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1.
Computational approach to contact fatigue damage initiation analysis of gear teeth flanks
Matjaž Šraml, Jože Flašker, 2006, izvirni znanstveni članek

Opis: The paper describes a general computational model for the simulation of contact fatigue-damage initiation in the contact area of meshing gears. The model considers the continuum mechanics approach, where the use of homogenous and elastic material is assumed. The stress field in the contact area and the relationship between the cyclic contact loading conditions and observed contact points on the tooth flank are simulated with moving Hertzian contact pressure in the framework of the finite element method analysis. An equivalentmodel of Hertzian contact between two cylinders is used for evaluating contact conditions at the major point of contact of meshing gears. For the purpose of fatigue-damage analysis, the model, which is used for prediction of the number of loading cycles required for initial fatigue damageto appear, is based on the Coffin-Manson relationship between deformations and loading cycles. On the basis of computational results, and with consideration of some particular geometrical and material parameters, theinitiation life of contacting spur gears in regard to contact fatigue damage can be estimated.
Ključne besede: machine elements, fracture mechanics, gears, contact fatigue, crack initiation, numerical modelling, teeth flanks
Objavljeno: 30.05.2012; Ogledov: 1752; Prenosov: 66
URL Povezava na celotno besedilo

2.
Pitting formation due to surface and subsurface initiated fatigue crack growth in contacting mechanical elements
Gorazd Fajdiga, Srečko Glodež, Janez Kramar, 2007, izvirni znanstveni članek

Opis: A computational model for simulation of surface and subsurface initiated fatigue crack growth due to contact loading is presented. The model is based on fracture mechanics theory where the required materials properties are obtained from common fatigue tests. For computational simulations an equivalent model of two contacting cylinders is used instead of simulating theactual contact of mechanical elements. The discretised model with the initial crack on or under the surface is then subjected to normal contact pressure, which takes into account the EHD-lubrication conditions, and tangential loading due to friction between contacting surfaces. The model considers also the moving contact of mechanical elements and for the surface initiated crack also the fluid trapped in the crack. The virtual crack extension method, implemented in the finite element method is then used for simulating the fatigue crack growth from the initial crack up to the formationof the surface pit. The numerical results correspond well with available experimental data. The described model can be used for simulation ofpitting phenomenon of contacting mechanical elements like gears, bearings, wheels, etc.
Ključne besede: fracture mechnaics, machine elements, gears, contact fatigue, pitting, subsurface crack initiation, computational analysis
Objavljeno: 31.05.2012; Ogledov: 1432; Prenosov: 81
URL Povezava na celotno besedilo

3.
Modelling of surface crack growth under lubricated rolling-sliding contact loading
Boštjan Zafošnik, Zoran Ren, Jože Flašker, Gennady Mishuris, 2005, izvirni znanstveni članek

Opis: The paper describes modelling approach to computational simulation of surface crack growth subjected to lubricated rolling-sliding contact conditions. The model considers the size and orientation of the initial crack, normal and tangential loading due to rolling-sliding contact and the influence of fluid trapped inside the crack by a hydraulic pressure mechanism. The motion of the contact sliding load is simulated with different load cases. The strain energy density (SED) and maximum tangential stress (MTS) crack propagation criteria are modified to account for the influence of internal pressure along the crack surfaces due to trapped fluid. The developed model is used to simulate surface crack growth on a gear tooth flank, which has been also experimentally tested. It is shown that the crack growth path, determined with modified crack propagation criteria, is more accurately predicted than by using the criteria in its classical form.
Ključne besede: fracture mechanics, numerical methods, contact loading, finite element analysis, internal pressure, maximum tangential stress, strain energy density, surface crack growth
Objavljeno: 01.06.2012; Ogledov: 1785; Prenosov: 68
URL Povezava na celotno besedilo

4.
The interaction ability of cellulosic materials as a function of fine structure and Helmholtz surface energy
Tatjana Kreže, Karin Stana-Kleinschek, Volker Ribitsch, Zdenka Peršin, Majda Sfiligoj-Smole, 2005, izvirni znanstveni članek

