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1.
Fatigue crack initiation and propagation in lotus-type porous material
Srečko Glodež, Sašo Dervarič, Janez Kramberger, Matjaž Šraml, 2016, izvirni znanstveni članek

Opis: The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation (Ni) and crack propagation (Np) period where the total fatigue life (N) is defined as: N = Ni + Np. The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, Ni, required for formation of initial cracks. The number of stress cycles, Np, required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction.
Ključne besede: lotus-type porous structures, fatigue crack initiation, fatigue crack propagation, numerical analysis
Objavljeno: 02.08.2017; Ogledov: 667; Prenosov: 312
.pdf Celotno besedilo (4,75 MB)
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2.
Compression pre-stress of tubular torsion springs
Vinko Močilnik, Nenad Gubeljak, Jožef Predan, Jože Flašker, 2010, izvirni znanstveni članek

Opis: This paper reports the results of a series of biaxial static compression and torsion experiments performed to evaluate the effects of static compression stress on the fatigue life of those smooth tubes made of high strength spring steel. The fatigue life of biaxial loaded springs depends, among others, on biaxial compression and torsion loading. A high shear loading ratio leads to low-cycle fatigue behaviour rather than high-cycle fatigue, because it was found that a crack was initiated at a local highly deformed area on surface of the specimen.The experimentally obtained results show a significant extension of fatigue strain life as a result of combining axial compression loading with torsion. Cracking behaviour was observed and it was noted that compression pre-stresses contribute to retardation of the fatigue crack initiation process and, consequently, contribute to the extension of fatigue life.
Ključne besede: fatigue, fatigue life, micro crack, multi axial stress state, pre-stressing, torsion, torsion bar springs, twist angle
Objavljeno: 11.07.2017; Ogledov: 459; Prenosov: 72
.pdf Celotno besedilo (1,10 MB)
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3.
Suitability of heat treatment for crack resistance of material in the connection part of heavy transporter for liquid slag
Tomaž Vuherer, Ljubica Milović, Milorad Zrilić, Ivan Samardžić, Vladimir Gliha, 2013, kratki znanstveni prispevek

Opis: The post weld heat treatment (PWHT) was used to reduce the level of the residual stresses and increase of the crack resistance of the materal in the cnnection part. The article presents the results of the residual stress measurements immediately after welding and after the stress relaxation by the PWHT.
Ključne besede: weld, heat treatment, residual stress, stress relaxation, crack resistance
Objavljeno: 03.07.2017; Ogledov: 1632; Prenosov: 61
.pdf Celotno besedilo (552,83 KB)
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4.
Fatigue crack initiation from microstructurally small Vickers indentations
Tomaž Vuherer, Andrej Godina, Zijah Burzić, Vladimir Gliha, 2007, izvirni znanstveni članek

Opis: The resistivity of coarse grain steel against crack initiation in the presence of micro defects is discussed. Samples of material with martensitic microstructure were prepared by adequate thermal treatment. Microstructurally small Vickers indentations were used as an artificial micro defect. The compressive residual stresses are due to the irreversibility of plastic deformation. The moment of indenting enables to prepare samples with and without effects of residual stresses. The stress level for crack initiation depends on the actual indentation size. The location of initiated cracks is affected by the presence and character of residual stresses.
Ključne besede: welded joints, micro defect, coarse grain HAZ, crack initiation, crack growth, stress concentration, fatigue limit, Vickers, residual stresses
Objavljeno: 03.07.2017; Ogledov: 673; Prenosov: 84
.pdf Celotno besedilo (1,02 MB)
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5.
Fatigue crack growth and fracture mechanics analysis of a working roll surface layer material
Matej Drobne, Tomaž Vuherer, Ivan Samardžić, Srečko Glodež, 2014, izvirni znanstveni članek

Opis: 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.
Ključne besede: fracture mechanics, fatigue crack growth, metal forming, rolling process, high chromium steel
Objavljeno: 03.07.2017; Ogledov: 524; Prenosov: 82
.pdf Celotno besedilo (1,11 MB)
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6.
Fracture toughness of a high-strength low-alloy steel weldment
Jelena Vojvodič-Tuma, Nenad Gubeljak, Borivoj Šuštaršič, Borut Bundara, 2006, izvirni znanstveni članek

