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This paper presents new methodology for determining the actual stress-strain diagram based on analytical equations, in combination with numerical and experimental data. The first step was to use the 3D digital image correlation (DIC) to estimate true stress-strain diagram by replacing common analytical expression for contraction with measured values. Next step was to estimate the stress concentration by using a new methodology, based on recently introduced analytical expressions and numerical verification by the finite element method (FEM), to obtain actual stress-strain diagrams, as named in this paper. The essence of new methodology is to introduce stress concentration factor into the procedure of actual stress evaluation. New methodology is then applied to determine actual stress-strain diagrams for two undermatched welded joints with different rectangular cross-section and groove shapes, made of martensitic steels X10 CrMoVNb 9-1 and Armox 500T. Results indicated that new methodology is a general one, since it is not dependent on welded joint material and geometry.

Određivanje stvarnog dijagrama napon-deformacija predstavlja višedecenijski problem jer metode određivanja deformacija, po sve tri ose, u realnom vremenu tek od nedavno predstavljaju realnost naučnog ispitivanja. Napretkom optičkih metoda za određivanje deformacija, prilikom ispitivanja zatezanjem došlo se do mogućnosti merenja deformacija uzorka u realnom vremenu. Ovu mogućnost je obezbedila primena stereometrijskog načina merenja deformacija. Dakle primenom dve kamere i softverskih paketa za obradu dobijenih rezultata, konačno postoji prilika da se odredi stvarni dijagram zatezanja tj. stvarni dijagram napon-deformacija. U okviru ove disertacije prikazana je metodologija određivanja stvarnog dijagrama napon-deformacija za ’’undermatching’’ zavareni spoj, na martenzitnom čeliku. Korišćena je korelacija digitalnih slika (DIC) i metoda konačnih elemenata (FEM) uz upotrebu softverskih paketa Aramis i Abaqus. Dijagram zatezanja koji se najčešće koristi naziva se ''inženjerski dijagram'', jer naponi i deformacije koji su predstavljeni na njemu nisu stvarni tj. dobijeni su upotrebom formula koje važe samo do trenutka kada na epruveti počne stvaranje ''vrata''. Pored ovog dijagrama često se koristi i ''stvarni'' dijagram zatezanja, koji se dobija upotrebom korigovanih formula kojima je izračunat inženjerski dijagram. Ni ovaj pristup ne daje tačne vrednosti napona niti deformacija jer ne uzima u obzir suženje epruvete i lokalne promene dimenzija kao ni koncentraciju napona koja se javlja na mestu suženja. U okviru ove disertacije, razmotren je pristup da troosno stanje napona, koje se javlja nakon stvaranja ''vrata'', dovodi do porasta napona kom je epruveta izložena i predstavljen je način dobijanja potrebnih parametara za određivanje stvarnog dijagrama koji uzima u obzir i koncentraciju napona na mestu suženja koja je, kako će se ispostaviti, dominantan parametar za porast napona pored smanjenja površine poprečnog preseka epruvete. Predstavljena naponska analiza je urađe, Determining the actual stress-strain diagram is a decades-long problem, because real-time strain determination methods on all three axes have only recently become a reality of scientific testing. Advances in optical methods for determining strains during tensile testing have made it possible to measure sample strains in real time. This possibility was provided by the application of the stereometric method for strain measurement. Therefore, by applying two cameras and a software package for processing the obtained results, we are finally able to determine the actual tensile diagram, ie. Actual stress-strain diagram. This disertation presents the methodology for determininig the actual stress-strain diagram for undermatching welded joints on martensitic steel. Used methods are digital image correlation (DIC) and FEM with software packages Aramis and Abaqus. The tensile diagram which is commonly used is called an engineering diagram, because the stresses and strains presented are not true, ie. they are obtained using formulas that are valid only until cross section of the specimen begin to change, necking of the sample ocurs. In addition to the engineering diagram, there is also a ’true’ diagram obtained by using corrected formulas. This principle to, does not provide true results because it does not take into account the reduction of cross section and stress concentration that occurs at the necking site of the specimen. This disertation considered an approach that the triaxial stress state that occurs after the formation of the neck leads to an increase in the stress to which the specimen is exposed and presented a method of obtaining the necessary parameters for determining the actual diagram, which includes the stress concentration at the necking site of the specimen. The presented stress analysis was done so that there is no influence of basic and filler materials on the obtained results of stresses. It can be concluded that the proposed method is suitable for dete

