Limit Load and Reference Stress for Curved Wide Plates

2010 ◽  
Author(s):  
Stijn Hertele´ ◽  
Wim De Waele ◽  
Rudi Denys ◽  
Jeroen Van Wittenberghe ◽  
Matthias Verstraete
Keyword(s):  
Experimental Validation ◽  
Limit Load ◽  
Plastic Collapse ◽  
Test Results ◽  
Finite Element Analyses ◽  
Flat Plates ◽  
Stress Equation ◽  
Reference Stress ◽  
Plate Curvature ◽  
Girth Welds

Curved wide plates are a valuable tool in the assessment of defective pipeline girth welds under tension. Throughout the years, Laboratory Soete collected an extensive database of curved wide plate test results. In an effort to investigate these results through FAD analysis, the authors recently developed a reference stress equation for curved plates. The approach followed is similar to the development of the Goodall and Webster equation for flat plates. This paper elaborates finite element analyses of the equation’s capability to predict plastic collapse. It is found that, although overestimated, the influence of plate curvature is correctly predicted in a qualitative way. For all simulations, the curved plate reference stress equation produced conservative estimations. This indicates that the proposed equation is suited to safely predict the plastic collapse of defective pipeline girth welds. An experimental validation is underway.

Elastic-Plastic Fracture Mechanics Assessment of Test Data for Circumferential Cracked Pipes

2003 ◽  
Author(s):  
Nam-Su Huh ◽  
Do-Jun Shim ◽  
Yun-Jae Kim ◽  
Young-Jin Kim
Keyword(s):  
Test Data ◽  
Experimental Validation ◽  
Limit Load ◽  
Maximum Load ◽  
The Other ◽  
Surface Cracks ◽  
Plastic Limit ◽  
Load Range ◽  
Reference Stress ◽  
Plastic Limit Load

This paper presents experimental validation of two reference stress based methods for circumferential cracked pipes. One is the R6 method where the reference stress is defined by the plastic limit load. The other is the enhanced reference stress method, recently proposed by the authors, where the reference stress is defined by the optimized reference load. Using thirty-eight published pipe test data, the predicted maximum instability loads according to both methods are compared with the experimental ones for pipes with circumferential through-thickness cracks and with part circumferential surface cracks. It is found that the R6 method gives conservative estimates of the maximum loads for all cases. Ratios of the experimental maximum load to the predicted load range from 0.54 to 0.98. On the other hand, the proposed method gives overall closer maximum loads than R6, compared to the experimental data. However, for part through-thickness surface cracks, the estimated loads were slightly non-conservative for four cases, and possible reasons were fully discussed.

Validation of Crack Growth for an Finite Element Based ECA Procedure for Reel-Laid Pipelines With Under-Matching Girth Welds

2015 ◽  
Author(s):  
T. Sriskandarajah ◽  
Daowu Zhou ◽  
Lingjun Cao
Keyword(s):  
Finite Element ◽  
Crack Growth ◽  
Large Scale ◽  
Limit Load ◽  
Bending Test ◽  
Offshore Pipelines ◽  
Reference Stress ◽  
Trial Test ◽  
3D Fea ◽  
Girth Welds

There is a concern on the fracture integrity of the partially over-matching or under-matching weld during reel-lay installation where there is large plastic strain in the pipe. Conventional ECA procedures such as BS7910 and DNV-OS-F101 are applicable for fully over-matching welds only, due to limitations in the reference stress solution (or limit load solutions). The ECA procedure based on 3D finite element (FE) analysis was developed for partially over-matching welds or under-matching. The methodology has been successfully applied to several projects of industry-wide significance, with partially over-matching welds in offshore pipelines. This paper provides a case study validating the crack growth from FE based ECA methodology against the large scale bending trial test where the pipe containing the notched defect was pre-strained under a series of straining cycles. A comparison of the crack growth between 3D FEA and the large scale bending test was presented.

Application of Limit Load Solutions for Engineering Critical Assessment of Embedded Flaws in Evenmatch Pipeline Girth Welds

2020 ◽  
Author(s):  
Aurélien Pépin ◽  
Tomasz Tkaczyk ◽  
Noel O’Dowd ◽  
Kamran Nikbin ◽  
Suresh V. Chettiar
Keyword(s):  
High Strain ◽  
Closed Form Solution ◽  
Limit Load ◽  
Form Solution ◽  
Embedment Depth ◽  
Plastic Collapse ◽  
J Integral ◽  
Crack Driving Force ◽  
Fracture Assessment ◽  
Girth Welds

