Steel Hemispheres Research Articles

Tribological behaviors and wettability evolution of phenolic resin-impregnated graphite materials under water immersion condition

This study investigated the tribological behaviors of two phenolic resin-impregnated graphite samples and one non-impregnated sample under dry and water immersion using a reciprocating testing device. The frictional configuration involved a steel hemisphere sliding against the surface of the graphite samples. The influence of water immersion time before each formal test on friction coefficients was discussed. The wettability evolution during the test processes and the difference in porosity, which could significantly affect water infiltrating into the sample matrix and frictional interfaces, were associated with the variation in tribological behaviors. The worn surface morphology and element analysis results indicated that the shallow valleys and pores on the surface of graphite were gradually filled by wear debris from both the graphite samples and the harder counterpart. Based on these experimental results, a tribological mechanism of the phenolic resin-impregnated graphite materials under water immersion was developed.

Methodology improvement of bulk compressive creep test: Deformation measurement and loading rate

Objectives(1) To identify improvements when bulk compressive creep testing of dental resin composite materials to reduce the sensitivity to the surface morphology and parallelism of specimens, to generate more accurate strain (displacement) measurement values. (2) To investigate the effect of loading rate on the creep and recovery behavior under bulk compressive creep test. MethodsCylindrical composite resin specimens were subjected to bulk compressive creep test with conventional and modified methodology (with/without introduction of stainless steel hemisphere and preload process). Furthermore, specimens undertook different loading rates ranging from 1 N/s to 50 N/s. Maximum deformation, creep deformation, permanent set as well as percentage of recovery during the creep and recovery procedure were compared, and surface topography changes before and after preload process was evaluated by laser scanning confocal. Burgers model was used to investigate the effect of improvements to each part of viscoelastic deformation of resin specimens. Results(1) The influence of surface evenness of resin specimens could be reduced by addition of preload process before the bulk compressive creep test resulting in significantly decreased permanent set (p = 0.002), and increased recovery to 91.7 % (p < 0.001). While the standard deviation of maximum deformation, permanent set and percentage of recovery had the smallest values when hemisphere was introduced to loading chain. (2) With increasing loading rate of bulk compressive creep tests, creep deformation increased and this trend became statistically significant when the loading rate reached 50 N/s. SignificanceThe accuracy of deformation measurement during bulk compressive creep test could be improved by means of introducing stainless steel hemisphere to the loading chain, and adding preload process to loading protocol.

Defect localization by an extended laser source on a hemisphere

The primary goal of this study is to localize a defect (cavity) in a curved geometry. Curved topologies exhibit multiple resonances and the presence of hotspots for acoustic waves. Launching acoustic waves along a specific direction e.g. by means of an extended laser source reduces the complexity of the scattering problem. We performed experiments to demonstrate the use of a laser line source and verified the experimental results in FEM simulations. In both cases, we could locate and determine the size of a pit in a steel hemisphere which allowed us to visualize the defect on a 3D model of the sample. Such an approach could benefit patients by enabling contactless inspection of acetabular cups.

Numerical Simulation of Tensile Residual Stresses in SWCNT-Reinforced Polymer Composites

Abstract The residual stresses play a significant role in the mechanical properties and strengthening capability of nanocomposites. The present research aims to numerically investigate the residual stress relaxation in nanotube-reinforced polymers in response to mechanical tensile loading. The systems under study consist of the armchair and zigzag single-walled carbon nanotubes (SWCNT) embedded in a polymer matrix. The nanotubes and polymer matrix are assumed to be bonded by van der Waals interactions based on the Lennard-Jones (L-J) potential at the interface. The interactions between carbon atoms in the nanotube and nodes in the polymer matrix are modelled by equivalent springs. In order to evaluate the analysis of elastic-perfectly plastic using finite element (FE) modelling, first, relaxation of the plastic residual stresses on steel hemisphere in contact with a rigid flat surface was examined in a loading-unloading cycle and verified with available data. Afterwards, the residual stress relaxation in nanotubes with different space-frame structures was computed due to displacement-controlled loading. Finally, the stress state and the plastic residual stresses in the nanocomposite for different carbon nanotube content were analyzed and discussed during loading and unloading. Regarding the effect of tensile stress, it was revealed that nanotube structures have significant effects on the residual stresses created in the nanocomposite.

