Shakedown, ratchet, and limit analyses of 90° back-to-back pipe bends under cyclic in-plane opening bending and steady internal pressure (original) (raw)
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Cyclic Plasticity Behavior of 90° Back-to-Back Pipe Bends Under Cyclic Bending and Steady Pressure
Volume 9: Student Paper Competition, 2018
Back-to-back pipe bends are widely adopted applications in many industries including nuclear sectors. Evaluation of their load bearing capability under complex cyclic loading is very important. Recently, a couple of research reported shakedown boundary of a 90° back-to-back pipe bends by adopting a conservative approach but no comprehensive post yield structural behaviors have been dealt with. In this research the concerning pipe bends subjected to cyclic opening in-plane (IP)/out-of-plane (OP) bending and steady internal pressures are analyzed to construct shakedown and ratchet limit boundary by means of the Linear Matching Method. Analyzed results present that the concerning pipe bends under out-of-plane bending has higher resistance to cyclic bending than under in-plane bending. In additions, the out-of-plane bending causes very small alternating plasticity areas, unlike the in-plane bending. Full cyclic incremental analyses known as step-by-step analysis are performed to verify ...
2005
To provide a data base for the confirmation of computational and classical residual strength analyses of corroded pipelines subjected to combined loads, full scale experiments of 48-inch diameter pipe sections with artificial corrosion were conducted. Design of the experiments was guided by the prerequisite of testing pipe sections in full scale such that subsequent corrections for the uniform depth and extent of the degraded region, and DA ratios were not required. The testing and analysis procedures were progres sively developed through three distinct phases of the program: 1) one proof of concept experiment performed on smaller diameter pipe with artificial corrosion subjected to internal pressure and axial bending, 2) five 48-inch diameter pipe tests, each with artificial corrosion, subjected to internal pressure and axial bending, and 3) eight 48-inch diameter pipe tests, each with artificial corrosion subjected to pressure, axial bending, and axial compression. Combined loading on the test specimens followed a predetermined path until failure by either rupture or global buckling occurred, while the elastic-plastic load-deflection and large strain behavior was recorded. The uniform depth, axial length, and circumferential length of the degraded region were selected to represent commonly observed general corrosion dimensions found among in-service pipelines, with the maximum and minimum extents reflecting the typical wall loss characteristics at the girth and seam weld locations. The pipe behavior during the experiments and analyses was ultimately modeled and verified by an elastic-shell model capable of defining failure pressure and curvature for a corroded pipe subjected to combined service loads. This paper presents details on the test procedures, specimen preparation and design, and complex data acquisition techniques utilized in the generation of required global and location response information. In addition, significant experimental results from the program which enabled the development and validation of a new procedure for the assessment of corroded pipes under combined loads are reviewed. IN T R O D U C T IO N When corrosion damage in the form of wall loss (referred to in this program as a corrosion patch in the context of a cabbage patch) is discovered by a pigging or excavation operation, a replace/repair/ignore decision must be made. This decision hinges crucially on a prediction of the failure pressure of the corroded pipe. Above all else, because environmental safety is paramount, the failure pressure that is predicted for the observed damage must be reliable and readily obtainable. But at the same time, unneces sary field maintenance operations should not be performed-not only to avoid unnecessary expenses and curtailment of service, but also to prevent the possibility of additional damage that sometimes occurs in field operations. Thus, the prediction must be accurate without being grossly conservative, and should not require a time consuming analysis procedure. The current and potential new ASME B31G guidelines satisfy these requirements for most existing pipelines. However, because the present guidelines are drawn from methods that are both empirically based and have a somewhat limited range of applicabil ity, there are pipeline service conditions for which they may not be entirely appropriate. These include damage regions having a large circumferential dimension, multiple nearby interacting discrete damage zones, damage zones at or near weldments, and combined loading conditions. This study was aimed at providing a theoretically-sound technology for a 48-inch pipeline, that will ultimately be adapted to pipelines in general, by conducting integrated research including (1) full-scale pipe burst experimentation, (2) finite element analyses (FEA), and (3) an engineering model for field use embodied in a PC program. Tests and analyses were conducted to delineate the effects of corrosion patch dimensions on the failure mode. Preliminary work [1] (Phase 1 of the study) consisted of a proof of concept test on a 20-inch (51 cm) diameter, X52 steel pipe with simulated corrosion metal loss under a combination of internal pressure and axial bending. Phase 2 of the study consisted of tests and analyses of full scale 48-inch (1.22 m) diameter X65 steel pipes
