Microindentation Experiments Research Articles

Macromechanical and micromechanical properties of polymers with reduced density of entanglements

AbstractThe macromechanical and micromechanical properties of polypropylene, polylactide, and polystyrene with different entanglement densities and different crystallinities were tested. The mechanical properties were characterized by macroscale compression tests and microindentation hardness measurements. The entanglement density modified the mechanical properties only in the high‐deformation region, above the yield point, when the strain‐hardening occurred, and ca 10% lower stresses were measured for disentangled polymers. In the low‐deformation region, the influence of the entanglement density was negligible. The stiffness of semicrystalline polymers increased with their crystallinity and the stiffness of amorphous polymers increased with their glass transition temperature. For example, the elastic modulus was equal to 1.59 MPa for entangled polylactide, 1.68 MPa for partially disentangled polylactide, and 2.20 MPa for semicrystalline, disentangled polylactide. The stiffness‐related properties in both macroscale (compressive modulus, E, and yield stress, Y) and microscale (indentation modulus, EIT and indentation hardness, HIT) showed very similar trends, which was in perfect agreement with theoretical predictions and confirmed the reliability of our results. The supplementary, viscosity‐related properties from microindentation experiments (indentation creep, CIT, and elastic part of indentation work, ηIT) supported the above‐mentioned conclusions in the sense that also these two low‐deformation region properties were independent on the entanglement density.

Indentation probe with optical fibre array‐based optical coherence tomography for material deformation

We present a new optomechanical probe for mechanical testing of soft matter. The probe consists of a micromachined cantilever equipped with an indenting sphere, and an array of 16 single‐mode optical fibres, which are connected to an optical coherence tomography (OCT) system that allows subsurface analysis of the sample during the indentation stroke. To test our device and its capability, we performed indentation on a PDMS‐based phantom. Our findings demonstrate that Common Path (CP)‐OCT via lensed optical fibres can be successfully combined with a microindentation sensor to visualise the phantom's deformation profile at different indentation depths and locations in real time.Lay DescriptionThis work presents a new approach to simultaneously perform micro‐indentation experiments and OCT imaging. An optical fiber array‐based sensor is used to develop a new hybrid tool where micro‐indentation is combined with optical coherence tomography. The sensor is therefore capable of compressing a sample with a small force and simultaneously collecting OCT depth profiles underneath and around the indentation point. This method offers the opportunity to characterize the mechanical properties of soft materials and simultaneously visualize their deformation profile. The ability to integrate OCT imaging with indentation technology is promising for the non‐invasive and precise characterization of different soft materials.

Characterization of meso-scale mechanical properties of Longmaxi shale using grid microindentation experiments

Abstract Mechanical properties, such as the hardness H, Young’s modulus E, creep modulus C, and fracture toughness Kc, are essential parameters in the design of hydraulic fracturing systems for prospective shale gas formations. In this study, a practical methodology is presented for obtaining these properties through microindentation experiments combined with quantitative observations of the mineralogical phases using X-ray diffraction (XRD), scanning electron microscopy (SEM) with backscattered electron (BSE) imaging, and energy-dispersive X-ray spectroscopy (EDS) analyses. We apply this method in the case of three types of Longmaxi shales with different mineralogies (i.e. carbonate-, clay-, and quartz-rich, respectively), which allows us to determine the characteristic indentation depth, hc = 8–10 μm, beyond which the mechanical response of the carbonate-rich shale is homogeneous and independent of its complex heterogeneous microstructure. Moreover, exploiting the results of a large number of indentation tests, we demonstrate that the indentation modulus M of the shale increases as a power-law of hardness H, and its creep modulus C increases linearly with H. We also compute the fracture toughness Kc from the indentation data by assuming a perfectly plastic behavior of the sample. Our results are in good agreement with independent measurements of Kc determined by microscratch tests. Finally, further tests on quartz- and clay-rich samples of the Longmaxi shale suggest further variations in the samples’ mechanical properties depending on their burial conditions and the mechanical properties of their dominant mineral phases.

