Search

This study aims at investigating the effects of local wall thinning on the cyclic plasticity behaviours of 90° back-to-back pipe bends, which are commonly utilised in flexible piping systems within plant facilities. Over time, the prolonged operation of these pipe bends may lead to wall thinning and a deterioration of their structural integrity. To understand the effects of such wall thinning, a 3D finite element analysis is conducted. Load interaction diagrams are generated using the Linear Matching Method, which considers the cyclic plasticity behaviours of shakedown and ratcheting. The investigation includes pipe bends with three different locations of local wall thinning: intrados, extrados, and crown. The pipe bends are subjected to steady internal pressure and various cyclic bending moments, including in-plane closing, in-plane opening, and out-of-plane bending. Moreover, comprehensive parametric studies are carried out for the pipe bend structures, varying the wall thinning rates. Semi-empirical formulas are derived based on the study results, enabling the prediction of limit load, shakedown limit, and ratchet limit concerning the position of the local wall thinning. Results and discussions have demonstrated the most detrimental combination of wall thinning type and loading on the structural integrity of the pipe bends and have provided valuable insights into the problems associated with deteriorated pipe bends. • The effects of local wall thinning on the cyclic plasticity behaviours of 90° back-to-back pipe bends are investigated. • The pipe bends with three different locations of local wall thinning is considered: intrados, extrados, and crown. • Shakedown and ratchet limit boundaries are analysed using the Linear Matching Method. • Critical loading combinations and the specific wall thinning locations that may lead to adverse effects are identified. [ABSTRACT FROM AUTHOR]

Local wall-thinning of pipe is a common volume defect in application, which would cause pipeline leakage or rupture under complicated pipeline loads. The finite element analysis method was used to analyze the limit load of Pipe Bend with Local Wall-Thinning defect (PB LWT) under internal pressure, bending moment, torsion moment, axial force, and their combinations. The limit load empirical equations of PB LWT under single load and complex loads were fitted. According to the allowable pipeline load on equipment nozzles, the influences of torsion moment and axial force on the load bearing capacity of PB LWT were quantitatively analyzed. It is found that the load bearing capacity is reduced by an average of 1 % for medium and low-pressure equipment. For high-pressure equipment, the load bearing capacity is reduced by 10.07 %∼20.90 % under the influence of torsion moment. Under the effect of axial force, the load bearing capacity is reduced by 2.01 %∼12.40 %. • The limit loads of bends with extrados local wall-thinning defect are analyzed under internal pressure, bending moment, torsion moment, axial force, and their combinations. • Empirical equations for limit load of bend with defects are worked out for complex loads. • Torsion and tension can decrease the load-bearing capacity of bends by up to 10 %∼20 %. [ABSTRACT FROM AUTHOR]

The mechanically lined pipe, which consists of a carbon steel carrier and a thin-walled corrosion-resistant alloy liner, has proved to be a practical way to prevent the pipeline from internal corrosion. However, the local dent caused by third-party falling objects severely threatens the integrity of the lined pipe, especially the more fragile liner. This work investigates the influence of a local dent on the plastic responses and stability of a lined pipe system during bending. After validation of the numerical framework with the experimental results of a 12-inch lined pipe, the indentation and bending responses of two typical scenarios, namely, TDI and LDI, are examined. It is found that the liner could maintain a certain moment level after reaching the maximum instead of collapsing promptly. Furthermore, classic diamond buckling mode is observed in the TDI case, while the LDI case exhibits a twin triangular-shaped buckling mode. In addition, extensive parametric studies regarding important variables of the full-scale problem are performed. Amongst other findings, the buckling and collapse of the dented lined pipe are found to be affected by the orientation of the dent, which should be taken into account for practical dent evaluation. • Validation of the FE model with four-point bending experimental results of a 12-inch lined pipe. • Buckling and collapse analysis of dented mechanically lined pipes under bending, including two types of dents, namely TDI and LDI. • An extensive parametric study is investigated, including dent length, angle, depth, and key factors of the full-scale problem. [ABSTRACT FROM AUTHOR]

In this paper, the ratcheting behavior of SS316L seamless straight pipes under cyclic internal pressure and in-plane cyclic bending is investigated experimentally and numerically. In order to applying the simultaneous cyclic bending and cyclic internal pressure, a special fixture was designed that can be mounted on the gripers of fatigue machine and so ability to apply cyclic pressure to inside pipe specimen and apply in-plane cyclic bending on pipe specimen simultaneously. According to experimental fatigue data, increasing the cyclic bending load and cyclic pressure, increases the accumulation of ratcheting strain in hoop and axial directions. So axial ratcheting strain creates due to cyclic bending and hoop ratcheting strain creates due to cyclic pressure inside pipe specimen. Finally, the finite element analysis was performed by ABAQUS software base on the nonlinear kinematic/isotropic hardening model and the experimental fatigue data were compared with the numerical results and it was found that the numerical results are in good agreement with experimental data. • Ratcheting behavior is studied on seamless SS316L straight pipes under cyclic internal pressure and cyclic bending. • A special fixture was designed that ability to apply cyclic pressure and in-plane cyclic bending on pipe simultaneously. • The cyclic bending affected on axial ratcheting strain and cyclic pressure affected on hoop ratcheting strain. • Ratcheting behavior of elbow pipes were simulated by FEM with nonlinear kinematic/isotropic hardening model. [ABSTRACT FROM AUTHOR]

Leak-Before-Break (LBB) requirement plays a vital role in demonstrating the structural integrity of sodium-cooled Fast Breeder Reactor (FBR) piping system. Crack size at penetration (2 C S ), crack size that corresponds to detectable coolant leak rate (2 C L ), and unstable crack size (2 C G ) are the primary crack parameters associated with the LBB design of FBR piping system. The detectable leak rate through a piping system is directly related to the differential pressure (Δ P) and crack size. The design pressure of FBR piping systems is relatively low, resulting in a high value of C L. Considering the need for an optimum LBB design of the FBR piping system, an experimental approach is adopted to arrive at a realistic estimate of C L. Based on the experiments, a power-law type correlation can be derived to predict the value of C L as a function of Δ P. The evidence from this study points towards the conservatism present in the current FBR LBB design philosophy as per RCC-MRx A16. These observations have several implications for achieving an optimum LBB design criterion for an FBR piping system. • A new approach to arrive realistic crack size corresponding to detectable leak rate. • Conservatism present in the current A16 philosophy is highlighted. • Simple correlation to predict C L as a function of differential pressure is proposed. • For a given leak rate, the crack size required as per A16 methodology is more. [ABSTRACT FROM AUTHOR]

