Effect Of Contact Geometry Research Articles

Numerical Investigation into the Effect of Contact Geometry on Fretting Fatigue Crack Propagation Lifetime

Fretting fatigue is a phenomenon in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. There is a need to analyze the effects of contact geometry on crack propagation under fretting fatigue conditions. In this investigation, a finite element modeling method was used to study the effects of different contact geometries along with crack–contact interaction on crack propagation lifetime. Different contacts geometries—that is, cylindrical on flat and flat on flat—along with different contact span widths were analyzed. In addition, the effects of different contact spans on stress distribution at the contact interface were investigated. The computed crack propagation life was compared with experimental results. It was found that the crack initiated near the contact trailing edge for all contact geometries, which agreed with experimental observations. In terms of crack propagation for different contact spans, the fretting fatigue life for a two-based cylindrical pad was shorter than that for a two-based flat pad. By increasing the contact span width for both flat and cylindrical pads, the crack propagation lifetime increased. A comparison between the experimental and numerical results demonstrated a difference of about 18% in crack propagation lifetime.

Computational study on the effect of contact geometry on fretting behaviour

Abstract A key challenge in the design of engineering couplings and contacting components relates to the development of an understanding of the comparative performance of contrasting contact geometries for a given application, including loading, applied deformations and geometrical space envelope. Although fretting is observed in many mechanical assemblies such as keyway-shaft couplings, shrink-fitted couplings, one specific example which has motivated the present work is the pressure armour layer of a marine flexible riser, where the groove and nub experience fretting contact damage. A Hertzian cylinder-on-flat contact geometry is commonly assumed for this groove-nub contact due to the ready availability of the contact (normal and tangential) analytical solutions for this geometry. In reality the contact geometry is closer to a rounded punch-on-flat. The present work adopts a finite element methodology to compare the significance of the Hertzian assumption to that of a rounded punch-on-flat, in terms of fretting behaviour.

Finite Element Analysis of Elastic-Plastic Contact Mechanic Considering the Effect of Contact Geometry and Material Properties

Each surface of roughness has different shape of asperity which is modeled with various shapes of analytical models. In this paper, the differences among various models of shape of asperity investigate using the Finite Element Method (FEM) and various analytical models. The contact stresses in rough surfaces are calculated analytically using various asperity shape models. Finite element analysis is also carried out assuming three types of material properties namely, the linear, the elastic-perfect plastic and the elastic-nonlinear hardening. The analytical results are compared with the results obtained by the finite element method. The results illustrate for using a deterministic approach which the numerical models are suitable. In hertz model, the result of force is very big in interface of causing deformation plastic, while Model Zhao has almost same result with FEM nonlinear property model. It is observed that the results obtained from Zhao’s model are generally in a better agreement with the results obtained from various finite element models especially in elastic-plastic and plastic zones, hence it may be concluded that Zhao’s model can be used for analyzing the rough surfaces in contact mechanics.

Improved quality nonpolar a ‐plane GaN/AlGaN UV LEDs grown with sidewall lateral epitaxial overgrowth (SLEO)

AbstractHigh quality nonpolar a ‐plane GaN templates were grown by utilizing sidewall lateral epitaxial overgrowth (SLEO) technique with threading dislocation density of ∼106–107 cm–2. 360 nm GaN/AlGaN multiple quantum well ultraviolet light emitting diodes were grown with metalorganic chemical vapor deposition. Reduced defect density SLEO a‐ plane, planar a ‐plane, and planar c ‐plane templates were co‐loaded for device growth and processed together in order to make relative device performance comparison. For SLEO a ‐plane LEDs E EL peak position was measured at 360 nm and it was independent of drive current. The linewidth of the emission was 7 nm. The series resistance of these diodes was as low as 16.5 Ω and the forward voltage was measured as 4.08 V at 20 mA. The external quantum efficiency of the high quality SLEO a ‐plane devices was ∼275× higher than planar a ‐plane devices. The highest output power was realized at 200 mA as 65 μW. The effect of contact geometry on electrical and optical characteristics of the devices was also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Contact Geometry on Pull-Off Force Measurements with a Colloidal Probe

