Y. J. Chao

Radial flexibility of welded-pad reinforced nozzles in ellipsoidal pressure vessel heads

Abstract A thin shell analysis was performed to investigate the radial flexibility of welded-pad reinforced nozzles in ellipsoidal pressure vessel heads. A comparison of the flexibility of such a nozzle to that of an integrally reinforced nozzle was made. The effect of size and thickness on flexibility was studied. A parametric study was performed for welded-pad reinforced nozzles in ASME 2:1 pressure vessel heads. Results are presented as flexibility factors which are functions of nondimensional nozzle-vessel geometries.

Stress concentration and flexibility factor of nozzles in ellipsoidal pressure vessel heads subject to external moment

Abstract Ellipsoidal heads are commonly used as end closures for cylindrical pressure vessels in chemical and process industries. In the design of nozzles in pressure vessels, stress and flexibility of the nozzle-vessel structure are important factors to be considered. In this paper, a parametric study of radial nozzles in ellipsoidal vessel heads, when the nozzles are subjected to overturning moments, is presented. The computer program KSHEL, which is based on elastic thin shell theory, was utilized. Stress concentration factors and flexibility factors were determined and are presented as graphs of dimensionless nozzle-vessel parameters.

Numerical Simulation of Cup-Cone Fracture in a Round Tensile Bar

Cup and cone fracture for a round tensile bar under tension is analyzed numerically using finite element method. Finite strain analysis with general solid elements is performed to simulate the development of necking, flat cracking at the center of the bar, propagation of the flat crack towards the free surface, and then slant 45° cracking leading to the final separation of the tensile bar. The key fracture stages and parameters in the simulation of this entire process include (a) a fracture criterion for the flat cracking at the center of the bar, (b) flat crack propagation using a constant crack-tip-opening-angle and/or displacement (CTOA/CTOD), (c) a criterion for determining the transition from flat cracking to slant cracking, (d) slant crack propagation using a constant shear CTOA/CTOD. Details of the numerical process are provided.Copyright

Thrust load on welded-pad reinforced nozzle in an ellipsoidal pressure vessel head

Abstract A thin shell analysis is performed to investigate the stress distribution of welded-pad reinforced nozzles in ellipsoidal pressure vessel heads. A comparison of the structural behavior of such a nozzle to an integrally reinforced nozzle shows that a sixty percent higher stress is present in a typical nozzle-vessel geometry. A parametric study is performed for welded-pad reinforced nozzles in ASME 2:1 pressure vessel heads. Results are presented as stress concentration factors and shear stress concentration factors as functions of nondimensional nozzle-vessel geometries.

An Engineering Method for Assessing the Effects of Neutron Irradiation on the Fracture Toughness of RPV Steels

Neutron irradiation degrades the reactor pressure vessel (RPV) steels. As the steel degrades, the mechanical properties of the material also change which affect the crack tip stress fields. In this article, we show that reduction of the “fracture toughness” of the RPV steels due to neutron irradiation can be interpreted by a change of an “equivalent constraint” due to material property changes. Using the J-A2 two-parameter fracture methodology to quantify the crack tip stress fields and a critical stress fracture criterion, the methodology is applied to the interpretation of fracture toughness test data from un-irradiated and irradiated RPV steels.Copyright

Elastic-Plastic Constraint Analysis of Semi-Elliptic Surface Cracks in X100 Pipeline Steel

In the framework of the J-A2 fracture theory, the crack driving force J and the crack tip constraint parameter A2 are used to describe the near crack tip stress and deformation fields. These two parameters, J and A2 , were calculated from three-dimensional finite element results for semi-elliptic surface cracks with various lengths and depths in X100 pipeline steel. It was found that, under a uniform far field tensile loading, A2 increases rapidly to a nearly constant value along the crack front from the free surface to the deepest part of the crack. A similar trend was found for the J-integral distribution except in the case of a semi-circular crack. In addition, for a given elliptic crack configuration, A2 showed significant J-integral dependence when the crack front approached the free surface, where a strong three-dimensional effect is apparent. On the other hand, at the deepest part of the crack, A2 converged to a constant value. Two-dimensional plane strain calculations were also performed for single edge-notched tension specimens (SENT), where the crack length corresponds to the depth of the surface crack. The constraint of these two configurations (semi-elliptic crack and SENT) were compared under the same crack driving force (J-integral). In general, the constraint at the deepest crack front of an elliptic crack is higher than that of the corresponding SENT, especially in mid- to large scale yielding condition where J-integral is relatively large. It can be concluded that using fracture toughness determined from SENT specimens to predict surface flaw stability may lead to non-conservative result.Copyright