Investigation of the mechanical properties and the influence of micro-structural characteristics of aggregates using micro-indentation and Weibull analysis

Micro-indentation experiments were performed to study the correlation, both qualitatively and quantitatively, between microstructural characteristics and mechanical properties of three types of aggregates, including basaltic crushed rock, vesicular volcanic rock composed of pumice and recycled concrete aggregates. Emphasis was placed on the measurements of micro-hardness and modulus, and the influence of number of loading cycles. Despite the highly vesicular structure leading to edge chipping during indentation, pumice showed lower variability in indentation hardness and modulus values compared to basalt and recycled concrete aggregate. The number of indentation cycles had different influence on the crystalline structured recycled concrete aggregate and basalt compared to the amorphous structured pumice in terms of hardness and modulus values. Weibull statistics was used to support the important influence of the microstructural and textural characteristics of the aggregates on the bulk properties of the specimens, and different statistical parameters were introduced to characterise the aggregates.

Tribological performance of high density polyethylene (HDPE) composites with low nanofiller loading

High density polyethylene (HDPE) nanocomposites were prepared by melt mixing of the polymer with 0.5–1.0 wt-% hydrophobic surface treated fumed silica and titanium nitride (TiN) nanopowders, fiber-shaped halloysite nanotubes (HNT), and graphene oxide (GO) nanoplatelets in the presence of trace amount of organic peroxide. Surface treatments of the nanofillers with hydrophobic vinyltrimethoxy silane (VTMS) were accomplished in boiling toluene solvents and in the case of fumed silica, most feasibly in ethanol solvent. Sliding friction and wear of the nanocomposites were characterized by pin-on-disk (POD) test against polished bearing steel disc. Microstructures of the nanocomposites, and the wear surfaces were characterized by scanning electron microscopy (SEM). Microindentation experiments were carried out to measure the surface hardness, elasticity and the plastic and elastic work of indentation. VTMS surface coverage was highest on fumed silica and GO powders, which also correlated with the most homogeneous and fine filler dispersion in HDPE polymer, and showed the best performance of the nanocomposites in the POD test. In all cases, the addition of silane treated nanopowders reduced the sliding wear of the HDPE against polished steel counter surface. Moreover, deviation of the wear rate data between parallel measurements was greatly reduced by the nanofiller addition indicating homogenous structure. The coefficient of friction (COF) of the neat polymer was reduced only in the case of TiN filler. Inverse linear trend was found between the specific wear rate of the nanocomposites and a parameter derived from micro-indentation data (HIT*total work of indentation)−1, which is formally similar to the well-known Ratner-Lancaster correlation.

Experimental study on ductile-to-brittle behavior and crystal orientation in micro-crack induced severing of 4H-SiC and sapphire

For the scribing-to-severing method, it is difficult to sever single-crystal SiC and sapphire without edge fracture and near-zero material removal due to material hardness, brittleness and crystal orientation. According to the micron-scale brittle behavior, a crack-free microgroove-apex is proposed on single-crystal SiC and sapphire surfaces to induce the micro-crack propagation. The objective is to understand how the ductile-to-brittle behavior and crystal orientation to influence the severing performance. In order to induce the micro-crack propagation in severing, the microgroove-apex was machined on the workpiece surface by the micro-grinding. First, the micro-indentation experiments were performed to explore the ductile-to-brittle behavior; then, the severing force, efficiency and accuracy were investigated in severing experiments, respectively. It is shown that the micro-crack propagation ratio derived from the micro-indentation may display the ductile-to-brittle behavior in relation to the severing performance. With larger harness and less micro-crack propagation ratio, the 4H-SiC leads to larger severing force, longer severing time and less form error than the sapphire does, respectively. Moreover, the severing force and the severing time are sensitive to the crystal orientation. Because the micro-crack propagation behavior is changed by the loading rate, the static severing is transferred to the dynamic severing at the critical loading rate, thus leading to different severing behavior. Along an accurate micro-crack propagation, the precision severing depends on a sharpness and smoothness microgroove-apex.