The fabrication of pipe bends by rotary bending results in geometrical imperfections as manifested by cross-sectional ovality and wall thickness variations which affect their load carrying capacity and hence their performance during service. Previous studies ignored the exact distribution of these imperfections and adopted either ideal pipe bend shape (IB) or assumed the distribution these imperfections in their analysis by a simplified assumed shape model (AS). The objective of the present work is to investigate the effect of the presence of the inherited geometrical imperfections as obtained from the rotary pipe bending process of 90° pipe bends including the presence of residual stresses on their load carrying capacities as compared with the IB and AS models. The present work is conducted by means of non-linear finite element modeling considering both material and geometrical non-linearities. Rotary pipe bending process is simulated with basic tooling configuration to obtain the as-fabricated 90° pipe bend after accounting for springback. Results of this step were verified against published experimental results and analytical solutions. The pipe bend was then subjected to different combinations of in-plane moment and internal pressure in order to construct a comparative limit load diagram. The IB model for the pipe bend results in non-conservative results, while AS model results in acceptable results compared to results of the as-fabricated shape. Results have also shown that presence of residual stresses improves the pipe performance under in-plane closing bending moment. • Ideal pipe bend model provides non-conservative results for limit loads. • Tensile tangential residual stresses prevent ovalization at low pressures. • Geometrical imperfections lowers limit loads of Annealed As-Fabricated pipe bends. • As-Fabricated pipe bend has higher Collapse and Instability Moments. [ABSTRACT FROM AUTHOR]

Flaws due to stress corrosion cracking have been detected in stainless steel or nickel-based alloy dissimilar metal weld joints of piping systems in nuclear power plants. Generally, stainless steel and nickel-based alloy are ductile materials with high toughness, and the failure mode of flawed pipes made of ductile materials is expected to be net-section plastic collapse. Failure bending moment of a ductile pipe containing a circumferential flaw is predicted using the net-section stress approach according to ASME Code Section XI as a limit load criterion. However, in the current code, the failure bending moment can only be adopted for a pipe containing a single circumferential flaw with constant depth, although many flaws detected actually are multiple flaws, and the flaw shapes are generally complex. In this study, a failure estimation method for pipes containing multiple circumferential flaws with complex shapes was proposed. Furthermore, failure experiments were performed for 8-inch diameter Schedule 80 stainless steel pipes containing two circular circumferential flaws. The failure bending moments obtained from the experiments were compared with the estimated results. Based on the experimental results, it was concluded that the proposed failure estimation method satisfactorily represents the failure behavior of the pipes and can be applied in engineering application. [ABSTRACT FROM AUTHOR]

Ratcheting behavior of pressurized straight pipes made of Z2CND18.10 austenitic stainless steel alloy was evaluated under in-plane bending cycles and under displacement- and force-controlled conditions by means of the Ahmadzadeh-Varvani (A-V) kinematic hardening rule. Ratcheting tests were conducted on straight pressurized steel pipes under bending cycles. Ratcheting response of steel pipes was also numerically simulated through inclusion of the A-V framework into ABAQUS software. The simulated ratcheting results along the axial and circumferential directions were found in good agreement with those of experimental values in the pressurized steel pipes. It was found that variables including the magnitude of internal pressure, cyclic bending moments, loading spectra, material properties, and FE element types, size and numbers noticeably impacted simulated ratcheting values. Ratcheting boundaries were further determined through coupling design codes of KTA/ASME and RCC-MR plus committee of three-dimensional finite element stress evaluation (C-TDF) with the A-V model. Both experimentally obtained ratcheting boundaries and those determined by design codes agreeably revealed the shakedown occurrence. • Ratcheting behavior of straight pipe under different controls is simulated by A-V model. • Influence of different factors on ratcheting behavior of pressurized straight pipe is studied. • Ratcheting boundary of shell and solid element types used for straight pipe is determined. [ABSTRACT FROM AUTHOR]

Plastic collapse loads of 90° pipe bends with structural defects subjected to in-plane closing bending are systematically examined in this study using three dimensional nonlinear finite element (FE) analyses. The analyses consider the large strain formulation option with the pipe bend material assumed to be of the elastic perfectly plastic (EPP) type. Pipe bends are analyzed by changing the ovality at the mid-section which is varied from 0% to 20% by 5% per step. The through-wall axial crack (TAC) parameter at the pipe bend's crown is varied from 0 to 1 by 0.2 per step. Moment rotation curves are drawn for all simulated pipe bend models and plastic collapse moment (PCM) loads are determined from the corresponding moment rotation curves using the Twice-Elastic-Slope (TES) methodology as recommended by clause NB-3213.25 in section III of ASME B&PV code. Both ovality and TAC of pipe bends significantly affects their plastic collapse moments (PCM) and this almost doubles when ovality and TAC act together. The observed effect of ovality and TAC on pipe bends having thin walled and short bend radius does not significantly change with increasing wall thickness and bend radius. The present FE analysis is validated using experimental PCM loads of pipe bends available in open literature. The crack closing effect of pipe bends during closing bending is examined. Based on the analysis, an improved structural integrity assessment equation for shape imperfect pipe bends with TAC is developed for in-plane closing bending moment loads. • The plastic collapse moments of shape imperfect and throughwall axially cracked pipe bends are examined. • The analysis of pipe bends considers the large strain formulation option with elastic perfectly plastic (EPP) type. • ABAQUS commercial nonlinear simulation software was used to perform the collapse load assessment of pipe bends. • Both ovality and TAC of pipe bends significantly affects their plastic collapse loads (PCM). • An improved structural integrity assessment equation for shape imperfect pipe bends with TAC is developed. [ABSTRACT FROM AUTHOR]

Elastic-plastic analyses based on finite element method are widely applied to simulate the nonlinear behavior of materials. When an analysis is conducted by implicit method, the stress values are generally updated with a time increment by using the so-called return mapping algorithm. This algorithm requires solving simultaneous nonlinear equations related to a constitutive model by an iterative solution scheme. In this study, we proposed a general method to reduce the number of unknowns in the return mapping algorithm based on the implicit function theorem. In addition, the proposed method was applied to the Gurson-Tvergaard-Needleman (GTN) model that considers the influence of damage due to nucleation and growth of microscopic void in materials in the simulation of the nonlinear behavior. After the GTN model with the proposed method was verified, elastic-plastic analysis was performed by the implicit method for a 4-point bending test of pipe with a through-wall crack. The numerical solution agreed well with the experimental result. In addition, the number of iterations to obtain converged solution was reduced. Thus, we concluded that the proposed method is useful for simulating nonlinear behavior, including void nucleation and growth in materials. • A method to reduce number of unknowns in return mapping algorithm was developed. • The method was validated by comparison of analysis with test of ductile fracture. • By using the method, finite element analysis can be conducted efficiently. [ABSTRACT FROM AUTHOR]

