Effects of Negative Biaxial Loadings and Notch on Failure Assessment Diagrams

2008 ◽  
Author(s):  
K. Ragupathy ◽  
K. Ramesh ◽  
Doug Hall
Keyword(s):  
Fracture Mechanics ◽  
Biaxial Loading ◽  
Failure Criteria ◽  
Small Scale ◽  
J Integral ◽  
Worst Case ◽  
Integral Methods ◽  
Failure Assessment ◽  
Cracked Structures ◽  
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 (LEFM) on one end and Plastic collapse on other end. This interaction is the realm of Elastic Plastic Fracture Mechanics (EPFM.) It is popularly known as the R6 approach which graphically characterizes the impact of plasticity on crack driving force. In the recent years, there has been continuous interest in using Failure Assessment Diagrams (FAD) 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 aircraft components operate under negative biaxial ratios up to −0.5. In this paper, a detailed study on FAD was conducted using FEA 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 and validated its practical application. Comparison studies were made between uniaxial and biaxial loading cases with FAD curves created using standard approaches for 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 biaxilaity. 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 non-conservative compared to the approach recommended here, particularly under the worst case condition. The proposed method is expected to predict lower failure loads relative to currently accepted analytical methods.

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.

scholarly journals 3D SSY Estimate of EPFM Constraint Parameter under Biaxial Loading for Sensor Structure Design

Sensors ◽  
2019 ◽  
Vol 19 (3) ◽  
pp. 735
Author(s):  
Ping Ding ◽  
Xin Wang
Keyword(s):  
Fracture Mechanics ◽  
Biaxial Loading ◽  
Structure Design ◽  
Theoretical Research ◽  
Small Scale ◽  
Validation Process ◽  
Wide Range ◽  
Sensor Structure ◽  
Estimate Method ◽  
Parameter Approach

Conventional sensor structure design and related fracture mechanics analysis are based on the single J-integral parameter approach of elastic-plastic fracture mechanics (EPFM). Under low crack constraint cases, the EPFM one-parameter approach generally gives a stress overestimate, which results in a great cost waste of labor and sensor components. The J-A two-parameter approach overcomes this limitation. To enable the extensive application of the J-A approach on theoretical research and sensor engineering problem, under small scale yielding (SSY) conditions, the authors developed an estimate method to conveniently and quickly obtain the constraint (second) parameter A values directly from T-stress. Practical engineering application of sensor structure analysis and design focuses on three-dimensional (3D) structures with biaxial external loading, while the estimate method was developed based on two-dimensional (2D) plain strain condition with uniaxial loading. In the current work, the estimate method was successfully extended to a 3D structure with biaxial loading cases, which is appropriate for practical sensor design. The estimate method extension and validation process was implemented through a thin 3D single edge cracked plate (SECP) specimen. The process implementation was completed in two specified planes of 3D SECP along model thickness. A wide range of material and geometrical properties were applied for the extension and validation process, with material hardening exponent value 3, 5 and 10, and crack length ratio 0.1, 0.3 and 0.7.

Failure Assessment Diagrams of Semi-Elliptical Surface Crack with Constraint Effect

2007 ◽  
Vol 353-358 ◽  
pp. 1952-1955
Author(s):  
Hyung Yil Lee ◽  
Jin Haeng Lee ◽  
Tae Hyung Kim
Keyword(s):  
Finite Element ◽  
Finite Element Modeling ◽  
Surface Crack ◽  
J Integral ◽  
Finite Element Analyses ◽  
Constraint Effect ◽  
Failure Assessment ◽  
T Stress ◽  
Cracked Structures ◽  
Constraint Effects

For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Gas Suspension Effects in Riser Unloading and Appropriate Modelling Approaches

2020 ◽  
Author(s):  
Dalila Gomes ◽  
Kjell Kåre Fjelde ◽  
Knut Steinar Bjørkevoll ◽  
Johnny Frøyen
Keyword(s):  
Drilling Fluid ◽  
Single Bubble ◽  
Small Scale ◽  
Gas Suspension ◽  
Worst Case ◽  
Drift Flux Model ◽  
Drift Flux ◽  
Flux Model ◽  
The Impact ◽  
Modelling Approaches

