Effect Of Tip Geometry Research Articles

Machined surface properties determined by nanoindentation: Experimental and FEA studies on the effects of surface integrity and tip geometry

An accurate characterization of subsurface properties is essential to understand machining induced surface integrity. However, little research has been conducted to investigate significant influences of surface integrity (hardness and residual stress) and tip geometry on the indentation process and the resulting load–depth curves. Nanoindentation was used to measure hardness and modulus in the subsurface of machined components by turning, grinding, and honing. The machining induced residual stresses can be estimated by matching the FEA simulated load–depth curves with the experimental ones. The simulation sensitivity analysis shows that the load–depth curves are significantly affected by residual stress and modulus, while friction, tip rounding, and yield strength have much smaller effects. The peak normal stresses occur on the top surface, while the peak shear stress is slightly under the surface. All peak strains are in the subsurface. The onset of material yielding during loading may be determined by tracing the evolution of loading/unloading loops at different indentation depths. A 3D simulation of nanoindentation with a spherical tip has shown that the indentation force induced by the spherical tip is less than 2% of the total force provided that the indentation depth is sufficiently large.

Indentation testing of human articular cartilage: Effects of probe tip geometry and indentation depth on intra-tissue strain

Experimental determination of intra-tissue deformation during clinically applicable rapid indentation testing would be useful for understanding indentation biomechanics and for designing safe indentation probes and protocols. The objectives of this study were to perform two-dimensional (2-D) indentation tests, using indenters and protocols that are analogous to those in clinically oriented probes, of normal adult-human articular cartilage in order to determine: (1) intra-tissue strain maps and regions of high strain magnitude, and (2) the effects on strain of indenter geometry (rectangular prismatic and cylindrical) and indentation depth (40–190 μm). Epifluorescence microscopy of samples undergoing indentation and subsequent video image correlation analysis allowed determination of strain maps. Regions of peak strain were near the “edges” of indenter contact with the cartilage surface, and the strain magnitude in these regions ranged from ∼0.05 to ∼0.30 in compression and shear, a range with known biological consequences. With increasing indentation displacement, strain magnitudes generally increased in all regions of the tissue. Compared to indentation using a rectangular prismatic tip, indentation with a cylindrical tip resulted in slightly higher peak strain magnitudes while influencing a smaller region of cartilage. These results may be used to refine clinical indenters and indentation protocols.

On the correlation between surface roughness and work function in copper

Both surface roughness (SR) and work function (WF) are important characteristics of a solid surface. Understanding the relationship between SR and WF is necessary in order to apply the Kelvin probe technique to characterize surface behavior. In this study, SR and WF of copper surfaces were measured using atomic force microscopy and scanning Kelvin probe, respectively. Experimental results showed that WF decreased with increase of SR. Using sine functions, a capacitor model was proposed and a correlation between WF and SR was established. The theoretical predictions of WF were in good agreement with experimental results. The model is also useful for analyzing the effect of tip geometry on measurements of WF.

Effect of Tip Gap and Squealer Geometry on Detailed Heat Transfer Measurements Over a High Pressure Turbine Rotor Blade Tip

The present study explores the effects of gap height and tip geometry on heat transfer distribution over the tip surface of a HPT first-stage rotor blade. The pressure ratio (inlet total pressure to exit static pressure for the cascade) used was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. A transient liquid crystal technique was used to obtain the tip heat transfer distributions. Pressure measurements were made on the blade surface and on the shroud for different tip geometries and tip gaps to characterize the leakage flow and understand the heat transfer distributions. Two different tip gap-to-blade span ratios of 1% and 2.6% are investigated for a plane tip, and a deep squealer with depth-to-blade span ratio of 0.0416. For a shallow squealer with depth-to-blade span ratio of 0.0104, only 1% gap-to-span ratio is considered. The presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients. The effects of different partial squealer arrangements are also investigated for the shallow squealer depth. These simulate partial burning off of the squealer in real turbine blades. Results show that some partial burning of squealers may be beneficial in terms of overall reduction in heat transfer coefficients over the tip surface.