Opis: Many chemical or physical modification processes significantly influence the accessibility of fiber forming polymers by causing structural changes. The wettability and sorption ability improvements of polymeric materials are major tasks during finishing processes. Different pre-treatment processes are used in order to improve the accessibility of dissociable groups, hydrophilicity, dyeability, and whiteness. These are usually alkaline purification, chemical bleaching and mercerization. In a previous paper we presented the data for structural characteristics (density, crystallinity index, molecular orientation, void volume, diameter and the specific inner surface of void, etc.) of untreated regenerated cellulose fibers (viscose, modal and lyocell) [41]. We now compare the influence of different pre-treatment processes on fiber structure and the accessibility of the chemical groups of these fibers. In order to improve the accessibility, two pre-treatment processes were used: chemical bleaching of fibers and tensionless alkali treatment. The influence of these pre-treatment processes on the structure parameters was evaluated using viscosity measurements (determination of polymerization degree (DIN 54 270)) and iodine sorption ability measurements according to the Schwertassek method (determination of crystallinity index) [13, 16]. The reactivity and accessibility in a polar environment was determined using tensiometry. Contact angles between the fibers and liquids of different polarities were determined using the powder contact angle method and calculated from a modified Washburn equation [26, 28]. The surface free (Helmholtz) energy of differently treated fibers was determined from the contact angle data using the Owens-Wendt-Raeble-Kaelble approximation [30, 33, 35]. The differences in the accessibility of raw and pre-treated regenerated cellulose fibers obtained using tensiometry are compared with the results of the conventional method used to determine moisture adsorption (DIN 54 351, DIN 53 802). In regard to raw fibers, viscose shows the most hydrophilic characteristic: adsorbs the highest amount of moisture, has the fastest penetration velocities (Fig. 6), the smallest contact angle, and the highest SFE (Fig. 8). Modal fibers have the largest contact angle, the lowest SFE, and they adsorb the smallest amount of water vapor. Pre-treatments increase the sorption ability and the surface free (Helmholtz) energy while they decrease the contact angle. This makes the material more accessible to water and chemicals used in the finishing processes although the crystallinity index increases. The main modification in polymer properties caused by the treatments is an increase in the fiber SFE caused by an increase of the fiber surfaces because of swelling in the alkaline medium (washing, slack-mercerization), and due to an increase of accessible OH- and COOH-groups (bleaching). This enables the formation of an increased number of hydrogen bridges between the water molecules and the OH- and COOH-groups. Our investigations confirm the results published earlier thatthe main property necessary for the proper sorption behavior of cellulose materials are the accessible, less ordered regions and not the degree of crystallinity.
Ključne besede: regenerated cellulose fibers, fiber pre-treatment, iodine sorption, cristallinity, tensiometry, contact angle, Helmholtz surface energy, water adsorption
Objavljeno: 01.06.2012; Ogledov: 1870; Prenosov: 49
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5.
Tuning of poly(ethylene terephtalate)(PET)surface properties by oxygen plasma treatment
Aleš Doliška, Metod Kolar, 2011, izvirni znanstveni članek

Opis: Modification of surface properties of poly(ethyleneterephtalate) (PET) thin films by treatment with weakly ionized oxygen plasma was studied by contact angles of water and diiodomethane (DIM) drops. Samples were exposed to oxygen plasma with the ion density of 5 x 1015/m[sup]3 and the neutral oxygen atom density of 3 x 1021/m[sup]3. Just after the treatment they were characterized by contact angle measurements. Results showed a quick decrease of the water contact angle in the first few seconds of plasma treatment, while prolonged treatment did not cause any substantiated modification. The contact angles of DIM, on the other hand, remained rather constant for the first several secondsof plasma treatment, and increased after prolonged treatment. It was found that the dispersion component of the surface free energy decreased with increasing treatment time, while the polar component increased with treatment time. The results were explained by surface functionalization as well as by roughness effects.
Ključne besede: poly(ethylene terephtalante), PET, oxygen plasma, contact angle, hydrophilic, functionalization
Objavljeno: 01.06.2012; Ogledov: 1290; Prenosov: 30
.pdf Celotno besedilo (329,87 KB)
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6.
Substrate-induced coagulation (SIC) of nano-disperse titania in non-aqueous media : the dispersibility and stability of titania in N-methyl-2-pyrrolidinone
Angelika Basch, Simona Strnad, Volker Ribitsch, 2009, izvirni znanstveni članek

Opis: Dispersions of 1 wt% titandioxide (titania or TiO2) were investigated with respect to their stability in non-aqueous media. The objective of this work was to find conditions for the substrate-induced coagulation (SIC) process in N-methyl-2-pyrrolidinone (NMP). The SIC process is a dip-coating process that enhances adsorption of fine dispersed particles on a pre-conditioned surface. The wetting behavior of titania and NMP was investigated by the powder contact angle method. The absorption process of the polar solvent NMP on the acid oxide TiO2and Aerosol OT (AOT) (bis-2-ethylhexyl sodium sulfosuccinate) was also investigated and a polar or hydrophilic interaction was found. The stability of titania in NMP dispersions and the influence of the solvent AOT and the electrolyte LiCl was investigated. By studying the electrophoretic mobilities of titania particles in NMP and the influence on solutes by the electrophoretic method phase analysis light scattering (PALS) and the electroacoustic method this paper explores suitable conditions for non-aqueous substrate-induced coagulation of titania.
Ključne besede: contact angle, non-aqueous dispersions, electrophoretic mobilities, substrate-induced coagulation, titandioxide, zeta potential
Objavljeno: 01.06.2012; Ogledov: 1449; Prenosov: 63
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7.
The influence of different parameters on surface pitting of contacting mechanical elements
Gorazd Fajdiga, Jože Flašker, Srečko Glodež, 2004, izvirni znanstveni članek