Considering the effect of carbide coarsening and cavity nucleation and growth on the creep damage of Fe–Cr–Ni-based austenitic stainless steels, an improved creep damage constitutive model is established based on the hyperbolic sine function. The material parameter optimisation procedure and the associated ABAQUS user subroutine CREEP of the model are also developed. In addition, the improved model is calibrated and validated by comparing the extrapolated failure times for the three creep damage models, and applied to an idealized pipe bend to evaluate the inelastic strain limits in the ASME-NH code. The results show that the proposed model is capable to reflect the creep behaviour of a hot outlet manifold steel under different stress levels at 890 °C. During the long-term creep exposure of the manifold component, the limits of 1% creep membrane strain and 2% creep membrane strain + bending strain ruled in ASME-NH code may not be conservative. If only the cumulative inelastic strain is considered in the structural design, membrane strain is the primary rule for investigating the failure of the pipe bending. • Tensile creep tests on 20Cr32Ni1Nb steel were performed at 890 °C. • An improved sine-hyperbolic CDM constitutive model was developed. • The strain linearization method was used to calculate the membrane strain and the membrane + bending strain. • The limits of 1% creep membrane strain and 2% creep membrane strain + bending strain ruled in ASME-NH code may not be conservative for the pipe bending. [ABSTRACT FROM AUTHOR]

This study is concerned with assessing the structural integrity of power plant pipes that operate under creep and fatigue conditions. In the process of manufacturing pipe bends, it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross-section of the bend also becomes non-circular due to the bending process, which tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of a pipe bend on stress-strain redistributions and creep-fatigue crack growth rates was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bends was performed for different stages of lifetime while considering various loading conditions. As a result, it was determined that the combined effect of ovality and thinning cross-sections is significant and is a function of creep time. The tensile and creep tests were performed for determination of the main mechanical properties of the material of the pipe bends after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross-sections, as well as operating time. The creep-fatigue crack growth rate tests were performed on a specially designed program test-cycle. A comparison demonstrates that the tensile properties of 12Cr1MoV steel were degraded by long-term high-temperature service, whereas a comparison of the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature was used in the form of a creep stress intensity factor. [ABSTRACT FROM AUTHOR]

Ratcheting deformation is studied on straight pipe made of Z2CND18.12 N stainless steel with local wall thinning subjected to constant internal pressure and reversed bending using finite element analysis. The local wall thinning is located at the center of straight pipe, whose geometry is rectangular cross-section. The effect of depth, axial length and circumferential length on the ratcheting behavior of straight pipe is studied in this paper. Three-dimensional elastic-plastic analyses with ANSYS employed Chen–Jiao–Kim (CJK) kinematic hardening model is carried out to evaluate structural ratcheting behaviors. Results indicate that ratcheting strain is along the center of straight pipe extending to the two ends. The ratcheting strain occurs mainly at hoop direction. Axial ratcheting strain is relatively small. The effects of the depth, axial length and circumferential length of local wall thinning on the ratcheting response are discussed by CJK model. [ABSTRACT FROM AUTHOR]

The combined effect of ovality and variable wall thickness on collapse load of pipe bends under in-plane opening bending moment with internal pressure was investigated using finite element limit analysis considering large geometric change effect. The material was assumed to be elastic–perfectly plastic. The collapse moments were obtained from moment-rotation curves drawn for each bend using twice-elastic-slope method. Thickening/thinning effect on collapse load is very minimal and can be neglected. The effect of ovality is significant with the reduction of collapse load by a maximum of 39.1% for the maximum internal pressure and ovality considered. Based on the finite element results, a mathematical expression is proposed to determine collapse load of the pipe bends with ovality and this equation has been verified with experimental collapse loads. [ABSTRACT FROM AUTHOR]

Abstract: Pipe bends or elbows are commonly used components for nuclear power plant piping system. In service, these piping components are subjected to internal pressure in addition to bending loads and the internal pressure is known to have a significant effect on the load carrying capacity of these components. Hence, a systematic study was carried out to investigate and quantify the effect of internal pressure on the fracture behaviour of elbows used in nuclear power plant piping system. Fracture studies were conducted on five 219 mm diameter carbon steel elbows with and without internal pressure under in-plane opening moment. The investigations have shown that the presence of a circumferential notch at the intrados has a more detrimental effect on the fracture behaviour of the elbow, when compared with the presence of an axial notch at the intrados. It is also found that internal pressure plays a significant role in reducing the ovalization. [Copyright &y& Elsevier]

In the present work, a systematic and detailed investigation on the plastic collapse load of 90° structurally defected pipe bends under in-plane opening bending was performed using three-dimensional finite element (FE) analyses. The analysis considers the material behavior as elastic-perfectly plastic (EPP) with a large strain formulation option. The structural defects in the pipe bend models included were ovality at mid-portion and throughwall axial crack (TAC) at the crown portion. Ovality varied from 0 to 20 % with 5 % increment at each step while the normalized throughwall axial crack (TAC) parameter length a/D from 0 to 1 with an increment of 0.2 in each step. As per the recommendation of clause NB-3213.25 in Section III of ASME B&PV code, Twice-elastic slope (TES) technique was extensively used to determine the plastic collapse moment (PCM) for all simulated pipe bend models from their corresponding reaction moment versus angular rotation curve. The analysis illustrated that both ovality and throughwall axial crack (TAC) significantly affected PCM load of pipe bends and when combined, the effect was almost double. In pipe bend models with thin walls and short bend radius, both ovality and TAC were found to be more vulnerable and the effect decreased with an increase in bend radius and thickness. The experimental results for pipe bend's plastic collapse moment available in open literature were used to validate the present finite element (FE) procedure and a new improved structural assessment correlation proposed for pipe bends with ovality and TAC defects. • A general non-linear finite element package ABAQUS was used to perform collapse load analysis of shape imperfect pipe bends. • The Twice-elastic-slope method was used to calculate the plastic load from moment rotation curves as recommended by ASME. • The behavior of pipe bends with throughwall axial crack length a/D = 0.1 was almost identical with the reference models. • A new improved structural assessment correlation proposed for pipe bends with ovality and throughwall axial crack defects. [ABSTRACT FROM AUTHOR]

