81. Effect of residual stresses on the fatigue crack propagation in welded jointsNenad Gubeljak, Jožef Predan, Reinhard Pippan, Maks Oblak, 2005, izvirni znanstveni članek Opis: 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.
Ključne besede: fracture mechnaics, fatigue crack, crack growth, residual stresses, fatigue crack growh rate, X-ray
Objavljeno v DKUM: 01.06.2012; Ogledov: 2029; Prenosov: 93
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82. Model for fatigue lifetime prediction of torsion bars subjected to plastic presettingVinko Močilnik, Nenad Gubeljak, Jožef Predan, 2011, izvirni znanstveni članek Opis: 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.
Ključne besede: presetting, torsion, spring-bars, fatigue, lifetime, cyclic fatigue, torque, spring steel
Objavljeno v DKUM: 01.06.2012; Ogledov: 1929; Prenosov: 101
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83. Tensile test models for low-carbon microalloyed steels with high niobium contentsMarcos Pérez-Bahillo, Nenad Gubeljak, David A. Porter, Beatriz López, Jožef Predan, Antonio Martín-Meizoso, 2011, izvirni znanstveni članek Opis: In the present investigation, the effect of both: rolling parameters (2 reduction rates and 3 cooling rates) and chemical elements such as: C, Mn, Nb,Ti, Mo, Ni, Cr, Cu and B, has been studied in relation to strength properties in low-carbon microalloyed steels with high niobium contents (up to 0,12 wt. % Nb). For this purpose, an experimental set-up was designed based on an intelligent design of experiments (DoE), resulting in 26 casts (laboratory casts).Acombination of metallography, Electron Back-Scattered Diffraction (EBSD) and tensile tests have been performed to study how processing parameters and chemical composition affect the strength. The results, where the proof stress, tensile strength, uniform and fracture elongations are the response variables, have been analysed statistically by means of multiple linear regression technique, leading to response equations. From the results, it was found that the effectiveness of niobium increasing the strength is reduced as carbon content increases.
Ključne besede: microalloyed steels, mechanical properties, niobium
Objavljeno v DKUM: 01.06.2012; Ogledov: 2342; Prenosov: 110
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86. Modern deformation measurement techniques and their comparisonBoštjan Kovačič, Rok Kamnik, Miroslav Premrov, Nenad Gubeljak, Jožef Predan, Zdravko Tišma, 2008, pregledni znanstveni članek Opis: Deformation measurement experiment has been undertaken where a concrete structure was subject to controlled loading. Concrete plate was loaded with hydraulic cylinder PZ 100 up to 42 kN. Nikon total station ser. 720 and camera Fuji Pro S3 have been used to make measurements at critical points during each additional load. A hydraulic computer operated cylinder was used for load increase. The use of total stations for deformation measurements is quite often, but the combination with digital photography is rather new and very suitable technique. It offers the capability of simultaneous monitoring of a large number of signalized or non-signalized points with very low root mean square error of 0.16 mm in geodetic method 0.7 mm in photogrammetry, respectively. The concrete plate was modeled and the comparison with the geodetic, photogrammetric and hydraulic method was made.
Ključne besede: deformation measurements, geodesy, digital photogrammetry
Objavljeno v DKUM: 31.05.2012; Ogledov: 3704; Prenosov: 54
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87. On the local variation of the crack driving force in a double mismatched weldJožef Predan, Nenad Gubeljak, Otmar Kolednik, 2007, izvirni znanstveni članek Opis: A material inhomogeneity in the direction of crack extension causes a difference between the near-tip crack driving force, Jtip, and the nominally applied far-field crack driving force, Jfar. This difference can be quantified by a material inhomogeneity term, Cinh, which is evaluated by a post-processing procedure to a conventional finite element stress analysis. The magnitude of the material inhomogeneity term is evaluated for cracks in an inhomogeneous welded joint made of a high-strength low-alloy steel. Both a crack proceeding from the under-matched (UM) to the over-matched (OM) and from the OM to the UM weld metal are treated. The effects of the inhomogeneity of the different material parameters (modulus of elasticity, yield strength, and strain hardening exponent) on Cinh and Jtip are systematically studied. The results demonstrate that the material inhomogeneity term is primarily influenced by the inhomogeneity of the yield strength. A crack growing towards an OM/UM interface experiences an accelerated crack growth rate or a pop-in, an UM/OM interface leads to a reduced crack growth rate or a crack arrest. The application of global assessment methods of the mismatch effect which are included in the Engineering Treatment Model (ETM) or in the Structural Integrity Assessment Procedures for European Industry (SINTAP) is discussed.
Ključne besede: crack driving force, material inhomogeneity, mismatched weld, interface, J-integral, finite element modeling
Objavljeno v DKUM: 31.05.2012; Ogledov: 1754; Prenosov: 88
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88. RAZVOJ POTISNEGA ČEVLJA IN DOLOČITEV MEJNE NOSILNOSTIGregor Strehar, 2011, diplomsko delo Opis: Ledinek Engineering d.o.o., za katerega sem opravljal diplomsko delo, izdeluje predvsem stroje in proizvodne linije za obdelavo lesa.