Experimental investigation of the integrity of casing pipes, used in the oil drilling rigs, withdrawn after about 70,000 h of service, has been conducted, together with numerical simulation. Pipes were manufactured by the high frequency contact welding (HF) of API J55 steel. The influence of corrosion damage is investigated by means of pressure test of a pipe with different damage levels made by machining the circular holes. The finite element analysis of the damaged pipe subjected to internal pressure is conducted to simulate the stress state in the pipe and to establish the criteria for the maximum pressure that a damaged pipe can withstand. Several analytical expressions were used to estimate the maximum pressure in a damaged pipeline, and the solutions were compared to predictions of finite element models and experimental results.

This paper presents an experimental and numerical study into the influence of elevated temperatures on mechanical properties of the heat treated high quality hot work tool steel H11. This steel belongs to a group of alloyed steels with extraordinary mechanical properties. The aim of this study was to determine the highest temperature at which these properties are still maintained. The experimental investigation focused on the tensile testing of specimens at seven different temperatures, including the room temperature. The highest testing temperature was 700 degrees C. The heat treatment of plates (specimens) consisted of quenching and tempering. Although the strain hardening of this type of materials is small, the strain hardening curves were calculated to show if there was a possibility for the material to increase its strength due to exploitation loads. Also, a numerical analysis of the tensile test by using the finite element method was done in order to define an appropriate model for numerical testing. The obtained results are then compared with the experimental results.

Influence of the contact pressure in the deep drawing processes is a very actual research topic in the metal forming technology. Within this research the tribological model of the strip sliding in the flat die was developed. The original experimental computerized apparatus for testing of the strip sliding in the variable contact pressure conditions was developed, as well. The complex, multi-factor experiment was performed by application of the Al thin sheet, with the contact elements of various roughnesses, with different lubricants application and with the variable contact pressure. Besides the description of the developed apparatus, the theoretical way for determination of the pressure dependences was also presented. Based on the theoretical changes of pressure, set in advance, the real influence of the contact pressure was obtained, for each of the given conditions. That makes possible to determine the influence of the tribological factors on the actual pressure. The obtained real contact pressure has verified the reliability of the experimental apparatus, namely the degree of the theoretical pressure dependences deviation from the real ones.

The deep drawing technology has a significant place in the contemporary industry. Intensive development of the automotive industry contributes a lot to that fact. With development of the new materials of the poorer machinability and with increase of the technological requirements to the higher level, the necessity for complete control of the forming process is increasing as well. There are significant difficulties in realizing any action during the process duration. In present conditions, only two possibilities are at hand: the blank holding force of the variable intensity and drawing beads of the variable height. Both actions require complex control systems. Contemporary technological problems in deep drawing of parts of the complex geometry can be successfully studied experimentally by means of the physical modeling. The physical model of the contact between the flat surfaces has the dominant space at the flange during the deep drawing process of thin sheets. The emphasis is on realizing the nonlinear dependencies of the contact pressure during the process, as one of the possibilities for the process control. The forming process is usually realized in such a way that all the process parameters and conditions (the blank holding force, deformation rate, etc.) are being set prior to the sliding path of the machine. The concrete investigation shows what effects are realized due to the variable intensities of the blank holding force during the deep drawing process. In addition, it is important to define the functional dependencies of the blank holding force. The basic idea is setting the variable pressure in the contact during the sliding process and monitoring the drawing force (the friction force), the friction coefficient, surface roughness of the thin sheet and the tool. Three different materials were applied, two types of lubricants and four versions of the tool's contact elements. The conducted investigations were of the theoretically-experimental chara, REZIME: Tehnologija dubokog izvlačenja zauzima značajno mesto u savremenoj industriji. Intenzivan razvoj automobilske industrije tome veoma doprinosi. Razvojem novih materijala lošije obradivosti i povećanjem tehnoloških zahteva na viši nivo, raste potreba za potpunim upravljanjem procesom oblikovanja. Postoje znatne teškoće u ostvarivanju bilo kakvih dejstava dok proces traje. U sadašnjim uslovima to su samo dve mogućnosti: sila držanja promenljivog intenziteta i zatezna rebra promenljive visine. Oba dejstva zahtevaju složene upravljačke sisteme. Savremena tehnološka problematika dubokog izvlačenja delova složene geometrije može se uspešno eksperimentalno izučavati putem fizičkog modeliranja. Fizički model kontakta između ravnih površina zauzima dominantan prostor na obodu komada pri dubokom izvlačenju tankih limova. Jedna od mogućnosti upravljanja procesom je zadavanje nelinearnih zavisnosti kontaktnog pritiska za vreme njegovog trajanja. Proces oblikovanja se najčešće realizuje tako što se svi parametri i uslovi procesa (sila držanja, brzina deformisanja, i dr.) podešavaju pre početka radnog hoda mašine. Konkretno istraživanje pokazuje kakve efekte ostvaruju promenljivi intenziteti sile držanja za vreme procesa dubokog izvlačenja. Takođe, od bitnog značaja je i definisanje funkcionalnih zavisnosti sile držanja. Osnovna ideja je zadavanje promenljivog pritiska u kontaktu za vreme procesa klizanja i praćenje promene vučne sile (sile trenja), koeficijenta trenja, površinske hrapavosti lima i alata. Primenjena su tri različita materijala, dve vrste maziva i četiri varijante kontaktnih elemenata alata. Sprovedena istraživanja su teorijsko-eksperimentalnog karaktera s težištem na eksperimentu. U prvom delu istraživanja izvodi se teorijska analiza i kritičko sagledavanje dosadašnjih rezultata. Zatim sledi formiranje nelinearnih i linearnih funkcionalnih zavisnosti simultane promene sile držanja (kontaktnog pritiska) tokom procesa, zavisno od hoda uzorka lima. Osobenost