Opis: The paper describes a general computational model for simulation of surface pitting of mechanical elements subjected to contact loading conditions. In the model it is assumed that the initial crack of length 0.015 mm is initiated at the contacting surfaces due to previously thermal, mechanical treatment of the material or running in process. The discretised model with the initial crack is then subjected to normal contact pressure, which takes into account the EHD lubrication conditions and tangential loading due to friction between contacting surfaces. The model accounts also for the influence of fluid trapped in the crack on crack prppagation. The virtual crack extension VCE method within FEM analysis is then used for 2 dimensional simulation of fatigue crack propagation from the initial crack up to the formation of the surface pit. The pit shapes and relationships between the stress intensity factor and crack length are determined for various combinations of contacting surface curvatures and loadings. The comparison of computational and available experimental results shows that the proposed model reliably simulates the surface fatigue crack growth under contact loading and can be used for computational predictions of surface pitting for various contacting mechanical elements.
Ključne besede: machine elements, gears, contact problems, pitting, fatigue crack growth, simulation, model
Objavljeno: 01.06.2012; Ogledov: 1267; Prenosov: 86
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8.
Critical plane modelling of fatigue initiation under rolling and sliding contact
Matjaž Šraml, Jože Flašker, Iztok Potrč, 2004, izvirni znanstveni članek

Opis: Contact fatigue is a phenomenon of important practical significance for engineering applications involving localized contacts, such as gears, rail wheel system and rolling bearings. The service lifetime of such components is related to damage, which results from the contact fatigue. The process in the material structure that causes this kind of failure is quite complicated. The aim of the present paper is to describe a contact fatigue initiation criterion, based on the critical plane approach for the general contact problem. On the basis of contact stress analysis with modified Hertzian boundary conditions, the loading cycle of characteristic material points in the contact area is determined. The Dang Van damage initiation criterion is based on the critical plane approach, which combines the largest allowable shearing and hydrostatic stresses (tensile and compressive), with an assumed elastic shakedown behaviour and it is used in this work. The material point of initial fatigue damage is then determined at the transition of the loading cycle stresses over the critical plane. The model assumed a homogeneous and elastic material model, without any imperfections or residual stresses, and elastic shakedown is considered. A proper determination of loading cycles and their characteristic values is of significance for contact fatigue initiation analysis. Finally, determination of the most critical material point on or under the contact surface and related number of loading cycles required for fatigue damage initiation is calculated with the strain-life (▫$epsilon$▫-N) method.
Ključne besede: machine elements, contact fatigue, crack initiation, strain life method, critical plane approach, numerical modeling
Objavljeno: 01.06.2012; Ogledov: 1322; Prenosov: 84
URL Povezava na celotno besedilo

9.
Symmetry preserving algorithm for large displacement frictionless contact by the pre-discretization penalty method
Dušan Gabriel, Jiři Plešek, Miran Ulbin, 2004, izvirni znanstveni članek

Opis: A three-dimensional contact algorithm based on the pre-discretization penalty method is presented. Using the pre-discretization formulation gives rise to contact searching performed at the surface Gaussian integration points. It is shown that the proposed method is consistent with the continuum formulation ofthe problem and allows an easy incorporation of higher-order elements with midside nodes to the analysis. Moreover, a symmetric treatment of mutually contacting surfaces is preserved even under large displacement increments. Theproposed algorithm utilizes the BFGS method modified for constrained non-linear systems. The effectiveness of quadratic isoparametric elements in contact analysis is tested in terms of numerical examples verified by analytical solutions and experimental measurements. The symmetry of the algorithm is clearly manifested in the problem of impact of two elastic cylinders.
Ključne besede: mechanics, numerical methods, contacting surfaces, contact problems, 3D contact algorithm, discretization, higher order elements, finite element method, Gauss point search, pre-discretization penalty method
Objavljeno: 01.06.2012; Ogledov: 1645; Prenosov: 56
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10.
Numerical procedure for predicting the rolling contact fatigue crack initiation
Matjaž Šraml, Jože Flašker, Iztok Potrč, 2003, izvirni znanstveni članek

Opis: A computational numerical model for contact fatigue damage analysis of mechanical elements is presented in this paper. The computational approach is based on continuum mechanics, where a homogenous and elastic material model isassumed in the framework of the finite element method analysis. Cyclic contact loading conditions are simulated with moving Hertzian contact pressure. The time-depending loading cycles are defined for each observed material point on and under the contact area. Furthermore, the influence of friction upon rolling-sliding contact loading cycles is analysed in detail, using Coulombćs friction law. The model for prediction of the number of loading cycles, required for initial fatigue damages to appear, is based on Coffin-Manson relations between deformations and loading cycles, and includes characteristic material fatigue parameters. As a general example, the model isused to analyse a fundamental contact problem of a cylinder and flat surface, which is usually a substitutional model for analysing real mechanicalproblems. However, the results concerning the identification of critical material points and the number of loading cycles, required for initial fatigue damages to appear at those points, are the main purpose of thepresented study.
Ključne besede: machine elements, gears, contact problems, contact fatigue, crack initiation, numerical modelling
Objavljeno: 01.06.2012; Ogledov: 1229; Prenosov: 56
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