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1.
Experimental determination of fatigue parameters of high chromium steel under different loading and temperature conditions
Matej Drobne, Nenad Gubeljak, Srečko Glodež, 2014, original scientific article

Abstract: Fatigue investigation of high chromium steel (HCS) at different loading ratios (R = 0, R = -1) and different temperatures (20 °C, 600 °C) is presented in this paper. Before fatigue testing, monotonic mechanical properties (ultimate compressive and ultimate tensile strength) are determined at different temperatures, using standardized testing procedures according to DIN 50125 standard. Moreover Charpy impact tests at different temperatures were done with specimens that comply with the standard ISO 14556. Fatigue testing is performed on a servo - hydraulic testing machine with consideration of different loading conditions as described above. On the basis of the experimental results the S - N curves are constructed from which typical fatigue parameters (the fatigue strength coefficient 0'f and the fatigue strength exponent b) are determined. After fatigue testing a comprehensive investigation of fracture surfaces is performed using the Scanning Electron Microscope (SEM). Experimental results presented in this paper will serve as a basis for further investigations related to fatigue behaviour of real working rolls in hot strip mills made of HCS.
Keywords: experiments, high chromium steel, high cycle fatigue, fracture mechanics
Published: 12.07.2017; Views: 558; Downloads: 104
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2.
Fatigue crack growth and fracture mechanics analysis of a working roll surface layer material
Matej Drobne, Tomaž Vuherer, Ivan Samardžić, Srečko Glodež, 2014, original scientific article

Abstract: Fatigue crack growth and fracture mechanics analysis of a working roll surface layer material is presented in this paper. The research is done on a hot strip mill working roll where High Chromium Steel is used for roll’s shell material. To obtain corresponding parameters, a rectangular single edge notched bend specimens – SENB, according to standard BS 7448, were used. The fatigue crack growth analysis was done on a resonant testing machine with use of special crack gauges, while for fracture mechanics parameters the electro–mechanical testing machine was used.
Keywords: fracture mechanics, fatigue crack growth, metal forming, rolling process, high chromium steel
Published: 03.07.2017; Views: 541; Downloads: 83
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3.
Factors influencing the yielding constraint by cracked welded components
Dražan Kozak, Jelena Vojvodič-Tuma, Nenad Gubeljak, Damir Semenski, 2005, original scientific article

Abstract: The effect of strength mismatch for welded joints performed with different geometries on the yielding constraint has been investigated in the context of single-edged fracture-toughness specimens subjected to bending SE(B) using the finite-element method. The crack was located in the centre of the weld. Two geometri cal parameters have been identified as being the most important: the crack-Iength ratio a/W and the sIenderness of the welded joint (W-a)/H. They were systematically varied as follows: a/W = 0.1; 0.2; 0.3; 0.4; 0.5 and W = 2H, 4H, 8H, 16H, 24H. Basic equations and plane-strain finite-element solutions for the overmatched SE(B) specimen with all configuration combinations are given. The results are in good agreement with those in literature. This paper aims to establish yield-Ioad solutions for the same weldment configurations, but with materials dissimilarity present within the weld. This situation is usually encountered during repair welding. For this purpose, a practical combination of filler materials, with the same portion of overmatched part with M = 1.19 and undermatched part with M = 0.86, has been selected. Plane-strain solutions for the heterogeneous weld with the cracklocated in the overmatched half were obtained. The influence of the yielding-constraint key parameters has al so been evaluated. Yield-Ioad results for the specimens performed with different weld widths have the greatest scattering for the a/W = 0.5. The transition from the overmatched to the undermatched solution with increasing H is evident. On the other hand, the behaviour of the specimen with a shallow crack is dictated by the overmatch region ahead of the crack tip and depends very little on the weld slenderness. An approximated 3-D area of the yield-Ioad solutions depending ona/W and (W-a)lH has been proposed. Furthermore, the stress triaxility parameter h has been calculated using 2-D and 3-D finite-element analysis, and given as a field in the spec imen to get an insight into yielding-constraint regions. It was found that the 3-D yield-Ioad solutions are very close to the plane-strain solutions. Also, the effect of a/W on the yielding constraint is more significant than the effect of Mand (W-a)/H.
Keywords: welded structures, welded joints, fracture mechanics, cracks, SE(B) specimens, yielding load, yielding constraint parameters
Published: 06.04.2017; Views: 876; Downloads: 75
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4.
Fracture toughness of a high-strength low-alloy steel weldment
Jelena Vojvodič-Tuma, Nenad Gubeljak, Borivoj Šuštaršič, Borut Bundara, 2006, original scientific article