Strength and chemical resistance of composites based on epoxy resins, filled with gypsum in the original and water­hardened forms

This paper reports the results of studying epoxy compositions with gypsum taken in the form of dispersed powders in the original and water-hardened form. The exact pattern has been shown in the way the introduction of a gypsum additive in the amount of 50 % by weight affects the strength, chemical stability, and morphology of the composites. Under conventional heat treatment (60‒110 °C) of the hardened composites, the maximum stress at compression σ m and the elasticity module at compression Е с , as well as wear resistance, decrease after the introduction of gypsums (of both types). At the same time, after a hard (destructive) heating at 250‒260 °C, the elasticity module Е с of the hardened composites increases. The maximum stress at compression σ m is also increased. The same applies to the wear resistance, which grows especially noticeably after 250 °C. The micro-hardness after filling is prone to increase but the fragility of epoxy-gypsum composites does not make it possible to measure it when a punch (a steel hemisphere) penetrates it deeper than 20 µm. However, after the heat treatment at 250‒260 °C, the unfilled polymer, on the contrary, is embrittled while the filled ones are plasticized, thus showing a high micro-hardness at significant (30‒50 µm) immersion. The composites with gypsum, in contrast to the unfilled ones, do not disintegrate in acetone and retain integrity at any aging duration (up to 75 days and beyond). In this case, the original gypsum produces a composite with less swelling in acetone than the hardened gypsum. Based on the data from atomic-strength microscopy (ASM) microscopy, the morphologies of the non-filled composite, the composites with the hardened gypsum and original gypsum are different. The original gypsum forms a composite with a more pronounced (possibly crystalline) filler structure; the morphology for the hardened composite reflects the distribution of inert particles; for the unfilled composite (H-composite), only pores are visible against the background of a relatively smooth relief

Study on dented hemispheres under external hydrostatic pressure

This paper is dedicated to the comprehensive investigation on dented hemispheres under external hydrostatic pressure. A total of 20 stainless steel hemispheres are fabricated, which have a nominal radius of 60 mm and a nominal wall thickness of 0.76 mm. These fabricated hemispheres are divided into four groups with five samples in each group. Wherein, three groups are dented using conical, spherical, and cylindrical indenters respectively, while the remaining group is designated as undented one. Both dented and undented hemispheres are hydrostatically tested into collapse to examine the effects of indention on the collapse characteristics of hemispheres. Additionally, all tested hemispheres are semi-analytically and numerically evaluated based on the measured geometric data and tested material properties. The experimental, semi-analytical, and numerical results are compared in figures and tables.

Effect of preparation design for all-ceramic restoration on maxillary premolar: a 3D finite element study

PurposeThe aim of this study was to investigate and quantify the effect of preparation design parameters on a premolar restored with two different CAD/CAM ceramic crowns by three-dimensional finite element analysis (FEA). MethodsA restored human first premolar was digitized by a micro-CT scanner and a 3D model was created by a medical image processing software (Mimics). Following segmentation, dentine and ceramic were extracted by a surface meshing software (3-matic). Models with different preparation designs with three convergence angles (6°, 12° and 20°) and two preparation heights (3.1mm and 4.1mm) were produced. Mesh generation for models was performed in IA-FEMesh software with a lithium disilicate glass ceramic (LD, E=95.9GPa) and a polymer-infiltrated ceramic (PIC, E=30.1GPa) as the restorative materials. A 5-mm diameter stainless steel hemisphere was employed as an indenter. Twelve models were analyzed numerically in Abaqus™. ResultsThe results indicated that preparation height was found to be a major factor affecting stress distribution in different components. In all models, the maximum principal stress of the ceramic crowns was found in contact area against the indenter. This stress was lesser in the longer abutment than the shorter one and it was greater for LD ceramic. Convergence angle had limited effect on stress distribution of ceramic crown in all models. ConclusionsThe preparation height appeared to play a more important role in the stress distribution of ceramic crown than the convergence angle.