Abstract A kick entering a drilling riser and expanding upwards uncontrolled can lead to severe consequences such as riser unloading and riser collapse, and in the worst case a blowout scenario may evolve. If the riser is filled with water-based mud, the kick will normally migrate on its own to surface, but it has been observed both in small scale experiments and in field tests that small amounts of gas are trapped by the mud during the kick migration. In some cases, the kick is not able to reach the surface without additional circulation. Hence, a certain kick size may or may not lead to an unloading scenario depending on the effect of gas suspension in the drilling fluid. In this paper, two different modelling approaches for describing the unloading scenario will be compared and the differences will be highlighted. In the first approach, the single bubble model will be considered. Here the gas bubble is assumed to occupy the whole cross-sectional area, and it is fully separated from the mud regions. This will be solved by two different calculations methods, one that is taken from literature and one that is based on a shooting technique. The second and recommended approach is to use a transient drift flux model, which includes friction, acceleration terms, and gas slip. For the gas slippage model, different flow patterns will be accounted for, as will the suspension effect that causes small amounts of gas to be trapped by the mud. The drift flux model will be solved numerically using the explicit AUSMV scheme. The impact of gas suspension will be studied by varying the onset for gas suspension to determine from simulations whether a riser will be unloaded or the kick become fully trapped in the riser. In addition, a sensitivity analysis will be presented where kick size, riser ID and riser length are varied to determine when the riser will be unloaded. The different simulations presented solves physical equations of the unloading scenario to calculate pressure at BOP, displaced mud volume (pit gain), liquid mass in well, surface rates, riser friction, and depth profiles of the gas distribution at a certain time. the tables provide a comprehensive overview of which combinations of parameters lead to a trapped gas scenario or and which lead to unloading the riser. It is shown that a fully transient drift flux model can cover a vast range of different situations e.g. gas becomes fully trapped in the riser, the riser becomes fully unloaded, and situations where only a very small part of the kick reaches the surface. The simulations show how the dynamics of the scenarios are quite different. A single bubble model will not have this capability.

Constraint and Failure Assessment Diagram Under Biaxial Loading on Semi-Elliptic Surface Cracks

2012 ◽  
Author(s):  
Zhongxian Wang ◽  
Ruifeng Zhang ◽  
Yuh J. Chao ◽  
Poh-Sang Lam
Keyword(s):  
Uniaxial Tension ◽  
Crack Front ◽  
Biaxial Loading ◽  
Pipeline Steel ◽  
Elliptic Surface ◽  
Elliptic Crack ◽  
J Integral ◽  
Near Surface ◽  
Failure Assessment ◽  
Constraint Level

Three-dimensional finite element analysis has been performed for several configurations of the semi-elliptic surface crack in an X100 pipeline steel plate under various biaxial loading conditions. The biaxial loading ratio (λ) is defined as the ratio of loading parallel to the crack face in the plate width direction to the loading perpendicular to the crack. The constraint level and the J-integral along the semi-elliptic crack front were calculated with J-A2 constraint theory in fracture mechanics, in which A2 is considered as the constraint parameter. It was found that λ influences the J values along the crack front. As λ varies from −1 to 0 then to +1, the location of the maximum J has a tendency to move from the deepest point of the crack to a location near the surface as the load increases, especially for a deeper crack. The constraint level (A2) along the crack front behaves similarly to the J-integral. At λ = −1, the value of A2 increases from the near surface to the deepest penetration when the load increases. In the case of uniaxial tension (λ = 0), the A2 values do not vary significantly except near the surface. When the equibiaxial condition is reached (λ = 1), the location of the highest constraint moves to the near surface. However, in this region the higher constraint level is unable to maintain as the load increases (i.e., the peak value of A2 decreases with increasing load). Finally, the failure assessment diagrams (FAD) at the deepest point of the semi-elliptic crack were constructed with the J-A2 fracture theory. The crack stability regions are apparently smaller in the cases of λ = −1 and 2 than those for λ = 0 (uniaxial tension) or λ = 1 (equibiaxial tension).

Failure Assessment Diagrams of Semi-Elliptical Surface Crack with Constraint Effect

2007 ◽  
Vol 353-358 ◽  
pp. 98-101
Author(s):  
Hyung Yil Lee ◽  
Jin Haeng Lee ◽  
Tae Hyung Kim
Keyword(s):  
Finite Element ◽  
Finite Element Modeling ◽  
Surface Crack ◽  
J Integral ◽  
Finite Element Analyses ◽  
Constraint Effect ◽  
Failure Assessment ◽  
T Stress ◽  
Cracked Structures ◽  
Constraint Effects

For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Study on Validity of Stress Intensity Factor in CT Specimen for Materials With Low Yield Stress

2013 ◽  
Author(s):  
Masahiro Takanashi ◽  
Yu Itabashi ◽  
Takashi Hirano
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Fracture Mechanics ◽  
Intensity Factor ◽  
Yield Stress ◽  
Small Scale ◽  
Low Alloy Steels ◽  
J Integral ◽  
Loading Conditions ◽  
Test Standards