This paper examines the effects of contact geometry on the pull-off (adhesion) force between a glass sphere (colloidal probe) and a silicon wafer in an environment with controlled relative humidity. An atomic force microscope is used to measure the pull-off force between the colloidal probe and the sample mounted at different tilt angles. The results show that the measured pull-off force is very sensitive to the tilt angle. Through the use of a newly developed direct scanning method, the exact contact geometry is determined for the zero-tilt angle case. The obtained digital image is then rotated to determine the contact geometry for the cases with other tilt angles. A detailed examination of the contact geometry, along with a magnitude analysis of the capillary force, suggests that the adhesion is most likely dominated by the capillary force from the meniscus formed between the probe and the sample. The strong dependence of the adhesion on the tilt angle may result from the change of meniscus dimensions associated with the probe-sample separation, which in turn is controlled by the highest peak on the probe sphere. Our observation emphasizes the combined role of microsurface shape near the contact and nanoroughness within the contact in determining the colloidal probe pull-off force and also microadhesion force in general.

Effect of Contact Geometry on the Pull-Off Force Evaluated under High-Vacuum and Humid Atmospheric Conditions

The effect of condensed water on pull-off forces under high vacuum (HV) and 0 to 83% relative humidity (RH) N2 atmospheric conditions was evaluated for different contact geometries using atomic force microscopy (AFM). The pull-off force was measured using two types of contact geometry: contact between hemispherical asperities and a flat silicon probe on an AFM cantilever (called a spherical-flat contact) and between a flat silicon substrate and a flat nickel probe on an AFM cantilever (called a flat-flat contact). The hemispherical asperities were fabricated using a focused ion beam (FIB) system, and each peak had a radius of curvature of between 70 and 610 nm. The flat nickel probe was fabricated by friction-induced wear. Measurement results showed that for the spherical-flat contact the pull-off force was proportional to the radius of curvature of the asperity peak and was slightly lower in HV than in humid 14% RH N2. For the flat-flat contact in HV, with increasing contact time, the pull-off force increased in HV but decreased in humid 62 and 83% RH N2. The pull-off force in HV was lower than that in humid N2 when the contact time was less than 10 s but was higher when the contact time was longer than 30 s. The estimated adhesion force based on the Laplace pressure from the contact geometry agreed reasonably well with the measured pull-off force.

Single-molecule field-effect transistors: A computational study of the effects of contact geometry and gating-field orientation on conductance-switching properties

The relation of geometric features to the effect of gating electric fields on the conductance through conjugated systems is investigated by electronic transmission calculations employing Green's function based modeling. Switching is only induced if the field is applied in an orientation which results in energy shifting of the molecular orbitals. This is found to depend on the orientation of the field with respect to the plane defined by the molecular conjugation. The switching can be quenched by structural rearrangement of the chemical bonds to the bulk, where the relative position of the electrodes is modified.

Effect of contact geometry on compressive failure processes in sandwich structures

Reducing the unladen weight of road and rail vehicles as well as ships and aircraft is often a requirement. This is to increase their performance, achieve better fuel economy, reduce direct operating costs and make other improvements as new suitable lightweight materials become available. However, at the design stage, often it is also needed to assess the lightweight or other candidate materials as to their ability to absorb impact energy in the event of a crash or other major impact. Related to the use of lightweight materials in racing car bodies, the Federation International de L’Automobile (FIA) has introduced stringent methods for assessing lightweight and other structural materials particularly as to their ability to protect racing car drivers in the advent of a crash. However, this FIA assessment method requires large sheets of material and a very powerful impact facility. One aim of this study was to devise a scaled down method that was able to provide data relatable to those from the full-scale FIA evaluation. This is together with providing for varying the impact conditions to study the variability of material properties in panels. Also, this is to explore the properties of lightweight materials for other applications as to their high-energy absorption ability.

A numerical procedure to establish a safe working pressure during excavation of a pipeline in a rock ditch

The purpose of this work is to describe a numerical procedure that was conducted to examine the effect of internal pressure on the possibility of puncturing and rupturing a high-pressure gas pipeline due to the impact of a falling rock. The mass and the height of the falling object were fixed. The analyses were performed in two parts: first by comparing an initial potential energy of the indenter with the energy needed to dent and possibly puncture the pipeline, then by determining if the resulting puncture would result in unstable crack extension leading to pipeline rupture. Finite element analysis was used to examine various shapes of the indenting surface to assess the effect of contact geometry on the puncture resistance. For cases where puncture is predicted, the dimensions of the hole are compared to the critical pressure to cause rupture, using industry standard methodology. The overall procedure can thus be utilized to determine the critical pressure to avoid a rupture due to a falling rock. By further application of an appropriate safety factor, the safe working pressure for a pipeline excavation can be established.

