Cyclic Indentation Research Articles

Human tibial cortical bone with high porosity in type 2 diabetes mellitus is accompanied by distinctive bone material properties.

Diabetes mellitus is a metabolic disease affecting bone tissue at different length-scales. Higher fracture risk in diabetic patients is difficult to detect with common clinical fracture risk assessment due to normal or high bone mineral density in diabetic patients. The observed higher fracture risk despite normal to high areal bone mineral density in diabetic patients points towards impaired bone material quality. Here, we analyze tibial bone from individuals with type 2 diabetes mellitus using a multiscale-approach, which includes clinical and laboratory-based bone quality measures. Tibial cortical bone tissue from individuals with type 2 diabetes mellitus (T2DM) and age-matched healthy controls (n=15 each) was analyzed with in situ impact indentation, dual energy X-ray absorptiometry (DXA), high resolution peripheral microcomputed tomography (HR-pQCT), micro-computed tomography (microCT), cyclic indentation, quantitative backscattered electron microscopy (qBEI), vibrational spectroscopy (Raman), nanoindentation, and fluorescence spectroscopy. With this approach, a high cortical porosity subgroup of individuals with T2DM was discriminated from two study groups: individuals with T2DM and individuals without T2DM, while both groups were associated with similar cortical porosity quantified by means of microCT. The high porosity T2DM group, but not the T2DM group, showed compromised bone quality expressed by altered cyclic indentation properties (transversal direction) in combination with a higher carbonate-to-amide I ratio in endocortical bone. In addition, in the T2DM group with high cortical porosity group, greater cortical pore diameter was identified with HR-pQCT and lower tissue mineral density using microCT, both compared to T2DM group. Micromechanical analyses of cross-sectioned osteons (longitudinal direction) with cyclic indentation, qBEI, and nanoindentation showed no differences between the three groups. High tibial cortical porosity in T2DM can be linked to locally altered bone material composition. As the tibia is an accessible skeletal site for fracture risk assessment in the clinics (CT, indentation), our findings may contribute to further understanding the site-specific structural and compositional factors forming the basis of bone quality in diabetes mellitus. Refined diagnostic strategies are needed for a comprehensive fracture risk assessment in diabetic bone disease.

Nano-sized Cu precipitation and microstructural evolution in aged ultralow and medium carbon steels

The nanoscale Cu precipitation exhibits considerable potential for enhancing strength and cyclic hardening potential in steels. In the present work, Cu-alloyed ferritic steels with different C contents (X0.5: 0.005 wt.%, X21: 0.21 wt.%) were investigated to systematically manifest the influences of C contents on the microstructural evolution, the precipitation behavior of nano-sized Cu precipitates and the resulting effects on cyclic hardening behavior. The investigated materials were subjected to austenitization at 900 °C for 10 min and subsequently aged at 600 °C for different time durations (120, 720, 7200 s). The 3D atom probe tomography rendered the investigation of nanoscale Cu precipitates in terms of average size, local chemistry, number density and spatial distribution. In X21 steel, the Cu precipitates exhibited a large nucleation density upon the onset of aging and further grew into a larger precipitate size. Compared to X0.5 steel, the higher nucleation density and the inhomogeneous precipitate distribution in X21 steel were attributed to the heterogeneous nucleation and high C content in the matrix. Cyclic indentation tests revealed a strong dependency of hardness and cyclic hardening potential on precipitation size and number density for both steels. Furthermore, for both steels aging had a similar effect on the mechanical properties. However, the increased C content induced a higher hardness and lower cyclic hardening exponentCHT of X21 steel.

