Showing papers on "Indentation published in 2011"

Indentation versus tensile measurements of Young's modulus for soft biological tissues.

Abstract: In this review, we compare the reported values of Young's modulus (YM) obtained from indentation and tensile deformations of soft biological tissues. When the method of deformation is ignored, YM values for any given tissue typically span several orders of magnitude. If the method of deformation is considered, then a consistent and less ambiguous result emerges. On average, YM values for soft tissues are consistently lower when obtained by indentation deformations. We discuss the implications and potential impact of this finding.

Nanoindentation strain-rate jump tests for determining the local strain-rate sensitivity in nanocrystalline Ni and ultrafine-grained Al

Abstract: A nanoindentation strain-rate jump technique has been developed for determining the local strain-rate sensitivity (SRS) of nanocrystalline and ultrafine-grained (UFG) materials. The results of the new method are compared to conventional constant strain-rate nanoindentation experiments, macroscopic compression tests, and finite element modeling (FEM) simulations. The FEM simulations showed that nanoindentation tests should yield a similar SRS as uniaxial testing and generally a good agreement is found between nanoindentation strain-rate jump experiments and compression tests. However, a higher SRS is found in constant indentation strain-rate tests, which could be caused by the long indentation times required for tests at low indentation strain rates. The nanoindentation strain-rate jump technique thus offers the possibility to use single indentations for determining the SRS at low strain rates with strongly reduced testing times. For UFG-Al, extremely fine-grained regions around a bond layer exhibit a substantial higher SRS than bulk material.

Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals

Abstract: Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elastic–plastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured load–displacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallography and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {1 1 0}〈0 0 1〉 slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to 〈1 1 1〉 and highest for those close to 〈0 0 1〉. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to 〈0 0 1〉. Because very high pop-in loads are measured for orientations close to 〈0 0 1〉, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near 〈0 0 1〉.

Evaluation of modulus of elasticity, nano-hardness and fracture toughness of TiB2–TiC–Al2O3 composite coating developed by SHS and laser cladding

Abstract: Modulus of elasticity (E) and nano-hardness (H) of the composite TiB2–TiC–Al2O3 coating deposited on AISI 1020 steel by combined self-propagating high-temperature synthesis (SHS) and laser cladding process have been measured from load–displacement curves, resulting from nano-indentation testing. Fracture toughness (KIC) of the coatings has been evaluated from the indentation method. A qualitative evaluation of interfacial strength between coating and substrate material was also performed by indentation method. Results indicated elastic modulus and fracture toughness of the composite coatings are in the range of 230–280 GPa and 5.46–6.12 MPa m1/2 those are respectively lower and higher than those of the individual constituent ceramics. Nano-hardness of the coating microstructure obtained was in the range of 16–22 GPa and was found to vary depending on the laser-processing parameter adopted. Indentation taken with high load at the coating–substrate interface indicated a strong interfacial bond between coating and steel substrate. An attempt has been made to co-relate the wear rate of the coating with H/E ratio.

Soda-lime silicate glass under hydrostatic pressure and indentation: a micro-Raman study

Abstract: Raman micro-spectroscopy is used to analyse the plastic behaviour of window glass (a soda-lime silicate glass) under high hydrostatic pressure and Vickers indentation. We show pressure-induced irreversible structural changes, notably an increase of Q2 species at the expense of Q3. For the first time, a very accurate calibration curve has been established. Local density variations of a Vickers indented window glass have been characterized by micro-Raman mapping using a high spatial resolution device. The effects of glass depolymerization on indentation and hydrostatic compression are discussed. Differences between window glass and pure SiO2 glass behaviour under high stresses are also highlighted and analysed at a local scale.

Extending the EGP constitutive model for polymer glasses to multiple relaxation times

Abstract: The one-mode EGP (Eindhoven glassy polymer) model captures the plastic flow at yield and post-yield quantitatively, but behaves poor in the non-linear viscoelastic pre-yield region. Since a proper description here is important in cases of complex loading and unloading situations, such as e.g. in indentation and scratching, an extension to non-linear modeling is required using a spectrum of relaxation times. It is shown that such a reference spectrum can be obtained from simple tensile tests. It shifts to shorter times under the influence of stress and is independent of the two important time-dependent processes in polymers: the strain rate applied during testing and the aging time during storage and use. The multi-mode model is critically tested and proves quantitative in describing the intrinsic polymer response and, based thereupon, in predicting the correct response in tensile testing, including necking, in flat tip indentation and in notched loading.

The relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls

Abstract: Nanoindentation is a well known tool for identification of mechanical properties at the micrometer scale of materials. When applied to study wood cell walls the commonly used isotropic indentation theory is not applicable. In this study, anisotropic nanoindentation theory was employed for analyzing nanoindentation test results on wood cell walls. The influence of elastic stiffness components, microfibril angle, and cell wall composition on the indentation modulus was studied. The indentation modulus was found to depend on longitudinal, transverse, and shear modulus to a similar extent. A significant influence of the microfibril angle on the indentation modulus was observed and discussed with respect to experimental scatter and sample preparation. It is concluded, that application of anisotropic nanoindentation theory provides a tool for quantitative instead of qualitative investigation of wood cell walls, with the goal of identifying all elastic properties of the transversely isotropic cell wall from nanoindentation tests.

Rolling Indentation Probe for Tissue Abnormality Identification During Minimally Invasive Surgery

Abstract: This paper presents a novel optical fiber-based rolling indentation probe designed to measure the stiffness distribution of a soft tissue while rolling over the tissue surface during minimally invasive surgery. By fusing the measurements along rolling paths, the probe can generalize a mechanical image to visualize the stiffness distribution within the internal tissue structure. Since tissue abnormalities are often firmer than the surrounding organ or parenchyma, a surgeon then can localize abnormalities by analyzing the image. The performance of the developed probe was validated using simulated soft tissues. Results show that the probe can measure both force and indentation depth accurately with different orientations when the probe approached and rolled on the tissue surface. In addition, experiments for tumor, identification through rolling indentation were conducted. The size and embedded depth of the tumor, as well as the stiffness ratio between the tumor and tissue, were varied during tests. Results demonstrate that the probe can effectively and accurately identify the embedded tumors.

Indentation of polydimethylsiloxane submerged in organic solvents

Abstract: This work uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory–Huggins theory. The measured permeability indicates that the solvent migrates in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers.

Materials Science and Engineering A

Abstract: a b s t r a c t Instrumented sharp indentation experiments using both conical and Vickers diamond pyramidal indenters were carried out to study deformation characteristics of Zr55Cu30Al10Ni5 bulk metallic glass. Finite element simulations of instrumented indentation were also performed to formulate an overall constitutive response. Comparing the experimentally obtained results with the finite element predictions, it can be stated that mechanical deformation of the bulk metallic glass can be described well by both Mohr–Coulomb and Drucker–Prager constitutive criteria. Using these criteria, the extent of material pile-up observed around the indenter was also estimated very well.

Experimental study of the indentation of sandwich panels with carbon fibre-reinforced polymer face sheets and polymeric foam core

Abstract: This paper presents experimental studies on the quasi-static indentation of a rigid indenter into sandwich panels with carbon fibre-reinforced polymer face and polymeric foam core. It was found that both nose shape and foam core density have large influence on the indentation response of the sandwich panels in terms of absorbed energy, indentation at failure and damage area. A dependency of the indentation load on the supporting condition was observed. It was also found that the difference in indentation resistance between the sandwich panel and its corresponding core material depends on the core density.

Local mechanical properties of the 6061-T6 aluminium weld using micro-traction and instrumented indentation

Abstract: The local mechanical properties of a weld zone, in a 6061-T6 aluminium alloy subjected to the modified indirect electric arc technique have been studied. The mechanical properties of the base metal, the weld metal and the heat affected zone were determined by means of usual and instrumented indentation testing, as well as micro-traction testing. To analyse the heat input effect resulting from the welding process, the evolution of the weld zone size was evaluated by means of classical indentation under a constant applied load. The results were presented using a Vickers hardness map representation. This allows monitoring exact hardness variation while leading to the identification of the different zones of the welded joint. Instrumented indentation testing was carried out to determine the local mechanical properties, such as the yield stress, the bulk modulus and the strain-hardening exponent. Obtained results are compared to those derived from tensile tests conducted on micro-specimen cuts taken from the weld zone. It was observed that yield stress values are directly comparable for indentation and micro-traction experiments. As for the elastic properties, no comparison was possible since the bulk modulus is measured by indentation, whereas it is the Young's modulus by tensile test. The micro-traction testing seems to be more sensitive to represent the work hardening of a material since the corresponding exponent is found to be constant by instrumented indentation.