Seismic performance of Transverse Steel Damper seismic system for long span bridges

To provide seismic resistance of long span bridges in transverse direction, fixed bearings are often installed between girders and piers (or towers). Although the fixed bearings and substructures can be designed extraordinarily strong to resist the design seismic loads, they may still be vulnerable when the design seismic loads are exceeded. To address this issue, this paper proposes a novel seismic system, which combines Transverse Steel Dampers (TSDs) with conventional sliding bearings, and is named the TSD seismic system. A TSD consists of several triangular steel plates outfitted with steel hemispheres at their upper vertices. The steel hemispheres not only allow free movements of the superstructures with respect to the piers in the longitudinal direction, but also provide reliable load paths in the transverse direction. Quasi-static tests have been conducted to investigate seismic behaviors of the TSD using two scaled and two prototype specimens. Test results show that the TSD has excellent performance in energy dissipation, large displacements, and synchronization of triangular plates under complex contact conditions. The load-displacement constitutive model of the TSD has been established using a bilinear model in ABAQUS, followed by a design method for the TSD seismic system. A 620m long-span cable-stayed bridge was selected for a case study of the TSD seismic system. Ground motions recorded at various site conditions were used as seismic inputs. Numerical results show that: (1) the TSD seismic system can achieve a desired balance of transverse seismic displacements and forces, which is not the case when a sliding bearing system (without TSDs) or a fixed bearing system is used; (2) TSDs contribute to most of the energy dissipation capacity of a TSD seismic system while the contribution of sliding bearings is negligible; and (3) the proposed TSD seismic system, compared with a sliding system, tends to be less sensitive to seismic input properties, such as peak ground accelerations and site conditions.

The effect of laser polishing on fretting wear between a hemisphere and a flat plate

Fretting wear between a hemisphere and a flat plate is investigated experimentally and the effect of laser polishing is studied. The energy dissipated between the hemisphere and the flat plate is determined as a function of operating conditions such as frequency and normal load and material properties such as surface roughness, and related to the wear observed at the sliding interface. Abrasive, adhesive and tribo-chemical wear are formed on the hemisphere and flat plate. No significant difference in wear production between a laser polished and a regular stainless steel hemisphere was observed.

Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

The relationship between friction, wear, and transfer films of three metal carbide-reinforced amorphous carbon coatings (TiC/a:C, TiC/a:C-H, and WC/a:C-H), sometimes referred to as metal-doped diamond-like carbon coatings, has been investigated. Tribological tests were performed in an in situ tribometer with sapphire or steel hemispheres run against coated flats in dry or ambient air. The sliding contact interface was observed and recorded by optical microscopy during reciprocating sliding tests. The friction and wear behavior during run-in depended on the number of sliding cycles to form a stationary transfer film on the hemisphere. Stationary transfer films formed rapidly (within ten cycles) and the friction coefficient fell to 0.2 (ambient air) or 0.1 (dry air), except with sapphire against WC/a:C-H in dry air; with the latter, a stationary transfer film required nearly 100 cycles to form, during which the friction remained high and the wear rate was from 10 to 100 times higher than the other two coatings. For all coatings, three velocity accommodation modes (VAM) were observed from run-in to steady-state sliding and were correlated with the friction and wear behavior. The delayed adherence of the transfer film to sapphire from WC/a:C-H coatings in dry air is discussed in terms of equilibrium thermochemistry. Friction and wear behavior during run-in, therefore, depended on transfer film adherence to the hemisphere and the VAM between transfer films and the coating.