Accumulation of fluorescent advanced glycation end products and carboxymethyl-lysine in human cortical and trabecular bone

Chemical crosslinks known as advanced glycation end-products (AGEs) are associated with increased bone fracture risk and deteriorated bone mechanical properties. However, measurement of bone AGEs via ex vivo and in vitro methods has been limited to quantification of bulk fluorescent AGEs (fAGEs) and pentosidine only, which is a crosslinking fluorescent AGE. However, a non-crosslinking and non-fluorescent AGE such as carboxymethyl-lysine (CML) is found to be 40–100 times higher in quantity than pentosidine, but only one previous study has reported it in cortical bone, and one study reported it in trabecular bone. In our study, we wanted to investigate if accumulation of CML differs in cortical and trabecular compartments and if they are more strongly associated with bone mechanical properties than with fAGEs. We hypothesized that CML and fAGEs level would be higher in the trabecular compartment and show negative correlations to mechanical properties in cortical and trabecular bone. We obtained human cadaveric cortical and trabecular bone specimens, induced the formation of AGEs via the established in vitro ribosylation method, imaged specimens by microcomputed tomography to assess specimen geometry and microarchitecture, and mechanically tested cortical specimens by cyclic reference point indentation and fracture toughness tests and trabecular specimens by compression tests, followed by measurement of fAGEs and CML. fAGEs were 22 % higher in cortical bone (687 ± 44.8 ng Q/mg collagen) compared to trabecular bone (859 ± 317.1 ng Q/mg collagen), whereas CML levels were found to be 148 % higher in trabecular bone (6189.9 ± 866 ng/mg of protein) compared to cortical bone (924.6 ± 576.3 ng/mg of protein). Pooling the specimens from both the control and ribose groups, Spearman correlation analysis indicated that CML levels, but not fAGEs, are moderately associated with cortical porosity (r = +0.505, p ≤ 0.05) and mechanical properties such indentation depth (r = +0.460, p ≤ 0.05), total indentation depth (r = +0.440, p ≤ 0.05), and average energy dissipated (r = +0.465, p ≤ 0.05) in cortical bone. fAGEs showed a trend towards negative association with crack propagation toughness in cortical bone (r = −0.365, p = 0.055). No significant correlations were observed between CML and microarchitecture or mechanical properties in trabecular bone. CML levels were also associated with fAGEs in cortical bone (r = +0.596, p ≤ 0.05) but not in trabecular bone. Our preliminary findings indicate that CML, a non-crosslinking AGE, may affect bone material and mechanical properties differently than bulk fluorescent AGEs, given the higher accumulation of CML in each bone compartment. This study provides direction to future studies to quantify crosslinking and non-crosslinking AGEs separately as their effect on material and mechanical properties may be different and it would help identify better biomarkers for bone strength prediction.

Indenting viscoelastic thin layers: A numerical assessment

Normal indentation of viscoelastic bodies is a technique widely employed to characterize the viscoelastic material properties. Here, a numerical Boundary Element methodology is developed to model the indentation process also when the solids in contact are thin layers. Specifically, two boundary conditions for the thin slabs are considered: the case of a confined layer, perfectly bonded to a rigid substrate, and that one of a free layer, supported by a constant pressure. Numerical analyses focus, firstly, on creep and relaxation indentation tests and show that finite values of the contacting layers thickness produce dramatic quantitative changes in comparison with what obtained under the half-plane (HP) assumption. Similar effects are found also in the case of loading and deformation indentation cycles: this results crucial for vibrational phenomena and confirms the opportunity of introducing the numerical technique here presented.

Thermally enhanced dislocation density improves both hardness and fracture toughness in single‐crystal SrTiO3

AbstractDislocation‐tuned functionality in ceramic oxides for potential versatile applications gains increasing attention. As the widespread chemical doping suffers from poor temperature stability, dislocations in well‐controlled mesoscopic structure may be an alternative to thermally stable intrinsic doping features. To this end, the dislocation density in plastic zones introduced by cyclic Brinell indentation is considered under thermal annealing conditions. The considerably enhanced dislocation density due to thermal treatment is found to impact both microhardness and fracture toughness, albeit only to a modest degree. The mechanistic understanding centers around enhanced mobility and multiplication of the pre‐engineered dislocations at elevated temperatures driven by the residual indentation stress, as well as the strengthened interaction of point defects and dislocations at high temperature.