Abstract: The use of high-strength low-alloy steels for high-performance structures, e.g., pressure vessels and pipelines, requires often high-strength consumables to produce an overmatched welded joint. This globally overmatched welded joint contains local mis-matched regions, which can affect the unstable fracture behaviour of the welded joint and the welded structure itself. If local mis-matched regions are present in the vicinity of a crack tip, then the fracture toughness of the weld metal can be significantly lower than that of the base metal. In this paper, the influence of the weld-metal microstructure on the fracture behaviour is estimated enabling an evaluation of the resistance to stable crack growth through different microstructures. The lower bound of the fracture toughness for different microstructures was evaluated using a modified Weibull distribution. The results, obtained using specimens with a through thickness crack front, indicated a low fracture toughness, caused by the strength mis-matching interaction along the crack front. In the case of through-the-thickness specimens, at least one local brittle zone (LBZ) or a local soft region is incorporated into the process zone in the vicinity of the crack tip. Hence, an unstable fracture occurred with small stable crack propagation, or without it. Despite the fact that the differences between the impact toughness of the weld metal and the base metal can be insignificant, the fracture toughness of a weld metal can be significantly lower.
Keywords: fracture mechanics, welded joint, crack-tip opening displacement, resistance curves
Published: 16.03.2017; Views: 699; Downloads: 62
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5.
Weld joint fracture behaviour of HSLA steels dissimilar in strength
Inoslav Rak, Arpad Treiber, 1998, original scientific article

Abstract: The effect of strength differences (mismatch) between weld metal and two base metals as well as local variations of strength within weld metal/HAZ zones on the toughness properties were discussed. The significance of local fracture toughness measurement technique was also discussed by comparing the CTOD results of [delta]5 and British Standard [delta]BS. Some differences between the two techniqes were discussed in particular for CGHAZ toughness of similar and dissimilar joints.
Keywords: welding, welded joints, fracture mechanics, HSLA steels, fracture toughness
Published: 10.07.2015; Views: 860; Downloads: 24
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6.
An integrity analysis of washing machine holders
Nenad Gubeljak, Matej Mejač, Jožef Predan, 2007, original scientific article

Abstract: The paper deals with structure integrity analysis of the holder for the carrying cross of a washing machine drum. Premature fracture of the holder occurred between mechanical sustainable tests of washing machine in the factory. In order to prevent fracture, the task was to determine the reasons for premature fracture of the holder and to estimate the suitability of the new design of the holder cross. Input data for structure integriry analysis were obtained by material by mechanical testing of used materials. Stress and strain analysis of holder limit load was performed by finite element modeling of holder. Dynamic tests of holders with two different thicknesses were done on a servo-hydraulic machine in order to find dynamical strength and endurance of holder. Fracture behaviour of holders is defined as initiation and propagation of crack. The obtained behaviour confirmed that the new design of holders reduces stress concentration in the critical region. Consequently, the new holder subjected to the same dynamic load can endure a higher number of cycles until breakage. The total number of cycles overcomes industrial testing requirements.
Keywords: lomna mehanika, visokociklično utrujanje, preizkušanje lomne žilavosti, nosilci kadi pralnega stroja, mejna obremenitev, ocena celovitosti konstrukcije, fracture mechanics, high cycle fatigue, fracture toughness testing, washing machine holders, washing machine drums, structure integrity assessment, limit load
Published: 10.07.2015; Views: 1015; Downloads: 75
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7.
Evaluation of stress intensity factors using finite elements
Boštjan Zafošnik, Zoran Ren, Miran Ulbin, Jože Flašker, 2002, published scientific conference contribution

