Aleksandar Sedmak

Prediction of ductile fracture initiation using micromechanical analysis

Abstract In the paper ductile fracture initiation analysis of low-alloyed ferritic steel has been made by application of two micromechanical models: the Rice–Tracey void growth model and the Gurson–Tvergaard–Needleman (GTN) model. The aim of the study was to analyse transferability of micromechanical parameters determined on specimens without initial crack to pre-cracked specimens. A significant part of the research has been carried out through participation in the round robin project organised by the European Structural Integrity Society (ESIS). Tensile tests have been performed on cylindrical smooth specimens and CT specimens. Critical values of micromechanical parameters determined on smooth specimen for both applied models, have been used for prediction of the crack growth initiation in CT specimen. Modelling of the first phase of ductile fracture––void nucleation––has been carried out using quantitative metallographic analysis of non-metallic inclusion content in tested steel. For determination of critical values of model parameters corresponding to ductile fracture initiation a simple procedure has been applied based on a combination of experimental and numerical results. Evaluated J -integral values corresponding to onset of crack growth, J i , are in good agreement with experimental result and both models have proved to be suitable for determination of the ductile fracture initiation in tested steel. The effect of FE size at a crack tip on J i -value has been particularly analysed: it has been established that the calculation with FE size corresponding to the mean free path λ between inclusions in steel gives results that are in accordance with the experimental ones.

Study of the Simulated Heat Affected Zone of Creep Resistant 9–12% Advanced Chromium Steel

Global warming leads to research-development attempts worldwide with a view to reduce CO2 emission. Therefore, in the energy supply sector, a special focus has been directed to further development of steels that can endure the ultra-supercritical steam conditions. Our article presents the basic study on weldability of the advanced 9–12% chromium steel applying the thermomechanical simulation of the heat affected zone (HAZ). The changes of microstructures and material properties of HAZ before and after postweld heat treatment (PWHT) have been analyzed and compared by light microscopy, scanning electron microscopy (SEM), hardness measurements, and impact toughness testing. Microstructures at representative points during typical welding cycle and PWHT were studied in details.

Hydrogen Embrittlement of Industrial Components: Prediction, Prevention, and Models

Hydrogen embrittlement is a common, dangerous, and poorly understood cause of failure in many metal alloys. In practice, it is observed that different types of damage to industrial components have been tied to the presence and localization of hydrogen in metals. Many efforts have been made at understanding the effects of hydrogen on materials, resulting in an abundance of theoretical models and papers. However, a fully developed and practically-applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen embrittlement. The connection of microstructure-based behaviors of materials and effects on the macroscopic measurable characteristics (stress levels, hardness, strength, and impact toughness) is of the utmost importance to achieve a unified model for hydrogen embrittlement. This paper gives an overview of the application of a model for structural integrity analysis of boiler tubes made of plain carbon steel exposed during operation to a local corrosion pro...

Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment

Abstract Local brittle zones (LBZ) cause the unstable fracture behaviour of weld metals. This threatens the safe service of welded structures and makes structural assessment procedures difficult. Therefore, the unstable fracture behaviour of an overmatched high strength low alloyed steel weldment was experimentally investigated. It showed that any interaction between two adjacent weld metal matrix and soft weld metal inclusions produces LBZ, causing local unstable fracture behaviour. The formation of a low hardness region is attributed to the multipass welding reheating process between Ac 1 and the self-tempering temperature. The presence of partly solid metallic inclusions with a high content of alloying elements and pro-eutectoid ferrite microstructure were found to be additional causes for the local unstable fracture behaviour of the weld metal. Local strength mis-match induced the yielding and strain hardening in the soft weld metal inclusions, contributing significantly to unstable fracture behaviour. Thus, a significantly different scatter of experimental results can be obtained. In the cases of specimens with through-the-thickness crack, not only is the scatter significantly lower, but the toughness itself.

Microstructural changes of Nimonic-263 superalloy caused by laser beam action

Pulsed laser treatments induce changes in the microstructure of materials. We have exposed the Nimonic-263 superalloy to pulsed laser beams of various repetition rates and wavelengths and investigated the changes in microstructure and microhardness. Two types of laser treatment of the material surface occurred: mechanical and thermomechanical. The microstructure changes have been comparatively analyzed by scanning electron microscopy and energo-dispersive spectrometry. The grain size was measured before and after the treatment and microhardness tests were performed. These investigations aim to contribute to the study of the level of improvement of microstructure and mechanical properties because of the interaction with laser beams.

Experimental in-vitro bone cements disintegration with ultrasonic pulsating water jet for revision arthroplasty

The paper deals with the study of using the selective property of ultrasonic pulsating water jet for the disintegration of the interface created by bone cement between cemented femoral stem and trabecular bone tissue as a potential technique for revision arthroplasty. Six types of commercial bone cements based on Polymethyl Methacrylate were used for investigation. The cements were mixed using the DePuy - SmartMix® CTS / vacuum mixing bowl. Mechanical properties of hardened bone cements were determined by nanoindentation. The bone cement samples were disintegrated using the pulsating water jet technology. The water pressure varied between 8÷20 MPa. A circular nozzle with an orifice diameter of 0, 7 mm was used for water jetting. The stand-off distance from the target material was 2 mm and the traverse speed 1 mm/s. The volume of material removal and depth of created traces were measured by MicroProf FRT optical profilometer. The results positively support an assumption that pulsating water jet has a potential to be a suitable technique for the quick and safe disintegration of bone cement during revision arthroplasty.

API J55 Steel Casing Pipe with an Initial Surface Crack under Internal Pressure - Determination of Fracture Parameters

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

Determination of ductile crack initiation by magnetic emission and potential drop techniques using pre-cracked Charpy specimens

Abstract Magnetic emission (ME) and potential drop (PD) techniques were used for instrumented Charpy impact testing in order to determine critical crack initiation properties of standard pre-cracked three-point bending specimens at room temperature. Results for high strength low-alloyed steel specimens, with ductile properties, were compared for determination of critical fracture mechanics parameters upon the onset of ductile crack growth. In the case of ductile, or mixed ductile/cleavage fracture at temperatures well above nil-ductile, or at lower impact energies, it is sometimes difficult to distinguish crack initiation from ME signal, while the integrated magnetic emission signal (MF) sometimes has a slower changing rate. Results were also obtained by applying the potential drop technique with single specimen HSLA steels and by evaluating the R-curve. Initiation of stable crack growth may also be depicted from local minimum (or maximum) of potential drop value and it may not give clear local extreme values when conditions of fracture change from brittle to ductile. Alternatively, if the change of slope in the PD-t diagram can be used to evaluate critical crack behaviour, compared to similar changes of slope in the MF-t diagram, it may provide a better understanding of both.

Application of Two Independent Measurement Techniques for Determination of Ductile Crack Growth Initiation

Charpy impact testing of the high strength low-alloyed steel has been performed by simultaneously recording two independent signals. The magnetic emission (ME) and potential drop (PD) techniques were used to determine critical crack initiation properties on standard three-point pre-cracked bending specimens at room temperature. Both signals (ME and PD) were recorded by digital oscilloscope with a high sampling rate and compared, with the purpose of more precise identification of critical fracture mechanics parameters determining the onset of ductile crack growth.

Finite element modelling of creep process - steady state stresses and strains

Finite element modelling of steady state creep process has been described. Using an analogy of visco-plastic problem with a described procedure, the finite element method has been used to calculate steady state stresses and strains in 2D problems. An example of application of such a procedure have been presented, using real life problem - cylindrical pipe with longitudinal crack at high temperature, under internal pressure, and estimating its residual life, based on the C*integral evaluation.