Diplomsko delo zajema postopek razvoja novega potisnega čevlja, ki je bil razvit v sodelovanju s Fakulteto za strojništvo Maribor. Primarni cilj razvoja je bil predvsem zmanjšanje teže potisnega čevlja ob zahtevani nespremenjeni nosilnosti stiskalnice Hyperpress, namenjene avtomatizirani izdelavi ukrivljenih lepljenih elementov, za približno 40%, kar nam je tudi uspelo. V diplomi so zajeta najpomembnejša področja razvoja novega, lažjega potisnega čevlja, od razvoja nove oblike do priprav na proizvodni proces, ki zajema razrez, pripravo, sestavo in varjenje potisnih čevljev. Velik poudarek je ravno na varjenju potisnih čevljev, saj so se pojavile neugodne zaostale napetosti in z njimi povezane deformacije, zaradi katerih je bilo potrebno razviti celoten tehnološki proces izdelave potisnih čevljev.
Ključne besede: konstruiranje, optimizacija, lameliran lepljen les, potisni čevelj, metoda končnih elementov, varjenje, zaostale napetosti, mejna nosilnost
Objavljeno v DKUM: 04.10.2011; Ogledov: 2782; Prenosov: 224
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89. VPLIV TERMIČNE OBDELAVE NA LOMNO ŽILAVOST VZMETNEGA JEKLAAndrej Likeb, 2011, diplomsko delo Opis: Visoko trdnostna jekla se uporabljajo na področjih velikih dinamičnih obremenitev. Zaradi česar se proizvajalci običajno soočajo s težavami, kako zagotoviti trajno dinamično trdnost. Za njeno zagotovitev največkrat posegajo po naknadnih termičnih obdelavah. Namen diplomskega dela je tako narediti primerjalno analizo utrujenostne rasti razpoke v skladu s standardom ASTM E 1820 - 01, za termično mehko in termično obdelano vzmetno jeklo 51CrMOV4. Diplomsko delo temelji na eksperimentalnih meritvah v LSEK laboratoriju na Fakulteti za strojništvo, kjer so se izvajali lomnomehanski preizkusi z metodo mehanike loma, določevale utrujenostne rasti razpoke in materialna parametra Parisovega zakona. Obdelava podatkov je potekala v programu Microsoft Excel 2010, s katerim so tudi bili izrisani diagrami. V diplomskem delu je v splošnem opisana mehanika loma ter področje utrujanja: njeni mehanizmi za nastanek, LCF in HCF utrujanje, določevanje hitrosti rasti razpoke in razni vplivi nastanka. Kratko poglavje v diplomskem delu je namenjeno tudi pripravi upogibnih lomnomehanskih preizkušancev v skladu s standardi.
Ključne besede: Vzmetno jelko 51CrMoV4, mehanika loma, LCF in HCF utrujanje, lomna žilavost, ASTM E1820 - 05, časovno spremenljivo obremenjevanje, mikromehanizmi utrujanja, širjenje kratkih razpok, ASTM E1820 - 01, Wӧhlerjeva krivulja, mehanske lastnosti, Kitagawa - Takahashijev diagram, Parisov zakon, utrujenostna rast razpoke
Objavljeno v DKUM: 02.09.2011; Ogledov: 4365; Prenosov: 373
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90. KONSTRUIRANJE PAKETA PRAVOKOTNIH TORZIJSKIH VZMETIMario Pešić, 2010, diplomsko delo/naloga Opis: Obravnavan paket pravokotnih torzijskih vzmeti se uporablja za dvigovanje loput na vojaškem vozilu, ki jih vgrajujejo v Sistemski tehniki ARMAS d.o.o. Izračunan torzijski moment v enačbah po literaturi, ni zagotovil potrebnega momenta v praksi. V različnih literaturah je prikazan postopek računanja torzijskega momenta, s katerim omogočimo dvig lopute določene teže. Na osnovi primerjave med različnimi literaturami in eksperimentalnega preizkušanja smo ugotovili, da so rezultati med seboj primerljivi. Eksperimentalno smo ugotovili, da se torzijski moment spreminja glede na način vpetja pravokotnih vzmeti, kar je lahko vzrok za razhajanje med izračunano karakteristiko in dejansko karakteristiko vzvojnega paketa vzmeti. Za zagotovitev torzijskega momenta, je potrebno opredeliti primerno vpetje pravokotnih vzmeti, saj v literaturi način vpetja ni natančno opredeljen, zaradi česar pride do razhajanja med izračunano karakteristiko in dejansko.
Ključne besede: TORZIJA, VZMET, EKSPERIMENTALNO PREIZKUŠANJE, VZMETNA KARAKTERISTIKA
Objavljeno v DKUM: 11.10.2010; Ogledov: 3135; Prenosov: 250
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