Effect of contact geometry on the magnetoresistance response of Ni80Fe20 antidot array

The effect of electrical contact geometry on the shape and sign of the magnetoresistance (MR) response in micron-size antirectangular array structures has been investigated. The MR response is strongly sensitive to the direction of the applied sense current. The results can be attributed to the competing anisotropic MR effects from two inhomogeneous orthogonal current flows in the structure. We have also investigated the effect of film thickness on the overall MR responses, and observed that as film thickness decreases, the switching field and MR ratio decrease accordingly and the competition between the anisotropic MR effects becomes more evident.

Fretting Fatigue and Stress Relaxation Behaviors of Shot-Peened Ti-6Al-4V

Abstract Relaxation behavior of residual stress and its effect on fretting fatigue response of shot-peened titanium alloy, Ti-6Al-4V were investigated at room and elevated temperatures under constant amplitude cycling condition using two contact configurations, cylinder on flat and flat on flat. Measurements by X-ray diffraction method before and after tests showed that residual compressive stress relaxed during fretting fatigue. Fretting fatigue life depended on the amount of stress relaxation as well as on the applied stress range. Elevated temperature induced more residual stress relaxation relative to that at room temperature, which, in turn, resulted in a reduction of fretting fatigue life. There was no effect of contact geometry on fretting fatigue life on the basis of the applied cyclic stress on the specimen, which did not account for any effects from contact stresses, even though less residual stress relaxation occurred with flat pad. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from finite element analysis for tests incorporating various levels of stress relaxation. It showed that not only crack initiation but also crack propagation should be considered to characterize fretting fatigue behavior of shot-peened specimens.

Force and weight distributions in granular media: effects of contact geometry.

The influence of the local contact network on interparticle forces and effective particle weights is studied in simulations of two-dimensional packings of frictionless, Hertzian spheres. The weight distribution P(w) changes qualitatively when approaching a boundary and differs for regular and irregular packings, while the interparticle force distribution P(f) is robust. We provide examples where P(w) at the boundary, which is the quantity probed experimentally, deviates substantially from P(f) in the bulk. Discrepancies between the P(w)'s predicted by the q model and measured in experiments are due to differences in the contact geometry.

Effect of contact geometry on 4H-SiC rectifiers with junction termination extension

SiC rectifiers with an on/off current ratio of 4×10 5 (at 1.5 V/−500 V) were fabricated using junction termination extension (JTE) and dielectric overlap. The reverse breakdown voltage was inversely dependent on contact area and was not a strong function of JTE length up to 40 μm. Similarly, for a given JTE length, the metal overlap did not have a strong influence on breakdown voltage. Oval- and circular-shaped contacts produced larger breakdown voltages than square rectifying contacts. The on-state resistance, R ON, was 4.2 m Ω cm 2, which is close to the theoretical minimum of these rectifiers using Ni Schottky contacts. The figure-of-merit ( V B) 2/ R ON was as high as 156 MW cm −2.

Fretting fatigue crack initiation mechanism in Ti–6Al–4V

ABSTRACTFretting fatigue crack initiation in titanium alloy, Ti−6Al−4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.

Effect of contact geometry on the failure modes of thin coatings in the scratch adhesion test

To use the scratch adhesion test for assessing the adhesion of a hard, thin coating to its substrate, it must be ensured that the failure event represents the loss of adhesion, since many coatings fail by non-adhesive modes in this test. This study demonstrates, theoretically and experimentally, that indenter-induced bending stress is very likely to cause cohesive failures, thus, the adhesion of a coating to its substrate cannot be assessed in many scratch adhesion tests. To induce adhesive failure but suppress cohesive failure in the scratch adhesion test, the compressive coating stress should be high but the bending-induced stress should be low. This can be achieved by using an indenter with large radius and a high normal load. To assess coatings with good adhesion to the substrate, the radius of the most commonly used Rockwell C indenter may not be large enough.