Evaluation of the plastic deformation behavior of modified 100Cr6 steels with increased fractions of retained austenite using cyclic indentation tests

Abstract In highly loaded components, such as roller bearings, early failures may occur due to microstructural defects. Thus, a higher defect tolerance of the material can improve the fatigue lifetime. To produce steels with high defect tolerance, innovative alloying concepts, and a sound knowledge of the influence of heat treatment parameters on the resultant mechanical properties is indispensable. Consequently, two bearing steels based on 100Cr6 with different silicon contents were investigated to achieve relatively high retained austenite (RA) fractions, which are assumed to increase the defect tolerance at cyclic loading. To understand the relation between bainitic heat treatment and mechanical characteristics, the resultant RA fractions, Vickers hardness, microhardness, and cyclic hardening exponent CHT eII, which correlates to defect tolerance of metallic materials, were determined for differently heat-treated conditions. Based on the statistical design of experiments and measurements obtained with X-ray diffraction and cyclic indentation testing, regression calculations were conducted. Pertaining to the results of the regression analyses, the heat treatment parameterizations of these steels were specifically determined for certain desired property combinations within the boundaries investigated. Therefore, the temperatures used in the austenitizing and bainitizing procedures have shown the highest influence on the mechanical characteristics and the RA fraction of both steels.

Investigation on the sensitivity of indentation devices for detection of fatigue loading induced damage in bovine cortical bone

Daily physiological activities subject our skeletal system to cyclic loading with varying frequencies and magnitudes. These loadings interact with the microstructure of bone and create microdamage, which can cause stress-induced injuries if not repaired on the time. The early detection is required to prevent the complications associated with these fractures. In the present study, to examine fatigue loading-induced damage in cortical bone, the sensitivity of four different indentation devices was investigated. For this, cortical bone samples were fatigued in four-point bending configuration at 0.5 Hz, 2 Hz and 4 Hz frequencies. Following the fatigue loading, cyclic reference point indentation (cRPI), impact reference point indentation (iRPI), Vickers microhardness and nanoindentation tests were performed on the bone samples. Results show that indentation devices are sensitive to detect fatigue loading induced damage only in 0.5 Hz group samples on compressive region. On the other hand, the sensitivity of indentation devices for tensile stress-induced damage is not clear. Also, histological examination of fatigued bone samples shows a significant increase in the crack density and crack length with fatigue loading only for the 0.5 Hz group samples. The present study provides insight into the sensitivity of different indentation devices to fatigue loading induced damage, which could be helpful in the development of new devices for the early diagnosis of stress induced injuries.

Reprint of: Viscoelastic dissipation in repeated normal indentation of an Hertzian profile

Simple exact solutions are known for the indentation problem of a viscoelastic halfspace by a rigid sphere only as long as the contact area is growing. We consider instead a more general cyclic repeated indentation with a pulsating load with a period of zero load. We show that a combination of exact with empirical relaxation solutions coming from simple uniaxial cases is sufficiently accurate to estimate the energy dissipated per cycle, which we report for the standard ”3-elements” solid and periodic half-sine loading for various parameters. The theoretical predictions favourably compare with boundary element numerical simulations. We find more energy is dissipated during the first indentation cycle with respect to the subsequent ones, due to the residual indentation left in the viscoelastic half-space. In load controlled systems, the maximum dissipation is reached at an angular frequency that is close to the reciprocal of the relaxation time of the material both for the first and subsequent cycles, but this is in general not true when displacement controlled systems are considered, when dissipation is much lower for subsequent cycles.

Crack Propagation Behavior of Fused Silica during Cyclic Indentation under Incremental Loads

Fused silica is an important optical material with important applications, where the surface must be precisely machined without subsurface damage. In this study, multi-cyclic indentations under incremental loads were performed on fused silica using two kinds of indenters to clarify the mechanisms of crack generation and propagation induced by precision grinding. It was found that incremental loading cyclic nanoindentation induced various patterns of subsurface cracking and surface spalling. Four kinds of surface spalling were identified at different locations around an indent, the temporal formation mechanisms of which were clarified by microscopic observation and topographical measurement. Load–displacement curve analysis demonstrated that incremental propagation of lateral cracks during early indentation cycles caused large-scale brittle fractures during later cycles. Compared with a Berkovich indenter, a cube-corner indenter caused more significant brittle fractures and surface spalling. The findings in this study will deepen the understanding of subsurface damaging mechanism of fused silica and other brittle solids caused by cyclic tool-workpiece interactions in grinding and other mechanical machining processes.

