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Orynyak I.V.,National Academy of Ukraine | Vlasenko N.I.,Scientific and Technical Center | Kozlov V.Ya.,Scientific and Technical Center | Andrieshin Ya.A.,Ltd | And 4 more authors.
Strength of Materials | Year: 2012

The experimental equipment has been upgraded and requirements to geometrical sizes of edge notches of pipe specimens are formulated. A series of tests until fracture have been performed for static loading by internal pressure and bending moment of straight pipe specimens having longitudinal and circumferential edge notches, as well as for a pipe bend with a longitudinal edge notch. Experimental data are compared with the theoretical calculation results. The conditional boundary line between leak and break has been theoretically predicted and experimentally verified for internal pressure loading of a straight pipe with a longitudinal surface defect. © 2012 Springer Science+Business Media New York. Source

Orynyak I.,Ltd | Oryniak A.,National Technical University of Ukraine
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2015

The development of powerful commercial computer programs made the concept of J-integral as computational parameter of fracture mechanics to be a very attractive one. It is equivalent to SIF in linear case, it converges in numerical calculation and the same results are obtained by different codes (programs). Besides, it is widely thought that elastic-plastic analysis gives bigger values than elastic SIF ones what is good from regulatory point of view. Such stand was reflected in the recommended by IAEA TECDOC 1627 (February 2010) devoted to pressurized thermal shock analysis of reactor pressure vessels, where the embedded crack in FEM mesh, elastic-plastic analysis with simultaneous determination of J-integral was stated as the best option of analysis. But at that time all the most widely used software were not able to treat the residual stresses, the thermal stresses in case of two different materials. Such a contradiction between requirements and the possibilities made a lot of problems for honest contractors especially in countries where the regulator had no own experience in calculation and completely relied on the authority of international documents. This means that at that time the said recommendations were harmful. The main reason of such a situation was the absence of the carefully elaborated examples. Now the capabilities and accuracy of such software are increasing. Nevertheless, some principal ambiguities and divergences of computations results in various J-integral contours around the crack tip still exist. They are exhibited when the large plastic zone emerges at the crack tip. Other problem is influence of the history of loading and the specification of the time of crack insertion in the mesh including the time of emergence of residual stress. This paper is invitation for discussion of the accuracy and restriction of computational J-integral. With this aim the detailed analysis of some simplified 2D examples of calculation of elastic-plastic J-integral for surface crack with accounting for residual stress, thermal stress and inner pressure are performed and commented. The attainment of consensus among the engineering society for treating the outcome results is the prerequisite for practical application of computational elastic plastic J-integral. Copyright © 2015 by ASME. Source

Orynyak I.,Ltd | Oryniak A.,National Technical University of Ukraine
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2015

The consideration of a geometrical nonlinearity is a common practice for the thin-walled structures. The relevance here are two well-known cases treated in ASME codes. First one is accounting for reduction of the pipe bends flexibility due to the inner pressure. The second one is the retarded increasing (and subsequent saturation) of additional local bending stress with increasing of inner pressure in a pipe with initial cross section form distortion. In both cases the rerounding effect and decreasing of local flexibilities take place. The crack can be treated as the concentrated flexibility and it is quite natural to expect that the stress intensity factor should grow nonlinearly with applied load. Two cases of SIF calculation for 1-D long axial surface crack in a pipe loaded by inner pressure are considered here: a) cross section has an ideal circular form: b) the form has a small distortion and crack is located in the place of maximal additional bending stresses. The theoretical analysis is based on: a) the well known crack compliance method [1] and b) analytical linearized solution obtained for deformation of the curved beam in case of action of fixed circumferential stress due to pressure written in the form convenient for transfer matrix method application. It was shown that for moderately deep crack (crack depth to the wall thickness ratio is 0.5 and bigger) and typical dimensions of pipes used for oil and gas transportation (radius to thickness ratio is 25-40) and loading which can reach up to 200 to 300 MPa, the effect investigated can be quite noticeable and can lead to 5-15 percent reduction of calculated SIF as compared with linear calculation. The analytical results are supported by nonlinear FEM calculation. Source

Batura A.,Ltd | Orynyak I.,Ltd | Oryniak A.,Kharkiv Polytechnic Institute
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2014

The exact analytical approach for stress intensity factor calculation for an arbitrary shape mode I crack loaded by the polynomial stresses is proposed. The approach is based on the calculation of the crack faces displacement at given loading. The displacement field is presented as a shape function multiplied by an adjustment polynomial. At that the key problem is the solution of well-known inverse task: obtaining the stresses field at the crack faces on the base of a given displacements field. Multiply solution of such task for a whole set of certain displacements base functions (e.g., for the single terms of the adjustment polynomial) allows to get analytical expression which connects stresses and displacements fields. The original semi-analytical technique for integration with subsequent differentiation of well-known singular integral equation of the flat crack problem is developed. The excellent accuracy of the method is confirmed for an elliptic crack as well as for a rectangular one in the infinite 3D body. New results are given for an inner semi-elliptic crack in the infinite body which surfaces are loaded by polynomial stresses up to the 6th order. The importance of choosing the appropriate shape function is demonstrated. Copyright © 2014 by ASME. Source

Oryniak A.,National Technical University of Ukraine | Radchenko S.,Ltd | Orynyak I.,Ltd
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2013

Brittle strength calculation of RPV nozzle is the central point of the integrity assessment of the reactor pressure vessel when extending its life. The important part of this calculation is a determination of the stress intensity factor, SIF, for the postulated crack of partly elliptical form in a nozzle under inner pressure, bending moments (from the main circulating pipe) and difference of temperatures. In this paper we use method of influence functions as the most convenient one for solution of similar tasks. Eight basic laws of the crack surface loading are introduced which account for real stress distribution in the depth and length direction of a crack including the jump of stress between cladding and main metal due to the difference in the thermal expansion factors. To determine the dimensionless SIF under chosen laws of loading were developed the FEM models of nozzle with crack of different ratios of axes. For all possible modes (regimes) of operation were carried the detailed calculations of the temperature field in the nozzle, which were used later for determining the stress state at each time point. The stress field defined in 120 discrete points of the crack surface was treated by the method of least squares for the presention as a linear combination of eight basic load laws with defined coefficients. The procedure for determination of the temperature brittle strength margin which employs the presentation of critical values of SIF (fracture toughness) in the exponential function form is described. Copyright © 2013 by ASME. Source

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