Purpose - The purpose of this paper is to point out the possibilities for friction welding of dissimilar steels which are used in various industries. In addition, friction welding is a welding method that is applied for executing the very responsible joints. This research is focused on friction and tribological processes in the friction plane of the two pieces during the welding. Design/methodology/approach - The present study research has been conducted based on the experimental testing of cylindrical specimens and results are analyzed. Findings - The austenite grain size is affected by several factors through the friction process phase and the compacting phase during the welding. The very fine grain is the consequence of the high degree of the plastic deformation of the near-the-contact layers even in the friction phase. The viscous layer, which is formed during the stable friction phase, is the area where the moving of matter occurs according to a very complex mechanism. Originality/value - The paper contains useful results from the area of conventional friction welding of dissimilar steels and it can be very useful to researchers and engineers who deal with similar problems.

In this paper is presented a theoretical-experimental analysis of the aluminum-copper joining by friction welding. Considering that such Al-Cu bimetal joints are widely applied in industrial practice, experimental analysis in this paper was performed on the concrete elements used in electronics. The fact that the joining is done of the two dissimilar materials points to complexity of the problem, since phenomena that appear in the joint zone extremely influence physical, mechanical and structural properties of the welded/base metals. Besides the theoretical analysis of the basic phases and mechanisms of the friction welding process, the research also included experimental analysis of the geometry changes due to the plastic deformation, the change of microstructure and hardness in the joint zone, as well as of the basic mechanical properties of the Al-Cu joint, such as tensile strength. This paper presents some significant results, which point to the possibility for realization of the reliable joints of the two dissimilar metals.

The problems related to welding of the high strength steel, aimed for manufacturing of welded structures operating in the prominent wear conditions, is considered in this paper. The paper presents an analysis of possibilities and prescribing the technology for welding the high strength steel HARDOX 450. The methodology for estimate the weldability of this steel was established in the theoretical part of the paper, as well as calculations of the welding parameters, while the hardness was investigated in details and the macro and micro structures of the individual welded joint zones were estimated in the experimental part. Obtained experimental results could be usefully applied for selecting and establishing the optimal welding technology for structures made of this steel. The technology thus established and the GMAW welding regime can enable successful execution of the welded joints and reduce the possibility for appearance of flaws and cracks to a minimum.

We present the experimental-numerical analysis of the influence of temperature on mechanical properties of structural high-strength steel class S690QL. Since the steel S690QL belongs to a group of steels with good mechanical properties, the aim of this paper is to determine the highest temperatures at which these good characteristics are kept. Experimental tensile testings of the specimens were performed at five different temperatures in the range from 20 to 550A degrees C. Beside experimental testing, strengthening curves were calculated and numerical analysis using finite element method was performed. Both the experimental and numerical results have shown that decrease of mecahanical properties occurs at approximately 450A degrees C.