A New Approach to Estimate the Fatigue Limit of Steels Based on Conventional and Cyclic Indentation Testing

For a reliable design of structural components, valid information about the fatigue strength of the material used is a prerequisite. As the determination of the fatigue properties, and especially the fatigue limit σw, requires a high experimental effort, efficient approaches to estimate the fatigue strength are of great interest. Available estimation approaches using monotonic properties, e.g., Vickers hardness (HV), and in some cases the cyclic yield strength, only allow a rough estimation of σw. The approaches solely based on monotonic properties lead to substantial deviations of the estimated σw in relation to the experimentally determined fatigue limit as they do not consider the cyclic deformation behavior. In this work, an estimation approach was developed, which is based on a correlation analysis of the fatigue limit σw, HV, and the cyclic hardening potential obtained in instrumented cyclic indentation tests (CIT). For this, eleven conditions from five different low-alloy steels were investigated. The CIT enable an efficient and quantitative determination of the cyclic hardening potential, i.e., the cyclic hardening exponentCHT eII, and thus, the consideration of the cyclic deformation behavior in an estimation approach. In this work, a strong correlation of σw with the product of HV and |eII| was observed. In relation to an existing estimation approach based solely on HV, considering the combination of HV and |eII| enables the estimation of σw with an enormously increased precision.

Multi-cycling nanoindentation in additively manufactured Inconel 625 before and after laser peening

In this research, a room temperature multicycle nanoindentation technique was implemented to evaluate the effects of the laser peening (LP) process on the surface mechanical behavior of additively manufactured (AM) Inconel 625. Repetitive deformation was introduced by loading-unloading during an instrumented nanoindentation test on the as-built (No LP), 1-layer, and 4-layer laser peened (1LP and 4LP) conditions. It was observed that laser-peened specimens had a significantly higher resistance to penetration of the indenter and lower permanent deformation. This is attributed to the pre-existing dislocation density induced by LP in the material which affects the dislocation interactions during the cyclic indentation. Moreover, high levels of compressive stresses, which are greater in the 4LP specimen than the 1LP specimen, lead to more effective improvement of surface fatigue properties. The transition of the material response from elastic-plastic to almost purely elastic in 4LP specimens was initiated much earlier than it did in the No LP, and 1LP specimens. In addition to the surface fatigue properties, hardness and elastic modulus were also evaluated and compared.

High-throughput characterization of the cyclic response of Ti-6Al-4V using spherical microindentation stress–strain protocols

This paper presents novel high-throughput spherical microindentation measurement and analyses protocols for the cyclic response of metal samples. These protocols are aimed at rapid screening of large design spaces encountered in the development of advanced metal alloys. They involve the measurement of cyclic load-depth hysteresis loops in spherical microindentation, their conversion to cyclic indentation stress–strain hysteresis loops, and the extraction of an indentation cyclic stress–strain curve. The proposed spherical microindentation measurement and analyses protocols are demonstrated on samples of Ti-6Al-4V. The indentation measurements are compared against the measurements obtained on the same material in standard uniaxial stress tests in both monotonic and cyclic loading. The cyclic indentation stress–strain measurements correlated well with the measurements from the standard uniaxial stress tests.

Correlation between microstructure and nanomechanical properties of 9Cr– 1Mo ferritic martensitic steel through instrumented indentation technique

Instrumented indentation technique (IIT) has been used to probe the phase-specific nanomechanical properties of heat treated 9Cr 1Mo ferritic martensitic (P9) steel. P9 steel was subjected to austenitization treatment, followed by water quenching, normalizing, furnace cooling, and subsequently tempering. Quantitative TEM analysis showed systematic changes in microstructure with variation in cooling rate in aforementioned heat treatments. Instrumented indentation tests were carried out at room temperature with a spherical indenter for evaluating hardness and elastic modulus. The yield strength of the steel from the load-displacement curves of cyclic indentation tests has been derived using neural networking method and yield parameter calculations. The properties derived from IIT using these methods were also validated with tensile test results. This work aims at correlating the local mechanical response measured by IIT with microstructural changes due to different heat treatments, which would be useful to extract mechanical properties of materials with heterogeneous microstructure.