Opis: 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.
Ključne besede: fracture mechanics, welded joint, crack-tip opening displacement, resistance curves
Objavljeno: 16.03.2017; Ogledov: 674; Prenosov: 62
.pdf Celotno besedilo (502,10 KB)
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7.
Probability function of fatigue crack growth in case of material overloading
Nenad Gubeljak, Janko Legat, 1998, izvirni znanstveni članek

Opis: Studies of structural components under a fatigue environment including overloads are rather interesting in engineering practice today. One important task is to determine the probability distribution function in consideration of two failure modes by a selected fatigue load and overload level. The probability density function permits to evaluate of overloading under fatigue environment on the residual life or to predict the failure of a component. The fatigue crack propagation in case of overloading can be modeled by the Markov's process. The obtained probability functions describe the probability of retarded crack growth propagation as a consequence of the overload cycle.
Ključne besede: utrujenostno širjenje razpoke, verjetnostna funkcija, Markov proces, fatigue crack propagation, probability function, Markov's process
Objavljeno: 10.07.2015; Ogledov: 1115; Prenosov: 32
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8.
Measurement of strain caused by residual stresses in a welded joint using neutron diffraction
Nenad Gubeljak, Jelena Vojvodič-Tuma, Hans-Georg Preismeyer, 1999, izvirni znanstveni članek

Opis: The distribution and amount of residual stresses can significantly contrubute to the fatigue fracture behaviour of welded joints in structures. Interior residual stresses, which interact with the plane strain state, are more dangerous than residual stresses at the surface of the welded joint. If the surface of the welded joint is mechanically treated (e.g. shapering), then significant differences between the stress stated at the surface and within the volume occur. Hence, different non-destructive methods (e.g. gamma radiation, neutron diffraction) have been developed to determine strains caused by residual stresses in the volume of polycrystalline materials. However, the measurement of strains caused by residual stresses becomes difficult for large samples. In this paper the measurement procedure on a sample taken from the weld joint is presented and the effecet of residual stresses on fatigue crack propagation is assessed for low and high cycle loading fatigue.
Ključne besede: zaostale napetosti, zvarni spoj, neutronski lom žarkov, utrujenostno širjenje razpoke, neutron diffraction method, residual stresses, fatigue crack propagation, weld joint
Objavljeno: 10.07.2015; Ogledov: 701; Prenosov: 17
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9.
Numerical modelling of crack growth in a gear tooth root
Srđan Podrug, Srečko Glodež, Damir Jelaska, 2011, izvirni znanstveni članek

Opis: A computational model for determination of crack growth in a gear tooth root is presented. Two loading conditions are taken into account: (i) normal pulsating force acting at the highest point of the single tooth contact and (ii) the moving load along the tooth flank. In numerical analysis it is assumed that the crack is initiated at the point of the largest stresses in a gear tooth root. The simple Paris equation is then used for a further simulation of the fatigue crack growth. The functional relationship between the stress intensity factor and crack length K=f(a), which is needed for determining the required number of loading cycles N for a crack propagation from the initial to the critical length, is obtained using a displacement correlation method in the framework of the FEM-method considering the effect of crack closure. The model is used for determining fatigue crack growth in a real gear made from case carburised and ground steel 14CiNiMo13-4, where the required material parameters were determined previously by appropriate test specimens. The results of the numerical analysis show that the prediction of crack propagation live and crack path in a gear tooth root are significantly different for both loading conditions considered.
Ključne besede: zobniki, utrujanje, širjenje razpoke, numerično modeliranje, gears, fatigue, crack growth, numerical modelling
Objavljeno: 10.07.2015; Ogledov: 666; Prenosov: 67
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10.
Behaviour of short cracks emanating from tiny drilled holes
Vladimir Gliha, P. O. Maruščak, Tomaž Vuherer, 2013, izvirni znanstveni članek

Opis: Specimens with martensitic microstructure were defected by tiny drilled holes with the existing local residual stresses induced by drilling and without them. The objective of this research was to determine the cyclic stress level for the crack initiation and fatigue limit of the defected and smooth specimens' dependence upon the residual-stress field. Compressive residual stresses retarded the crack initiation. Immediately after the crack initiation, residual stresses decelerated the short-crack propagation, but later, when the residual-stress sign was changed, they accelerated it. Non-propagating short-crack size and fatigue limit also depend on the residual-stress field.
Ključne besede: small defect, drilled hole, crack initiation, crack propagation, short crack, long crack, anomalous fast-crack propagation
Objavljeno: 10.07.2015; Ogledov: 749; Prenosov: 79
.pdf Celotno besedilo (1,42 MB)
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