Finding Minimal Optimal Indent Separation for Polystyrene via Instrumental Nanoindentation and FEA Method

Nanoindentation became a standard non-destructive technique to measure mechanical properties at the submicron scale of various materials. A set of empirical rules were established to guarantee the validity of the results. One of those rules is the separation between individual indents that should be 20–30 times maximum indentation depth. This paper investigates the influence of the distance between indents on the accuracy of mechanical properties for polystyrene with a view to determine minimum optimal separation that is needed to measure various material properties. A series of different depths with three different orientations was considered through both the experimental and finite element method to explore the relationship between the distance and indentation depth. Both methods demonstrated that hardness and modulus values for polystyrene keep stable with the distance approximately 15 times the maximum indentation depth for the matrix type set up, and nominal separation of 10 is enough when indents are executed in a single row or column.

Morphology, Thermal and Mechanical Properties of Co-Continuous Porous Structure of PLA/PVA Blends by Phase Separation

Poly (lactic acid) (PLA) was blended with poly (vinyl alcohol) (PVA) in the composition of 70/30 (L7V3), 60/40 (L6V4), and 50/50 (L5V5) wt.%. L7V3 exhibits a sea–island morphology, while L6V4 and L5V5 show co-continuous phase morphologies. These polymers exhibited a solitary glass transition temperature, which obeyed the Fox equation. Thereafter, the blends were made porous by an etching process in hot water (35 °C) for 0–7 days, to remove PVA. The maximum etched PVA content of L7V3, L6V4, and L5V5 was 0.5%, 13.4%, and 36.1%, respectively; hence, L5V5 exhibited a co-continuous porous morphology with the porosity of 43.4%, the degree of swelling of 47.5%, and the pore size of 2 µm. The degree of crystallinity of PLA, exposed PLA, and L7V3 showed an insignificant change. L5V5, having the highest porosity, demonstrated the highest increase in the degree of crystallinity of approximately two times, because water induced the crystallization of PLA. The high porosity of L5V5 exhibited an excellent absorption property by increasing absorption energy more than two times, as obtained by micro indention. It had the maximum indentation depth more than 250 µm. Flexural and tensile properties considerably decreased with an increase in the porosity.

Determination of an Objective Criterion for the Assessment of the Feasibility of an Instrumented Indentation Test on Rough Surfaces

The influence of roughness on the results of indentation testing was investigated using a semianalytical model. This model used simulated surfaces that were described using three standard roughness parameters: the root-mean-square deviation Sq, the wavelength (or cut-off of Gaussian high-pass filter), and the fractal dimension. It was shown that Sq had the largest effect on the determination of the macrohardness, while the surface wavelength and fractal dimension had negligible effects at the scale of investigation. The error of determination of the macrohardness rose with the increase of the ratio Sq/hmax where hmax was the maximum indentation depth: Sq/hmax ratios lower than 0.02 were required to obtain a systematic error of the macrohardness lower than 5%, whatever the examined material mechanical properties (in elasticity and plasticity).

Determination of the Mechanical Properties of PIN–PMN–PT Bulk Single Crystals by Nanoindentation

The present study aimed to experimentally evaluate the mechanical properties of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) bulk single crystals with different crystallographic directions using the nanoindentation technique. The load–indentation depth curves, elastic and plastic deformations, hardnesses, and Young’s moduli of [100]- and [110]-oriented 0.28PIN–0.43PMN–0.29PT bulk single crystals were investigated. Our results show that with an increase in the maximum indentation depth hmax, the plastic residual percentage increased for both the [100]- and the [110]-oriented single crystals. At each hmax, the plastic residual percentage of the [100]-oriented PIN–PMN–PT single crystals was less than that of the [110]-oriented PIN–PMN–PT single crystals. At hmax from 500 nm to 2000 nm, the plastic deformation was larger than the elastic deformation, and the plastic residual percentage was larger than 50% for both the [100]- and the [110]-oriented single crystals. This means that the plastic deformation dominated in the indentation process of PIN–PMN–PT single crystals. The indentation size effect on the hardness of the PIN–PMN–PT single crystals was apparent in the nanoindentation process. Both the hardness and the Young’s modulus of the [100]-PIN–PMN–PT single crystals were greater than those of the [110]-PIN–PMN–PT single crystals, which indicates that the PIN–PMN–PT single crystals had anisotropic mechanical characteristics.

Диссипация механической энергии в осциллирующем адгезионном контакте между жестким индентором и эластомером

A model of hysteresis in an adhesive contact under oscillating loading is proposed, based on the chemical inhomogeneity of the indenter surface. Results of numerical simulation are compared with experimental data obtained with a setup allowing direct observation of the dynamics of contact area. It is shown that the hysteresis almost disappears if the amplitude of the oscillating load is smaller than a critical value depending on the maximum indentation depth.

