Secondary Stress Weighting Factor and Plastic Reduction Factor of Surface Cracked Pipes Under Bending

2021 ◽  
Author(s):  
Sushma Pothana ◽  
Gery Wilkowski ◽  
Sureshkumar Kalyanam ◽  
Yunior Hioe ◽  
Gary Hattery ◽  
...  
Keyword(s):  
Stress Analysis ◽  
Elastic Stress ◽  
Weighting Factor ◽  
Reduction Factor ◽  
Piping System ◽  
Crack Plane ◽  
Secondary Stress ◽  
Pipe Failure ◽  
Elastic Plastic ◽  
Two Parameters

Abstract In piping design analysis, the secondary stresses (displacement controlled) may have different design limits than primary stresses (load-controlled stresses). The current design limits for secondary stresses are based on elastic stress analysis. But realistically a flaw in the piping system can cause non-linear behavior due to the plasticity at the crack plane as well as in the adjacent uncracked-piping material. Hence, the actual stresses in a cracked piping system which are elastic-plastic are different than the design stresses which are elastically calculated. To assess margins in the secondary stresses calculated using elastic stress analysis, two parameters are defined in this paper. The first one is the Secondary Stress Weighting Factor (SSWF) on total stress which is defined as the ratio of actual elastic-plastic stresses in a system to the elastic design stress. An alternative approach to applying margins on secondary stresses is to a use a reduction factor only on stresses above the yield stress. This reduced factor is called Plastic Reduction Factor (PRF). In this paper, a methodology developed to determine these factors for circumferential surface-cracked TP304 stainless steel pipes subjected to bending loads at room temperature is described. Four-point-bend tests are conducted on pipes with varying circumferential surface-crack lengths and depths. The moment and rotations needed for the pipe failure for different crack sizes are determined and compared to elastically calculated moments and rotations to establish margins.

Development of Secondary Stress Weighting Factor and Plastic Reduction Factor From Moment-Rotation Curves of Surface Cracked Pipe Tests

2017 ◽  
Author(s):  
S. Pothana ◽  
G. Wilkowski ◽  
S. Kalyanam ◽  
Y. Hioe ◽  
G. Hattery ◽  
...  
Keyword(s):  
Lower Bound ◽  
Stress Analysis ◽  
Elastic Stress ◽  
Weighting Factor ◽  
Reduction Factor ◽  
Bending Moments ◽  
Secondary Stress ◽  
Pipe System ◽  
Bend Tests

In flaw evaluation criteria, the secondary stresses (displacement controlled) may have different design limits than primary stresses (load-controlled stress components). The design limits are based on elastic stress analysis. Traditionally the elastic design stresses are used in the flaw evaluation procedures. But realistically a flaw in the piping system can cause non-linear behavior due to the plasticity at the crack plane as well as in the adjacent uncracked-piping material. A Secondary Stress Weighting Factor (SSWF) was established which is the ratio of elastic-plastic moment to the elastic moment calculated through an elastic stress analysis. As long as the remote uncracked pipe stresses are below yield, the SSWF is 1.0, and if the uncracked pipe plastic stresses are above the yield stress, the SSWF reaches a limit which is called the Plastic Reduction Factor (PRF). Four-point-bend tests were conducted on pipes with varying circumferential surface-crack lengths and depths. The moment-rotation plots obtained from various pipe tests were used in the determination of PRF. A lower-bound limiting PRF can be calculated from a tensile test, but pipe systems are not uniformly loaded like a tensile specimen. The actual PRF value for a cracked pipe was shown to have a lower bound, which occurs when the test section of interest is at a uniform stress (such as the center region in a four-point pipe bend tests). When multiple plastic hinges develop in a pipe system (a “balanced system” by ASME Section III NB-3650 design rules), this gives a greater reduction to the elastically calculated stresses since there is more plasticity. It was found that the plastic reduction is less when most parts of the pipe system remains elastic, or if the crack is located in the high strength/ lower toughness pipe or welds, or if the pipe size is large enough that elastic-plastic conditions occur even for a higher toughness material. Interestingly, it was shown that the same system with different loading directions could exhibit different actual PRF values if the change in the loading direction changes how much of the pipe system experiences plastic stresses. For smaller cracks, where the bending moments are high, the actual PRF is controlled by plasticity of the uncracked pipe, which is much larger than the plasticity that occurs locally at the crack. However, for large cracks where the bending moments are lower (closer to design conditions), the plasticity at the crack is equally important to the smaller amount of plasticity in the uncracked pipe for the actual PRF. Hence the plasticity of both the uncracked pipe and at the cracked sections is important to include in the determination of actual PRF values.

A Comparison of Design by Analysis Techniques for Evaluating Nozzle-to-Shell Junctions per ASME Section VIII Division 2

2015 ◽  
Author(s):  
Phillip E. Prueter ◽  
Robert G. Brown
Keyword(s):  
Stress Analysis ◽  
Failure Modes ◽  
Elastic Stress ◽  
Limit Load ◽  
Plastic Collapse ◽  
Elastic Plastic ◽  
Plastic Analysis ◽  
Non Linear ◽  
Section Viii ◽  
Division 2