수조모델을 이용한 메쉬접지극의 접지저항에 관한 연구

Recently, a number of equipments related with electricity, electronics, and communication in the same building are needed to the grounding system for safety from unexpected accidents. When the faulted electric current flows into a certain grounding system, the potential rise in that system takes place and it might induce the potential rise to other grounding system. This potential interference was strongly affected by the surface potential, which was deeply related with the electrode shape. In this paper, the fundamental formula was deduced on the basis of surface potential of two grounding electrodes. Which corresponds to source of the potential interference and other grounding electrode, respectively. Therefore, the degree of potential interference in this mesh grounding electrode system was verified by the simple model simulation. In addition, in order to identify the difference between the grounding resistance in the realistic construction site and the expected value from the corresponding simulation, the experiment was performed with model on a reduced scale about the realistic grounding system. It consists of stainless steel hemisphere electrodes in a water tank. From this work, the grounding resistance in the mesh grounding electrode showed the good coincidence results between those. Consequently, it is confirmed that the grounding resistance in the mesh electrode is possible to be estimated by performing the experiment using the water tank model.

Design and feasibility tests of a flexible gripper based on inflatable rubber pockets

In this paper, we present feasibility test results of a flexible gripper design following a literature survey on various types, design and control strategies of the existing grippers. A flexible gripper based on the use of compliant materials (i.e., rubber) with pneumatic inflation was designed, analyzed, built and tested. Parametric FE analyses were conducted to investigate the effects of process and design parameters, such as rubber material, pressure, initial jaw displacement and friction. Based on the FEA results, a simple, single rubber-pocketed flexible gripper was designed and built. Feasibility experiments were performed to demonstrate and obtain an overall understanding about the capability and limitations of the gripper. It was found that objects with different shapes (cylindrical, prismatic and complex), weight (50 g–20 kg.), and types (egg, steel hemi-spheres, wax cylinders, etc.) could be picked and placed without any loss of control of the object. The range of positioning error for two different part shapes (i.e., prismatic or cylindrical) was found to be 20–90 μm (translational) and 0.03–0.9° (rotational).

Role of Third Bodies in Friction Behavior of Diamond-like Nanocomposite Coatings Studied by In Situ Tribometry

The friction and wear behavior of amorphous diamond-like nanocomposite (DLN) coatings (C:H:Si:0) in low speed, dry sliding contact has been investigated using a home-built in situ tribometer. Tests were performed under reciprocating sliding against sapphire and steel hemispheres in dry (∼4% RH) and moist (35 to 52% RH) air at contact stresses of 0.7 and 1.1 GPa. In situ visual observations identified how third body processes affected the friction behavior of DLN coatings. For all test conditions, a transfer film began to form on the sapphire hemisphere during the first cycle, and the coefficient of friction dropped during further buildup of the transfer film. Video analysis showed most of the sliding motion took place between the transfer film on the hemisphere and the DLN coating. This velocity accommodation mode, interfacial sliding, was responsible for steady-state friction coefficients between 0.03 and 0.05 with lower values obtained at high stress and low RH. When the transfer film broke up the friction began to spike (0.1 ≤ μ ≤ 0.5); during the spikes, shearing of third bodies also contributed to velocity accommodation, as debris attached, detached and recirculated between the hemisphere and the track. Subsequently, the friction coefficient fluctuated between low and high values due to reformation and breakup of the transfer film, respectively.

