Residual Indentation Research Articles

Determination of Ratio of Yield Strength and Reduced Modulus during Cold Work Processing in 304 Austenitic Stainless Steel

304 austenitic stainless steel is a common structural material used in the nuclear power plant, and its basic mechanical property would be changed by work hardening, which seriously affects the evaluation of the integrity of the nuclear power structure. The relationships of between the yield strength and the reduced modulus ratio (σ0.2/Er) with the residual indentation depth and maximum depth ratio (hr/hm) during cold work processing of 304 austenitic stainless steel is analyzed by using theoretical analysis, experiment and finite element simulation, and an approximate analytical model to expressσ0.2/Erbyhr/hmis established in this paper. The investigating results indicate that yield strength, Vickers hardness, andσ0.2/Erwill increase, andhr/hmdecreases with the material deformation increasing, and the analytical result of theσ0.2/Erbasically consistent with the experiment result.

Hexagonal germanium formation at room temperature using controlled penetration depth nano-indentation

Thin Ge films directly grown on Si substrate using two-step low temperature growth technique are subjected to low load nano-indentation at room temperature. The nano-indentation is carried out using a Berkovich diamond tip (R ~ 20 nm). The residual impressions are studied using ex-situ Raman Micro-Spectroscopy, Atomic Force Microscopy combined system, and Transmission Electron Microscopy. The analysis of residual indentation impressions and displacement-load curves show evidence of deformation by phase transformation at room temperature under a critical pressure ranging from 4.9GPa–8.1GPa. Furthermore, the formation of additional Ge phases such as r8-Ge, hd-Ge, and amorphous Ge as a function of indentation depth have been realized. The inelastic deformation mechanism is found to depend critically on the indentation penetration depth. The non-uniform spatial distribution of the shear stress depends on the indentation depth and plays a crucial role in determining which phase is formed. Similarly, nano-indentation fracture response depends on indentation penetration depth. This opens the potential of tuning the contact response of Ge and other semiconductors thin films by varying indentation depth and indenter geometry. Furthermore, this observed effect can be reliably used to induce phase transformation in Ge-on-Si with technological interest as a narrow band gap material for mid-wavelength infrared detection.

A novel method to extract the equi-biaxial residual stress and mechanical properties of metal materials by continuous spherical indentation test

In this present paper, a novel method was proposed to extract the equi-biaxial residual stress, yield strength and strain hardening exponent of metal materials from the load-displacement curve by continuous spherical indentation test. A representative strain independent on strain hardening exponent was introduced to establish the relationship between residual stress, material properties and indentation parameters by dimensional analysis. The representative strain and dimensionless function at different penetration depths were determined through analyzing the results obtained from finite element (FE) simulations. Then based on the reverse analysis algorithm, three unknown parameters were estimated from the corresponding dimensionless functions. The optimal combination of three different indentation depths was confirmed by comparing the relative errors of each dimensionless equation at different depths. At last the effectiveness and reliability of the proposed method were verified by analyzing the numerical simulation data and the actual results of two typical materials (S45C and API X42).

On the indentation of elastoplastic functionally graded materials

In this paper, the contact problem of an elastoplastic functionally graded substrate and a rigid spherical indenter is investigated using finite element method. For the first time, the proposed finite element model accounts for the smooth variation in all elastic and plastic properties through the thickness direction of the substrate according to power law and Tamura, Tomota and Ozawa (TTO) model. The proposed model is implemented into the ANSYS FE package. The results obtained from the FE analysis are verified and compared with those available in literature. An extensive parametric analysis is conducted to explore the influence of the indenter radius, gradient index, Poisson's ratio and hardening of the indented substrate on the load-indentation response, contact pressure distribution, stress and plastic strain distributions and displacement beneath the indenter. The results illustrated that the indentation responses of elastic, elastoplastic and elasto-perfectly-plastic functionally graded materials are significantly affected by the material property distribution. Finally, a simple empirical equation is presented to relate the residual indentation depth of elastoplastic functionally graded materials with the gradient index using a single material parameter. The results of this study can guide tribology engineers in determining the contact stress and indentation depth that have a great effect on the hardness and the damage of indented body.

