Indentation Loading Rate Research Articles

Effect of nanoindentation experimental parameters on the estimation of the plastic events in metallic glasses employing various analysis methods

The term Shear Transformation Zone (STZ) refers to the atomic cluster that dissipates external stress through cooperative shear. The STZ is the process that describes the local atomic rearrangement mechanism of metallic glasses (MGs). Based on the cooperative shear model (CSM), the size of an STZ has been estimated using the rate jump method (RJM), nano-scale creep method (NCM), first pop-in statistical method (FSM) and serration-flow statistic method (SSM), respectively. In the nanoindentation process, different sizes of STZs can be elicited by employing different experimental conditions (indentation depth and loading rate). In addition, different characterization methods exhibit various degrees of response to changes in the experimental conditions. In order to study the heterogeneity of the STZ of MGs using different nanoindentation methods, in this study, three types of Zr-based MGs with different Poisson's ratios were used as the research objects, and the sizes of the STZs of these amorphous alloys were calculated using the previously mentioned methods. By applying different maximum loads and loading rates, the effect of the nanoindentation test parameters on the size of the metallic glasses' STZs were characterized using different nanoindentation methods. Under the same testing conditions, the volume of the STZs was calculated using each method that exhibited improvement with the increase of the Poisson's ratio. The calculated results obtained using the RJM, NCM and SSM were found to be much greater than those obtained using the FSM. With the increase in the loading rate, the results obtained using NCM decreased, whereas the statistic method results slightly increased. At the maximum load, the STZ volume calculated using the JRM, NCM and SSM increased significantly. The FSM was not affected by the maximum load. Finally, these differences are discussed from the aspects of the calculation principles and the activation and evolution of the STZ.

Impact resistance of cork-skinned marine PVC / GRP sandwich laminates

The introduction of a thin cork layer was investigated as a means to improve the impact resistance of composite foam sandwich laminates whilst maintaining structural performance. Six configurations of this ‘cork-skinned’ sandwich architecture were evaluated via extensive experimental comparisons with a ’baseline’ GRP/ PVC foam sandwich laminate as typically used in marine structures. The concept improved perforation resistance (by up to 60%) for both quasi-static indentation and impact loading rates, albeit with an increase in laminate weight. However, initial (slight) damage resistance and bending strength can be compromised if the detailed laminate architecture is not designed with care.

Strain rate dependent micromechanical properties of NiTi shape memory alloys: Laser powder bed fusion versus casting

As the strain rate influences the loading sensitivity of the metallic materials over time, it is very important to assess the associated plastic behavior exposed to a diverse strain rate. Strain rate sensitivity is a fundamental property used to assess the controlling mechanisms of plastic deformation and the susceptibility of a material to creep. In materials with high strain rate sensitivity, plastic deformation can occur even under the application of a small stress value with relatively low rates of strain. Because of the diverse use of NiTi shape memory alloys in small size applications, it would be crucial to understand how strain rates, locally and on small scale, influence plastic deformation, especially in superelastic conditions. Thus, proper and optimal implementation of their functional properties can be achieved. In this paper, the local strain rate sensitivity of heat-treated NiTi samples in the additive manufactured and cast conditions have been investigated employing depth-sensing indentation testing technique at ambient temperature subjected to various indentation loading rates. To this end, using a self-similar pyramidal (Berkovich) indenter, the materials are loaded with different indentation loading rates of 1, 5, 10, and 50 mN/s to a peak load of 200 mN and then unloaded. Upon conducting the indentation tests, the extracted results including indentation load-depth curves, indentation stress-depth curves, and indentation strain rate sensitivity values have been analyzed and discussed for the cast and additive manufactured materials. Experimental results demonstrate that the hardness values increase linearly with the increase in indentation loading rate while the elastic modulus stays relatively constant. These findings are important in understanding the local and small-scale deformation behavior of additively manufactured NiTi in which properties are location-dependent, as compared with the cast counterparts.

Indentation Deformation of a Cu47.5Zr19Hf28.5Al5 Bulk-Metallic Glass-Matrix Nanocomposite

In this work, we study the indentation deformation of a Cu47.5Zr19Hf28.5Al5 bulk-metallic glass-matrix composite and characterize the effects of the indentation-loading rate and the holding time at the peak-indentation load. For the same peak-indentation load, increasing the holding time and/or decreasing the indentation-loading rate cause the increase of the indentation depth. There exists the "bulge" of the unloading curve at the onset of the unloading for small indentation-loading rates. The Vickers hardness is a monotonically increasing function of the indentation-loading rate for the same peak-indentation load. For the indentations with the same loading and unloading time of 30 s and without an intermediate stage at the peak-indentation load, the Vickers hardness of the Cu47.5Zr19Hf28.5Al5 bulk-metallic glass-matrix composite decreases with the increase of the indentation load. The strain energy dissipated through plastic deformation during the indentation is a power-law function of the indentation load with a power index of 3/2, and the energy ratio (total energy/plastic energy) linearly increases with the depth ratio (residual indentation depth/maximum indentation depth).

