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Behaviour of coarse grain heat affected zone (HAZ) during cycle loading
Tomaž Vuherer, P. O. Maruščak, Ivan Samardžić, 2012, original scientific article

Abstract: This paper presents results of a study on martensitic coarse grain heat affected zone that can appear in welded joints. Mechanical properties of martensitic coarse grain heat affected zone and its microstructure were investigated. Special attention was given to its behaviour during the cycle loading under stress concentration. Stress concentration was similar to the one in real welds. The S-N curve and the fatigue limit were determined. The Paris curve and the threshold for crack propagating were also determined.
Keywords: welded joints, coarse grain HAZ, microstructure, fatigue limit, Paris curve
Published: 01.06.2012; Views: 913; Downloads: 55
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Method for detecting fatigue crack in gears
Aleš Belšak, Jože Flašker, 2006, original scientific article

Abstract: The most undesirable damage that can occur in gear units is crack in the tooth root as it often makes gear unit operation impossible. Monitoring vibrations can be used to detect defects. Time signals are acquired experimentally and afterwards. Different methods can be used to analyse them. The changes in tooth stiffness caused by a fatigue crack in the tooth root are of significance. The dynamic response of a gear unit with a damaged tooth differs from the one of an undamaged tooth. Amplitudes of time signal are, by time-frequency analysis, presented as a function of frequencies in spectrum.
Keywords: gear drives, fatigue failures, crack initiation, crack propagation, vibrations
Published: 31.05.2012; Views: 1097; Downloads: 59
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Computational approach to contact fatigue damage initiation analysis of gear teeth flanks
Matjaž Šraml, Jože Flašker, 2006, original scientific article

Abstract: 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.
Keywords: machine elements, fracture mechanics, gears, contact fatigue, crack initiation, numerical modelling, teeth flanks
Published: 30.05.2012; Views: 1602; Downloads: 58
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Pitting formation due to surface and subsurface initiated fatigue crack growth in contacting mechanical elements
Gorazd Fajdiga, Srečko Glodež, Janez Kramar, 2007, original scientific article

Abstract: 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.
Keywords: fracture mechnaics, machine elements, gears, contact fatigue, pitting, subsurface crack initiation, computational analysis
Published: 31.05.2012; Views: 1301; Downloads: 72
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Numerical analysis of the crack growth in a high loaded bolt connection
Marko Knez, Srečko Glodež, Janez Kramberger, 2007, original scientific article

Abstract: The paper deals with the research on the crack growth in a bolt connection of a lug for crane counter weight bars. Counter weight bars are structural elements subjected to very heavy loads and therefore need special attention. The main purpose of this research is to determine the number of load cycles required for a crack to propagate from initial to criticakl crack length, whenthe final failure can be expected to occur. All required material parameters and experimental results were determined in our previous research. The influence of the initial crack siye upon the remaining life of the lug is reserched numericallu by means of FE analysis and analytically by use of the corrected analystical model.
Keywords: fracture mechanics, cyclic loading, fatigue crack growth, service life prediction, numerical analysis, bolt connection
Published: 31.05.2012; Views: 1495; Downloads: 64
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Crack identification in gear tooth root using adaptive analysis
Aleš Belšak, Jože Flašker, 2007, original scientific article

Abstract: Problems concerning gear unit operation can result from various typical damages and faults. A crack in the tooth root, which often leads to failure in gear unit operation, is the most undesirable damage caused to gear units. This article deals with fault analyses of gear units with real damages. A laboratory test plant has been prepared. It has been possible to identify certain damages by monitoring vibrations. In concern to a fatigue crack in the tooth root significant changes in tooth stiffness are more expressed. When other faults are present, other dynamic parameters prevail. Signal analysis has been performed also in concern to a non-stationary signal, using the adaptive transformation to signal analysis.
Keywords: machine elements, gears, fatigue crack, fault detection, vibrations, adaptive signal analysis, engineering diagnostics
Published: 31.05.2012; Views: 1196; Downloads: 51
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Some factors affecting fatigue resistance of welds
Vladimir Gliha, Zijah Burzić, Tomaž Vuherer, 2010, published scientific conference contribution