Journal of Pressure Vessel Technology, 2011
A simplified technique for determining the shakedown limit load for a long radius 90 deg pipe bend was previously developed (Abdalla, H. ). The simplified technique utilizes the finite element (FE) method and employs the small displacement formulation to determine the shakedown limit load (moment) without performing lengthy time consuming full cyclic loading finite element simulations or utilizing conventional iterative elastic techniques. The shakedown limit load is determined through the calculation of residual stresses developed within the pipe bend structure. In the current paper, a parametric study is conducted through applying the simplified technique on three scheduled pipe bends, namely, nominal pipe size (NPS) 10 in. Sch. 20, NPS 10 in. Sch. 40 STD, and NPS 10 in. Sch. 80. Two material models are assigned, namely, an elastic perfectly plastic (EPP) material and an idealized elastic-linear strain hardening material obeying Ziegler's linear kinematic hardening (KH) rule. This type of material model is termed in the current study as the KH-material. The pipe bends are subjected to a spectrum of steady internal pressure magnitudes and cyclic bending moments. The cyclic bending includes three different loading patterns, namely, in-plane closing, in-plane opening, and out-ofplane bending moment loadings of the pipe bends. The shakedown limit moments outputted by the simplified technique are used to generate shakedown diagrams of the scheduled pipe bends for the spectrum of steady internal pressure magnitudes. A comparison between the generated shakedown diagrams for the pipe bends employing the EPP-and the KH-materials is presented. Relatively higher shakedown limit moments were recorded for the pipe bends employing the KH-material at the medium to high internal pressure magnitudes.
Journal of Pressure Vessel Technology
Pipe bends are generally employed for routing piping systems by connecting to straight pipes but back-to-back pipe bends are often necessary for confined space applications. In order to achieve safe operation under complex loading, it requires a thorough pipeline integrity assessment to be commenced. This paper investigates the effects of cyclic thermo-mechanical loading on cyclic plastic behaviour of a ninety-degree back-to-back pipe bend system, including temperature-dependent yield stress effects. Structural response interaction boundaries are determined for various different combinations of cyclic and steady loading. Constructed structural responses are verified by full cyclic incremental, step-by-step, Finite Element Analysis. The numerical studies provide a comprehensive description of the cyclic plastic behaviour of the pipe bends, and semi-empirical equations for predicting the elastic shakedown limit boundary are developed to aid pipeline designers in the effective assessme...
The Influence of the Internal Pressure and In-Plane Bending Moment Loadings on Pipe Bends
2021
Circular thin-walled pipe bends are frequently used as a key part in pipeline connection either in the vertical direction or the horizontal direction due to their high flexibility. The high flexibility of pipe bends is due to the ability of their cross-section to ovalize when subjected to internal pressure and/ or bending moments that lead to high-stress concentrations at bend locations within the pipeline system. Moreover, the surface geometric characteristics of bends may cause some unbalanced outward forces caused by the induced internal pressure loading only which leads to an outward resultant force that tends to straighten out the bend causing a rise within the deformations and stress levels. This phenomenon was known as "The Bourdon effect". In addition to that, external bending moment load acting on the pipe bends may result from either occasional loadings such as; seismic loads, soil settlement, and/ or secondary loadings exerted on the pipe due to thermal expansions resulted in additional stresses. These additional stresses resulting from bending loads acting on the pipe bend are accounted for in the design codes using stress intensification factors (i) and flexibility factors (K). These factors are presented in the current American code ASME B31.3.Although they have been derived for a 90-degree pipe bend subjected to in-plane closing bending moment with long bend radius(R), they cannot be used for other loading cases such as in-plane opening moment or out-of-plane bending moment. Previous studies showed that the direction of bending moment affected the distribution and magnitude of stress levels found on the bend. However, previous studies considered only small pipe sizes of NPS 16 (406mm) and smaller under bend angles of 90 degrees or less. This paper extended the investigation on smooth pipe bends with initial circular cross-sections and uniform wall thickness with large pipe size from NPS20 (508mm) up to NPS 72 (1829mm) under a wide range of bend angles (Ø)(from 30° up to 160°). The loading considered in this study is the internal pressure and the in-plane opening/closing bending moment. In this respect, an extensive parametric study is conducted using a numerical finite element analysis (FEA) simulation using ABAQUS software to model Pipe bends with different nominal pipe sizes (NPS), bend angles (Ø), bend wall thickness (t), and various bend radius (R). The results showed that as the bend angle increases, the flexibility of the bend increases as well leading to higher stresses on the pipe bend. Finally, from the finite element analysis results depicted through curves, it could be concluded that the codes do not cover the stress distribution for large pipe bends accurately.