Experiments on Pool Boiling of Water from Downward-Facing Hemispheres

Quenching experiments were conducted to investigate pool boiling of saturated water on downward-facing aluminum and 303e stainless steel hemispheres. Test sections had an outer diameter of 0.152 m and a wall thickness of 0.020 m. Destabilization of film boiling and wetting of the stainless steel surface occurred earlier than with aluminum (15 s versus 92 s), at 20 K higher wall superheat and ~10% higher minimum film boiling heat flux qmin. Wetting of the stainless steel surface occurred first near the edge of the test section and then gradually propagated azimuthally inward, followed by the maximum heat flux (MHF) front. Conversely, wetting of the aluminum surface occurred first at the lowermost position (θ = 0 deg) and then propagated azimuthally outward. The azimuthal propagation of the MHF front on the stainless steel surface (~5 deg/s for 60 deg < θ < 90 deg decreasing to ~1.8 deg/s for 10 deg < θ < 60 deg and then increasing slightly to ~2 deg/s for 0 deg < θ < 60 deg) was much slower than on aluminum (~22.5 deg/s on average). The MHF front traversed the entire stainless steel boiling surface in ~40 s versus only 4 s for aluminum. When MHF for the latter first occurred at θ = 0 deg, the radial and near-boiling surface azimuthal temperature gradients were 4 K/mm and 0.6 K/deg, respectively, compared to 12 to 15 K/mm and 1.5 K/deg for stainless steel. For both surfaces, the MHF and qmin values displayed parabolic dependencies on azimuthal angle. The local MHF at θ = 0 deg was 0.81 and 0.40 MW/m2 for aluminum and stainless steel, respectively, decreasing with increased azimuthal angle to minimums of 0.47 and 0.256 MW/m2 at θ = 45 deg. Beyond 45 deg, the local MHF increased with increased azimuthal angle to 0.76 and 0.47 MW/m2, respectively, near the edge of the surface (θ = 80 deg). However, the wall superheats corresponding to the MHF (30 K for aluminum and 80 K for stainless steel) and the qmin (125 K for aluminum and 145 K for stainless steel) were independent of azimuthal angle.

Some Problems on the Measurement of 222Rn Concentrations by Passive Cup Method

A passive cup monitor has been constructed by using a 50 mm radius stainless steel hemisphere. The conductive housing has reduced the scatter in track densities. In dwellings with high 220Rn concentrations, it is particularly necessary to measure 222Rn concentration with a monitor having a radon exchange rate less than 0.1 h-1.

Measurements of Thoron Concentration by Passive Cup Method and Its Application to Dose, Assessment

A new type of passive integrating cup monitors was developed by using a 50 mm radius stainless steel hemisphere to measured indoor radon (222Rn) and thoron (220Rn) concentrations. By placing a pair of cup monitors with an air exchange opening of diameter 5 mm and four openings of diameter 20mm at a 20cm distance from wall during about three months, the concentrations of both gases could be assessed from the a track densities on the cellulose nitrate (CN) films. The 222Rn and 220Rn concentrations were surveyed with the cup monitors in the different types of dwellings around Nagoya in Japan over three years. The 220Rn concentrations were rather high in the dwellings with soil wall, and the mean 220Rn concentration was 160±12 Bq·m−3. The 220Rn exhalation rate from wall and the 220Rn diffusion were evaluated from the distribution of 220Rn concentrations in the dwellings. The results of the surveys have also clarified the relationship between the 220Rn concentrations a t a 20cm distance from wall and the 220Rn...

Compression cracking of steel ball bearings

Abstract Quasi-static compression cracking of 3 mm diameter steel ball bearings by uniaxial loading between tungsten carbide flats has been investigated experimentally. It is shown that the cracking occurs only after the bearings have been deformed plastically at the contact zones. Moreover, it is also shown that the cracking occurs not only during loading, but also after complete unloading if the maximum applied load is at least 65–70% of the load required for cracking during loading. The time delay between complete unloading and cracking has been found to be in the range of a few seconds to several hours. The fragments formed by cracking during loading are like orange segments in shape, whereas only two fragments of nearly equal masses are always formed when the cracking occurs after complete unloading. The unloading cracks are thought to form by the residual tensile circumferential stresses in the plastically deformed spheres. Separate experiments on specially made mild steel hemispheres have shown that the displaced material is pushed into the bulk of the sphere parallel to the load axis, and this leads to unloading residual stresses. It is suggested that the cracking during loading is initiated sub-surface at the load axis due to tensile radial stresses, whereas unloading cracks are formed at the surface due to tensile circumferential stresses. Experimental evidence in support of the latter is also presented.