Abstract Engineering Critical Assessment (ECA) is commonly undertaken to derive the acceptance criteria for girth weld flaws in rigid pipelines deployed subsea by low-strain installation methods, such as S-Lay or J-Lay, or high-strain installation methods, such as Reel-Lay. The ECA generally considers the whole load history seen by the pipeline from fabrication to the end of service, and involves fracture and fatigue assessments. Fracture, which is the main focus of this paper, is deemed to have initiated when either (i) the crack driving force, expressed in terms of the J-integral or the Crack Tip Opening Displacement (CTOD), δ, is greater than the materials resistance, or (ii) the applied load exceeds the bearing capacity of the ligament of a cracked structure, also referred to as the plastic collapse or limit load. The robustness of the ECA procedure relies on the accuracy of the assessment solutions. Most flaws in pipeline girth welds are embedded. Unlike surface breaking flaws, embedded flaws are typically not directly assessed in a high-strain fracture ECA because the available assessment solutions are too conservative. A work-around approach is often followed, where the maximum acceptable surface breaking flaw sizes are also considered acceptable below the surface if the embedment depth is equal to or greater than half of the flaw height. Otherwise, an embedded flaw must be reclassified as a surface breaking flaw with a height equal to the sum of the embedded flaw height and embedment depth. To enable the direct fracture assessment of embedded flaws, the authors undertook in a previous work a parametric finite-element (FE) study on the effect of the embedment depth, the crack height and the crack length on the plastic collapse load of the shorter ligament of embedded flaws. Subsequently, a new limit load solution was proposed for the fracture assessment of embedded flaws in evenmatch pipeline girth welds subjected to tension and/or bending. This closed-form solution was shown to be significantly more accurate for estimating the crack driving force and the ligament plastic collapse load than other solutions available in the literature. For some geometries, however, the predicted limit load still needs to be significantly adjusted (increased) to correctly evaluate the J-integral, in a combined tearing and collapse assessment. This suggests that further enhancement of the solution is possible. This paper describes small-scale fracture tests which were undertaken to determine the load required to collapse a smaller ligament of embedded flaws in a modified middle crack tension (MMCT) specimen. A closed-form solution, which can also be used as a flaw reclassification criterion, is fitted to the test results and then compared to the FE-based solution. Finally, recommendations are made for the direct fracture assessment of embedded flaws in evenmatch pipeline girth welds subjected to load or displacement-controlled conditions.

Validation of Plastic Collapse Assessments Using BS 7910:2013 and R6 Procedures

2013 ◽  
Author(s):  
Şefika Elvin Eren ◽  
Tyler London ◽  
Yang Yang ◽  
Isabel Hadley
Keyword(s):  
Finite Element Analysis ◽  
Limit Load ◽  
Plastic Collapse ◽  
Element Analysis ◽  
Metallic Structures ◽  
British Standard ◽  
Reference Stress ◽  
Fracture Assessment ◽  
The Difference ◽  
Reference Stress Approach

The British Standard, BS 7910 Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures is currently under revision [1]. Major changes have been undertaken, especially in the fracture assessment routes, and this paper specifically addresses the assessment of proximity to plastic collapse, usually expressed as the parameter Lr via either a reference stress or limit load approach. In the new edition of BS 7910, the reference stress approach has been retained for the assessment of many geometries, mainly for reasons of continuity. However, new limit load solutions (originating in the R6 procedure) are given for use in the assessments of strength mismatched structures or clad plates. In general, a reference stress solution and a limit load solution for the same geometry should deliver the same value of Lr. However, recent comparative studies have shown differences in the assessment of plastic collapse depending on whether the reference stress solutions in BS 7910:2013 or the limit load solutions in R6 are used for the calculation of Lr. In this paper, the extent of the difference in the assessment results with respect to the choice of solutions and boundary conditions are discussed. The results of the assessments in accordance with BS 7910 and R6 are compared with the results of numerical assessments obtained via Finite Element Analysis (FEA). The collapse loads observed in various wide plate tests conducted in the last 20 years are also compared with the collapse loads predicted by BS 910:2013, R6 and FEA. Finally, observations regarding the accuracy of different Codes and FEA are discussed.