Part 5 of ASME Section VIII Division 2 offers several design by analysis (DBA) techniques for evaluating pressure retaining equipment for Code compliance using detailed computational stress analysis results. These procedures can be used to check components for protection against multiple failure modes, including plastic collapse, local failure, buckling, and cyclic loading. Furthermore, these procedures provide guidance for establishing consistent loading conditions, selecting material properties, developing post-processing techniques, and comparing analysis results to the appropriate acceptance criteria for a given failure mode. In particular, this study investigates the use of these methods for evaluating nozzle-to-shell junctions subjected to internal pressure and nozzle end loads. Specifically, elastic stress analysis, limit load analysis, and elastic-plastic stress analysis are utilized to check for protection against plastic collapse, and computational results for a given load case are compared. Additionally, the twice elastic slope method for evaluating protection against plastic collapse is utilized as an alternate failure criterion to supplement elastic-plastic analysis results. The goal of these comparisons is to highlight the difference between elastic stress checks and the non-linear analysis methodologies outlined in ASME Section VIII Division 2; particularly, the conservatism associated with employing the elastic stress criterion for nozzle end loads compared to limit load and elastic-plastic analysis methodologies is discussed. Finally, commentary on the applicability of performing the Code-mandated check for protection against ratcheting for vessels that do not operate in cyclic service is provided. The intent of this paper is to provide a broad comparison of the available DBA techniques for evaluating the acceptability of nozzle-to-shell junctions subjected to different types of loading for protection against plastic collapse. Predicted deformations and stresses are quantified for each technique using linear and non-linear, three-dimensional finite element analysis (FEA) methodologies.

A Comparison Study of Pressure Vessel Design Using Different Standards

2013 ◽  
Author(s):  
Frode Tjelta Askestrand ◽  
Ove Tobias Gudmestad
Keyword(s):  
Stress Analysis ◽  
Elastic Stress ◽  
Limit Load ◽  
Technical Committee ◽  
Pressure Vessels ◽  
Analysis Method ◽  
Elastic Plastic ◽  
Load Analysis ◽  
American Society ◽  
Division 2

Several codes are currently available for design and analysis of pressure vessels. Two of the main contributors are the American Society of Mechanical Engineers providing the ASME VIII code, Ref /4/ and the Technical Committee for standardization in Brussels providing the European Standard, Ref /2/. Methods written in bold letters will be considered in the discussion presented in this paper. The ASME VIII code, Ref /4/, contains three divisions covering different pressure ranges: Division 1: up to 200 bar (3000 psi) Division 2: in general Division 3: for pressure above 690 bar (10000 psi) In this paper the ASME division 2, Part 5, “design by analysis” will be considered. This part is also referred to in the DNV-OS-F101, Ref /3/, for offshore pressure containing components. Here different analysis methods are described, such as: Elastic Stress Analysis Limit Load Analysis Elastic Plastic Analysis The Elastic Stress Analysis method with stress categorization has been introduced to the industry for many years and has been widely used in design of pressure vessels. However, in the latest issue (2007/2010) of ASME VIII div. 2, this method is not recommended for heavy wall constructions as it might generate non-conservative analysis results. Heavy wall constructions are defined by: (R/t ≤ 4) with dimensions as illustrated in Figure 1. In the case of heavy wall constructions the Limit Load Analysis or the Elastic-plastic method shall be used. In this paper focus will be on the Elastic-plastic method while the Limit Load Analysis will not be considered. Experience from recent projects at IKM Ocean Design indicates that the industry has not been fully aware of the new analysis philosophy mentioned in the 2007 issue of ASME VIII div.2. The Elastic Stress Analysis method is still (2012) being used for heavy wall constructions. The NS-EN 13445-3; 2009, Ref /2/, provides two different methodologies for design by analysis: Direct Route Method based on stress categories. The method based on stress categories is similar to the Elastic Stress Analysis method from ASME VIII div. 2 and it will therefore not be considered in this paper.

A Simplified Method to Estimate Elastic-Plastic Fracture Mechanics Parameters under Combined Primary and Secondary Stresses

2006 ◽  
Vol 306-308 ◽  
pp. 655-660 ◽  
Author(s):  
Yun Jae Kim ◽  
Chang Gyun Oh ◽  
Jong Sung Kim ◽  
Tae Eun Jin
Keyword(s):  
Three Dimensional ◽  
Relative Magnitude ◽  
Engineering Method ◽  
Secondary Stress ◽  
Factors Affecting ◽  
Elastic Plastic ◽  
Wide Range ◽  
Cracked Structures ◽  
Two Parameters ◽  
Three Dimensional Finite Element

Detailed two- and three-dimensional finite element analyses are performed to develop an engineering method to estimate elastic-plastic J for cracked structures under combined primary and secondary stresses, based on the V-factor. Extensive analyses with a wide range of geometry and load combination are considered. The results suggest important factors affecting the value of the V-factor. The most important one is the load magnitude, parameterized by the proximity of plastic yielding. The second one is the relative magnitude of the secondary stress to the primary stress. Although the effect of material, in particular materials with Lüders strain seems to be present, such an effect could be neglected, compared to those of the above two parameters. Based on the present results, an engineering method to estimate J for combined primary and secondary stress can be proposed using bilinear equations in terms of the above two parameters.

Stress Analysis and Structure Optimization for the Opening Flat Cover of Pressure Vessels

2012 ◽  
Vol 538-541 ◽  
pp. 3253-3258 ◽  
Author(s):  
Jun Jian Xiao
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Stress Concentration ◽  
Stress Analysis ◽  
Structure Optimization ◽  
Pressure Vessels ◽  
Secondary Stress ◽  
Element Analysis ◽  
Primary Stress ◽  
Flat Cover

According to the results of finite element analysis (FEA), when the diameter of opening of the flat cover is no more than 0.5D (d≤0.5D), there is obvious stress concentration at the edge of opening, but only existed within the region of 2d. Increasing the thickness of flat covers could not relieve the stress concentration at the edge of opening. It is recommended that reinforcing element being installed within the region of 2d should be used. When the diameter of openings is larger than 0.5D (d>0.5D), conical or round angle transitions could be employed at connecting location, with which the edge stress decreased remarkably. However, the primary stress plus the secondary stress would be valued by 3[σ].

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