Architected metamaterials with tailored 3D buckling mechanisms at the microscale

Attaining architected structures with unprecedented properties not found in natural materials is a long-sought goal for both materials science and additive manufacturing. Improving the mechanical behavior of engineered materials such as auxetics for bioengineering applications or bistable structures for mechanical actuation depends on the buckling mechanisms governing their response. Despite the fact that the design principles for two-dimensional buckling have been elucidated in previous studies, comprehending three dimensional buckling mechanisms is still tenuous. In this study, we aim to illuminate the buckling mechanisms governing the response of three-dimensional structures at the microscale, providing the critical guidelines for the design of novel metamaterials. In addition, finite element analysis and in-situ SEM — microindentation experiments are performed to investigate how scale effects affect the design principles to architect a controlled 3D buckling mechanism and reflect the mechanical response to the deformation of the structure in-situ. Our findings elicit the design process of tailored three dimensional buckling structures based on the geometric architecture instead of the behavior of the bulk material, while setting a broader framework to fabricate and characterize these structures at the microscale.

Automated analysis of soft hydrogel microindentation: Impact of various indentation parameters on the measurement of Young's modulus.

Measurements of Young’s moduli are mostly evaluated using strong assumptions, such as sample homogeneity and isotropy. At the same time, descriptions of measurement parameters often lack detailed specifications. Many of these assumptions are, for soft hydrogels especially, not completely valid and the complexity of hydrogel microindentation demands more sophisticated experimental procedures in order to describe their elastic properties more accurately. We created an algorithm that automates indentation data analysis as a basis for the evaluation of large data sets with consideration of the influence of indentation depth on the measured Young’s modulus. The algorithm automatically determines the Young’s modulus in indentation regions where it becomes independent of the indentation depth and furthermore minimizes the error from fitting an elastic model to the data. This approach is independent of the chosen elastic fitting model and indentation device. With this, we are able to evaluate large amounts of indentation curves recorded on many different sample positions and can therefore apply statistical methods to overcome deviations due to sample inhomogeneities. To prove the applicability of our algorithm, we carried out a systematic analysis of how the indentation speed, indenter size and sample thickness affect the determination of Young’s modulus from atomic force microscope (AFM) indentation curves on polyacrylamide (PAAm) samples. We chose the Hertz model as the elastic fitting model for this proof of principle of our algorithm and found that all of these parameters influence the measured Young’s moduli to a certain extent. Hence, it is essential to clearly state the experimental parameters used in microindentation experiments to ensure reproducibility and comparability of data.

Effects of ion irradiation on chemical and mechanical properties of magnetron sputtered amorphous SiOC

Amorphous silicon oxycarbide (SiOC) films, fabricated by magnetron sputtering, were irradiated at room temperature with 3.5 MeV Fe ions to damage levels of 10, 20, and 50 displacements per atom (dpa). Irradiation-induced changes in the nature of the atomic bonds were studied using Raman spectroscopy and X-ray photoelectron spectroscopy. The chemical composition of the surfaces of the films remained relatively unchanged after ion irradiation. Surface topography of the films was studied by atomic force microscopy and it was found that ordered topographic patterns were formed on the surface of the irradiated films. Deformation behavior of the films was studied by a combination of nanoscratch experiments and in-situ scanning probe microscopy. No signs of cracks or chipping were observed around the scratches on the as-deposited and irradiated films. Furthermore, microindentation experiments with a Vickers indenter were performed on selected films and the impressions were studied by scanning electron microscopy. The film irradiated to 20 dpa showed an increased resistance to indentation cracking compared to the as-deposited film. The study demonstrates that amorphous SiOC, with desirable structural stability and mechanical properties, can be a promising candidate for irradiation tolerant materials.

Softening Mechanisms and Microstructural Evolution in a Low Stacking Fault Energy Material Between Warm and Hot Working Temperatures

A low stacking fault energy material is subjected to warm working as well as hot working, i.e. in the temperature range 873 K (600 °C)–1173 K (900 °C). The evolution of microstructure is quantified using average grain size and grain size distributions. The microstructural transitions resulting from continuous dynamic recrystallization and discontinuous dynamic recrystallization are examined. The resultant softening is studied using macro- and micro-indentation experiments. The applicability of empirical relations between hardness and microstructural parameters is critically examined. The SFE at each of these temperatures is evaluated using a thermodynamic model and correlated with the softening mechanism.

