Papers by Author: Aleksandar Sedmak

Abstract: The role of welding in fracture mechanics development is considered from historical point of view. Starting point was analysis of Schenectady ship failure during the II World War, leading to development of linear elastic, as well as elastic-plastic fracture mechanics, soon afterwards. Two case studies are described to illustrate weldment fracture mechanics and structural integrity assessment.

Abstract: This paper presents the development of numerical models which were used to simulate the behaviour of welded joints containing different combinations of multiple defects under tensile loads. Four representative combinations of defects were selected (including undercuts, incomplete root penetration, misalignments...), based on practical experience. In order to create accurate and functional models, this research involve a number of stages. This paper will focus on the various improvements made to the models, which started in relatively simple form. For this purpose, initial experimental and numerical analyses were carried out on specimens made of low-alloyed low-carbon steel S235, and after their accuracy was verified, the same methodology was applied to specimens made of higher quality material, steel S275. Improvements made to the models involved geometry, different combinations of boundary conditions and loads, and some were based on stress-strain states obtained by a combination of tensile testing and digital image correlation. The final result was a set of detailed numerical models which accurately simulated the behaviour of welded joints with multiple defects in them.

Abstract: Using austenitic steel in welding structures requires both destructive and non-destructive testing (NDT) to approve the use of intended welding specification, but also testing of material properties, even if they are guaranteed by manufacturer. In this particular case, X2CrNiMo17-12-2 (commercial trademark AISI 316L) is the material that has to be partly replaced by NiCr21Mo (commercial trademark Incoloy 825) in a piece of process equipment exposed to elevated temperature and highly corrosive environment. This research presents an investigation of overmatching V groove welded joint made of these two austenitic steels. Due to materials properties and specific application conditions, low heat input TIG welding was utilized. Mechanical properties of welded joint have been examined to check criteria which guarantees safe work and integrity of welded components. Chemical composition of used filler material was close to the one of NiCr21Mo alloy, providing weld overmatching, which needs to be checked as well. Mechanical testing (tensile, bending, impact, hardness), macro-and microstructure analysis provided detailed insight into welded joint characteristics and a ground for further investigation concerning heterogeneous austenitic welded joint, taking into account overmatching effect.

Abstract: Structural integrity and life of welded joints made of a micro-alloyed low-carbon fine-grained normalised high strength pressure vessel steel, P460NL1 is presented [1]. The researach performed within the scope of this topic involved a large number of experiments, including tensile and bending tests, hardness and toughness, as well as metallography and fractography tests, in order to determine the mechanical properties of the materials and the welded joints in detail, along with their microstructures and their influence on the obtained test results. Specimens cut out of a welded plate with dimensions of 500x500x14 mm were used for the experiments, whereas certain tests required the making of notches in the specimens, inside the heat affected zone, and this welded joint region was the focus of the research. Fatigue experimental tests were based on the assumption that fatigue crack growth rate changes depending on the regions through which the crack passed during its propagation. For this purpose, specimens used in toughness and fatigue tests were divided into four groups, depending on the part of the plate from which they were taken. Numerical calculations were performed using the extended finite element method (XFEM) [2]. Simulations were based on the experimentally determined values of Paris law coefficients, C i m [3-5], for every region through which the crack propagated during each test. Obtained results have indicated good agreement with the experimental ones, which verified the application of extended finite element method in this case.

Abstract: Finite elements analysis has been performed to compute a J-integral values in a notched pressure vessel. A semi-elliptical notch was introduced in a wall of cylindrical pressure vessel with aspect ratio of notch depth to wall thickness a/w = 1/6 to 5/6, where the wall thickness was 3 mm. Two cases were examined – inner and outer notch. Numerical examinations were revealed that outer notch has much higher values of J-integral (3-4 times), all other being the same, (i.e. inner pressure and dimensions of the notch). That was explained in terms of triaxality stress state in front of notch. Namely, it was shown that inner notch has positive triaxiality stress ratio and vice-versa, outer notch revealed highly negative hydrostatic stress component ahead the notch.

Abstract: Comparative analysis of a repaired and a new crane wheel, was performed in Steelworks Smederevo, including an economic analysis and technological procedures. The repair procedure for a crane wheel is presented, along with the selection of filler materials, as well as testing of mechanical properties performed on samples taken from hard faced welds. The advantages of repair techniques compared to the manufacturing of a new wheel are shown, but also the flaws that may affect the worklife and integrity of wear-damaged elements and components.

