubes/epoxy nanocomposite

High temperature instrumented indentation testing was used to evaluate the mechanical properties of multiwall carbon nanotubes/epoxy nanocomposite system. Reference neat epoxy samples were also tested and compared with the results obtained for the nanocomposite. The nanoindentation creep tests were utilized to provide the creep strain rate sensitivity parameter, the contact creep compliance and the time-dependent deformation under constant loads. Different thermo-mechanical conditions comprising three temperatures of 25, 40 and 55 °C and three loads of 1, 2 and 3 mN were utilized. The improvements in the properties were not as high as anticipated through the use of mixture rule, indicating insufficient dispersion. However, variations in modulus, hardness and creep strain rate sensitivity parameter obtained using nanoindentation showed quantifiable differences between the MWCNTs nanocomposite and epoxy specimens. The comparison of the creep strain rate sensitivity A/d(0) from short term, 60 s, creep tests and the creep compliance J( t) from the long term, 1800 s, creep tests suggests that former parameter is a more useful comparative creep parameter than the creep compliance. The analysis of the creep strain rate sensitivity clearly revealed that the addition of MWCNTs to a commercial epoxy reduced the creep rate. This reduction of creep rate sensitivity parameter was observed particularly at thermal environments just below the glass transition temperature.

Plastic instability in amorphous selenium near its glass transition temperature

The deformation behavior of amorphous selenium near its glass transition temperature (31 °C) has been investigated by uniaxial compression and nanoindentation creep tests. Cylindrical specimens compressed at high temperatures and low strain rates deform stably into barrel-like shapes, while tests at low temperatures and high strain rates lead to fragmentation. These results agree well with stress exponent and kinetic activation parameters extracted from nanoindentation creep tests using a similarity analysis. The dependence of the deformation modes on temperature and strain rate can be understood as a consequence of material instability and strain localization in rate-dependent solids.

Surface-effect territory in small volume creep deformation

It is yet unclear how far surface effects can dominate small volume creep deformation in the surface layer of a metallic solid. We report experimental results of the apparent activation volume of single, ultrafine-grained, and nanocrystalline Cu over a range of nanoscale displacements. The dependence of the apparent activation volume on the depth and grain size was determined using nanoindentation creep tests. The surface-affected deformation regimen, within which interfacial diffusion between the nanoindenter tip and the sample totally dominates the creep behavior, was quantitatively determined to be below ∼12 nm. As the initial creep depth is increased, the dominant mechanism is shifted from interfacial diffusion to grain-boundary diffusion as the contribution of the surface effects gradually vanishes when the indenter penetrates deeper into the sample (i.e., further away from the external surface).