Indentation size and loading rate sensitivities on mechanical properties and creep behavior of solid bitumen

Creep behavior of rocks could impair fracture conductivity and wellbore stability during gas production from highly matured organic-rich shales in South China, of which the organic matter is mainly in the form as solid bitumen and is thought to be a major contributor for the creep deformation. To get a better insight into this phenomenon, this paper for the first time characterizes the mechanical properties and creep behavior of a millimeter-sized solid bitumen sample by using quasi-static state creep tests and Dynamic Mechanical Analysis in nanoindentation, and reports their dependences on indentation size and loading rate, respectively. Mechanical properties (including hardness and Young's modulus) are found to be negatively related with both indentation size and loading rate. The extremely small creep strain rate sensitivity (m) of solid bitumen indicates a localized shear flow inside. And m exhibits slightly positive dependences on indentation size and loading rate. The potential mechanisms controlling the deformation of solid bitumen under indentation are also discussed.

Depth sensing indentation of magnesium/boron nitride nanocomposites

Magnesium nanocomposites, considered as energy-saving lightweight materials of future, are a fairly new family of composite materials with enhanced specific strength and ductility compared to pure magnesium and/or magnesium alloys. In the present study, time-dependent plastic deformation of novel light-weight magnesium/boron nitride nanocomposites containing 0.5, 1.5 and 2.5 vol% of nano-boron nitride particulates is studied through a depth-sensing indentation approach against monolithic pure magnesium. The synthesis of magnesium–boron nitride nanocomposites was accomplished using powder metallurgy technique coupled with microwave sintering, followed by hot extrusion (end products are 8-mm diameter rods). The depth sensing indentation creep tests were conducted at room temperature (∼0.32Tm of magnesium) using an instrumented indentation platform via a self-similar pyramidal (Berkovich) indenter. To assess the influence of loading rate on the indentation-induced deformation behavior of the materials, a dual stage indentation creep including a constant loading rate followed by a constant load-holding scheme was used; indentation tests were performed on the specimens. The specimens were loaded at rates of 0.05, 0.5, 5, and 50 mN/s to a peak load of 50 mN then force was held constant for 400 s while load/displacement/time data were recorded continuously. The results of the depth sensing indentation tests were correlated and explained using the microstructural characteristics placing special emphasis on the volume fraction of reinforcement and the indentation loading rate. Finally, the controlling creep mechanisms of the magnesium–boron nitride nanocomposites and the base metal (pure magnesium) were discussed in the present paper. The results of this paper can be used as a baseline for high-temperature creep analysis of magnesium–boron nitride nanocomposites which is of engineering significance.

Structure relaxation effect on hardness and shear transformation zone volume of a Ni[sbnd]Nb metallic glassy film

The annealing effect below glass transition temperature (Tg) on nanoindentation hardness and shear transformation zone (STZ) volume was studied in a NiNb metallic glassy thin film. The indentation size effect (ISE) and loading rate effect on hardness were systematically investigated. It was found that ISE was less pronounced and strain rate sensitivity (SRS) was higher in the sample suffered longer annealing time. As annealing time reached a critical value, hardness value, ISE and SRS changed quite limited which could be due to full structure relaxation in samples. The hardness spatial distribution was more scattered with increasing annealing time, indicating a more pronounced mechanical heterogeneity at microscale. Based on SRS values and cumulative probability distribution of hardness, STZ volumes were estimated upon two distinct methods, respectively. A tight correlation between STZ volume and structure state was revealed. As increasing annealing time, STZ volume was continuously decreased and approached down to a stable value.