Predictive model for spherical indentation on elastoplastic nanocomposites: Loading and unloading behavior

The response of elastoplastic nanocomposites in contact with rigid indenter has a lot of importance in engineering applications such as material characterization, Young modulus, Poisson's ratio and strain hardening parameters. With the aim of presenting a predictive model for the loading and unloading response, the problem of contact between elastoplastic composites with stiff spherical indenter was simulated using 2D axisymmetric model implemented in ANSYS. Parametric studies were performed numerically to investigate effects of geometrical and material parameters on the behavior of nanocomposites. Based on the parametric study observation, a normalization procedure is presented to express the loading and unloading responses in a nondimensional form. Three equations were numerically derived by fitting the FE normalized data to predict the loading and unloading responses and the residual indentation depth after unloading for elastoplastic nanocomposite with wide range strain hardening exponent. The predictions of proposed equations were in excellent agreement with experimental results for Al-Al2O3 nanocomposites and also with other experiments available in the literature for nanocomposites and pure metals. Moreover, derived equations were exploited to predict the Rockwell hardness of nanocomposites.

Comment on van Casteren et al. (2018): softer metallic spheres do abrade harder enamel.

Comment on van Casteren et al. (2018): softer metallic spheres do abrade harder enamel.

Characterization of Hardness and Stiffness of Ceramic Matrix Composites through Instrumented Indentation Test

The determination of the hardness allows useful qualitative conclusions regarding the mechanical properties of materials. It is one of the most economical characterization methods, since this can be carried out with little effort on small specimens. However, due to the heterogeneous structure containing hard and brittle ceramic phases, the determination of hardness of ceramic matrix composites (CMCs) is a challenge and has thus rarely been investigated. In this work, the hardness and the stiffness in out of plane orientation are investigated on Liquid Silicon Infiltrated (LSI) C/C–SiC composites. For hardness tests, a hemispherical indenter is used in order to evaluate residual indentation. The load–unload curve generated during the indentation process is analyzed to determine the hardness and stiffness. The employed experimental procedure is expected to be applicable to other CMCs with similar microstructure.

In Situ Surface Imaging Through a Transparent Diamond Tip

A new approach to the design of transparent diamond indenters is proposed, which allows one to obtain a full optical image of the investigated area of the sample surface, including images directly during measurements by applying the indentation and scratching methods. In this case, it is not required to use immersion liquids that fill the region between the sample and the indenter. The measured area is observed with an optical microscope with illumination through an objective lens. Using an indenter in the form of a trihedral Berkovich pyramid, images of the surface of a test structure and the residual indentation are reconstructed and an image of the surface during its deformation under the application of a load to the indenter is obtained.

Multi-scale dimple creation on metallic glass by a two-step method involving nanoindentation and polishing

Creation of micro dimples on material surface is significant for their functional applications. Because of unique properties, metallic glasses are regarded as emerging structural and functional materials but they are also a kind of difficult-to-cut materials due to high hardness and easy to crystallization and oxidation. In this study, a new method was proposed to fabricate micro dimples on metallic glass surface, which consisted of two steps, nanoindentation experiments to form the pre-indent array followed by polishing. Via this two-step method, square and rectangle dimples were successfully formed which exhibited multi-scale features with one relatively large dimple with width in micron-scale and depth in nanometer-scale, together with relatively small residual indents in nanometer-scale distributed on its bottom surface. The experimental results indicated that the depth of multi-scale dimples was affected by the spatial interval between two indents and the polishing time. Under the used experimental conditions, for the same polishing time, the depth of the large dimple was decreased when increasing the interval; for the same interval, with increase in the polishing time, the depth of the large dimple was also decreased. The possible formation mechanism of the large dimple was discussed, which was mainly ascribed to the enhanced material removal at the position of pre-indent array because of the concentration of polishing particles by the pre-indents.

