Application of fracture mechanics to welds with crack origin at the weld toe: a review Part 1: Consequences of inhomogeneous microstructure for materials testing and failure assessment

2019 ◽  
Vol 63 (6) ◽  
pp. 1715-1732 ◽  
Author(s):  
U Zerbst
Keyword(s):  
Fracture Mechanics ◽  
Materials Testing ◽  
Crack Origin ◽  
Failure Assessment ◽  
Weld Toe ◽  
Inhomogeneous Microstructure

Implementation of CSA Z662-15 Annex K Option 2 on a 610mm Liquid Pipeline

2016 ◽  
Author(s):  
Rory Belanger ◽  
Derrick Sarafinchan
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Test Specimen ◽  
Critical Assessment ◽  
Fracture Criteria ◽  
Weld Profile ◽  
Failure Assessment ◽  
Toughness Testing ◽  
Considerable Benefit ◽  
Brittle Fracture Criteria

For more than two decades, CSA Z662 Annex K has provided a method for developing alternative acceptance criteria for weld flaws in mechanized welded pipelines. Increasingly, over the years, fracture mechanics practitioners have found the method overly conservative and restrictive with respect to brittle fracture criteria when compared to other accepted fracture mechanics-based engineering critical assessment ECA codes and methods. These limitations rendered the CSA Annex K method difficult to implement on pipelines constructed with materials not possessing optimal toughness and in cases requiring consideration of fracture toughness at temperatures lower than the typical minimum design metal temperature (MDMT) of −5°C. This paper presents experiences implementing CSA Z662-15 Annex K Option 2 methodology on a 610 mm diameter liquids pipeline and compares and contrasts the utility and benefits of the code revision. This pipeline required consideration for installation during winter months, necessitating installation temperatures as low as −30°C. In addition to evaluation of actual ECA results, analytical evaluations of the Option 2 methodology were also conducted considering parameters outside those used on the project. The new Annex K Option 2 method was found to be of considerable benefit in preparation of a practical ECA. Since fracture toughness testing was conducted at the anticipated lowest installation temperature, the flaw criteria were, as expected, principally controlled by elastic/plastic crack growth consideration. The failure assessment diagram implemented into the CSA Z662-15 Annex K Option 2 provided tolerance for both longer and deeper flaws than that afforded by Option 1 (which resorts to the former 2011 Annex K method). Furthermore, the reduced restriction to the surface interaction ligament (p distance) offers additional advantages including increased flexibility in weld profile design and weld pass sequencing. Fracture toughness (CTOD) testing of TMP pipeline steels used in the project at −30°C often produced transitional fracture toughness results. It was found that the particular project materials were quite sensitive to the level of test specimen pre-compression (an acceptable plastic straining method to reduce residual stress gradients) applied to the CTOD specimens to enhance fatigue crack-front straightness. It was found that optimizing the level of pre-compression (to achieve acceptable pre-crack straightness while minimizing plastic pre-strain) achieved a balance between fully satisfying testing requirements, providing a conservative assessment of CTOD, and facilitating a functional Annex K ECA.

Brittle Fracture Assessment and Failure Assessment Diagrams

2021 ◽  
pp. 1-8
Author(s):  
Brian Macejko
Keyword(s):  
Fracture Mechanics ◽  
Brittle Fracture ◽  
Flaw Size ◽  
Process Equipment ◽  
Reliable Prediction ◽  
Fracture Failure ◽  
Failure Assessment ◽  
Fracture Assessment ◽  
Assessment Procedures

Abstract A detailed fracture mechanics evaluation is the most accurate and reliable prediction of process equipment susceptibility to brittle fracture. This article provides an overview and discussion on brittle fracture. The discussion covers the purpose for evaluating, provides a brief summary of historical failures that were found to be a result of brittle fracture, and describes key components that drive susceptibility to a brittle fracture failure, namely stress, toughness/temperature, and flaw size. It also presents industry codes and standards that assess susceptibility to brittle fracture. Additionally, a series of case study examples are presented that demonstrate assessment procedures used to mitigate the risk of brittle fracture in process equipment.

Advanced Probabilistic Fracture Mechanics Using the R6 Procedure

2010 ◽  
Author(s):  
David W. Beardsmore ◽  
Karen Stone ◽  
Huaguo Teng
Keyword(s):  
Fracture Toughness ◽  
Monte Carlo ◽  
Fracture Mechanics ◽  
Monte Carlo Methods ◽  
Probability Of Failure ◽  
Defect Size ◽  
Probabilistic Fracture Mechanics ◽  
Safety Margins ◽  
Reliability Method ◽  
Failure Assessment

Deterministic Fracture Mechanics (DFM) assessments of structural components (e.g. pressure vessels and piping used in the nuclear industry) containing defects can usually be carried out using the R6 procedure. The aim of such an assessment is to demonstrate that there are sufficient safety margins on the applied loads, defect size and fracture toughness for the safe continual operation of the component. To ensure a conservative assessment is made, a lower-bound fracture toughness, and upper-bound defect sizes and applied loads are used. In some cases, this approach will be too conservative and will provide insufficient safety margins. Probabilistic Fracture Mechanics (PFM) allow a way forward in such cases by allowing for the inherent scatter in material properties, defect size and applied loads explicitly. Basic Monte Carlo Methods (MCM) allow an estimate of the probability of failure to be calculated by carrying out a large number of fracture mechanics assessments, each using a random sample of the different random variables (loads, defect size, fracture toughness etc). The probability of failure is obtained by counting the proportion of simulations which lead to assessment points that lie outside the R6 failure assessment curve. This approach can give good results for probabilities greater than 10−5. However, for smaller probabilities, the calculation may be inefficient and a very large number of assessments may be necessary to obtain an accurate result, which may be prohibitive. Engineering Reliability Methods (ERM), such as the First Order Reliability method (FORM) and the Second Order Reliability Method (SORM), can be used to estimate the probability of failure in such cases, but these methods can be difficult to implement, do not always give the correct result, and are not always robust enough for general use. Advanced Monte Carlo Methods (AMCM) combine the two approaches to provide an accurate and efficient calculation of probability of failure in all cases. These methods aim to carry out Importance Sampling so that only assessment points that lie close to or outside the failure assessment curve are calculated. Two methods are described in this paper: (1) orthogonal sampling, and (2) spherical sampling. The power behind these methods is demonstrated by carrying out calculations of probability of failure for semi-elliptical, surface breaking, circumferential cracks in the inside of a pressure vessel. The results are compared with the results of Basic Monte Carlo and Engineering Reliability calculations. The calculations use the R6 assessment procedure.