This paper discusses the validity of a stress intensity factor K for compact tension (CT) specimens of stainless and low-alloy steels by computing J-integral values. In fracture mechanics testing, specimen configurations and loading conditions are strictly regulated to meet the small-scale yielding (SSY) condition and obtain valid data. For some materials, particularly those without an obvious yield point, it is hard to meet the SSY condition. The regulations specified in fracture mechanics test standards are based on experimental results. The theoretical reason for the SSY condition is not necessarily clear and the condition seems conservative. This study focuses on a correlation with the stress intensity factors K and the J-integral values under the SSY condition. J-integral values of CT specimens were analyzed by elasto-plastic finite element analysis. Subsequently, the validity of the stress intensity factor K was checked by comparing K and J–integral values to determine whether CT specimens were under the SSY condition or not. To simplify the comparison, J-integral values were converted to K values, equivalent stress intensity factor KJ values. When K values were low enough to meet the SYY condition of the specimen, they equaled KJ values. Meanwhile, KJ values exceed K values in a high K region, which means the specimen is no longer under a SSY condition. It is possible to determine whether the specimen is under the SSY condition or not by comparing the values of KJ and K. This paper evaluated the validity of K values using the correlative relationship and showed that they were valid to some extent even if loading conditions and specimen configurations were outside the scope of the specification by the test standards. The validity of K values with 0.2% offset strength or flow strength instead of yield stress is also confirmed by this method.

A Failure Assessment Method for a Pipe Bend Subjected to Both a Bending Moment and Internal Pressure

2005 ◽  
Vol 128 (4) ◽  
pp. 605-617 ◽  
Author(s):  
Masaki Yoshikawa ◽  
Akihiko Katoh ◽  
Kazuaki Sasaki
Keyword(s):  
Internal Pressure ◽  
Failure Criterion ◽  
Equivalent Plastic Strain ◽  
Bending Moment ◽  
Failure Criteria ◽  
Assessment Method ◽  
Small Scale ◽  
Pipe Bend ◽  
Critical Deformation ◽  
Failure Assessment

This paper proposes a new failure assessment method for a steel pipe bend subjected to both a bending moment and internal pressure. Consistent with previous studies, it was shown that the maximum bending moment of a pipe bend subjected to a bending moment increases with the addition of internal pressure. However, it was experimentally confirmed that the addition of this internal pressure has the detrimental effect of significantly reducing the critical deformation (maximum bending angle) of the pipe bend. In addition, it was found that, subsequent to the application of a large deflection, cracks initiate at the most deformed part of the pipe bend during the process of unloading the internal pressure and then the applied load. Herein, the authors propose a practical failure assessment method which uses small-scale tests and nonlinear finite element (FE) analyses to predict the critical deformation and crack initiation position for a full-scale pipe bend. The failure criterion, which uses principal stress, mean stress, and equivalent plastic strain, was developed using small-scale tests. A failure assessment was conducted by comparing the predictions of this criterion with stress and strain histories obtained from FE analyses. Also, the authors’ failure criterion was compared with previous failure criteria, and the advantages/disadvantages discussed.

Small scale fracture mechanics of ductile materials: Advantage of fatigue precracks and comparison of J-integral evaluations

Materialia ◽  
2018 ◽  
Vol 4 ◽  
pp. 104-108 ◽  
Author(s):  
Patrick Gruenewald ◽  
Florian Schaefer ◽  
Matthias Thielen ◽  
Michael Marx ◽  
Christian Motz
Keyword(s):  
Fracture Mechanics ◽  
Small Scale ◽  
J Integral ◽  
Ductile Materials

Two-Parameter Elastic-Plastic Fracture Mechanics Analysis of Surface Cracked Plates Under Uniaxial and Biaxial Loading

2011 ◽  
Author(s):  
Xin Wang
Keyword(s):  
Fracture Mechanics ◽  
Cleavage Fracture ◽  
Crack Front ◽  
Biaxial Loading ◽  
Small Scale ◽  
Elastic Plastic ◽  
Cracked Plates ◽  
Scale Yielding ◽  
Two Parameter ◽  
Uniaxial And Biaxial Loading

In this paper, the J-Q two-parameter elastic-plastic fracture mechanics approach is used to analyse the surface cracked plates under uniaxial and biaxial loading. First, the J-Q characterization of crack front stress fields of surface cracked plates under uniaxial and biaxial tension loadings are discussed. The complete J-Q trajectories for points along the crack fronts as load increases from small-scale yielding to large-scale yielding were obtained. Based on the materials toughness locus, (resistance to fracture JC as a function of Q), the assessments of the onset of cleavage fracture are conducted. The critical location along the 3D crack front, and the corresponding maximum load carrying capacity are obtained. The results are consistent with experimental observations. It is demonstrated the J-Q two-parameter approach is capable of providing comprehensive assessments of cleavage fracture of surface cracked plates under uniaxial/biaxial loadings, capturing all the important aspects of the problem.

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