Indentation-based rate-dependent plastic deformation of polycrystalline pure magnesium

Using a Berkovich pyramidal indenter and an instrumented nanoindentation platform, dual stage nanoindentation creep tests, including loading to a pre-set load and holding at the constant load then unloading, were performed on polycrystalline pure magnesium at 300K. Indentation tests were performed at four indentation loading rates of 0.05, 0.5, 5, and 50mN/s at constant load of 45mN and holding time of 400s. These were performed to assess indentation force-displacement response along with average indentation stress, indentation strain rate sensitivity and ambient temperature rate dependent plastic deformation response of the material. To this end, activation volume, a kinetic characteristic of plastic deformation, and density of mobile dislocations in plastically deforming material in the plastic zone around the indenter are discussed in detail. Uncertainties and sources of error, i.e. indentation size effect, surface roughness, and thermal drift, and pile-up/sink-in measuring creep response through nanoindentation are provided in the current paper as well. The microstructure of the material was also studied through optical and scanning electron microscopy to further investigate the microstructure/property correlations in the tested polycrystalline pure magnesium. The results show the dependency of indentation stress, strain rate sensitivity, and activation volume upon depth and loading rate. According to the creep stress exponent measurements, the dominant mechanism of rate dependent plastic deformation for polycrystalline pure Mg at ambient temperature is attributed to obstacle limited dislocation glide. Finally, the contribution of mechanical twins in the plastic zone around the indenter in the studied polycrystalline pure magnesium is briefly discussed.

Water film facilitating plastic deformation of Cu thin film under different nanoindentation modes: A molecular dynamics study

To emphasize the effect of humid or liquid circumstances on the mechanical behavior of metallic thin films with dimensions down to the nanoscale, atomistic simulations were employed to study the water film affecting the plastic deformation of Cu thin film during nanoindentation with different loading rates and indentation speeds. The results indicate that the largest load increases with the water film thickness and indentation rate for displacement-controlled indentation, contrary to the trend of the largest penetration depth for load-controlled indentation. The dislocation evolution reveals that the raised indentation speed or loading rate results in larger and rapider plastic deformation. The presence and thickening of water film as a media of force transmission from indenter to Cu thin film lead to a wide range of plasticity within Cu, large load dissipation and surface damage. The plastic deformation of Cu thin film under different indentation modes shows similarly changed trend. Our observations provide a better understanding of the mechanism of plastic deformation under the effect of water film.

Nanoindentation Creep Behavior of an Al0.3CoCrFeNi High-Entropy Alloy

Nanoindentation creep behavior was studied on a coarse-grained Al0.3CoCrFeNi high-entropy alloy with a single face-centered cubic structure. The effects of the indentation size and loading rate on creep behavior were investigated. The experimental results show that the hardness, creep depth, creep strain rate, and stress exponent are all dependent on the holding load and loading rate. The creep behavior shows a remarkable indentation size effect at different maximum indentation loads. The dominant creep mechanism is dislocation creep at high indentation loads and self-diffusion at low indentation loads. An obvious loading rate sensitivity of creep behavior is found under different loading rates for the alloy. A high loading rate can lead to a high strain gradient, and numerous dislocations emerge and entangle together. Then during the holding time, a large creep deformation characteristic with a high stress exponent will happen.

Indentation creep analysis of T22 and T91 chromium based steels

Classical creep tests are very time-consuming, they usually take more than one year for steels, whereas creep analysis by instrumented indentation tests (IIT) only requires few hundred minutes. By IIT, creep is observed when the indenter continues to penetrate into the material at constant indentation load which leads to a horizontal plate in the load–depth curve. However, an indentation test cannot reasonably replace the conventional creep test. This is the reason why the objective of this work is to propose a methodology to classify the materials according to their creep sensitivity by means of the indentation test. Within this objective, the indentation testing conditions (dwell-time, maximum indentation load, loading rate and indenter shape) are optimized regarding the ability of the different models to adequately represent the time-indenter displacement variation. Applied to the study of T22 and T91 chromium based steels, the proposed methodology by indentation allows the identification of the creep behavior of these two materials. Furthermore, it is found an analogy with classical creep test through the stress exponent.

Loading rate sensitivity of nanoindentation creep behavior in a Fe-based bulk metallic glass

Effect of loading rate on room temperature creep deformation of {[(Fe0.6Co0.4)0.75B0.2Si0.05]0.96Nb0.04}96Cr4 bulk metallic glass (BMG) was investigated by performing nanoindentation creep experiments. Experimental results show that the creep displacement of the Fe-based BMG increases with increasing the loading rate. Furthermore, based on an empirical power-law equation, the stress exponent for room temperature creep of this BMG was calculated. As the indentation loading rate increases from 1 mN/s to 50 mN/s, the stress exponent decreases gradually from 28.1 down to 4.9, exhibiting significant indentation loading rate sensitivity. Finally, nanoindentation creep behavior in this Fe-based BMG was demonstrated in terms of free volume and “shear transformation zone” (STZ) theories, which could provide semi-quantitative explanation as for the observed experimental and analytical results.