Abstract: The paper describes a suitability of Virtual Crack Extension (VCE) method for numerical simulation of crack propagation under mixed mode loading. The suitability of VCE method was determined using The Compact-Tension-Shear (CTS), which is subjected to fracture Mode I, Mode II or mixed mode (Mode I + Mode II) loading. In mixed mode the crack kinks when fracture crack growth occurs. Kink angle is determined with VCE method, which is an energy method, maximum tangential stress (MTS) criterion, strain energy density (SED) criterion and experimental results. Determination of crack propagation angle using MTS and SED criterion is based in regard to the stress intensity factors KI and KII. Stress intensity factors are determined using the maximum energy release rate theory using complex J integral, crack opening displacement method and method of stresses extrapolation. Bigger is the rate between KII and KI, crack path, determined with VCE method, propagates more slowly to the kink angle, determined with experiment. The computational results show that the VCE method is suitable for crack propagation, when KI around the crack tip is dominant. In case where KII is dominant, crack does not kink immediately as in other two criteria. When KII dominates around a crack tip, MTS criterion is the most appropriate for determination of kink angle.
Keywords: fracture mechanics, stress intensity factors, CTS specimen, crack propagation, virtual crack extension method, strain energy density criterion, maximum tangential stress criterion
Published: 01.06.2012; Views: 1862; Downloads: 30
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8.
9.
Numerical modelling of micro-pitting of gear teeth flanks
Gorazd Fajdiga, Jože Flašker, Srečko Glodež, Trevor K. Hellen, 2003, original scientific article

Abstract: A two-dimensional computational model for simulation of contact fatigue of gear teeth flanks is presented. In the model, it is assumed that the initial crack of length 0.015 mm is initiated at the surface due to previous mechanical or heat treatment of the material as well as a consequence of the running in process. The discretized model with the initial crack is then subjected to normal contact pressure, which takes into account the elasto-hydro-dynamic (EHD) lubrication conditions, and tangential loading due to friction between contacting surfaces. The model also considers the moving contact of gear flanks, fluid trapped in the crack and residual stresses due to heat treatment of the material on crack propagation. The virtual crack extension (VCE) method, implemented in the finite element method, is then usedfor simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The computational results show that the initial surface crack of length 15 m and the considered boundary conditions lead to the appearance of very small surface pits, which can be termed as micro-pitting on gear teeth flanks. The numerical results correspond well withavailable experimental data.
Keywords: machine elements, gearing, fracture mechanics, numerical analysis, micro pitting, pitting simulation, matematical model, fatigue, contact mechanics, crack propagation
Published: 01.06.2012; Views: 1895; Downloads: 0

10.
Strength mismatch effect on yield load in X-shaped weldment with centre crack
Dražan Kozak, Nenad Gubeljak, Jožef Predan, Franjo Matejiček, 2004, published scientific conference contribution

Abstract: This paper provides yield load solutions for single edged fracture toughness specimen subjected to bending SE (B) with present X-shaped weld joint. The weld centre crack is located in the overmatch weld part. The corresponding fully plastic yield loads were obtained directly by plain strain FEM analysis for five characteristic a/W ratios: 0,1;0,2;0,3;0,4 and 0,5. Also, the influence of the sistematically varied weld root width 2H on the fracture behaviour has been evaluated. It was found that yield load decreases with the increasing weld root, because the undermatched region ahead the crack tip plays the dominant role.
Keywords: fracture mechanics, X-shaped welded joints, centre crack, strength mismatching, plastic yield load, constraint effects
Published: 01.06.2012; Views: 1251; Downloads: 30
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