Abstract: The fatigue strength of polycrystalline metals is hardness and defect size dependent. The fatigue strength of welds depends on the size of grains and actual local mean stress, too. Local mean stress is influenced by the global R-ratio and static preloading. The welds in the as-welded condition are pre-stressed due to the existence of welding residual stresses. The sign and magnitude of residual stresses depend on the welding conditions. Changes of residual stresses, hardness and defect size are experimentally determined in the present work. Samples of studied metals are prepared by using either thermal cycle simulator or laboratory furnace and water quenching. The methods of microstructure preparation and type of load result in actual local R-ratio at the used weld toe models. Specimens are cyclic loaded in the similar way as steel at the weld toe. The bottom of the notch is either defect-free or defected by Vickers indentation. Final results show that coarser grain and higher local R-ratios lower the fatigue strength of welds.
Keywords: CrNiMo steels, weld, fatigue resistance, heat-affected zone, small weld defects
Published: 31.05.2012; Views: 886; Downloads: 10
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Model for fatigue lifetime prediction of torsion bars subjected to plastic presetting
Vinko Močilnik, Nenad Gubeljak, Jožef Predan, 2011, original scientific article

Abstract: The torsion presetting of a spring-bar is a common technique for extending the elastic twist-angle and increasing torque, but it reduces fatigue lifetime.Spring-bars are used as shock absorbers in caterpillar machines. Spring-bars, made from high strength fine-grain steel, grade VCN, provide a high level of absorbed energy. Any slight change in the presetting procedure has a strong influence on fatigue-lifetime. Within the same loading range, springs can exhibit low-cyclic or high-cyclic fatigue behavior regarding different presetting levels. A model for fatigue-lifetime and shear-stress levels is proposed on the basis of performed tests. Considering prescribed fatigue tests, it is possible to optimize a spring bar's geometry, in order to survive the required number of cycles.
Keywords: presetting, torsion, spring-bars, fatigue, lifetime, cyclic fatigue, torque, spring steel
Published: 01.06.2012; Views: 924; Downloads: 46
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Microcrack initiation and growth in heat-risistant 15Kh2MFA steel under cyclic loading
P. Yasniy, V.B. Hlado, V. Hutsaylyuk, Tomaž Vuherer, 2005, original scientific article

Abstract: This paper presents the results of investigation of a nuclear reactor pressurevessel steel 15Kh2MFA of two strength levels under cyclic loading. Themechanism of microcrack formation on the surface and in the bulk of 15Kh2MFA steel under cyclic deformation was investigated. Analysis of the specimen surfaces has shown that microcracks are caused by cyclic sliding in grains most favourably oriented with respect to the direction of the maximum shear stresses. Transmission electron microscope investigations show that microcracks in the material inside the grains are formed mainly along the band-type dislocation structure parallel to the dislocation subboundary. During cyclic deformation, the dislocation density on the subboundaries increases, in the local areas the dislocation density becomes limiting and it reaches the plasticity limit and causes microcrack formation. The interrelation of the average length of microcracks and their surface density with the energy density of inelastic deformation has been found.
Keywords: material testing, nuclear reactor, pressure vessel, cyclic loading, cyclic deformation, dislocation structure, fatigue crack growth, TEM, fracture mechanics
Published: 01.06.2012; Views: 1008; Downloads: 18
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Effect of residual stresses on the fatigue crack propagation in welded joints
Nenad Gubeljak, Jožef Predan, Reinhard Pippan, Maks Oblak, 2005, original scientific article

Abstract: The effect of residual stress on the fatigue crack growth was investigated forcyclic tension. Three-point bend specimens were used with through thicknessnotches at the center line of the welded joint. The experiments were performed for different load amplitudes and load ratios under conditions of small-scale yielding. The influence of the residual stresses on fatigue crack growth was estimated by experimentally observed fatigue crack growth rate and measurement of residual stresses at the surface of specimen. This paper describes an approach taken in two models to estimate the residual stress effect on fatigue crack propagation and to predict remaining service life of awelded structure.
Keywords: fracture mechnaics, fatigue crack, crack growth, residual stresses, fatigue crack growh rate, X-ray
Published: 01.06.2012; Views: 1187; Downloads: 58
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