Dent Resistance and Effect of Indentation Loading Rate on Superelastic TiNi Alloy

The large recoverable deformation associated with reversible stress-induced martensitic transformation for superelastic TiNi alloys has been widely exploited in many applications. However, to employ superelastic TiNi in applications where high impact loading is expected, as in bearings, the effect of loading rate on superelasticity needs to be understood. In the current article, the effect of indentation loading rate on dent resistance and superelasticity of TiNi is studied. Indentation tests are performed, at different loading rates on superelastic TiNi alloy and correlated to tensile stress–strain data. It is found that the reversible deformation drops as loading rate is increased and superelasticity diminishes. Based on data collected and results analysis it is proposed that the loss in superelastic behavior under high indentation loading rate is related to retardation of the stress-induced martensitic transformation. Furthermore, a simple heat model was proposed and showed that the temperature rise during indentation is not significant.

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.

Analyse de marche après arthroplastie totale de hanche. Apport de la voie antérieure mini-invasive avec planification tridimensionnelle

The effects of indentation size and loading rate on micro-hardness of the P/M nickel-base superalloy FGH96 were investigated by the micro-indentation tests performed in the indenter load rang from 100 mN and 450 mN and the loading rate range from 1.94 mN/s to 19.37 mN/s at room temperature. The results show that the micro-hardness obviously decreased with the increase of indentation depth, while the influence of indentation depth on elastic modulus was slight. The average value of elastic modulus was 193.97 GPa. Moreover, the rate-dependence phenomenon was prominent during indentation tests as well. It was found that the micro-hardness increased with the increasing loading rate. By means of Nix and Gao model, the size effect during indentation tests was studied. The discipline between H2 and 1/h exhibited a better linear relationship. Further, the density of statistically stored dislocations (SSDs) obtained from the model exhibited a decreasing tendency with the increase of loading rate. In contrast, the density of geometrically necessary dislocations (GNDs) showed an inverse relationship. According to the dynamic material model, the equivalent strain rates during indentation tests were obtained. It was found that the equivalent strain rate decreased with the increase of indentation depth. This phenomenon was considered to be related with the plastic power density (PPD), which represented the kinetic energy of GNDs. By means of Zener–Hollomon parameter, the relationship between activation energy and kinetic energy was established. Furthermore, the dislocation-power theory was developed and applied to interpret the deformation mechanism and the influence of indentation size and loading rate during indentation tests. It should be pointed that the decreasing kinetic energy caused by the decrease of PPD was closely related to the geometry changes, and the decrease of equivalent strain rate would lead to the increase of activation energy which exhibited ascending deformation resistance. In this paper, the influences of indentation size on micro-hardness could be attributed to the variation of equivalent strain rate.

606 純アルミニウムの微小押込試験に及ぼす圧子角度および押込速度の影響

606 純アルミニウムの微小押込試験に及ぼす圧子角度および押込速度の影響

Investigation of micro-indentation characteristics of P/M nickel-base superalloy FGH96 using dislocation-power theory

The effects of indentation size and loading rate on micro-hardness of the P/M nickel-base superalloy FGH96 were investigated by the micro-indentation tests performed in the indenter load rang from 100 mN and 450 mN and the loading rate range from 1.94 mN/s to 19.37 mN/s at room temperature. The results show that the micro-hardness obviously decreased with the increase of indentation depth, while the influence of indentation depth on elastic modulus was slight. The average value of elastic modulus was 193.97 GPa. Moreover, the rate-dependence phenomenon was prominent during indentation tests as well. It was found that the micro-hardness increased with the increasing loading rate. By means of Nix and Gao model, the size effect during indentation tests was studied. The discipline between H2 and 1/h exhibited a better linear relationship. Further, the density of statistically stored dislocations (SSDs) obtained from the model exhibited a decreasing tendency with the increase of loading rate. In contrast, the density of geometrically necessary dislocations (GNDs) showed an inverse relationship. According to the dynamic material model, the equivalent strain rates during indentation tests were obtained. It was found that the equivalent strain rate decreased with the increase of indentation depth. This phenomenon was considered to be related with the plastic power density (PPD), which represented the kinetic energy of GNDs. By means of Zener–Hollomon parameter, the relationship between activation energy and kinetic energy was established. Furthermore, the dislocation-power theory was developed and applied to interpret the deformation mechanism and the influence of indentation size and loading rate during indentation tests. It should be pointed that the decreasing kinetic energy caused by the decrease of PPD was closely related to the geometry changes, and the decrease of equivalent strain rate would lead to the increase of activation energy which exhibited ascending deformation resistance. In this paper, the influences of indentation size on micro-hardness could be attributed to the variation of equivalent strain rate.