Abstract: Pipe bends represent geometric discontinuities in the steam pipe systems of power plants, therefore understanding the behaviour of these potential locations of weakness is of great industrial importance for component inspection, design and analysis. Due to the high operating temperatures encountered, the failure mechanism of creep is a justified concern. Furthermore, while the geometry of pipe bends appears to be simplistic, the manufacturing process employed results in variations to the critical dimensions of the pipe bends. It is these variations in geometry that can cause potentially significant differences in peak steady-state rupture stress magnitude (approximately 48% in some of the cases considered in the present work). Through analysis of industrial data, several novel non-dimensional parameters have been established, allowing for (with suitable constraint equations depending on the type of bends analysed) the approximation of the complexity of pipe bend geometry in only a few dimension factors. Using these factors, systematic finite element analysis (FEA) studies may be completed with these non-dimensional parameters taking account of a range of geometry variation. Using this philosophy, the stress states and failure lives of pipe bends of the same type (i.e. Hot Reheat or Main Steam) with similar, but not identical, dimensions may be estimated and compared using approximations of the peak rupture stress function. By way of example, this procedure is applied to Main Steam and Hot Reheat type 90° pipe bend geometries. The accuracy of interpolation for the stress function is also analysed, along with comments on failure locations and possible future improvements. [Copyright &y& Elsevier]

Abstract: The comparison of limit load based on small displacement limit analysis and collapse load based on large displacement analysis for shape-imperfect pipe bends, under combined in-plane closing bending and an internal pressure, were carried out using finite element method. The limit and collapse moments were obtained from moment–rotation curves drawn for each model. Twice-elastic-slope method was used to obtain collapse load. The effect of thinning on limit and collapse moments are minimal and hence thinning need not be considered for the analysis of pipe bends. The influence of ovality on both limit and collapse loads for most of the cases considered are significant. Comparison of effect of ovality on limit and collapse loads reveals that the determination of collapse load is preferable when ovality is included in the analysis of pipe bend. A closed-form solution is presented to include ovality in the determination of the collapse load of pipe bends. [Copyright &y& Elsevier]

Abstract: Approximate limit and plastic collapse load solutions for un-reinforced mitred bends under internal pressure and under bending are proposed in this paper, based on three-dimensional finite element analysis and approximate solutions for smooth bends. Solutions are given for single- and multi-mitred bends (mainly for single and double segmented bends) with the pipe mean radius-to-thickness ratio (r/t) ranging from r/t = 5 to r/t = 50, and the bend radius-to-mean radius ratio (R/r) from R/r = 2 to R/r = 4. Internal pressure, in-plane bending and out-of-plane bending loads are considered, but not their combination. It is found that the essential features of limit and plastic collapse loads for mitred bends are similar to those for smooth bends, and thus existing solutions for smooth elbows can be used to construct limit loads and plastic collapse for mitred bends. [Copyright &y& Elsevier]

Abstract: A global limit load solution for rectangular surface cracks in plates under combined end force and cross-thickness bending is derived, which allows any combination of positive/negative end force and positive/negative cross-thickness moment. The solution is based on the net-section plastic collapse concept and, therefore, gives limit load values based on the Tresca yielding criterion. Solutions for both cases with and without crack face contact are derived when whole or part of the crack is located in the compressive stress zone. From the solution, particular global limit load solutions for plates with extended surface cracks and through-thickness cracks under the same loading conditions are obtained. The solution is consistent with the limit load solution for surface cracks in plates under combined tension and positive bending due to Goodall & Webster and Lei when both the applied end force and bending moment are positive. The solution reduces to the limit load solution for plain plates under combined end force and cross-thickness bending when the crack vanishes. [Copyright &y& Elsevier]

Abstract: Piping elbows under in-plane bending moment are vulnerable to cracking. The crack initiates at the surface and eventually reaches through the thickness and may lead to failure. The structural integrity assessment requires knowledge of the limit load. Limit load solutions for elbows with through-wall crack configurations are available in the open literature. But solutions for surface crack are not available. This paper presents a closed form expression for the plastic collapse moment (PCM) of 90°, long radius elbows with circumferential surface cracks at the intrados, under in-plane bending moment. The expression is derived, based on the results of non-linear (geometric and material) FE analyses covering a wide range of geometries and crack sizes. These plastic collapse moments evaluated herein will help in structural integrity assessment. [Copyright &y& Elsevier]

Abstract: The Linear Matching Method is used to create the shakedown limit and limit load interaction curves of 90° pipe bends for a range of bend factors. Two load cases are considered i) internal pressure and in-plane bending (which includes opening, closing and reversed bending) and ii) internal pressure and a cyclic through wall temperature difference giving rise to thermal stresses. The effects of the ratios of bend radius to pipe mean radius (R/r) and mean radius to wall thickness (r/t) on the limit load and shakedown behaviour are presented. [Copyright &y& Elsevier]

Abstract: In this paper, the J integral was calculated for semi-elliptical surface cracks in pipes under bending using three-dimensional finite element analysis. The computations were performed for elastic and elastic-plastic behaviours. For the elastic case, the numerical results allowed the extrapolation of shape functions for analytical determination of the J integral. The results are in a good agreement with those in the literature if the ratio between the radius and the thickness of the pipe (R/t) is from 1 to 10. The analysis was extended to values of the ratio R/t higher than 10. For the elastic-plastic, the numerical results are in good agreement with the analytical solution found in the literature for thick pipes (R/t ≥ 10). The effect of the ratio R/t becomes sensible when the ratio of the applied moment to the moment of reference (M/Mor) exceeds 0.9. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a comparison of the plastic collapse loads from experimental in-plane bending tests on three 90° single un-reinforced mitred pipe bends, with the results from various 3D solid finite element models. The bending load applied reduced the bend angle and in turn, the resulting cross-sectional ovalisation led to a recognised weakening mechanism. In addition, at maximum load there was a reversal in stiffness, characteristic of buckling. This reversal in stiffness was accompanied by significant ovalisation and plasticity at the mitre intersection. Both the weakening mechanism and the post-buckling behaviour are only observable by testing or by including large displacement effects in the plastic finite element solution. A small displacement limit solution with an elastic-perfectly plastic material model overestimated the collapse load by more than 40% and could not reproduce the buckling behaviour. The plastic collapse finite element solution, with large displacements, produced excellent agreement with the experiment. Sufficient experimental detail is presented for these results to be used as a benchmark for analysts in this area. Given the robustness of non-linear solutions in commercial finite element codes and the ready availability of computing resources, it is argued that pressure vessel code developers should now be recommending large displacement analysis as the default position for limit and plastic collapse analyses, rather than expecting engineers to anticipate weakening mechanisms and related non-linear phenomena. [ABSTRACT FROM AUTHOR]