Effects of Contact Geometry of Faults on Transmission Waves

— In order to clarify the effects of contact geometry of faults on transmission waves, we have performed a series of experiments in which P and S waves with known wavelength were transmitted through an artificial fault. A pair of piezo-electric transducers (PZT) with various resonant frequency were used for the transmitter and the receiver. Parallel grooves were cut on disk surfaces and two disks were placed face to face with the grooves on one disk being perpendicular to those on the other disk. This yields evenly spaced square contacts on the fault. We regard the square contacts as asperity contacts, the size and the height of which were controlled by changing the width and the depth of the grooves. We found that the transmissivity of the waves is solely determined by the ratio of the groove depth/width to wavelength. The shallower and the narrower the groove depth and width are, the larger the amplitude of first arrival is for both P and S waves. When the groove depth is shallower than a quarter of wavelength, the effect of groove depth is negligible; deeper grooves significantly reduce the amplitude. We have made a mathematical model based on the stiffness of fault. By comparing the model calculations with the observation we found that the model has a limit at which the prediction by the model deviates from the data. The deviation occurs when the ratio of the groove depth/width to wavelength becomes 0.25. We refer to the wavelength as the critical wavelength. When the wavelength is larger than the critical wavelength, the observed data can be well explained by the model. Above this threshold, the model no longer fits the data. In this range, the amplitude of transmitted waves is found to be proportional to the real contact area. Although it is a kind of paradox that the amplitude, not the energy, is proportional to the real contact area, it is possibly explained by taking a non-uniform distribution of stress on the surface of the receiver PZT into account.

A photoelastic study of the effect on subsurface stresses of the contact geometry and friction

Three-dimensional photoelasticity has been used to investigate the effects of contact geometry and interfacial friction on the distribution of stress in a punch acting on a semi-infinite plate and a cup-cone contact. The analysis of the data was performed using an automated polariscope based on the method of phase stepping. The results show that the included half-angle of the punch has a significant effect on the magnitude of the stress which decreases with the angle. The effect of increasing the fillet radius is to lower the maximum subsurface shear stress. Reducing the inclination of the contact interface to the applied stress in the cup-cone model had the effect of raising the peak shear stress. By varying the interfacial friction no significant change in the subsurface stress was observed. It is demonstrated that the stress field around a crack is similar to that at the edge of contact and that the use of a normalized stress intensity factor is relevant to the stress field generated at the boundary of a contact. This parametric study has demonstrated that certain design parameters can reduce the local peak stresses around contact and therefore this would lead to a lower probability of crack initiation and propagation.

Effects of contact geometry in wide Hall bars

We determine numerically the current distribution in a rectangular Hall bar of arbitrary aspect ratio embedded in an infinite strip, without using a common approximation such as perfectly conducting leads or a long sample. For a short sample with large enough difference between sample and contact Hall voltages, closed current loops occur.

Unstable Neck Formation during Initial‐Stage Sintering

By observing pairs of single‐crystal cubic‐faceted small particles of MgO in contact along faces, edges, and corners, it was possible to determine the effects of contact geometry on the stability of neck regions formed during the initial stages of sintering. Changes in the neck morphology were monitored during the sintering process by continuous TEM/ video observations of particles heated in situ between 1100° and 1260°C. Particles in face‐to‐face contact were found to coalesce in the classic manner to form a single particle, Particles in contact along either cube corners or edges, however, developed unstable necks that initially grew, then shrunk, and eventually broke so that such particle pairs effectively “de‐sintered.”

Static and Dynamic Stresses during Valve Closure of a Bileaflet Mechanical Heart Valve Prosthesis

The effect of contact geometry and component compliance on the magnitude, distribution, and state of various types of stresses on a bileaflet mechanical heart valve prosthesis during valve closure was analyzed using an Edwards-Duromedics mitral valve as example. Static and dynamic stresses developing on both the leaflet and pivot ball during valve closure were modeled using finite element analysis (FEA). Uniform contact between the leaflet and housing as well as between the pivot ball and pivot slot can significantly reduce both static and dynamic stresses around the contact area. The level of the dynamic flexural stresses can be an order of magnitude higher than that of the static stresses. When both the radial and axial compliance of the housing are taken into consideration, peak dynamic stress was more than 40% less than that generated through the impact between a moving leaflet and a non-compliant rigid housing.