Quantitative mechanical analysis of indentations on layered, soft elastic materials.

Atomic force microscopy (AFM) is becoming an increasingly popular method for studying cell mechanics, however the existing analysis tools for determining the elastic modulus from indentation experiments are unable to quantitatively account for mechanical heterogeneity commonly found in biological samples. In this work, we numerically calculated force-indentation curves onto two-layered elastic materials using an analytic model. We found that the effect of the underlying substrate can be quantitatively predicted by the mismatch in elastic moduli and the homogeneous-case contact radius relative to the layer height for all tested probe geometries. The effect is analogous to one-dimensional Hookean springs in series and was phenomenologically modeled to obtain an approximate closed-form equation for the indentation force onto a two-layered elastic material which is accurate for up to two orders of magnitude mismatch in Young's modulus when the contact radius is less than the layer height. We performed finite element analysis simulations to verify the model and AFM microindentation experiments and macroindentation experiments to demonstrate its ability to deconvolute the Young's modulus of each layer. The model can be broadly used to quantify and serve as a guideline for designing and interpreting indentation experiments into mechanically heterogeneous samples.

A continuum dislocation-based model of wedge microindentation of single crystals

Recent Electron Backscatter Diffraction (EBSD) experiments have revealed the emergence of heterogeneous dislocation microstructures forming under a wedge indenter in fcc crystals, where micro-meter dislocation patterns challenge the predictions of traditional models of plasticity. In order to explain the formation of these features and develop a relationship between the force-displacement curve and the dislocation substructure, we present here a model of wedge indentation based on the continuum theory of dislocations. The model accounts for large deformation kinematics through the multiplicative split of the deformation gradient tensor, where the incompatible plastic component of deformation results from the flux of dislocations on different and interacting slips systems. Constitutive equations for dislocation fluxes are determined from a dissipative variational principle. As a result, each dislocation density satisfies an initial-boundary value problem with convective-diffusive character, which is coupled to the macroscopic stress and displacement fields governing the deformation process. Solution to the self-consistent continuum formulation is found using the finite element method. Computer simulations mimic the experimental conditions of wedge micro-indentation experiments into Ni single-crystals used by Kysar et al. (2010a). A comparison of overall dislocation density distribution and macroscopic mechanical response shows good overall agreement with the experimental results in terms of the detailed features of dislocation patterns and lattice rotations as well as the macroscopic force-displacement response.

A large deformation elastic–viscoplastic model for lithium

An essential ingredient in modeling the response of lithium in an all-solid-state lithium battery in which a lithium metal anode is paired with a non-flammable inorganic solid electrolyte (SE), is a constitutive model for the large deformation elastic–viscoplastic response of lithium. Wang and Cheng (2017) have recently published indentation load-versus-depth data from their microindentation experiments conducted in an argon-filled glove box to study the viscoplastic behavior of lithium at room temperature. In this paper we report on a large deformation isotropic elastic–viscoplastic theory for modeling the response of lithium. We have implemented the theory as a user-material-subroutine in a finite element program, and the material parameters for the theory have been calibrated using the data from the microindentation experiments of Wang and Cheng. To exhibit the utility of our finite element simulation capability, we show results from two representative numerical simulations of relevance to modeling the mechanical interaction between a lithium anode and a SE: (a) flattening of an asperity on the surface of lithium when it is mechanically impressed by a flat ceramic SE; and (b) intrusion of lithium into a cavity on the surface of SE, when the SE is mechanically impressed into lithium with a flat surface.

Calibration of colloidal probes with atomic force microscopy for micromechanical assessment.

