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Indentation tests with large penetration depths have been used to study the plastic deformation behavior of materials. In this work, finite element simulations of wedge indentation into face-centered cubic single crystals were performed. Numerical solutions to the stresses and shear strains within the single crystals indented with a relatively large penetration depth were obtained. The crystal lattice rotation map of the indented crystals was also shown. Indentation experiments were conducted on copper crystals and the results were used to validate the numerical predictions. Comparison of the numerical solutions to the crystal lattice rotation with the experimentally measured lattice rotation map was made. The main features of the crystal lattice in-plane rotation map from the finite element simulations are also found on the map developed from the electron backscatter diffraction measurements. Both simulations and experimental measurements reveal the same dislocation structures as evidenced by the slip sectors underneath the wedge indentation zone.

Nanoindentation has been widely used to evaluate material properties. In this study, we propose a method that utilizes only the loading curves of an indentation test to extract the elastoplastic properties of an elastic-perfectly plastic thin film as well as the plastic properties of a work hardening thin film. The use of loading curve circumvents some common difficulties encountered during the post-processing of experimental unloading curves. Measurements are taken at two different indentation depths, which have different levels of substrate effects and lead to the establishment of independent equations that correlate the material properties with indentation responses. Effective reverse analysis algorithms are proposed by following which the desired film properties can be determined from a sharp indentation test. The extracted material properties agree well with that measured from a bulge test.

Nanoindentation is an effective technique for deducing the elastic property of single-walled carbon nanotubes (SWCNTs). Following an atomistic study of the nanoindentation mechanism, reverse analysis algorithms are proposed by utilizing the indentation force-depth data measured from the initial uniaxial compression and post-buckling regimes, respectively, which lead to stretching stiffness of 382 Pa m and 429 Pa m, that are very close to those in the literature. Parallel finite element simulations incorporating atomic interactions are also carried out, which closely duplicates the indentation response of SWCNTs in atomistic simulations. The numerical studies carried out in this paper may be used to guide the nanoindentation experiments, explain and extract useful data from the test, as well as stimulate new experiments.

Based on microindentation experiments of three different metals, Guelorget et al. [J. Mater. Res. 22, 1512 (2007)] have compared the performance of five different indentation methods on extracting material plastic properties—among them, three papers were proposed by Cao and Lu [Acta Mater.52, 4023 (2004); J. Mater. Res.20, 1194 (2005); J. Mech. Phys. Solids53, 49 (2005)] and two papers were published by our group [Ogasawara et al., Scripta Mater.54, 65 (2006); Zhao et al. Acta Mater.54, 23 (2006)]. They argued that the performances of our techniques in [Ogasawara et al., Scripta Mater.54, 65 (2006); Zhao et al. Acta Mater.54, 23 (2006)] were quite poor. Here we show that Guelorget et al. [J. Mater. Res. 22, 1512 (2007)] have made quite a few mistakes and problematic steps when they handled the experiment data and performed reverse analysis. Indeed, the material plastic properties extracted from the correct procedures based on our papers [Ogasawara et al., Scripta Mater.54, 65 (2006); Zhao et al. Acta Mater.54, 23 (2006)] are much better and more stable than that reported in Guelorget et al. [J. Mater. Res. 22, 1512 (2007)]. Several general issues related to interpreting microindentation data and reverse analysis are also discussed, which may serve as important guidelines for similar studies in the future.

Nanoindentation is a technique commonly used for measuring material elastoplastic properties. In this study, we extend the indentation analysis to an elastoplastic thin film on an elastic substrate. With the assistance of the substrate effect, the film elastic modulus, yield stress and work hardening exponent can be measured from one simple sharp indentation test with moderate penetration depth. The film elastoplastic properties and film/substrate mismatches are varied over a large range in the forward analysis, and an effective reverse analysis algorithm is established to deduce film properties from the indentation load–displacement curve. Comprehensive analyses are carried out to examine the accuracy, error sensitivity, and effectiveness of the proposed method. Both nanoindentation and bulge tests are carried out on a Cu film/Si substrate system, and the good agreement shows that the new film indentation technique may be applied in practice.