The response of yttrium aluminum garnet (YAG) grains and grain boundaries to nanoindentation

A systematic investigation on the mechanical response of the interior grain and grain boundary of yttrium aluminum garnet is carried out using nanoindentation and microstructure characterization. The nano-hardness is calculated, while the residual indent is analyzed in terms of surface profiles, plastic deformation and elastic recovery. The results show that the nano-hardness at the center of grain is higher than that in the grain boundary region. All indents display an anisotropic elastic recovery, mainly occurring in the loading direction. The grain boundary has a better plasticity than the center of grain, which is probably due to a weaker inter-atomic bonding force at the grain boundary rather than the formation of glassy phase at intergranular region. In addition, indentation size effect in the center of grain and at grain boundary region are determined by the analysis of nano-hardness using Meyer’s law. A weaker indentation size effect in the grain boundary region is observed due to a smaller elastic recovery.

Influence of Powder Morphology and Microstructure on the Cold Spray and Mechanical Properties of Ti6Al4V Coatings

The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.

Superplastic deformation behavior of Zn-22% Al alloy investigated by nanoindentation at elevated temperatures

Due to their high ductility, superplastic metallic alloys are promising candidates for microforming processes. However, it is not yet clear if the macroscopically observed superplastic material behavior also persists at micrometric length scales. For this reason, the superplastic deformation behavior of the eutectoid alloy Zn-22% Al was investigated at the microscale as a function of temperature, strain-rate and grain-size. Nanoindentation strain-rate jump tests at room temperature and elevated temperatures were correlated with the deformed microstructure. A sigmoidal relationship between the hardness and the strain-rate is evidenced, which is typical for superplastic alloys at the macroscale. In the central part of the sigmoidal curve strong indications for superplastic flow at the microscale are found. Microstructural investigations of the residual indents reveal noticeable intercrystalline deformation processes in combination with some intracrystalline dislocation activity regardless of the applied testing conditions. These experimental findings were further coupled with finite element analysis to determine the size of the plastic zone beneath the indents as a function of the penetration depth. Based on this, a critical cubic material volume of 8 grains over the edge length is found to be sufficient for superplastic flow at local length scales.

A Novel Approach to Modelling Nanoindentation Instabilities

We review the recently developed models for load fluctuations in the displacement controlled mode and displacement jumps in the load controlled mode of indentation. To do this, we devise a method for calculating plastic contribution to load drops and displacement jumps by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities by including relevant dislocation mechanisms. These equations are then coupled to the equation defining constant displacement rate or load rate. The model for the displacement controlled mode using a spherical indenter predicts all the generic features of nanoindentation such as the elastic branch followed by several force drops of decreasing magnitudes and residual indentation depth after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted numbers for all the quantities match experiments on single crystals of Au using a spherical indenter. We extend the approach to model the load controlled nanoindentation experiments that employ a Berkovich indenter. We first identify the dislocation mechanisms contributing to different regions of the F − z curve as a first step for obtaining a good fit to a given experimental F − z curve. This is done by studying the influence of the parameters associated with various dislocation mechanisms on the model F − z curves. The study also demonstrates that the model predicts all the generic features of nanoindentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes and residual plasticity after unloading for a range of model parameter values. Furthermore, an optimized set of parameter values can be easily determined that give a good fit to the experimental load–displacement curves for Al single crystals of ( 110 ) and ( 133 ) orientations. Our model also predicts the indentation size effect in a region where the displacement jumps disappear. The good agreement of the results of the models with experiments supports our view that the present approach can be used as an alternate method to simulations. The approach also provides insights into several open questions.

Mechanical properties of carbon fibre-reinforced polymer/magnesium alloy hybrid laminates

In this study, we prepared fibre metal laminates (FMLs) consisting of high-modulus carbon fibre-reinforced polymer (CFRP) prepregs and thin AZ31 alloy sheets by using hot-pressing technology. Tensile and low-velocity impact tests were performed to evaluate the mechanical properties and fracture behaviour of the magnesium alloy-based FMLs (Mg-FMLs) and to investigate the differences in the fracture behaviour between the Mg-FMLs and traditional Mg-FMLs. Results show that the Mg-FMLs exhibit higher specific tensile strength and specific tensile modulus than traditional Mg-FMLs and that the tensile behaviour of the Mg-FMLs is mainly governed by the CFRP because of the combination of high interlaminar shear properties and thin magnesium alloy layers. The Mg-FMLs exhibit excellent bending stiffness. Hence, no significant difference between the residual displacement dr and indentation depth di, and the permanent deformation is mainly limited to a small zone surrounding the impact location after the impact tests.

