Stojan Sedmak

The resistance to crack growth of different regions of weldments in a real structure

Abstract The growth of a crack in regions of welded joints, made from high-strength low-alloy steel, has been analysed on specimens and full-scale structures. The effect of such crack growth on the safety of welded steel pressure vessels has been considered.

Numerical and Analytical Modelling of Elastic-Plastic Fracture Mechanics Parameters

Structural integrity and service reliability depend on the fracture resistance of a material. Cracks in the material are the locations of stress concentration, and elastic-plastic deformation can occur causing the development of mixed-mode type of fracture ahead the crack tip. Crack behavior in the elastic-plastic region is analyzed applying numerical and analytical simulation based on fracture mechanics parameters, characterizing the response of the material at the crack tip. Numerical and analytical results are compared with the corresponding experimental results obtained in previously performed fracture mechanics tests with standard single-edge notch bending – SEN(B) specimens. The comparison shows an acceptable level of agreement, enabling application of the proposed numerical model of crack growth in the mixed-mode fracture analysis for structural integrity assessment.

Crack significance evaluation with special reference to welded structures

Fracture mechanics parameters (stress intensity factorKI and its critical valueKIc, crack opening displacement, and the contourJ integral) are originally defined for static and quasistatic loading conditions. On the basis of theoretical background, standard test methods for the experimental determination of their specific values were developed. Structural integrity analysis requires the extension of application of these parameters to other types of loading. We propose new parameters for stress corrosion (stress corrosion cracking thresholdK1sco), for cyclic loading (stress intensity factor range ΔK and fatigue threshold ΔKth), and for creeping at elevated temperatures (C* andCt integrals). The structural integrity of welded structures is mainly affected by cracks in welded joints. We demonstrate the practical application of fracture mechanics parameters to the evaluation of structural integrity under the above-mentioned loading conditions.

Fracture Mechanics and Non-Destructive Testing for Structural Integrity Assessment

Fracture mechanics parameters can be applied for the analysis of failures of structures, and also for prevention of failures when defects in a structure are detected and defined. The approach is presented through stages: detection of defects, stress-strain analysis of loaded component, characterization of material properties required for structural integrity assessment and application of convenient procedure. In this way the decision about next use of defective component can be made (to continue the operation, increased care by inspection, exclusion the component from next service, with eventual repair, if possible). Special attention is paid to the most popular testing procedures for crack resistance parameters.

Structural Integrity at Elevated Temperatures - Residual Service Life Evaluation

The essence of creep deformation is that the plastic strain is time dependent and can occur at stresses which in an ordinary tensile test would be below the yield stress. The mechanism of creep relies on the thermally activated passage of atoms over an activation barrier which is characterized by an activation free energy.

Structural Integrity Assessment by Local Approach to Fracture

Local approach to fracture has been developed for complete understanding of fracture mechanism [1,2], including the material degradation process. This approach combines theoretical, experimental and numerical solutions, enabling less conservative assessment of crack significance and residual stress.

Micromechanical Coupled Study of Crack Growth Initiation Criterion in Pressure Vessel Steel

We present results of the combined design-theoretical investigation of the mechanism of crack growth at the onset of ductile fracture of NPP reactor pressure vessels. Micromechanical approach to the prediction of ductile fracture has been applied, according to which the volume fraction of voids in the deformed material is determined by the finite-element method. On the basis of CT-specimen tests and known damage parameters, determined for smooth spherical specimens, we propose a micromechanical criterion of crack growth initiation for ductile fracture.

ANALYSIS OF THE TRANSFERABILITY OF MICROMECHANICAL PARAMETERS OF DAMAGE OF STEEL UNDER THE CONDITIONS OF DUCTILE-FRACTURE INITIATION

For the description of the mechanical behavior of a cracked material, we use a macromechanical model of fracture based on the well-known criterion of plastic yield for porous materials. This model is realized as a computational scheme with the help of the finite-element method. The critical values of the micromechanical parameters of damage are experimentally measured and numerically evaluated for smooth specimens with an aim to predict the onset of plastic yield in a compact cracked specimen. For a low-alloy pressure-vessel steel considered as an example, we show that the proposed micromechanical model can be used for the prediction of the onset of fracture processes in both smooth and compact specimens.

Finite element method of thin shell J integral evaluation

ABSTRACT The finite element method has been applied on thin shell J integral evaluation. Thin shell J integral has been recently derived using Gurtins approach, which has been shown to be path dependent, unless the crack is placed along a generatrix of a cylindrical shell. In any other case two line and one surface integral should be added to the Rices J integral in order to regain its path independency. Anyhow, even in the case of path dependency of Rices J integral, evaluation of the complete integral expression is relatively an easy task in the computational mechanics, having in mind finite element method. Therefore, in this paper the finite element method analysis of the thin shell J integral has been presented, together with an example regarding axial crack in the cylindrical shell.

Integrity and life estimation of turbine runner cover in a hydro power plant

This paper presents integrity and life estimation of turbine runner cover in a vertical pipe turbines, Kaplan 200 MW nominal output power, produced in Russia, and built in six hydro-generation units of hydroelectric power plant „Đerdap 1” in Serbia. Fatigue and corrosion-fatigue interaction have been taken into account using experimentally obtained material properties, as well as analytical and numerical calculations of stress state, to estimate appropriate safety factors. Fatigue crack growth rate, da/dN, was also calculated, indicated that internal defects of circular or elliptical shape, found out by ultrasonic testing, do not affect reliable operation of runner cover.