Mechanical assessment of biological materials and tissue-engineered scaffolds is increasingly focusing at lower length scale levels. Amongst other techniques, atomic force microscopy (AFM) has gained popularity as an instrument to interrogate material properties, such as the indentation modulus, at the microscale via cantilever-based indentation tests equipped with colloidal probes. Current analysis approaches of the indentation modulus from such tests require the size and shape of the colloidal probe as well as the spring constant of the cantilever. To make this technique reproducible, there still exist the challenge of proper calibration and validation of such mechanical assessment. Here, we present a method to (a) fabricate and characterize cantilevers with colloidal probes and (b) provide a guide for estimating the spring constant and the sphere diameter that should be used for a given sample to achieve the highest possible measurement sensitivity. We validated our method by testing agarose samples with indentation moduli ranging over three orders of magnitude via AFM and compared these results with bulk compression tests. Our results show that quantitative measurements of indentation modulus is achieved over three orders of magnitude ranging from 1 kPa to 1000 kPa via AFM cantilever-based microindentation experiments. Therefore, our approach could be used for quantitative micromechanical measurements without the need to perform further validation via bulk compression experiments.

Evaluation of shale fracture toughness based on micrometer indentation test

It is an important mechanical parameter for fracture toughness that affects hydraulic fracturing. Some methods such as indoor test core and logging data interpretation are commonly used to obtain fracture toughness. These methods, however, have their own limitations. Liu's method has focused the mechanical properties of shale including fracture toughness based on micro-/nano-indentation experiments. Zeng's method has pointed that the former method of calculating fracture toughness ignores the maximum holding stage in the indentation-displacement curves, they point out and correct the fracture stress intensity factor model, but the final calculation of the fracture toughness of the numerical difference is of two orders of magnitude. In this paper, micro-indentation experiments were carried out to further analyze and correct the model by collecting and analyzing Longmaxi shale cuttings. After Zeng and Liu's methods are combined, the irreversible elastic energy by energy area actually was measured. To some extent, the difference between the two kinds of fracture toughness is reduced.

Micromechanical Study of the Forged Ti-1023 Titanium Alloy by Micro-Indentation

In order to study the micromechanical behaviour of the forged Ti-1023 titanium alloy, micro-indentation experiments of the forged Ti-1023 titanium alloy were performed with various maximum indentation loads from 500 mN to 4000 mN and various loading speeds from 5.06 mN/s to 51.85 mN/s. Using the experimental data, the non-destructive instrumental approach was applied to indicate the mechanical properties just like the Young’s modulusE, microhardnessH, initial yield stressσyand strain hardening exponentnusing theP-hcurves from the tests. The result showed that the value of the indentation Young’s modulus basically remain unchanged in the range from 110 GPa to 150 GPa andHdecreased with the increase of the load, the micro-indentaion plasticity constitutive equations were obtained by using Hookean elastic and power-law plastic stress-strain equations.

Modulation of Endothelial Membrane Biomechanics by Oxidized Phospholipids: A Combined Experimental and Computational Approach

Elevated concentrations of biologically active oxidized phospholipids (oxPCs) are known to exert various pathophysiological effects, including disruption of the endothelial barrier and induction of monocyte adhesion to endothelial cells. These effects are further implicated in a range of disorders, including certain cardiovascular, metabolic, and immunological diseases. The specific mechanisms by which oxPCs modulate membrane barrier function are poorly understood.We investigated the differential influence of two bioactive oxidized phospholipids on model bilayer properties, membrane packing, fluidity, elastic properties, and endothelial cell biomechanics both experimentally, using Laurdan multi‐photon imaging and atomic force microscopy (AFM) microindentation experiments, and computationally, using coarse‐grained molecular dynamics simulations. Specifically, we focused on two major oxidation products of 1‐palmitoyl‐2‐arachidonoyl‐sn‐glycero‐3‐phosphocholine (PAPC), which are known to exert multiple pathophysiological effects. The oxidized phospholipids 1‐palmitoyl‐2‐(5‐oxovaleroyl)‐sn‐glycero‐3‐phosphocholine (POVPC ‐ aldehyde‐terminated) and 1‐palmitoyl‐2‐glutaroyl‐sn‐glycero‐3‐phosphocholine (PGPC ‐ carboxyl‐terminated) have been found to induce inflammation, cell proliferation, and cell death.Our molecular simulations of multicomponent phospholipid bilayers predicted differential bilayer perturbation effects of the two oxidized phospholipids based on the chemical identities of their truncated tails. We showed that for the analogous oxPCs incorporated in a phosphatidylcholine bilayer containing sphingomyelin, the carboxylic analog, PGPC, experienced more significant reorientation of the truncated tail and displacement of the oxidized lipid from the plane of the bilayer. These phenomena induced greater perturbation of the structure of the lipid bilayer than the aldehyde analog, POVPC. Decreased bilayer packing and increased water permeation was observed, which was consistent with Laurdan imaging results. Spectral analysis of bilayer undulations further indicated increased elasticity of bilayer systems containing both oxPCs. Computational predictions of larger membrane perturbation by PGPC corresponded with greater stiffness observed for PGPC‐treated endothelial cells measured by cellular elastic moduli using AFM. These analyses confirmed the paradoxical relationship observed between membrane softening by oxPCs and increased overall cell stiffness in the presence of POVPC and PGPC.Support or Funding InformationNIH grants: T32 HL‐82547, HL‐073965, HL‐083298; AHA MWA Pre‐doctoral Fellowship: 14PRE20490156