Abstract: In order to obtain a precise failure assessment curve (FAC) in the R6 defect assessment procedure, it is necessary to evaluate the J-value of cracked components. The reference stress method can be used for estimating J-values. However, the accuracy of estimation depends on the limit load used for evaluating the reference stress. In this study, the applicability of several limit load solutions was investigated through comparison with the results of elastic-plastic finite element analyses (FEA). A pipe containing a circumferential surface crack was analyzed under pure bending load. Six materials used in nuclear power plants were assumed. It was shown that the reference stress method is valid for FAC evaluation. The maximum non-conservativeness caused by using the reference stress method is less than 20% compared to the results obtained by FEA. [Copyright &y& Elsevier]

Abstract: Stress-based life prediction techniques are commonly used to estimate the failure life of pressurised pipe-related components, such as welds and bends, under creep conditions. Previous research has shown that reasonable life predictions can be obtained, based on the steady-state peak stresses, compared with the life predictions obtained from creep damage modelling. In this work, a series of parametric steady-state peak rupture stress functions of right-angled pipe bends with ovality are presented, which are based on the results obtained from finite element (FE) analyses, covering a number of material property and geometry parameters in practical ranges. Methods used to determine the stress functions are described. The FE analyses have been performed using axisymmetric models, subjected to internal pressure only, with a Norton creep law. Typical examples of parametric peak stress curve fitting are shown. In particular, the accuracy of the interpolation and extrapolation abilities of the stress functions is assessed. The results show that in most cases the interpolated and extrapolated peak stresses are accurate to within ±3% and ±5%, respectively. [Copyright &y& Elsevier]

This work provides detailed analyses of creep behaviors considering the influence of geometry change for pipe bends under combined bending and internal pressure loads based on the finite element method (FE). A modified Karchanov‒Rabotnov (K‒R) equation for creep damage is used in the ABAQUS user defined material subroutine (UMAT) compiled by FORTRAN. Typical results of creep stress, damage, and deformation for pipe bends are presented. Creep relaxation and damage evolution will happen during the creep procedure. The damage evolution procedure seems contrary to that of stress redistribution. There are some differences between the deformation distributions when using geometric non-linearity (GNL) and no GNL, and also the results of stress, damage, and creep failure life for pipe bends show a corresponding distinction. Phenomena of the differences of pipe ovality for pipe bends under different loading types can supply some evidence to explain the failure trait. Pipe bends are stiffened as the GNL choice hinders the deformation, so GNL has a positive effect on creep failure life prediction for pipe bends under combined loadings. • A modified Karchanov-Rabotnov equation for creep damage is used. • Creep relaxation and damage evolution happens during creep procedure. • There are some differences using GNL and no GNL. • GNL has a positive effect on creep failure life prediction for combined loading. [ABSTRACT FROM AUTHOR]

Solid particles entrained in the low liquid loading pipelines will cause pits or scratches on the pipe wall, and further lead to wall thinning or even leaks. Annular flow is a common flow pattern in pipeline. The particles in annular flow distribute in the liquid film and the gas core, and there are two existence forms for the solid particles in the gas core: wrapped in droplets and not be wrapped. In this paper, gas-particle slip was considered in a slip model in order to calculate the erosion of the particles wrapped in droplets in the gas core and a no-slip model was proposed to calculate the erosion of other particles in the gas core. The cushion effect of gas-liquid interface and the liquid film on the solid particles was equivalent to wall roughness. Finally, a simplified numerical simulation method considering the cushion effect and the gas-particle slip was developed to predict the erosion rate of vertical pipe bends. It is found that the predictions are in good agreement with the experimental data. The ratio of the solid particles wrapped in droplets to the total number of solid particles in the gas core is 0.626. Additionally, the relationship among liquid entrainment, erosion rate and liquid film thickness is investigated. The liquid entrainment and erosion rate show the same trend when the liquid film thickness changes. This trend is more obvious especially when the liquid film thickness is less than the particle diameter. [Display omitted] • The cushion effect of gas-liquid interface and the liquid film on the particles is equivalent to wall roughness. • The ratio of the particles wrapped in droplets to the total number of particles in the gas core is determined. • A simplified numerical simulation method is developed to predict the erosion rate of vertical pipe bends under annular flow. [ABSTRACT FROM AUTHOR]

The present study addresses outcomes of extensive nonlinear finite element analyses concerning a diverse geometric spectrum ranging from 30° to 180° pipe bend configurations accounting for the initial geometric deformations of the cross-sections resulting from the bending fabrication process. Pipe bends are modeled with the realistic and accurate cross-section by incorporating initial ovality and thinning/thickening (together called initial geometric imperfection) up to 20%. The modeled pipe bends are loaded by in-plane opening moment and internal pressure, considering material models of elastic-perfectly-plastic (EPP) and strain-hardening (SH). The detailed finite element results show that the presence of initial imperfection has a pronounced impact on the plastic collapse moment. At the lower magnitude of applied pressure, the initial imperfection decreases collapse moment of deformed pipe bends significantly as compared to that of circular cross-sectioned pipe bends. At the higher pressure values, the negative effect of initial imperfection decreases with increased pressure for thinner pipe bends. Based on the results of the conducted finite element analyses, governing empirical equations are formulated to predict the collapse moments of pressurized 30° to 180° thin-walled pipe bends with more realistic cross-section employing EPP and SH models. • Nonlinear analysis of 30°to 180°pipe bends with initial geometric imperfection. • Elastic-perfectly plastic and strain-hardening materials are used for analysis. • The initial geometric imperfection produces negative effect on collapse moment. • Collapse moment equations are proposed for 30°to 180°pressurized pipe bends. [ABSTRACT FROM AUTHOR]

Abstract: Approximate plastic limit load solutions for pipe bends under combined internal pressure and bending are obtained from detailed three-dimensional (3-D) FE limit analyses based on elastic-perfectly plastic materials with the small geometry change option. The FE results show that existing limit load solutions for pipe bends are lower bounds but can be very different from the present FE results in some cases, particularly for bending. Accordingly closed-form approximations are proposed for pipe bends under combined pressure and in-plane bending based on the FE results. [Copyright &y& Elsevier]