Effects of Tip Geometry on Unsteady Cavitation and Backflow in Inducer

The effect of blade tip geometry on unsteady cavitation was investigated in a 3-bladed inducer. Experiments in inducers with and without the blade tip rounding on the pressure side were carried out and the results were compared. Through the measurement of inlet pressure fluctuation, it is found that unsteady cavitation is suppressed in the inducer with rounded tip compared with that with flat tip at relatively high flow rates. Backflow in each inducer was also investigated for various cavitation numbers because backflow may be also affected by tip geometry. It is clarified that backflow in the inducer with rounded tip is a little greater than that with flat tip and the backflow becomes weaker with the decrease of cavitation number.

Amplitude or frequency modulation-detection in Kelvin probe force microscopy

Kelvin probe force microscopy in ultrahigh vacuum is a sensitive tool to determine the absolute work function of sample surfaces. For the detection of the electrostatic force, an a.c.-voltage with a frequency ω is applied between tip and sample. Two different methods can be employed for the measurement: the frequency modulation (FM) mode or amplitude modulation (AM) mode detection. In FM-mode the oscillation of the frequency shift at ω is measured, which is proportional to the gradient of the electrostatic force. In AM-mode the amplitude of the cantilever oscillation at ω is measured which is proportional to the electrostatic force itself. A detailed comparison between AM and FM detection modes is presented, using measurements of gold islands on highly oriented pyrolytic graphite. The effect of tip geometry was studied for both detection modes. In the FM-mode mainly the tip apex contributes to the measured signal due to the detection of the force gradient, however, a.c.-voltages of around 2 V are necessary to perform stable measurements. A much stronger influence of the tip shape was determined for AM detection, reducing the contrast in the measured contact potential difference images. Nevertheless, the main advantage of the AM-mode is the possibility to use a.c.-voltages as low as 100 mV, which allows absolute work function measurements also on semiconductor surfaces.

Effects of Tip Geometry and Tip Clearance on the Mass/Heat Transfer From a Large-Scale Gas Turbine Blade

An experimental investigation has been performed to measure average and local mass transfer coefficients on the tip of a gas turbine blade using the naphthalene sublimation technique. The heat/mass transfer analogy can be applied to obtain heat transfer coefficients from the measured mass transfer data. Flow visualization on the tip surface is provided using an oil dot technique. Two different tip geometries are considered: a squealer tip and a winglet-squealer tip having a winglet on the pressure side and a squealer on the suction side of the blade. Measurements have been taken at tip clearance levels ranging from 0.6 to 3.6% of actual chord. The exit Reynolds number based on actual chord is approximately 7.2×105 for all measurements. Flow visualization shows impingement and recirculation regions on the blade tip surface, providing an interpretation of the mass transfer distributions and offering insight into the fluid dynamics within the gap. For both tip geometries the tip clearance level has a significant effect on the mass transfer distribution. The squealer tip has a higher average mass transfer that sensibly decreases with gap level, whereas a more limited variation with gap level is observed for the average mass transfer from the winglet-squealer tip.

The effect of specimen roughness and indenter tip geometry on the determination accuracy of thin hard coatings stress–strain laws by nanoindentation

Nanoindentation, conducted using devices with enhanced accuracy and capabilities, contributes to the reliable characterization of coatings and other material mechanical properties. Herein there are many parameters that significantly affect measurement reliability and thus may lead to errors in the evaluation of results. Such parameters include specimen roughness and geometrical deviations (due to manufacturing tolerances) in the indenter tip form. In earlier investigations, the actual tip geometries of various indenter types were approached with the aid of a continuous simulation of the nanoindentation using finite element method modeling techniques that enable the determination of material stress–strain characteristics. In the present paper, the application of this method—further improved to define the constitutive laws of thin hard coatings—is described, wherein the effect of specimen roughness on the accuracy of the indentation results is taken into account. Furthermore, the results introduced show that the defined material elastoplastic deformation characteristics, based on the nanoindentation test results, are independent of the indenter type (Vickers or Berkovich) when deviations in the indenter tip geometry from the ideal are considered.

Effects of characteristic dendritic tip geometry on the electrical properties of teleost thalamic neurons.