Alternate approach for calculating hardness based on residual indentation depth: Comparison with experiments

It is well known that plastic deformation is a highly nonlinear dissipative irreversible phenomenon of considerable complexity. As a consequence, little progress has been made in modeling some well-known size-dependent properties of plastic deformation, for instance, calculating hardness as a function of indentation depth independently. Here, we devise a method of calculating hardness by calculating the residual indentation depth and then calculate the hardness as the ratio of the load to the residual imprint area. Recognizing the fact that dislocations are the basic defects controlling the plastic component of the indentation depth, we set up a system of coupled nonlinear time evolution equations for the mobile, forest, and geometrically necessary dislocation densities. Within our approach, we consider the geometrically necessary dislocations to be immobile since they contribute to additional hardness. The model includes dislocation multiplication, storage, and recovery mechanisms. The growth of the geometrically necessary dislocation density is controlled by the number of loops that can be activated under the contact area and the mean strain gradient. The equations are then coupled to the load rate equation. Our approach has the ability to adopt experimental parameters such as the indentation rates, the geometrical parameters defining the Berkovich indenter, including the nominal tip radius. The residual indentation depth is obtained by integrating the Orowan expression for the plastic strain rate, which is then used to calculate the hardness. Consistent with the experimental observations, the increasing hardness with decreasing indentation depth in our model arises from limited dislocation sources at small indentation depths and therefore avoids divergence in the limit of small depths reported in the Nix-Gao model. We demonstrate that for a range of parameter values that physically represent different materials, the model predicts the three characteristic features of hardness, namely, increase in the hardness with decreasing indentation depth, and the linear relation between the square of the hardness and the inverse of the indentation depth, for all but 150 nm, deviating for smaller depths. In addition, we also show that it is straightforward to obtain optimized parameter values that give good fit to the hardness data for polycrystalline cold worked copper and single crystals of silver.

Explicit dynamics simulation of shot peening process induced by various types of shots impacts

Shot peening is a cold-working process that induces a small indentation on a metallic surface by impacting shots onto it causing residual compressive stresses which delays the initiation of fatigue cracking. The experimental assessment of shot-peening process parameters is not only very complex but costly as well. The most promising technique is the explicit dynamics finite-element (FE) analysis capable of determining the shot-peening process parameters subject to the selection of a suitable material’s constitutive model and numerical technique. In this work, Ansys/LS-Dyna code was employed to simulate the impact of shots of various sizes made of steel and glass on an aluminium alloy plate defined with strain-rate-sensitive elasto-plastic material model. The impacts were carried out at various incident velocities. The effects of velocity, size, and material of the shots on the compressive residual stress, indentation, and force–time response were investigated. The results showed that impact with the steel shots caused more indentation and larger area of plastic deformation in target material than the glass shots due to higher kinetic energy. Furthermore, increasing the shot velocity and size resulted in an increase in indentation. However, a negligible effect of the shot velocity, size, and material was observed on target’s surface and subsurface residual stress. The obtained results demonstrated that the proposed FE models simulated the shot-peening process in a realistic way. The study can be used to determine and optimise the shot-peening parameters such as shot size, material and velocity, and amount indentation and residual stresses in the aluminium alloy plate.