Abstract: Evaluation of semi-circular surface crack influence on threaded connection fatigue behavior, made of 42CrMo4 heat treatable steel, was carried out. Crack diameters were defined as 0.02, 0.05, 0.1, and 0.15 mm. Influence of semi-circular surface cracks was investigated by means of Kitagawa–Takahashi diagram and El Haddad equation. Assessments were done for survival probability of 99% on detailed FE model with normal metric thread profile and preload force at 70% of force at bolt yield point. The most critical location on threaded connections usually are thread roots which contain a very high notch effect. In order to accurately consider multiaxial stress field in thread root, multiaxial fatigue stress criterion based on a critical plane theory for fatigue assessment, was used. Mean stress influence was taken into account by means of Haigh diagram. Variable amplitude loading history of low-high (Lo-Hi) sequence spectrum was analyzed with the numerical algorithm of Rainflow cycle counting in the time domain. Fatigue damage was calculated according to the modified Palmgren-Miner linear damage accumulation hypothesis.

Abstract: The subject of this work is examination of API J55 steel casing pipes manufactured by high frequency (HF) contact welding. Since the pipes were withdrawn after about 70 000 hours of service in an oil drilling rig, the influence of the exploitation conditions on the material properties is determined. Experimental analysis includes tensile, impact toughness and fatigue testing of the specimens cut from the exploited pipe and the new pipe manufactured from the same-grade material. Additionally, pipe with flaws (stress concentrators) is tested by subjecting to internal pressure. The analysed stress concentrators represent defects which are often found in the exploitation: localized corrosion damages (corrosion pits). They were simulated by machining on the external surface of the pipe wall. The behavior of the pipes with these defects is determined by strain measurement and finite element analysis in software package Abaqus. The dimensions of the defects are varied, in order to determine the dependence of load carrying capacity on their depth and length. Several expressions from the literature are applied for estimation of the maximum pressure in damaged pipelines, and the solutions are compared with the predictions of finite element models. Additionally, triaxiality values at the bottom of the defects are compared with those obtained on tensile specimens, having in mind the key role of this parameter in ductile fracture initiation.

Abstract: Globe valves have widespread application in various industrial sectors. Due to this variety, valve is subjected to different working loads (static, dynamic, thermal etc.) that can cause valve failures. In this paper, the analysis is performed on globe valve subjected to external axial loading caused by pipeline dilatations. Experimental analysis was focused on determining local mechanical properties of critical areas, i.e. on local areas of globe valve housing with high geometrical discontinuities. Experimental analysis and numerical model verification were performed using system Aramis based on Digital Image Correlation method. Using full-field experimental results, numerical model behavior was improved and enabled acquiring more accurate results in transition areas that cannot be found in currently available literature. Determining local mechanical properties of critical areas enables better understanding of complex structures such as valve housing and provides an opportunity for further development and improvement for practical industrial application.

Abstract: Seam casing pipe used in an oil drilling rig, manufactured by high frequency (HF) contact welding of API J55 steel, is tested. The influence of an initial defect (machined surface crack) is analysed, by performing pressure test of a pipe segment closed at both ends. Besides the damages at the internal surface, casing pipes are exposed to damage at the external surface, which is why such configuration is analysed here. Measurement of strains and crack mouth opening displacement (CMOD) enabled the application of direct method for J integral evaluation. This procedure is based on the path independence of the J integral and can be applied both in laboratory conditions (on specimens) and on structures. However, it requires a demanding experimental - computational procedure, which is accomplished here using the developed routine. Additionally, the behaviour of the pipe under internal pressure, including fracture mechanics parameters determination, is modelled numerically (by finite element method) in software package Abaqus. The pressure is applied as distributed load acting on the inner surface of the three-dimensional model, and axial tension is applied at the end of the pipe to simulate the closed end. J integral values determined numerically and using direct method are used for estimation of the critical pressure corresponding to the crack growth initiation. Additionally, plastic limit load, i.e. pressure which causes yielding of the ligament, is determined. Based on the results, criteria for pipe integrity assessment are discussed