Abstract: With a quasi-three point bending apparatus, ratchetting and the ratchetting boundary were studied experimentally for a pressurized low carbon steel pipe under reversed bending. The ratchetting strain occurs mainly in the hoop direction. Multi-step bending revealed that the ratchetting rate increases with increasing loading level but reduces or even vanishes at a lower bending level imposed after a higher bending level. Ratchetting simulation was performed by elasto–plastic finite element analysis with ANSYS in which the Ohno–Wang model and some modified Ohno–Wang models were applied. By comparison with the experimental data, it is found that the Jiang–Sheitoglu model with a minor modification gives reasonable simulation. The ratchetting boundary was determined by the KTA/ASME, RCC-MR and C-TDF with the lightly modified Jiang–Sehitoglu model. By comparison with the experimental data range in this study, it is shown that the ratchetting boundary determined by the C-TDF method, with the lightly modified Jiang–Sehitoglu model, divides the shakedown region well. [Copyright &y& Elsevier]

Abstract: Two recently proposed design by analysis criteria of plastic collapse based on plastic work concepts, the plastic work (PW) criterion and the plastic work curvature (PWC) criterion, are applied to a strain hardening pipe bend arrangement subject to combined pressure and in-plane moment loading. Calculated plastic pressure–moment interaction surfaces are compared with limit surfaces, large deformation analysis instability surfaces and plastic load surfaces given by the ASME Twice Elastic Slope criterion and the tangent intersection criterion. The results show that both large deformation theory and material strain hardening have a significant effect on the elastic–plastic response and calculated static strength of the component. The PW criterion is relatively simple to apply in practice and gives plastic load values similar to the tangent intersection criterion. The PWC criterion is more subjective to apply in practice but it allows the designer to follow the development of the gross plastic deformation mechanism in more detail. The PWC criterion indicates a more significant strain hardening strength enhancement effect than the other criteria considered, leading to a higher calculated plastic load. [Copyright &y& Elsevier]

Abstract: Results are presented from an FE numerical study of the capacity of a dented pipe to withstand combined pressure and moment loading. The denting process was modelled with internal pressure applied at the design level. The pipe support was modelled by a saddle-type arrangement. The strength of the dented pipe was first assessed under pure bending, applied in such a way that the dent was either on the tension side or the compression side. The strength of the dented pipe was then assessed under internal pressure loading. Finally, the behaviour of the dented pipe under combined bending and pressure loading was assessed and interaction diagrams prepared. [Copyright &y& Elsevier]

Abstract: To investigate the relevance of the definition of the reference stress to estimate J and C* for surface crack problems, this paper compares finite element (FE) J and C* results for surface cracked pipes with those estimated according to the reference stress approach using various definitions of the reference stress. Pipes with part circumferential inner surface cracks and finite internal axial cracks are considered, subject to internal pressure and global bending. The crack depth and aspect ratio are systematically varied. The reference stress is defined in four different ways using (i) a local limit load, (ii) a global limit load, (iii) a global limit load determined from the FE limit analysis, and (iv) the optimised reference load. It is found that the reference stress based on a local limit load gives overall excessively conservative estimates of J and C*. Use of a global limit load clearly reduces the conservatism, compared to that of a local limit load, although it can sometimes provide non-conservative estimates of J and C*. The use of the FE global limit load gives overall non-conservative estimates of J and C*. The reference stress based on the optimised reference load gives overall accurate estimates of J and C*, compared to other definitions of the reference stress. Based on the present findings, general guidance on the choice of the reference stress for surface crack problems is given. [Copyright &y& Elsevier]

Piping systems are structural sets used in the chemical industry, conventional or nuclear power plants and fluid transport in general-purpose process equipment. They include curved elements built as parts of toroidal thin-walled structures. The mechanical behaviour of such structural assemblies is of leading importance for satisfactory performance and safety standards of the installations. This paper presents a semi-analytic formulation based on Fourier trigonometric series for solving the pure bending problem in curved pipes. A pipe element is considered as a part of a toroidal shell. A displacement formulation pipe element was developed with Fourier series. The solution of this problem is solved from a system of differential equations using mathematical software. To build-up the solution, a simple but efficient deformation model, from a semi-membrane behaviour, was followed here, given the geometry and thin shell assumption. The flexibility factors are compared with the ASME code for some elbow dimensions adopted from ISO 1127. The stress field distribution was also calculated. [Copyright &y& Elsevier]

Steady-state reference rupture stresses were obtained for a range of 90° pipe bends, subjected to internal pressure only, using simplified 2D axisymmetric finite element (FE) models. The bends were considered to be circular in shape and not include any ovality. Creep damage FE analyses were performed to obtain realistic failure lives and to determine the skeletal point rupture stresses, using the material properties, obtained at 640 °C, for a service-exposed CrMoV pipe steel. The effects of the normalised pipe bend dimension on the reference rupture stresses are presented. The results obtained confirm the validity of the use of the steady-state reference rupture stress in life estimation for a wide range of pressurised pipe bend geometries. The life predictions were compared with those of the corresponding straight pipes and their relevance considered. [Copyright &y& Elsevier]

The problem of limit analysis for a cylinder–cylinder intersection forming a single mitred pipe bend subject to in-plane bending has been investigated. Lower bound analysis with new equations of force and moment equilibrium together with a higher number of parameters resulted in more stability as compared to a previous analysis of the same problem [PhD Thesis, The University of Manchester, 1991]. Concurrently, abaqus finite element plastic collapse moments were obtained as upper bounds to the problem. Two sets of results were compared, showing good agreement with each other. It could be finally concluded that the true limit moments are bounded in between. [Copyright &y& Elsevier]

This paper describes the results of a finite element investigation into the effects of bending on the creep failure behaviour of an internally pressurised thick walled pipe weld. The material properties used were those for a service-exposed 1/2Cr1/2Mo1/4V:2 1/4Cr1Mo weldment at 640 °C. The peak rupture stresses in each material zone, which were obtained from steady-state analyses, were used to predict failure lives for a range of bending load levels. The rupture stresses showed very little variation across the heat-affected zone, in the dominant stress regions, for low and moderate bending loads. However, the variation is more significant when the bending load is high but still within the allowable limits of design codes. The results obtained indicate that, in addition to the pressure, there is a higher probability of type IV cracking occurring, when an external (system) bending load is present. The results obtained were compared with those obtained for the uniform end load cases. [Copyright &y& Elsevier]

Approximate J-estimation methods including the effects of weld mis-match, for a circumferentially surface-cracked pipe with an internal, constant-depth and finite-length surface flaw in pure bending are developed. The methods are based on principles described by Rahman and Brust. Closed-form J-solutions in terms of elementary functions are derived from classical beam theory combined with an equivalence criterion. Several numerical examples of welded joints are presented to verify the proposed methods. [Copyright &y& Elsevier]