Buckling strength of imperfect steel hemispheres

Experimental and numerical results on seven 580 mm diameter, spun steel, hemispherical shells subjected to external pressure are discussed in the paper. The average wall thickness of the shells varied from 0·37mm to 2·5mm. Careful shape and thickness measurements on all the shells were obtained and utilised in several types of analysis (2-D Finite Element, bestfit axisymmetric, axisymmetric with a local fattening, etc.). None of the analysis techniques employed proved to be entirely reliable insofar as predicting the collapse strength of the spun steel hemispheres. For example, the ratios of the experimental to the 2-D FE numerical collapse pressures were between 0·56 and 1·21. The test results were also compared with the ECCS design curve and it is shown that one should use the minimum shell thickness for design purposes and not rely on the average wall thickness (three test results plotted below the design curve when the average wall thickness was used).

Externally Pressurized Hemispherical Fibre-Reinforced Plastic Shells

The results of external pressure tests on ten 0.8 m diameter torispherical shells made from FRP (fibre-reinforced plastic) were discussed in a recent paper by the authors. In the present paper, the results of similar tests on six hemispherical FRP shells are given. Four of the hemispheres were made from single sheets of woven pre-preg (three of carbon-fibre-reinforced plastic and one of glass-fibre-reinforced plastic): two of those made from carbon-fibre-reinforced plastic (CFRP) were petalled, that is they were made by butt-jointing together pieces of composite cloth. The diameter-average thickness (D/tav) ratios of the hemispherical domes varied from 66 to 108. The BOSOR 4 program was used to predict the stresses in the composite shells and their buckling pressures; the Tsai-Wu equation (in stress space) was utilized for the material failure criterion. For all the shells in this investigation, the controlling failure mode appeared to be first-ply failure (FPF). The latter is, of course, influenced by the compressive strength of the composite. Assuming a reasonable value for this (for example 570 N/mm2 for CFRP) gave ratios of Pexpt/PFPF which were in the range 1.05–1.19. In all cases, the experimental result was higher than the theoretical prediction, that is it was on the safe side. The authors measured the compressive strength of the CFRP composite experimentally. They obtained values in the range 510 ± 100 N/mm2. These were less than values obtained at Imperial College (664 ± 44 N/mm2). At the moment, it is not known why this wide variation in compressive strength was obtained. More work needs to be done on this aspect of the problem. Each of the butt-jointed CFRP hemispherical domes was weaker than its unjointed counterpart. In part, this is due to the increase in thickness of the unjointed hemispheres near the clamped edge, where the failures originated. With the CFRP torispherical domes, the failure location was at the spherical cap/knuckle junction (that is away from the clamped edge) and the butt-jointed torispherical dome turned out to be stronger than its unjointed counterpart. The 30-ply CFRP hemisphere (D/tav = 66) was 1.4 times as strong as the 36-ply GFRP hemisphere (D/tav = 73). In addition, the 30-ply CFRP hemisphere had the same collapse pressure as a geometrically similar welded steel hemisphere which had D/t ≈ 113 and a = σYP = 645 N/mm2. The CFRP dome would be much ligher than the steel one, in the ratio 1:2.8.

Comments on the paper by Md. W. Uddin: Buckling of general spherical shells under external pressure: Int. J. Mech. Sci. 29, 469–481 (1987)

Experimental and numerical results on seven 580 mm diameter, spun steel, hemispherical shells subjected to external pressure are discussed in the paper. The average wall thickness of the shells varied from 0·37mm to 2·5mm. Careful shape and thickness measurements on all the shells were obtained and utilised in several types of analysis (2-D Finite Element, bestfit axisymmetric, axisymmetric with a local fattening, etc.). None of the analysis techniques employed proved to be entirely reliable insofar as predicting the collapse strength of the spun steel hemispheres. For example, the ratios of the experimental to the 2-D FE numerical collapse pressures were between 0·56 and 1·21.The test results were also compared with the ECCS design curve and it is shown that one should use the minimum shell thickness for design purposes and not rely on the average wall thickness (three test results plotted below the design curve when the average wall thickness was used).