Significance of Ms and Validation of Reference Stress Solutions for Crack Like Flaws: Part 1

2020 ◽  
Author(s):  
Yoichi Ishizaki ◽  
Greg Thorwald
Keyword(s):  
Flat Plate ◽  
Correction Factor ◽  
Fem Analysis ◽  
Limit Load ◽  
Metal Loss ◽  
Loss Assessment ◽  
Flat Plates ◽  
Reference Stress ◽  
Similar Factor ◽  
Surface Correction

Abstract This is Part 1 of two papers discussing the significance of two key factors of crack like flaw assessment in the Fitness for Service assessment. While FEM analysis technology has been advancing amazingly in recent years, and FEM based fitness-for-service assessment of damaged components, such as crack like flaws and local metal loss assessment, has become mainstream in assessments, it is still important to understand the reference stress solution and the role of each factor in the failure mode to operate the damaged component safely until the end of its life. In API 579-1/ASME FFS-1[1], Part 9, Assessment of Crack like Flaws, those reference stress solutions were developed based on the limit load analysis using Folias factor Mt and surface correction factor Ms. Folias factor Mt and surface correction factor Ms, are factors that account for the bulging effect around flaws. Those factors enable prediction of a maximum allowable pressure of a damaged cylindrical shell from a simple flat plate model that contain same size of defected area. As for Folias factor, Mt, it is well known to express the relationship between the reference stress of a through-wall crack flat plate and a through-wall crack cylinder. The application of Mt is clearly defined in ASME/API 579 FFS-1 part 9C [1], as well as papers by Folias et al. [2][3]. The significance of the surface correction factor for surface flaw, Ms, has not been commonly understood well enough in general. Unfortunately, API 579-1/ASME FFS-1[1] also does not clearly mention its significance and how Ms is to be applied in the stress analysis. Also the detailed discussion of the derivation process of each reference solution was rooted in several papers with different nomenclature and slightly different definition of factors, which can be very confusing. At a glance, surface correction factor, Ms, looks like a similar factor to Mt, and it is tempting to simply apply Ms to primary membrane stress term like Mt, but that is not correct. Eventually, an incorrect application of Ms would lead to an incorrect discussion of a flaw characterization. Often, there is a question about ASME/API 579 FFS-1[1] Part 9C reference stress solutions, especially for ASME/API 579 FFS-1[1] eq.9C.76, from the misunderstanding meaning of the Ms factor. Addressing this issue is important to maintain the understanding and integrity of the Fitness-For-Service technology. In this Part 1 of two papers, authors reviewed and reorganized step by step procedure of each reference stress solutions for flat plates and cylinders. Through this discussion, authors clarified the significance of Mt and Ms that are defined in ASME/API 579 FFS-1[1] Part 9C. In part 2, validation of equations obtained in this paper is discussed based on FEM analysis.

Differences in the Assessment of Plastic Collapse in BS 7910:2005 and R6/FITNET FFS Procedures

2011 ◽  
Author(s):  
S¸efika Elvin Eren ◽  
Isabel Hadley ◽  
Kamran Nikbin
Keyword(s):  
Boundary Conditions ◽  
Comparative Studies ◽  
Limit Load ◽  
Plastic Collapse ◽  
Reference Stress ◽  
Fracture Assessment

At present within the fracture assessment routes of different codes and standards, two different options for the assessment of plastic collapse, Lr, are available, namely reference stress and limit load approaches. Recent comparative studies have shown significant differences in the assessment of plastic collapse depending on whether the reference stress solutions in BS 7910:2005 or the limit load solutions in R6/FITNET are used for the calculation of Lr. In this paper, differences with respect to the choice of solutions and boundary conditions will be illustrated and observations regarding the route that the Codes should take with respect to a unified assessment will be discussed.

Limit Loads of Mechanical Components and Structures Using the GLOSS R-Node Method

1992 ◽  
Vol 114 (2) ◽  
pp. 201-208 ◽  
Author(s):  
R. Seshadri ◽  
C. P. D. Fernando
Keyword(s):  
Practical Interest ◽  
Limit Load ◽  
Load Control ◽  
Plastic Collapse ◽  
Element Analysis ◽  
Limit Loads ◽  
Linear Elastic ◽  
Reference Stress ◽  
Stress Classification ◽  
Mechanical Components

A method for determining plastic collapse loads of mechanical components and structures on the basis of two linear elastic finite element analysis is presented in this paper. The r-nodes, which are essentially statically determinate locations, are obtained by GLOSS analysis. The plastic collapse loads are determined for statically determinate and indeterminate components and structures by using the single-bar and the multibar models, respectively. The paper also attempts to unify the concepts of load-control, limit load, reference stress and stress-classification. The GLOSS R-Node method is applied to several component configurations of practical interest.

LIMIT LOADS OF FRAMED STRUCTURES AND ARCHES USING THE GLOSS R-NODE METHOD

1993 ◽  
Vol 17 (2) ◽  
pp. 197-214
Author(s):  
C.P.D. Fernando ◽  
R. Seshadri
Keyword(s):  
Finite Element ◽  
Approximate Method ◽  
Equivalent Stress ◽  
Limit Load ◽  
Load Control ◽  
Finite Element Analyses ◽  
Limit Loads ◽  
Framed Structures ◽  
Linear Elastic ◽  
Reference Stress

An approximate method for determining limit loads of mechanical components and structures on the basis of two linear elastic finite element analyses is described. The load-control nature of the redistribution nodes (r-nodes) leads to considerable simplifications. The combined r-node equivalent stress, which can be obtained by invoking an appropriate multibar mode, can be identified with the reference stress. The method is applied to beam, framed and arched structures, and the limit load estimates obtained are reasonably accurate.