To a first approximation the end effects are considered, which arise due to the elastic junction of a pipe bend with other elements of a piping system. It is emphasized that Karman''s assumption about inextensibility of the mid-surface in the circumferential direction can be applied only after exclusion of the circumferential force from the equilibrium equations. For the first time, the general solution for the end effect at a pipe bend is derived analytically, allowing the elastic pipe bend to be considered as only one element in a piping system. Examples are given for calculating the quarter of a torus with different boundary conditions. [Copyright &y& Elsevier]

Most existing limit load solutions for cracked pipe bends under in-plane bending have been developed following the experimental work by Griffiths on bends with through-wall defects or by extrapolation of solutions developed for cracked straight pipes. No data exists for part-penetrating defects.This contribution summarises recently obtained experimental and finite element results from 13 tests on axially (at the crown) and circumferentially (at the intrados) cracked carbon steel pipe bends under opening bending loads. Comparisons with predictions by existing solutions for the cases investigated are reported. The solutions are shown to be excessively conservative and, on occasions, non-applicable to the cases for which they are intended. The presented data, together with results more recently made available in the open literature, could be used to form a working basis for revising the existing solutions. [Copyright &y& Elsevier]

The present work quantifies the effect of ovality and wall thinning on plastic, shakedown and elastic limit load of pipe bends under cyclic in-plane closing bending load and internal pressure. Three-dimensional finite element analyses with large strain (geometric nonlinear analysis) formulation was performed by considering the pipe bend material to be elastic-perfectly plastic. Plastic limit loads are determined from the plot of reaction moment versus angular rotation curve. Abdalla's simplified technique (ST) was used to calculate elastic and shakedown limit loads. The presence of ovality shows significant effect on plastic, shakedown and elastic limit loads and their boundaries for all the models. Thinning has minimal effect on plastic limit loads for large strain analysis while it has a remarkable effect on elastic and shakedown limit loads for ovality up to 5%. Experimental plastic limit moments in open literature was used to validate the present finite element (FE) plastic limit loads. Plastic, shakedown and elastic limit boundaries of large strain analysis or geometric non linear (GNL) analysis were compared with the respective boundaries of small strain analysis or Geometric linear (GL) analysis. Mathematical equations are formulated to express the generated limit moment boundaries for shape imperfect pipe bends. • Large strain FE analysis was performed on pipe bends under cyclic closing bending and steady internal pressure. • Ovality and thinning are considered as shape imperfection in the pipe bends. • Bree diagrams are constructed for reference pipe bend models. • Failure of pipe bends is either due to revered plasticity or ratcheting. • Mathematical equations are proposed to generate the plastic, shakedown and elastic limit boundaries. [ABSTRACT FROM AUTHOR]

Pipe elbows (bends) are considered critical pressurized components in the piping systems and pipelines due to their stress intensification and the effect of bend curvature. They are prone and hence more exposed to different corrosion failure modes than straight pipes. Late detection of such elbow damages can lead to different dangerous and emergency situations which cause environmental disasters, pollution, substantial consumer losses and a serious threat to human life. A comprehensive safety and reliability assessment of pipe elbows, including usage of prediction models, can provide significant increases in the service life of pipelines. It is well known that the limit pressure is an important parameter to assess the piping integrity. In this paper, the integrity assessment of damaged pipeline elbows made of API 5L X52 steel was done within the framework of numerical modeling using the finite element method (FEM) and finite element analysis (FEA). The evaluation of numerically FEM modeled limit pressure in the corroded elbow containing a rectangular parallelepiped-shaped corrosion defect with rounded corners at the intrados section was done and compared to different codes for calculating limit pressure. Moreover, the area with the corrosion defects with different relative defect depth to wall thickness ratios was FEM modeled at the intrados section of the pipe elbow where the highest hoop stress exists. The results showed that the codes for straight pipes could not be applied for the pipe elbows due to the significantly higher error in the obtained limit pressure value compared with numerically FEM obtained results. However, the results for modified codes, adapted for the pipe elbow case using the Goodall formula for calculation of the hoop stress in pipe elbows with defects are pretty consistent with the numerical FEA results. The notch failure assessment diagram (NFAD) was also used for the straight pipe and pipe bends with different corrosion defect depth ratios, while the obtained critical defect depth ratios further highlighted the criticality of pipe elbows as an essential pipeline component. • Integrity of corroded pipe elbow was investigated based on the limit pressure. • A parallelepiped rounded borders corrosion defect was created in the intrados section. • The numerical finite element analyses (FEA) were done to modify the codes. • Two models (modified B31G and DNV RP-F101) agrees well with FEA results. • The notch failure assessment diagram (NFAD) confirms the pipe elbow criticality. [ABSTRACT FROM AUTHOR]

The present study uses advanced numerical tools to examine the mechanical behavior of steel pipe bends (elbows), subjected to strong cyclic loading, associated with repeated plastic deformations of alternate sign. The elbows are modeled with finite elements, accounting for the measured elbow geometry and the actual properties of steel elbow material. The main feature of the present work is the simulation of material response using an advanced cyclic-plasticity model, based on the bounding-surface concept, implemented through an in-house material subroutine. Upon appropriate calibration with material tests in strip specimens, extracted from the bends under consideration, the constitutive model is employed for predicting the mechanical response of large-scale physical laboratory experiments. Very good predictions are obtained in terms of global load-displacement response, as well as in terms of local strains and their accumulation over the loading cycles (ratcheting) at specific elbow locations. This indicates that the bounding-surface model can be an efficient tool for predicting accurately the mechanical response of piping components under severe cyclic loading conditions. Finally, using the validated numerical models, extensive results are obtained, focusing on the effects of internal pressure on strain accumulation at specific locations of the elbow, for three values of pipe thickness. • We simulate elbows using rigorous finite elements and bounding-surface plasticity. • Upon calibration from small-scale testing, we simulate large-scale physical tests. • Our results compare very well with tests in terms of strain range and ratcheting. • We examine pressure and thickness effects on local strain at various locations. • We demonstrate the capability of the bounding-surface model to predict local strain. [ABSTRACT FROM AUTHOR]