Constraint Analysis and Failure Prediction of Line Pipes

The crack tip stress fields in X80 pipeline steels were analyzed with the J-A2 constraint theory in fracture mechanics. The relationship between the constrain parameter, A2 , and the ratio of crack length to specimen width (a/W) or pipe wall thickness (a/t) was obtained, respectively, for three-point bend, single edge-notched tension, and a part-through wall axial crack in a line pipe subject to internal pressure. For engineering purposes, the equivalent values of a/W (or a/t) between these geometries at the same constraint level can be calculated by a conversion formula based on curve fitting. In addition, an empirical factor, which is a function of a/t and is defined as the ratio of plastic limit load to the fracture load based on J-A2 theory, was shown to be an effective method to estimate the pipeline failure stress without complicated elastic-plastic stress analysis.© 2009 ASME

Crack Growth in 18G2A Steels With Different Constraint

A constraint theory in fracture mechanics is used to analyze the test data of 18G2A steels using single edge-notched bend (SENB) specimens with various crack depth to specimen width ratios (a/W). A bending correction factor is included in the two-parameter (J-A2 ) asymptotic solution to improve the theoretical prediction of the stress field for deep cracks under large scale yielding condition, where J is the J-integral and A2 is the constraint parameter, which depends on the in-plane geometry of the cracked body (a/W). As a result, the valid region for a traditional J-controlled crack growth is extended, and the ASTM specimen size requirements for fracture toughness testing can be relaxed. In addition, it is shown that the functional dependence of J-R curves on A2 for 18G2A steels is established with test data; and the predicted J-R curves agree very well with the experimental curves. This ensures the transferability of laboratory test data to an actual structure provided the constraint level (A2 ) of the cracked structure is known or determined. This allows an appropriate J-R curve with the same constraint level to be constructed and used in flaw stability analysis of any cracked body.Copyright

Determination of J-R Curves for A285 Carbon Steel Using Normalization Method

The normalization method is adopted and extended in this paper to develop J-R curves for standard and nonstandard specimens directly from load versus load-line displacement test data without the need for online crack length measurement. A set of single edge notched bend (SENB) specimens with different crack lengths for an A285 carbon steel are tested according to the guideline of ASTM standard E1820, and the ability of normalization method is then demonstrated in determination of crack-tip constraint-dependent J-R curves for the SENB specimens. The results show that the normalization method can be used to determine J-R curves for the standard as well as nonstandard specimens. The resistance curve procedure and the basic procedure specified in ASTM E1820 are evaluated, and a modified basic procedure is also presented for determining the J-integral. Comparisons of the resulting J-R curves indicate that the modified basic procedure can be equivalent to the resistance curve procedure. To validate the normalization method, the conventional electric potential drop method is used to monitor crack growth and to determine the J-R curves for the A285 steel. Two equations used in the potential drop method, i.e. linear and nonlinear relationships between the crack length and the electric potential, are employed to determine the crack length during the fracture testing. It shows that the J-R curves determined with the normalization method are in good agreement with those based on the potential drop methods for all specimens considered.© 2006 ASME

Determination of Constraint-Modified J-R Curves for Carbon Steel Storage Tanks

Mechanical testing of A285 carbon steel, a storage tank material, was performed to develop fracture properties based on the constraint theory of fracture mechanics. A series of single edge-notched bend (SENB) specimen designs with various levels of crack tip constraint were used. The variation of crack tip constraint was achieved by changing the ratio of the initial crack length to the specimen depth. The test data show that the J-R curves are specimen-design-dependent, which is known as the constraint effect. A two-parameter fracture methodology is adopted to construct a constraint-modified J-R curve, which is a function of the constraint parameter, A2 , while J remains the loading parameter. This additional fracture parameter is derived from a closed form solution and can be extracted from the finite element analysis for a specific crack configuration. Using this set of SENB test data, a mathematical expression representing a family of the J-R curves for A285 carbon steel can be developed. It is shown that the predicted J-R curves match well with the SENB data over an extensive amount of crack growth. In addition, this expression is used to predict the J-R curve of a compact tension specimen (CT), and reasonable agreement to the actual test data is achieved. To demonstrate its application in a flaw stability evaluation, a generic A285 storage tank with a postulated axial flaw is used. For a flaw length of 10% of the tank height, the predicted J-R curve is found to be similar to that for a SENB specimen with a short notch, which is in a state of low constraint. This implies that the use of a J-R curve from the ASTM (American Society for Testing and Materials) standard designs, which typically are high constraint specimens, may be overly conservative for analysis of fracture resistance of large structures.Copyright