Compression, crumpling and collapse of spherical shells and capsules

Abstract: The deformation of thin spherical shells by applying an external pressure or by reducing the volume is studied by computer simulations and scaling arguments. The shape of the deformed shells depends on the deformation rate, the reduced volume V/V0 and the Foppl–von Karman number γ. For slow deformations the shell attains its ground state, a shell with a single indentation, whereas for large deformation rates the shell appears crumpled with many indentations. The rim of the single indentation undergoes a shape transition from smooth to polygonal for γ7000(ΔV/V0)− 3/4. For the smooth rim the elastic energy scales like γ1/4 whereas for the polygonal indentation we find a much smaller exponent, even smaller than the exponent 1/6 that is predicted for stretching ridges. The relaxation of a shell with multiple indentations towards the ground state follows an Ostwald ripening type of pathway and depends on the compression rate and on the Foppl–von Karman number. The number of indentations decreases as a power law with time t following Nind~t− 0.375 for γ=8×103 and γ=8×104 whereas for γ=8×105 the relaxation time is longer than the simulation time.

Indentation of polydimethylsiloxane submerged in organic solvents

Abstract: This work uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory–Huggins theory. The measured permeability indicates that the solvent migrates in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers.

High temperature nanoindentation – the importance of isothermal contact

Abstract: The suitability of high temperature techniques was investigated by finite element analysis modelling to give a qualitative view of how the thermal picture develops under a diamond indenter without controlled heating of the diamond. In the case of a low-conductivity sample such as fused silica, the thermal gradient below the indenter tip is relatively diffuse, whereas with a high-conductivity sample such as gold, most of the sample is able to equilibrate at the set temperature, leading to a very steep thermal gradient in the volume of material that must accommodate the deformation. However, in both cases indentation is occurring in material that is at a lower, and unknown, temperature than the bulk sample. The results of the model are validated by comparing results obtained by heating the indenter either indirectly by contact with the sample or utilising a separate heater for the indenter (an isothermal contact method). Nanoindentation results are presented for experiments using a cubic boron nitride Berko...

Nanoindentation of hard multilayer coatings: Finite element modelling

Abstract: Stress concentrations undermine the load-bearing ability of superhard TiSiN coatings. Experimental studies have shown that multilayer coatings that contain TiSiN layers alternating with ceramic layers with dissimilar mechanical properties suppress contact damage during nanoindentation. In this work, nanoindentation-induced deformation in TiSiN-based multilayer coatings was simulated by means of finite element modelling (FEM). Stress distributions under moderate indentation loading in the structure were quantified with particular emphasis on the relationship between stress concentrations and crack initiation. The results showed that the structural layering can be used to modify the stress distribution, and lower the overall stress level within the coating. In the case of radial tensile stresses at the coating/substrate interface, a reduction ∼50% has been achieved through layering. The resistance to shear damage can also be improved by optimising the multilayer structure.

Carbon nanotubes: do they toughen brittle matrices?

Abstract: The development of a model CNT-brittle matrix composite system, based on SiO2 glass containing well-dispersed CNTs at up to 15 wt%, allows a direct assessment of the effect of the nanoscale filler on fracture toughness (KIC) Samples were prepared by colloidal heterocoagulation followed by spark plasma sintering Detailed KIC measurements, using both indentation and notched beam techniques, show a linear improvement with CNT content, with up to a twofold increase of fracture toughness at maximum loading The results from the two methods used in this study show equivalent trends but differing absolute values; the relative merits of these two approaches to measuring nanocomposite toughness are compared Possible toughening mechanisms associated with CNT pull-out, crack bridging, and crack deflection are identified, and discussed quantitatively, drawing on conventional short-fibre composite theory and the potential effects of scaling fibre diameter