Indentation is widely used to extract material elastoplastic properties from the measured force–displacement curves. One of the most well-established indentation techniques utilizes dual (or plural) sharp indenters (which have different apex angles) to deduce key parameters such as the elastic modulus, yield stress, and work-hardening exponent for materials that obey the power-law constitutive relationship. However, the uniqueness of such analysis is not yet systematically studied or challenged. Here we show the existence of “mystical materials”, which have distinct elastoplastic properties yet they yield almost identical indentation behaviors, even when the indenter angle is varied in a large range. These mystical materials are, therefore, indistinguishable by many existing indentation analyses unless extreme (and often impractical) indenter angles are used. Explicit procedures of deriving these mystical materials are established, and the general characteristics of the mystical materials are discussed. In many cases, for a given indenter angle range, a material would have infinite numbers of mystical siblings, and the existence maps of the mystical materials are also obtained. Furthermore, we propose two alternative techniques to effectively distinguish these mystical materials. The study in this paper addresses the important question of the uniqueness of indentation test, as well as providing useful guidelines to properly use the indentation technique to measure material elastoplastic properties.

Cao and Huber [J. Mater. Res., 21, 1810 (2006)] proposed parameter-dependent representative strains, which have the potential to measure plastic properties from one conical indentation test. However, the potential performance of such a technique was not systematically analyzed. In this commentary, through a comprehensive numerical investigation, it is found that the two basic variables in Cao and Huber’s formulation are not completely independent, and it is difficult to obtain the two independent (sufficiently separated) representative stress–strain points needed for determining the plastic properties. Consequently, systematic errors (which could well exceed 100%) are generated for a wide range of materials, and the results are quite sensitive to small perturbations. As a complementary and critical examination of the original article, this commentary suggests that it is unreliable to use the representative strains proposed by Cao and Huber [J. Mater. Res., 21, 1810 (2006)] to measure the material plastic properties from one indentation.

The mechanical properties of nanowires consisting of nanoporous gold are studied experimentally. The nanoporous gold is synthesized from dealloyed “white gold” leaf (Au37.4 and Ag62.6 in at.%) into a free-standing dog-bone-shaped specimen with a gauge section 7 μm long, 300 nm wide, and 100 nm thick. The elastic modulus, residual stress, and yield stress are measured by deflective tensile testing and nanoindentation, and are about 9 GPa, 65 MPa, and 110 MPa, respectively.

The results of two sets of experiments to measure the elastic?plastic behaviour of gold at the nanometre length scale are reported. One set of experiments was on free-standing nanoscale single crystals of gold, and the other was on free-standing nanoscale specimens of open-celled nanoporous gold. Both types of specimens were fabricated from commercially available leaf which was either pure Au or a Au/Ag alloy following by dealloying of the Ag. Mechanical testing specimens of a 'dog-bone' shape were fabricated from the leaf using standard lithographic procedures after the leaf had been glued onto a silicon wafer. The thickness of the gauge portion of the specimens was about 100?nm, the width between 250?nm and 300?nm and the length 7??m. The specimens were mechanically loaded with a nanoindenter (MTS) at the approximate midpoint of the gauge length. The resulting force?displacement curve of the single crystal gold was serrated and it was evident that slip localization occurred on individual slip systems; however, the early stages of the plastic deformation occurred in a non-localized manner. The results of detailed finite element analyses of the specimen suggest that the critical resolved shear stress of the gold single crystal was as high as 135?MPa which would lead to a maximum uniaxial stress of about 500?MPa after several per cent strain. The behaviour of the nanoporous gold was substantially different. It exhibited an apparent elastic behaviour until the point where it failed in an apparently brittle manner, although it is assumed that plastic deformation occurred in the ligaments prior to failure. The average elastic stiffness of three specimens was measured to be Enp = 8.8?GPa and the stress at ultimate failure averaged 190?MPa for the three specimens tested. Scaling arguments suggest that the stress in the individual ligaments could approach the theoretical shear strength.

We propose a new theory with the potential for measuring the elastoplastic properties of compliant and soft materials using one sharp indentation test. The method makes use of the substrate effect, which is usually intended to be avoided during indentation tests. For indentation on a compliant and soft specimen of finite thickness bonded to a stiff and hard testing platform (or a compliant/soft thin film deposited on a stiff/hard substrate), the presence of the substrate significantly enhances the loading curvature which, theoretically, enables the determination of the material power-law elastic-plastic properties by using just one conical indentation test. Extensive finite element simulations are carried out to correlate the indentation characteristics with material properties. Based on these relationships, an effective reverse analysis algorithm is established to extract the material elastoplastic properties. By utilizing the substrate effect, the new technique has the potential to identify plastic materials with indistinguishable indentation behaviors in bulk forms. The error sensitivity and uniqueness of the solution are carefully investigated. Validity and application range of the proposed theory are discussed. In the limit where the substrate is taken to be rigid, the fundamental research is one of the first steps toward understanding the substrate effect during indentation on thin films deposited on deformable substrates.