Prediction of Collapse Load Reduction due to Non-Aligned Multiple Flaws

2020 ◽  
Author(s):  
Fuminori Iwamatsu ◽  
Katsumasa Miyazaki ◽  
Koichi Saito
Keyword(s):  
Plastic Strain ◽  
Fracture Strain ◽  
Equivalent Plastic Strain ◽  
Plastic Collapse ◽  
Load Reduction ◽  
Test Results ◽  
Collapse Load ◽  
Flat Plates ◽  
Fracture Tests ◽  
Flaw Characterization

Abstract Fitness-for-Service (FFS) codes, such as ASME Boiler and Pressure Vessels Code, Section XI, have flaw characterization rules for evaluation of structural integrity. Since stress corrosion cracking (SCC) and thermal fatigue frequently cause multiple flaws, FFS codes should have proximity rules as a part of flaw characterization rules. The flaw characterization rules should consider fracture modes, such as brittle fracture, ductile fracture, and plastic collapse. Those in the current codes are not divided by the fracture modes. Especially, application of the current proximity rules to plastic collapse of non-aligned multiple flaws should be validated because there are few studies for this issue. Thus, fracture tests of flat plates with through-wall flaws and finite element analysis (FEA) were conducted for predicting collapse loads due to plastic collapse. A series of the fracture tests of flat plates with non-aligned two flaws has been conducted, and a trend between the load reduction and the flaw locations was shown from the results. This trend shows that the defined net-section for non-aligned multiple flaw dominate the collapse load. For the validation the trend shown by the fracture tests, FEA was performed for predicting the measured collapse load. Equivalent plastic strain around a flaw tip dominates a collapse behavior, and an equivalent plastic strain at collapse called as fracture strain was determined for FEA. The collapse loads predicted by the fracture strain are correspond with the test results for any flaw locations. FEA conditions can interpolate and cover a wide range of flaw locations conducted by the tests. The load ratios which represent effect of flaw interaction on a collapse load were estimated by parametric FEA. The ratios were mapped to investigate the trend of the effect on a collapse load. The mapped results show that the load ratio depends on a shorter flaw length of two flaws. This trend shown by the analysis results is corresponds with the fracture test results. These results are fundamental idea to make a flaw characterization rule in the FFS codes, such as ASME BPVC Section XI, for ductile fracture evaluation.

Limit Load Solution of Non-Aligned Multiple Flaws

2018 ◽  
Author(s):  
Fuminori Iwamatsu ◽  
Katsumasa Miyazaki ◽  
Koichi Saito
Keyword(s):  
Power Plants ◽  
Limit Load ◽  
Pressure Vessels ◽  
Ductile Material ◽  
Evaluation Procedure ◽  
Test Results ◽  
Collapse Load ◽  
Flat Plates ◽  
Fracture Tests ◽  
Single Flaw

Limit load solutions have been applied to estimate the collapse load of a component made of ductile material. Worldwide maintenance codes for power plants, such as ASME Boiler and Pressure Vessels Code, Section XI, and JSME fitness-for-service code, describe limit load solutions under the assumption of a single flaw. Detected flaws are, however, not always a single flaw, and adjacent flaws due to stress corrosion cracking have been detected in power plants. Thus, development of a limit load solution to estimate the collapse load in the case of multiple flaws remains an issue of structural integrity evaluation. Under the aim of developing a method for evaluating the effect of multiple flaws on collapse load as a part of a limit load solution, fracture tests of flat plates and pipes with multiple flaws were conducted. Although experimental approaches have been attempted to establish the evaluation method, further efforts are required to incorporate the evaluation procedure into a code rule. Effective parameters for considering reduction of collapse load on the basis of test results for specimens with multiple flaws were identified. Test results clearly show a correlation between collapse load and ratios of net-section areas. This correlation leads to the conclusion that distance parameters and flaw length of a smaller flaw determine the existence of an effect on the collapse load by multiple flaws. To investigate the physical sense of the correlation, finite element analysis (FEA) was performed. The FEA results show that strain distributions at the flaw tip under several conditions correspond at the time of maximum load of the fracture tests regardless of the effect of multiple flaws. Also according to the FEA results, the extent of the strain field is linearly proportional to flaw length. These FEA results are consistent with the correlation obtained by the test results.

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