2014
The behaviour of smooth 90-degree pipe bends under combined internal pressure and cyclic bending loads has received a substantial attention in recent few years where shakedown and ratcheting boundaries have been determined. However such data are scarcely found for those pipe bends suffering from local wall thinning. This paper quantifies the effect of local wall thinning parameters on both elastic and shakedown boundaries via a non cyclic-numerical technique developed by Abdalla et al. [1]. The effect of local wall thinning parameters is investigated also on limit loads of pipe bends under the internal pressure and bending moments based on a systematic FE limit loads using elastic perfect plastic material model. The limit load study has been verified with the proposed approximations developed by Oh et al. [2] and Kim et al. [3]. The thinning geometry is assumed to be rectangular rather than circular but the geometrical nonlinearities are considered. Finite and fully circumferential ...
2007
A simplified technique for determining the lower bound shakedown limit load of a structure, employing an elastic-perfectly plastic (EPP) material model, was previously developed and successfully applied to a long radius 90 deg pipe bend (Abdalla et al., 2006, "Determination of Shakedown Limit Load for a 90 Degree Pipe Bend Using a Simplified Technique," ASME J. Pressure Vessel Technol., 128, pp. 618-624). The pipe bend is subjected to steady internal pressure magnitudes and cyclic bending moments. The cyclic bending includes three different loading patterns, namely, in-plane closing, in-plane opening, and out-of-plane bending moment loadings. The simplified technique utilizes the finite element (FE) method and employs a small displacement formulation to determine the shakedown limit load without performing lengthy time consuming full elastic-plastic (ELPL) cyclic loading FE simulations or conventional iterative elastic techniques. In the present research, the simplified technique is further modified to handle structures employing an elastic-linear strain hardening material model following Ziegler's linear kinematic hardening (KH) rule. The shakedown limit load is determined through the calculation of residual stresses developed within the pipe bend structure accounting for the back stresses, determined from the KH shift tensor, responsible for the rigid translation of the yield surface. The outcomes of the simplified technique showed an excellent correlation with the results of full ELPL cyclic loading FE simulations. The shakedown limit moments output by the simplified technique are utilized to generate shakedown diagrams (Bree diagrams) of the pipe bend for a spectrum of steady internal pressure magnitudes. The generated Bree diagrams are compared with the ones previously generated employing the EPP material model. These indicated relatively conservative shakedown limit moments compared with the ones employing the KH rule.
Shakedown Limits of a 90-Degree Pipe Bend Using Small and Large Displacement Formulations
Journal of Pressure Vessel Technology, 2007
In this paper the shakedown limit load is determined for a long radius 90-deg pipe bend using two different techniques. The first technique is a simplified technique which utilizes small displacement formulation and elastic-perfectly plastic material model. The second technique is an iterative based technique which uses the same elastic-perfectly plastic material model, but incorporates large displacement effects accounting for geometric nonlinearity. Both techniques use the finite element method for analysis. The pipe bend is subjected to constant internal pressure magnitudes and cyclic bending moments. The cyclic bending loading includes three different loading patterns, namely, in-plane closing, in-plane opening, and out-of-plane bending. The simplified technique determines the shakedown limit load (moment) without the need to perform full cyclic loading simulations or conventional iterative elastic techniques. Instead, the shakedown limit moment is determined by performing two analyses, namely, an elastic analysis and an elasticplastic analysis. By extracting the results of the two analyses, the shakedown limit moment is determined through the calculation of the residual stresses developed in the pipe bend. The iterative large displacement technique determines the shakedown limit moment in an iterative manner by performing a series of full elastic-plastic cyclic loading simulations. The shakedown limit moment output by the simplified technique (small displacement) is used by the iterative large displacement technique as an initial iterative value. The iterations proceed until an applied moment guarantees a structure developed residual stress, at load removal, equal to or slightly less than the material yield strength. The shakedown limit moments output by both techniques are used to generate shakedown diagrams of the pipe bend for a spectrum of constant internal pressure magnitudes for the three loading patterns stated earlier. The maximum moment carrying capacity (limit moment) the pipe bend can withstand and the elastic limit are also determined and imposed on the shakedown diagram of the pipe bend. Comparison between the shakedown diagrams generated by the two techniques, for the three loading patterns, is presented.