Severe sink-in occurrence in thermally sprayed hydroxyapatite coating

The performance of orthopaedic and dental implants counts on the ability to appraise the mechanical properties of biomaterials. In the present study, nanoindentation was used to characterize the mechanical properties of thermally sprayed hydroxyapatite-coated commercial pure titanium (HA/CP-Ti). From the XRD patterns, the as-sprayed HA coating showed an inhomogeneous mixture of polycrystalline and amorphous phases along with the formation of the decomposed constituents including tricalcium phosphate (?-TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO). The SEM micrographs of HA coating indicated some incompletely melted particles on the surface of flattened droplets, where the average pore size was around 0.8 ?m. After the loadunload indentation experiment, a clear severe sink-in deformation feature was observed around the residual indentation impression of the amorphous part. In contrast, pile-up only occurs during loading for the crystalline part of the coating. In addition, the crystalline part of the coating exhibited a Young?s modulus and hardness of around 120.46 and 7.91GPa, respectively, which were almost 8 and 259% higher than that of the bare CP-Ti substrate. This is the first report on the nanoindentation characterization of the inhomogeneous mixture of various phase structures in thermally sprayed HA coatings.

Nanoindentation-Induced Pile-Up in the Residual Impression of Crystalline Cu with Different Grain Size

Nanoindentation morphologies of crystalline copper have been probed at the grain scale. Experimental tests have been conducted on nanocrystalline (NC), ultrafine-grained (UFG), and coarse-grained (CG) copper samples with a new Berkvoich indenter at the strain rate of 0.04/s without holding time at an indentation depth of 2000 nm at room temperature. As the grain size increases, the height of the pile-up around the residual indentation increases and then exhibits a slightly decrease in the CG Cu. The maximum of the pile-up in the CG Cu obviously deviates from the center of the indenter sides. Our analysis has revealed that the dislocation motion and GB activities in the NC Cu, some cross- and multiple-slip dislocations inside the larger grain in the UFG Cu, and forest dislocations from the intragranular Frank-Read sources in the CG Cu would directly induce this distinct pile-up effect.

Effects of Crystal Orientation on the Crack Propagation of Sapphire by Sequential Indentation Testing

Single-crystal sapphire (α-Al2O3) is a hard and brittle material. Due to its highly crystalline nature, the fracture behavior of sapphire is strongly related to its crystal structure, and understanding the effects of crystal structure on the crack propagation of sapphire is essential for the successful application of this important material (e.g., as wafers in the electronics industry). In the present work, crack propagation that is induced by sequential indentation was investigated on the A-plane and C-plane of sapphire using a Vickers indenter on a micrometer scale. It was found that increasing indentation depth obviously increases the rate of crack propagation on the A-plane, but this effect is not so obvious on the C-plane because of the different slip systems induced by indentation on the different crystal planes of sapphire. Moreover, some parallel linear traces along the A-plane, which fracture with increasing indentation depth, are observed from the residual indentation on the A-plane. The fracture toughness of both A-plane and C-plane sapphire is smaller after indentation testing than that obtained using conventional testing methods. The subsurface damage was detected by transmission electron microscopy (TEM).

Nanofriction of Graphene/Ionic Liquid-Infused Block Copolymer Homoporous Membranes.

We have infused graphene/ionic liquid into block copolymer homoporous membranes (HOMEs), which have highly ordered uniform cylindrical nanopores, to form compact, dense, and continuous graphene/ionic liquid (Gr/IL) lubricating layers at interfaces, enabling a reduction in the friction coefficient. Raman and XPS analyses, confirmed the parallel alignment of the cation of ILs on graphene by the π-π stacking interaction of the imidazolium ring with the graphene layer. This alignment loosens the lattice spacing of Gr in Gr/ILs, leading to a larger lattice spacing of 0.36 nm in Gr of Gr/ILs hybrids than the pristine Gr (0.33 nm). The loose graphene layers, which are caused by the coexistence of graphene and ILs, would make the sliding easier, and favor the lubrication. An increase in the friction coefficient was observed on ILs-infused block copolymer HOMEs, as compared to Gr/ILs-infused ones, due to the absence of Gr and the unstably formed ILs film. Gr/ILs-infused block copolymer HOMEs also exhibit much smaller residual indentation depth and peak indentation depth in comparison with ILs-infused ones. This indicates that the existence of stably supported Gr/ILs hybrid liquid films aids the reduction of the friction coefficient by preventing the thinning of the lubricant layer and exposure of the underlying block copolymer HOMEs.