An experimental study has been performed on a representative prototype-sized pipe bend made of SS 316 LN material to generate the relevant material parameters required for fatigue crack growth (FCG) simulation. Traditionally, FCG numerical simulation material parameters are generated based on specimen level test. In the literature there are many prototype pipe bend test results can be found. However, this paper takes a new look for generating FCG material parameters based on prototype-sized pipe bend test results. A semi-elliptical shaped pre-cracked pipe bend is used for the crack growth studies. Advancement of the crack profile has been measured in a periodic interval of cyclic testing, and the essential crack dimensions are derived to extract the FCG material parameters. FCG response curve derived, based on the prototype sized pipe bend test result, is well below the similar response curve obtained from the conventional material properties. The major reason behind this difference in FCG response is attributed to the influence of the residual stress locked during pipe bend fabrication. Based on these findings, it is recommended to adopt the material parameters derived based on the component-level test for more accurate prediction. Further studies are required to generalise the role of fabrication assisted effects in the material parameters in the case of prototype sized components. • Fatigue crack growth (FCG) experiments on a full-scale prototype-sized SS 316 LN pipe bend have been conducted. • Advancement of the crack profile is measured at periodic intervals and the required crack growth data is extracted. • Material parameters required for FCG simulation are derived based on the test data and FCG response is computed. • This response is compared with those computed based on conventional material parameters. • An approach for predicting a realistic FCG response of power plant piping components is proposed. [ABSTRACT FROM AUTHOR]

This paper is dedicated to providing a detailed limit load analysis for 180° pipe bends under bending moment using three dimensional finite element (FE) method considering geometric nonlinearity. Results show that FE results for 180° pipe bends are all lower compared to those for 90° pipe bends, especially for pipe bends with thin wall and small bend radius (R / r = 1.5, r / t = 50). However, for pipe bends with thick wall and large bend radius (R / r = 4, r / t = 5), results seem very close. New predicted equations are proposed based on the FE results. If the geometric nonlinearity effect is considered, when the material parameters of yield strain ε s = 0, results still obey the estimating solutions without geometric nonlinearity considered. But the geometry change should have some significant influence on the failure with the material's behavior changing. With the applied bending load increasing, the ovalization parameter C will also continue to increase. The change of the parameter C is significant for the thin pipe bends (r / t = 50), while is not obvious for thick pipe bends (r / t = 5). The influence of the bend radius on the parameter C is not significant. Geometric effect is significant for a high yield strain value that the pipes will have enough ability to be deformed. • Three-dimensional non-linear finite element method considering geometric nonlinearity is used. • Limit loads are analyzed for 180° pipe bends under bending moment. • Parameter r / t is the main factor affecting the limit load of the pipe bends. [ABSTRACT FROM AUTHOR]

The present study determines the effect of shape imperfections on the B 2 index of pipe bends subjected to in-plane opening moment using 3D nonlinear finite element (FE) methodology. The analysis considers the large strain formulation option and the material behavior is elastic-perfectly plastic. The reaction moment versus angular rotation curves are plotted for all the simulated pipe bends and the plastic collapsing moments are determined using Twice-elastic slope technique as recommended in ASME B&PV, section III (clause NB-3213.25). The B 2 stress index was computed for each model from their respective collapse moment. The variable wall thickening in the pipe bend's cross section yields negligible effect and the presence of ovality has a momentous effect on B 2 stress indices. A mathematical equation is suggested to compute B 2 stress index for pipe bends with ovality subjected to in-plane opening bending moment. • Non-linear finite element analysis was performed on pipe bends under in-plane opening moment. • The B 2 stress index was evaluated in the pipe bends having shape imperfections. • Presence of ovality in the pipe bends cross section produces significant effect. • The effects of thinning on B 2 stress index are minimal. • Closed form solution is proposed to compute B 2 stress index of pipe bends with ovality. [ABSTRACT FROM AUTHOR]

This study determines the plastic, shakedown and elastic limit loads and the corresponding regimes of 90° pressurized pipe bends with shape imperfections subjected to cyclic in-plane closing bending moment. Three dimensional FE analyses with small strain formulation was performed. The induced shape imperfections are ovality and thinning, which were varied from 0% to 20% with a 5% increment at every step. Plastic limit load was determined from the moment rotation curve while shakedown and elastic limit loads were determined by using Abdalla's simplified technique (ST). The analysis indicates that the presence of ovality produces considerable effect on plastic limit loads. Thinning alters plastic limit loads only for long bend radius models with no ovality when normalised internal pressure ratio is unity and in other cases its effect is minimal. Presence of ovality and thinning has a remarkable effect on shakedown and elastic limit loads and their regimes though the thinning effect is insignificant at higher ovality (10%–20%). Bree diagram was constructed for the reference pipe bend models. It was observed that the failure of pipe bends is due to either reversed plasticity or ratcheting. Mathematical equations are proposed to express the generated plastic, shakedown and elastic limit moment boundaries for the analyzed pipe bend models. • Plastic, shakedown and elastic limit loads for shape imperfect 90ᴼ pipe bends subjected to internal pressure and cyclic in-plane closing bending moment was studied. • Shape imperfections in the pipe bends considered for the analysis are ovality and wall thinning. • Moment rotation curve is used for determining plastic limit load and Abdalla's simplified technique is employed to determine the shakedown and elastic limit loads. • Premature failure cause of pipe bends either due to reversed plasticity or ratcheting is studied by using bree diagram. • Beyond a certain pressure limit, ovality increases the pipe bend's performance. [ABSTRACT FROM AUTHOR]

The focus of this paper is to bring out the conservatism present in the fast reactor design standard RCC MRx A16 in predicting fatigue crack initiation for a component that has a crack-like defect. Full-scale pipe bend test results were used for comparing the fatigue life predicted based on the RCC MRx A16 approach. A representative pipe bend of size 570 mm outer diameter and 15 mm thickness has been used for the purpose. The selected pipe bend is made up of austenitic stainless steel (SS 316 LN) and is provided with a surface notch 22.5 mm long and 2.1 mm in depth to simulate the potential flaw, which could be present in the piping system. Cyclic testing has been carried out to determine fatigue life. These tests were conducted in two stages. During the first stage, the finite-sized notch was sharpened into a crack-like defect allowing it to develop into a natural crack (85,150 cycles). In the second stage, the testing was continued to estimate the number of cycles required to advance the crack by > 50 μm and thereby compare prediction capability of the sigma-d approach given in the RCC MRx A16. The number of cycles required for crack incubation and advancement was obtained by periodic measurement of the measured readings obtained from the crack depth gauge. A comparison of the number of cycles was predicted by the RCC MRx A16 method, and the experimental results show that the A16 prediction was very conservative. Useful suggestions were proposed in this paper for improving the fatigue crack initiation prediction capability. • Proposal to improve fatigue crack initiation prediction capability of the present design code philosophy is addressed. • Fatigue crack life predicted as per RCC MRx A16 Sigma-d approach compared with the test results. • The degree of conservatism present in A16 Sigma-d approach has been quantified. • Recommended a more realistic factor of safety. [ABSTRACT FROM AUTHOR]