Method of Estimated Unloading Curve for Evaluation of Interfacial Strength by Nanoindentation Test

A new evaluation method was proposed for the interfacial strength between a thin film and its substrate by nanoindenation test. In this method, the released energy due to a coating delamination is obtained by estimating the shapes of the unloading curve at the points just after and before delamination on the load-displacement curve. Shape of unloading curves can be assumed by obtaining the contact stiffness of thin films. To confirm the validity of the method, nanoindentation tests were conducted for polyimide (PI) coating / Si substrate specimens and the interfacial strength was evaluated. Three specimens that have different thickness were prepared. Contact stiffness of PI film bears a linear relationship with indentation load. Irrespective of the coating thickness, the values of the interfacial strengths converge. In addition, the values of strengths agree with the results obtained by the authors in the previous paper for the case that small scale yielding conditions are satisfied. For the case that indentation loading rate is high and the delamination occurs before the relaxation of film, it is necessary to measure the contact stiffness with each indentation loading rate.

Indentation creep of an Fe-based bulk metallic glass

abstract In this study, the room temperature creep behavior of Fe 41 Co 7 Cr 15 Mo 14 C 15 B 6 Y 2 bulk metallic glass wasinvestigated using nanoindentation technique with the maximum applied load ranging from 1 mN to100 mN under different loading rates (0.01–2.5 mNs 1 ). The creep stress exponent was derived from therecorded displacement–holding time curve. It was found that the stress exponent increases rapidly from2.87 to 6.37 with increasing indentation size, i.e. exhibiting a positive indentation size dependence.Furthermore, as the indentation loading rate increases from 0.01 mNs 1 to 2.5 mNs 1 , the stressexponent decreases gradually from 4.93 to 0.94. The deformation mechanism causing the nano-indentation creep is discussed in the light of the ‘‘shear transformation zone’’ (STZ) which providesqualitative explanation for the observed plasticity in amorphous alloy. 2008 Elsevier Ltd. All rights reserved. 1. IntroductionThe last twodecades havewitnessed rapid development of bulkmetallic glasses (BMGs), which exhibit some unique mechanicalproperties and high potentiality for application as advancedstructural and functional materials [1–3]. While extensive plasticstrain can be reached in few localized shear bands, BMGs usuallyexhibit little or no macroscopic plasticity at room temperature[2–4]. Recently, several researchers have studied the plastic defor-mation mechanisms of BMGs using nanoindentation (see forexample Refs. [5,6]). Study of time-dependent deformation (i.e.creep) of metallic glasses using nanoindentation has been rare[7–9]. Apart from its scientific interest [10], creep under sub-micrometercontact is an issue of practical engineering significancefor the applications of MEMS and NEMS. From the indentationcreep studies of crystalline materials, a few indentation creepmethods have been proposed and used [11]. The constant loadholding method is the most widely used one, where the indentertip is maintained at a prescribed load, for an extended time period,while the displacement of the indenter into the surface iscontinuouslyrecorded,akintotheconstantloadcompressioncreeptests [11–13].The purpose of the present work is to investigate the creepbehaviors of an Fe-based bulk metallic glass. The stress exponentvalues were derived fromthe displacement–holding time curves. Itwas found that the stress exponent values obtained are dependenton the applied peak loads and indentation loading rates. Thedeformation mechanisms of metallic glasses are discussed.2. Experimental procedureAlloy ingots with nominal composition of Fe

Creep behavior of a Fe-based bulk amorphous alloy using nanoindentation

The creep behavior of a {［（Fe0.6Co0.4）0.75B0.2Si0.05］0.96Nb0.04}96Cr4 bulk amorphous alloy and effects of different loading rates on its creep deformation behavior were investigated using nanoindentation technique at room temperature. It is found that the creep deformation of this alloy is strongly dependent on the indentation loading rate: when the loading rate ranges from 3 to 24 mN/s, distinct creep deformation occurs; but when the loading rate decreases to 1 mN/s or 0.75 mN/s, the creep deformation is completely suppressed. The creep deformation behavior of the {［（Fe0.6Co0.4）0.75B0.2Si0.05］0.96Nb0.04}96Cr4 bulk amorphous alloy was modeled using the elastic-viscoelastic-viscous （EVEV） model and a high correlation coefficient （R） of 0.9392 was obtained, implying that this creep behavior studied can be well described by the EVEV model. Based on the EVEV model, the creep compliance and retardation spectrum, which are related to the interior structure of the bulk amorphous alloy, were further calculated by the EVEV model. Finally, the creep mechanism at room temperature was discussed in detail according to the analysis of the creep rate sensitivity index （m） as well as the observation on the morphology of indents using the atomic force microscope （AFM）.