Failure Assessment of Nuclear Piping Components Due to Combined Degradation Mechanisms by an Advanced Probabilistic Fracture Mechanics Code

2009 ◽  
Author(s):  
Debashis Datta ◽  
Changheui Jang
Keyword(s):  
Fracture Mechanics ◽  
Development Process ◽  
Failure Mechanisms ◽  
Piping System ◽  
Degradation Mechanisms ◽  
Probabilistic Fracture Mechanics ◽  
Start Up ◽  
Failure Assessment ◽  
Probabilistic Failure Analysis ◽  
Short Incubation Time

Probabilistic failure analysis of nuclear piping components due to a combined degradation mechanisms is a challenging issue. At present the majority of analyses were done by assuming a single failure mechanism for a specific location of a piping system. But in reality, this might not be an accurate approach. A tiny crack might be present in a weld location due to fabrication defect or initiated due to fatigue after a short incubation time of plant’s start up. This pre-existing or initiated crack then may be further matured by the synergistic effect of different probable degradation mechanisms e.g. fatigue, stress corrosion cracking, etc. In this study the development process of an advanced probabilistic fracture mechanics code has been described which can handle this combined failure mechanisms. Numerical examples are also presented to rationalize the use of such methodology.

Effects of Negative Biaxial Loadings and Notch on Failure Assessment Diagrams

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
K. Ragupathy ◽  
K. Ramesh ◽  
D. Hall
Keyword(s):  
Finite Element ◽  
Fracture Mechanics ◽  
Biaxial Loading ◽  
Failure Criteria ◽  
Small Scale ◽  
J Integral ◽  
Element Analysis ◽  
Worst Case ◽  
Failure Assessment ◽  
The Impact

The failure assessment diagram (FAD) is a simplified and robust flaw assessment methodology, which simultaneously connects two dominant failure criteria: linear elastic fracture mechanics on one end and plastic collapse on the other end. This interaction is in the realm of elastic-plastic fracture mechanics. It is popularly known as the R6 approach, which graphically characterizes the impact of plasticity on crack driving force. In recent years, there has been continuous interest in using FADs to assess the failure of cracked structures subjected to biaxial loadings. Biaxiality is defined as the ratio of stress applied parallel and normal to the crack. Some pressure loaded aircraft components operate under negative biaxial ratios up to −0.5. In this paper, a detailed study on FAD was conducted using finite element analysis computed J-integral methods to investigate the effect of biaxial loading using different FAD approaches for geometries with notches. Geometries with a crack that emanates at a fillet region were simulated with various biaxial loading ratios from −0.5 to +0.5 using 2014-T6 material. FAD curves were numerically generated for cracks at notched regions subjected to various biaxial loadings using J-integral values from finite element analyses. These results were compared with standard FAD approaches. All comparison studies were made between uniaxial and biaxial loading cases with FAD curves created using four different crack sizes. Under small scale yielding, this study clearly shows that FAD curves are not influenced by negative biaxial loading at low load (up to 40% of yield strength). It was clearly confirmed that the majority of previously developed analytical FAD curves do not effectively account for notch and plasticity effects due to negative biaxiality. Based on this study, tension normal to the crack and compression parallel to the crack is the worst combination, and it has a very pronounced effect on FAD curve shapes. The standard analytical FAD curves are nonconservative compared with the approach recommended here, particularly under the worst case condition. FAD curves developed are shown to predict lower failure loads as compared with the currently accepted analytical FAD approaches defined in existing standards, e.g., R6 and API 579. The impact of negative biaxial loading can be investigated directly using a J-integral FAD approach but can be compared with ease by plotting both approaches in a FAD format.

Safety Margin Characterization for In-Service Pressure Vessels Containing Crack Defects

2014 ◽  
Vol 607 ◽  
pp. 717-720
Author(s):  
Si Jian Lin ◽  
Wei Long ◽  
Da Qing Tian
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Fracture Mechanics ◽  
Safety Margin ◽  
Assessment Method ◽  
Pressure Vessels ◽  
Residual Lifetime ◽  
The Finite Element Method ◽  
Failure Path ◽  
Failure Assessment

It’s significant and necessary to assess the safety of the in-service pressure vessels containing crack defects, and there are so many methods that can do, for example, the finite element method and probabilistic fracture mechanics assessment method. However, knowing the safety of the pressure vessels containing crack defects is not enough. For the residual lifetime reason, we are eager to get the safety margin of the pressure vessels. That is how secure they are. Aiming at this problem, we put forward the concept of failure path based on the Failure Assessment Diagram (FAD) and fracture mechanics to help to characterize safety margin. Facts proved that this method was original and useful which can provide a new way in solving the residual lifetime assessment problem of the in-service pressure vessels containing crack defects.

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