On the micro-indentation of plant cells in a tissue context

The effect of geometry on cell stiffness measured with micro-indentation techniques has been explored in single cells, however it is unclear if results on single cells can be readily transferred to indentation experiments performed on a tissue in vivo. Here we explored this question by using simulation models of osmotic treatments and micro-indentation experiments on 3D multicellular tissues with the finite element method. We found that the cellular context does affect measured cell stiffness, and that several cells of context in each direction are required for optimal results. We applied the model to micro-indentation data obtained with cellular force microscopy on the sepal of A. thaliana, and found that differences in measured stiffness could be explained by cellular geometry, and do not necessarily indicate differences in cell wall material properties or turgor pressure.

Molecular-Scale Biophysical Modulation of an Endothelial Membrane by Oxidized Phospholipids

The influence of two bioactive oxidized phospholipids on model bilayer properties, membrane packing, and endothelial cell biomechanics was investigated computationally and experimentally. The truncated tail phospholipids, 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC), are two major oxidation products of the unsaturated phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-phosphocholine. A combination of coarse-grained molecular dynamics simulations, Laurdan multiphoton imaging, and atomic force microscopy microindentation experiments was used to determine the impact of POVPC and PGPC on the structure of a multicomponent phospholipid bilayer and to assess the consequences of their incorporation on membrane packing and endothelial cell stiffness. Molecular simulations predicted differential bilayer perturbation effects of the two oxidized phospholipids based on the chemical identities of their truncated tails, including decreased bilayer packing, decreased bilayer bending modulus, and increased water penetration. Disruption of lipid order was consistent with Laurdan imaging results indicating that POVPC and PGPC decrease the lipid packing of both ordered and disordered membrane domains. Computational predictions of a larger membrane perturbation effect by PGPC correspond to greater stiffness of PGPC-treated endothelial cells observed by measuring cellular elastic moduli using atomic force microscopy. Our results suggest that disruptions in membrane structure by oxidized phospholipids play a role in the regulation of overall endothelial cell stiffness.

A dissolution-precipitation mechanism is at the origin of concrete creep in moist environments.

Long-term creep (i.e., deformation under sustained load) is a significant material response that needs to be accounted for in concrete structural design. However, the nature and origin of concrete creep remain poorly understood and controversial. Here, we propose that concrete creep at relative humidity ≥ 50%, but fixed moisture content (i.e., basic creep), arises from a dissolution-precipitation mechanism, active at nanoscale grain contacts, as has been extensively observed in a geological context, e.g., when rocks are exposed to sustained loads, in liquid-bearing environments. Based on micro-indentation and vertical scanning interferometry data and molecular dynamics simulations carried out on calcium-silicate-hydrate (C-S-H), the major binding phase in concrete, of different compositions, we show that creep rates are correlated with dissolution rates-an observation which suggests a dissolution-precipitation mechanism as being at the origin of concrete creep. C-S-H compositions featuring high resistance to dissolution, and, hence, creep are identified. Analyses of the atomic networks of such C-S-H compositions using topological constraint theory indicate that these compositions present limited relaxation modes on account of their optimally connected (i.e., constrained) atomic networks.