Characterization of strut indentation during mechanical thrombectomy in acute ischemic stroke clot analogs

BackgroundAlthough it is common practice to wait for an ‘embedding time’ during mechanical thrombectomy (MT) to allow strut integration of a stentriever device into an occluding thromboembolic clot, there is a scarcity of evidence demonstrating the value or optimal timing for the wide range of thrombus compositions. This work characterizes the behavior of clot analogs of varying fibrin and cellular compositions subject to indentation forces and embedding times representative of those imparted by a stentriever during MT. The purpose of this study is to quantify the effect of thrombus composition on device strut embedding, and to examine the precise nature of clot integration into a stentriever device at a microstructural level.MethodClot analogs with 0% (varying densities), 5%, 40%, and 80% red blood cell (RBC) content were created using ovine blood. Clot indentation behavior during an initial load application (loading phase) followed by a 5-min embedding time (creep phase) was analyzed using a mechanical tester under physiologically relevant conditions. The mechanism of strut integration was examined using micro-computed tomography (µCT) with an EmboTrap MT device (Cerenovus, Galway, Ireland) deployed in each clot type. Microstructural clot characteristics were identified using scanning electron microscopy (SEM).ResultsCompressive clot stiffness measured during the initial loading phase was shown to be lowest in RBC-rich clots, with a corresponding greatest maximum indentation depth. Meanwhile, additional depth achieved during the simulated embedding time was most pronounced in fibrin-rich clots. SEM imaging identified variations in microstructural mechanisms (fibrin stretching vs rupturing) which was dependent on fibrin:cellular content, while µCT analysis demonstrated the mechanism of strut integration was predominantly the formation of surface undulations rather than clot penetration.ConclusionsDisparities in indentation behavior between clot analogs were attributed to varying microstructural features induced by the cellular:fibrin content. Greater indentation was identified in clots with higher RBC content, but with an increased level of fibrin rupture, suggesting an increased propensity for fragmentation. Additional embedding time improves strut integration, especially in fibrin-rich clots, through the mechanism of fibrin stretching with the majority of additional integration occurring after 3 mins. The level of thrombus incorporation into the EmboTrap MT device (Cerenovus, Galway, Ireland) was primarily influenced by the stentriever design, with increased integration in regions of open architecture.

DETERMINATION OF PARAMETERS FOR THE PLASTICITY MODEL OF VT-22 ALLOY POWDER PARTICLES

A computational and experimental method is proposed to determine the medium model for describing the mechanical properties of powder particles of a high-strength VT-22 titanium alloy obtained by plasma spraying in an inert gas jet. The method is based on the indirect determination of the σs ~ ε strain hardening diagram (yield stress dependence on plastic deformation). A specimen for fullscale tests was prepared as a section of powder particles filled into a special resin. The paper provides the results of Vickers indentation into VT-22 powder particles and the results of computer simulation of this process by the finite element method. The average value of the maximum indentation depth was hmax = 6,56 μm with a maximum loading value of 2 N. The Johnson-Cook elasto-plastic model of a material with nonlinear hardening was used for the volume element considered during the computer simulation. An algorithm for searching coefficients by computational experiment multistage planning was proposed to identify the parameters of the equation sought. Estimated maximum coincidences of experimental and calculated data were chosen as selection criteria, in particular, based on maximum indentation depth. As a result of the study, material model coefficient values that meet search conditions best were chosen from a possible set of such values. According to the algorithm proposed, the result was achieved in 4 calculation cycles. Powder metallographic study was carried out. It was found that particles have a coarse intragranular structure with the dominating b phase formed during plasma spraying. This probably led to a decrease in VT-22 alloy deformation resistance in the powder particles.

Deconvolution of design parameters from elasto-plastic energy data acquired by nanoindentation in ceramic coatings

The elasto-plastic energies and the RID parameter (Relative Indentation Depth b=hmax/t), defined as the ratio between the maximum indentation depth and the coating thickness, were used to evaluate the ability of the ceramic coated systems to withstand load. The study was conducted on M2 and 304L steels coated with titanium nitride by PAPVD. Both substrates were coated with TiN of about 2,0 and 3,0 µm thickness. The hardness vs. (b) and the ratio of plastic to total energy (Wp/Wt) vs. (b) graphs were analyzed. There were found correlations between the indentation depths at which the transition from mechanical response due to the coating to the mechanical response of substrate-coating and to substrate dominated behavior and the depths at which the efficiency of using the coating falls 33% and 50 %. Results show that it is possible to get useful information from the energy taken from the mechanical response of the coated systems to avoid the Oliver and Pharr method for measuring the hardness and elastic modulus.

Effect of the Injection Moulding Parameters on the Creep Properties in Polypropylene/Carbon Nanotubes Composites

The goal of this paper is to study the time-dependent properties of multi-walls carbon nanotubes (MWCNTs)-filled polypropylene (PP) composites using the instrumented indentation technique. Two types of the indentation test, the 3-step and the 5-step indentation tests, were considered to investigate the creep response during sharp indentation. In order to characterize the state of distribution of MWCNTs, the Scanning Electron Microscopy (SEM) technique was used. It was found that the maximum indentation depth decreases with the increase in the MWCNTs concentration. At lower MWCNTs concentration, the effect of injection pressure on the creep displacement is not significant. Comparison of the creep displacements from 3-step and 5-step loading histories indicates a noticeable decrease in creep displacement when the 5-step loading history is used.

WEAR BEHAVIOR OF NITI THIN FILM AT MICRO-SCALE

This paper reports experimental study on the hardness and wear behavior of NiTi Thin Film Shape Memory Alloy (SMA) at micrometer scales. A triboindenter (Hysitron Inc., Minneapolis, USA) was used to conduct a series of indentations under various loads (the corresponding maximum indentation depth from 18.52nm to 333.53nm) and wear by scanning scratch method at temperatures from 25°C to 120°C. It was found that with increasing temperature, the hardness of NiTi thin film increased while its wear resistance decreased. The observed anomalous variation of wear resistance with hardness value is further analyzed by the interplay of phase transition and plasticity.