We propose an improved technique to determine the uniaxial residual stress, elastic modulus, and yield stress of a linear elastic, perfectly plastic bulk material from the force–displacement curve of one conical indentation test. Explicit relationships between the indentation loading–unloading parameters, material properties, and residual stress are established through extensive finite element analyses. Good agreement is found between the input material parameters used in numerical indentation tests and the properties identified from the reverse analysis, with an error of less than 10% in most cases. The technique is applied to a nanoindentation experiment on the cross-section of a thermal barrier system, to measure the elastic–plastic behavior and the residual stress in the bond coat. Likewise, the improved method may be used to measure effectively the material properties and uniaxial residual stress of a multilayer system.

Spherical indentation has the potential for measuring the elastic–plastic properties of a power-law hardening bulk specimen with just one simple test. In this paper, based on a new definition of representative strain, a set of dimensionless functions that relate the indentation characteristics and material properties are established by means of extensive finite element analyses. A new numerical algorithm of reverse analysis is proposed to extract the elastic–plastic properties of bulk materials from one indentation test. This method is applicable to a wide range of power-law hardening materials. Good agreement is found between the material properties obtained from the reverse analysis of the new approach and experimental data.

Nanoindentation test at scale of hundreds of nanometers has shown that measured hardness increases strongly with decreasing indent depth, which is frequently referred to as the size effect. Usually, the size effect is displayed in the hardness-depth curves. In this study, the size effect is characterized in both the load–displacement curves and the hardness–depth curves. The experimental measurements were performed for single-crystal copper specimen and for surface-nanocrystallized Al-alloy specimen. Moreover, the size effect was characterized using the dislocation density theory. To investigate effects of some environmental factors, such as the effect of surface roughness and the effect of indenter tip curvature, the specimen surface profile and the indentation imprint profile for single-crystal copper specimen were scanned and measured using the atomic force microscopy technique. Furthermore, the size effect was characterized and analyzed when the effect of the specimen surface roughness was considered.

The interface adhesion strength (or interface toughness) of a thin film/substrate system is often assessed by the micro-scratch test. For a brittle film material, the interface adhesion strength is easily obtained through measuring the scratch driving forces. However, to measure the interface adhesion strength (or interface toughness) for a metal thin film material (the ductile material) by the micro-scratch test is very difficult, because intense plastic deformation is involved and the problem is a three-dimensional elastic-plastic one. In the present research, using a double-cohesive zone model, the failure characteristics of the thin film/substrate system can be described and further simulated. For a steady-state scratching process, a three-dimensional elastic-plastic finite element method based on the double cohesive zone model is developed and adopted, and the steady-state fracture work of the total system is calculated. The parameter relations between the horizontal driving forces (or energy release rate of the scratching process) and the separation strength of thin film/substrate interface, and the material shear strength, as well as the material parameters are developed. Furthermore, a scratch experiment for the Al/Si film/substrate system is carried out and the failure mechanisms are explored. Finally, the prediction results are applied to a scratch experiment for the Pt/NiO material system given in the literature.

In the present research, the adhesion properties and failure mechanisms for a ductile thin film on a silicon substrate (Ni/Si) are studied experimentally, and are simulated theoretically. In the experimental research, the relations of the horizontal driving force, vertical displacement and the frictional coefficients with horizontal displacement are measured. Furthermore, the variation of the total energy release rate and the frictional coefficient between contact surfaces are measured through obtaining a frictional effect law. The law displays that the frictional influences on the energy release rate of the total system weakly depend on the thin film thickness. This conclusion leads to that the frictional effect can be eliminated in the toughness ratio relation approximately. So that one can directly obtain the interfacial adhesion toughness from measurements in the micro-scratching test. In addition, the micro-scratching process for the ductile thin film/brittle substrate systems is simulated using the double cohesive zone model. Prediction results of the energy release rate are obtained, and are compared with the experimental results obtained in the present research.

Freestanding single crystal nanowires of gold were fabricated from a single grain of pure gold leaf by standard lithographic techniques, with center section of 7μm in length, 250nm in width, and 100nm in thickness. The ends remained anchored to a silicon substrate. The specimens were deflected via nanoindenter until plastic deformation was achieved. Nonlocalized and localized plastic deformations were observed. The resulting force-displacement curves were simulated using continuum single crystal plasticity. A set of material parameters which closely reproduce the experimental results suggests that the initial critical resolved shear stress was as high as 135MPa.