Dynamic indentation measurements on amorphous materials

Abstract An alternative indentation method to study materials that present a time-dependent behaviour (e. g. viscoelasticity) is the dynamic stiffness measurement technique, which provides fast and accurate values of the elastic properties through the calculation of the storage modulus (related to the elastic recovery of the material), and the loss modulus (related to the damping), using indentation cycles with oscillating load or displacement. We have made dynamic measurements on four amorphous materials (CNx, amorphous carbon, fused silica and polypropylene). Such materials have a relatively low density compared to crystalline materials of similar composition and atoms are able to move into internal free space during deformation. The results suggest that all the amorphous materials have a similar time-dependant behaviour associated with the restrictions to movement of atoms (or chains) by neighbouring atoms in the amorphous structure.

In-situ SEM cyclic nanoindentation of pre-sintered and sintered zirconia materials

Efficient diamond machining of zirconia requires a comprehensive understanding of repetitive diamond indentation mechanics. This paper reports on in-situ cyclic nanoindentations of pre-sintered and sintered zirconia materials performed inside a scanning electron microscope (SEM). In-situ SEM imaging of cyclic indentation processes and high-magnification SEM mapping of indentation imprints were conducted. The elastic and plastic behaviors of pre-sintered and sintered zirconia materials were investigated as a function of the cyclic nanoindentation number using the Sakai and Sakai-Nowak models. For pre-sintered zirconia, cyclic nanoindentation induced quasi-plastic deformation, causing localized agglomeration of zirconia crystals with microcracks and large cracking along the indentation edge. Severely compressed, fragmented, and pulverized zirconia crystals and smeared surfaces were also observed. For sintered zirconia, shear bands dominated quasi-plastic deformation with the formation of edge pile-ups and localized microfractures occurred at indentation apex and diagonals. All elastic and plastic behaviors for pre-sintered and sintered zirconia materials revealed significantly microstructure-dependent. Pre-sintered zirconia yielded significantly lower contact hardness, Young's moduli, resistance to plasticity, elastic deformation components, and resistance to machining-induced cracking, and higher elastic and plastic displacements than sintered state. Meanwhile, all the behaviors for the two materials were independent from the cyclic nanoindentation number. A model was proposed for cyclic nanoindentation mechanics, revealing their cyclic indentation-induced microstructural changes in the two zirconia materials. This study advances the fundamental understanding of nanoindentation mechanics of zirconia materials.

Effect of repeated nanoindentations on the deformation of potassium dihydrogen phosphate crystals

In many surfacing processes such as precision mechanical polishing, grinding and cutting, a material inevitably experiences repeated interactions with a surfacing tool, which often brings about significant surface integrity problems of the material. This paper aims to explore the deformation mechanisms of potassium dihydrogen phosphate (KDP) crystals subjected to repeated nanoindentations. The molecular dynamics (MD) method was employed to investigate the atomic scale distortion and possible phase transformation under the external stresses due to the indenter-KDP interactions. A machine learning-based method was used to facilitate the structural characterization. The results showed that the response of KDP to the repeated nanoindentations is significantly anisotropic, depending on the atomic lattice orientation to the indentation direction. On the (001) surface of KDP, the indentation will first create an irreversible amorphous phase which will remain stable in the subsequent indentation cycles. While indenting on the (100) surface, however, the amorphous phase emerged in the first indentation will transfer to a combination of orthorhombic and monoclinic phases during the subsequent indentation cycles.