Of the factors that characterize the properties and functions of neurons, dendritic geometry is one of the most critical. We used simulations employing the multi-compartment model to study the effects of dendritic tip geometry on the electrical properties of the "large cell" in a teleost thalamic nucleus from the corpus glomerulosum. We demonstrated a dramatic geometrical "boosting" effect; passive propagation of the synaptic inputs from the dendritic tip to the soma through the dendritic stalk is less attenuated in cells with larger tips.

Nanoscale variation in electric potential at oxide bicrystal and polycrystal interfaces

Scanning surface potential microscopy (SSPM), has been used to measure spatial variations in grain boundary properties in SrTiO 3 and ZnO. Experimental measurements of a Fe-doped SrTiO 3 Σ3 bicrystal are compared to finite element calculations to quantify the effects of tip geometry and sample-tip separation. Experimental and numerical treatments include realistic tip interactions and lateral inhomogeneity in sample properties. A procedure for extracting actual interface potentials from separation dependence is proposed. Both the sign and magnitude of the grain boundary potential barrier measured with SSPM agree with macroscopic measurements. For experimentally available tips, the effect of tip geometry was found not to contribute to uncertainty. In application to polycrystalline materials, the voltage dependence of individual interface properties has been determined in micropatterned, ZnO-based, polycrystalline varistor devices.

Analysis of indentation: implications for measuring mechanical properties with atomic force microscopy.

Indentation using the atomic force microscope (AFM) has potential to measure detailed micromechanical properties of soft biological samples. However, interpretation of the results is complicated by the tapered shape of the AFM probe tip, and its small size relative to the depth of indentation. Finite element models (FEMs) were used to examine effects of indentation depth, tip geometry, and material nonlinearity and heterogeneity on the finite indentation response. Widely applied infinitesimal strain models agreed with FEM results for linear elastic materials, but yielded substantial errors in the estimated properties for nonlinear elastic materials. By accounting for the indenter geometry to compute an apparent elastic modulus as a function of indentation depth, nonlinearity and heterogeneity of material properties may be identified. Furthermore, combined finite indentation and biaxial stretch may reveal the specific functional form of the constitutive law--a requirement for quantitative estimates of material constants to be extracted from AFM indentation data.

Scanning Electrochemical Microscopy (SECM): An Investigation of the Effects of Tip Geometry on Amperometric Tip Response

Transient and steady-state amperometric tip responses were simulated with an alternating direction implicit algorithm. Compared with previous publications, the simulation domain was designed to account for the diffusion of the redox species around the corner of the insulating sheath. Expanding space and time grids were used to optimize the algorithm, and the simulation was validated by comparison with published data for the microdisk electrode. Tip responses were simulated for a wide range of tip substrate distances over conducting and insulating substrates. The shape of the approach curves was investigated for several electroactive disk to insulator radii ratios. Diffusion around the edge of the insulating sheath was found to have a pronounced effect on the approach curves. In contrast to the findings of earlier studies, tip currents for conducting substrates were found to significantly depend on the tip geometry. The parameters of functions used to describe approach curves in the SECM literature were studied for several tip geometries commonly used experimentally. Simulated results were also used to assess the topographical sensitivity (the rate of change of tip current with respect to tip-substrate distance) and spatial resolution (the ability of the microdisk to distinguish two conducting islands inlaid into an insulating substrate) of the scanning electrochemical microscope (SECM).

On the effect of notch radius and local friction on the mode I and mode II fracture toughness of a high-strength steel

This paper reports on the results of an experimental and analytical investigation on the mode I and mode II fracture mechanics behaviour of a high-strength steel. The aim was to identify the effect of crack tip geometry and loading system on the loading capacity of the material. Notched as well as pre-cracked single edge notched specimens were loaded in symmetric and anti-symmetric bending. The experimental results show that crack tip and loading parameters have a significant influence on the apparent critical fracture toughness values. The influence of a finite notch radius on the mode I and mode II fracture toughness is quantified by a J-integral analysis which indicates that for both loading systems crack initiation can be estimated by a local strain energy density criterion. A weight function analysis of the mode II loading shows that crack tip shielding due to contact between the crack faces becomes important if the distance from the first support roller to the crack plane is less than the thickness of the specimen. This result helps to explain the large scatter which is present in one of the experimental loading configurations.

Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility

We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60$^\circ$ corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by several orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness in some materials where a sharp crack would seem to be able to propagate in a brittle manner. Results for blunt cracks loaded in mode II are also presented.

Lateral translation of an Xe atom on metal surfaces

This work presents the theoretical study of the controlled lateral translation of an Xe atom physisorbed on the Ni(110) surface. The motion of the Xe is manipulated by the tip of the scanning tunnelling microscope. The interaction of the physisorbed atom with the tip and sample surface is described by empirical potentials. Using molecular statics and dynamics, the energetics and different modes of the translation are revealed. Important effects of electrode relaxation, tip geometry and material parameters are briefly discussed.

Scanning force microscope as a tool for studying optical surfaces

The scanning force microscope (SFM) is used to study the characteristics of optical surfaces, such as polished and precision-machined surfaces and thin-film structures. Previously unreported images of raised surface scratches and clumpiness on the surface of extremely smooth dielectric films are presented. The characteristics of SFM's that are important in studying optical surfaces are discussed. They include the effects of tip geometry, surface charging, particulate contamination, scanner artifacts, and instrument calibration.

A new approach to adaptive-wall windtunnel testing

SummaryA technique is described whereby experiments may be conducted on models which are geometrically similar to only part of the full scale configuration with the windtunnel walls being contoured to simulate the flow over that part. In this way, higher spatial resolutions can be achieved in a windtunnel of moderate size. The method is demonstrated by studying the effects of tip geometry on the flow over a wing of aspect ratio 15 using a half wing model of chord 500 mm projecting 1031 mm into a test section of width 1200 mm.

Imaging of cracks in semiconductors using scanning tunneling microscopy

Scanning tunneling microscopy (STM) has developed into a useful tool for atomic-scale characterization of material surfaces. It has a great advantage over other high-resolution techniques in that no extensive sample modification, such as thinning or polishing, is required to obtain atomic resolution images. This is especially useful in the field of fracture, as the vast majority of high-resolution images of fracture processes are made in the transmission electron microscope, where thin-film effects may greatly modify crack tip stresses and dislocation structures. This investigation involved imaging of cracks introduced in single crystals of galena (PbS) by rapid indentation at 77 K. Images were obtained both at the arrested crack tip and along the flanks of the crack in the dynamic growth region. Measurements were made of both crack-tip morphology and upsets observed along the flanks of the crack. The latter is discussed in terms of plasticity upsets associated with dislocation emission from rapidly growing cracks. Effects of STM tip geometry and scan conditions on the resulting image of the cleavage crack, as well as work in progress on other material systems, is discussed.

Imaging of Cracks in Semiconductor Surfaces Using Scanning Tunneling Microscopy

ABSTRACTScanning Tunneling Microscopy (STM) has developed into a useful tool for atomic-scale characterization of material surfaces. It is proving especially useful in the field of fracture, as the vast majority of high-resolution images of fracture processes are made in the transmission electron microscope, where thin film effects and sample preparation may greatly modify crack tip stresses and dislocation structures. This investigation involved imaging of cracks introduced in Si at room temperature and single crystals of galena (PbS) by rapid indentation at 77K. Images were obtained at the arrested cracktip, around the indentation, and along the flanks of the crack in the dynamic growth region. Measurements were made of both crack-tip morphology and upsets observed along the flanks of the cracks in PbS. Interesting results concerning crack tip geometry in Si, the effect of STM tip geometry and scan conditions on the resulting image of the cleavage crack, as well as work in progress on other material systems, will be discussed.

Behavior of diorite under impact by variously-shaped projectiles

Behavior of Diorite Under Impact by Variously-Shaped Projectiles The effects of striker diameter and tip geometry on the crater and crack network produced in diorite by normal projectile impact in the energy range from 4–30 J was investigated. Ejecta kinematics were determined by high-speed photography; elastic strain wave propagation was measured by embedded gages in a composite specimen; and the damage pattern was ascertained from an examination of the sectioned specimen. It was found that the projectile nose shape exerts a strong influence on the shape of the elastic transient, on the crater geometry, on the extent of the crack network and on the average size of the ejecta. The crater depth was found to be the most repeatable parameter in identical tests using the same striker and initial kinetic energy.