Viscoelastic dissipation in repeated normal indentation of an Hertzian profile

Simple exact solutions are known for the indentation problem of a viscoelastic halfspace by a rigid sphere only as long as the contact area is growing. We consider instead a more general cyclic repeated indentation with a pulsating load with a period of zero load. We show that a combination of exact with empirical relaxation solutions coming from simple uniaxial cases is sufficiently accurate to estimate the energy dissipated per cycle, which we report for the standard ”3-elements” solid and periodic half-sine loading for various parameters. The theoretical predictions favourably compare with boundary element numerical simulations. We find more energy is dissipated during the first indentation cycle with respect to the subsequent ones, due to the residual indentation left in the viscoelastic half-space. In load controlled systems, the maximum dissipation is reached at an angular frequency that is close to the reciprocal of the relaxation time of the material both for the first and subsequent cycles, but this is in general not true when displacement controlled systems are considered, when dissipation is much lower for subsequent cycles.

ANALYSIS OF THE REASONS FOR SPECIFIC DEFORMATION BEHAVIOR OF TOPOCOMPOSITE OF THE ALN-D16T SYSTEM DURING INSTRUMENTAL INDENTATION

The reasons for obtaining the unloading curves of the indentation diagrams of topocomposites coated with aluminum nitride with a nonlinear section of the end of the unloading curve are investigated. The influence of the plasticity of the base and the structure of the coating of aluminum nitride of coatings deposited by the magnetron method on the base made of aluminum alloy D16T on the mechanism of deformation and destruction of the topocomposite was evaluated. The assessment of these characteristics was carried out based on the results of the analysis of model and experimental indentation diagrams of topocomposites with coatings of aluminum nitride and titanium nitride, as well as using the results of materials science studies of the behavior of materials of topocomposite components at high specific loads known in the literature.
 It was found that the nonlinear shape of the end of the unloading curves is associated with solid-phase structural changes in the coating material. The level of solid-phase structural changes in the coating can be estimated from the width of the hysteresis loops during repeated indentation cycles.

Nanoindentation Reveals Crosslinking Behavior of Solar Encapsulants-The Methodological Advantages over Bulk Methods.

The power degradation and failure of photovoltaic (PV) modules can be caused by changes in the mechanical properties of the polymeric components during the module lifetime. This paper introduces instrumented nanoindentation as a method to investigate the mechanical properties of module materials such as polymeric encapsulants. To this end, nanoindentation tests were carried out on ethylene vinyl acetate (EVA) surfaces, which have been separated from the glass panel. Two types of time-dependent indentation cycle modes, the time domain (creep mode) and frequency domain (dynamic mode) were performed to determine the viscoelastic behavior. For each mode, a corresponding model was applied to calculate the main mechanical properties. The general capability of nanoindentation as cross-linking determination method is investigated with the methodological advantages over bulk mechanical characterization methods. A large number of Glass/EVA/Backsheet laminates were built using different lamination conditions resulting in different degrees of curing. Both indentation modes indicate good modulus sensitivity for following the EVA crosslinking in its early stages but could not reliably differentiate between samples with higher EVA branching. Additional dynamic mechanical analysis (DMA) characterization was used as an established method to validate the indentation measurements. Both nanoindentation and DMA tensile mode produce similar quantitative viscoelastic responses, in the form of the damping factor parameter, demonstrated for three different frequencies at room temperature. A statistical study of the data reveals the advantages for the investigation of multilayer PV laminates by using nanoindenation as a surface method while also being applicable to field aged modules.

Gradients of cyclic indentation mechanical properties in PR520 epoxy and its 3D carbon fiber composite induced by aging at 150 °C

A cyclic indentation test was carried out on PR520 epoxy resin and its 3D interlock carbon fiber reinforced composite in order to study the effects of thermo-oxidative aging on the material local properties. Thermal aging has been performed in air at 150 °C, which is close to the service temperature in warm zones of aircrafts, for different durations. Two parameters have been considered in this study: the reload indentation modulus, representative of the elastic response, and the ratio between irreversible and total indentation energy, representative of plastic (first cycle) and viscoelastic response. The analysis of cyclic indentation curves shows that elastic and plastic behavior are affected by thermal aging, while viscoelastic behavior is not. Moreover, a gradient of local properties has been measured through the thickness of aged samples. The affected zone extends of about 150 µm from the surface exposed to the oxidative environment, while the properties of the sample core are similar to those of the virgin material. Property gradient profiles measured on resin and composite aged in the same conditions are well superposed.