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).

Elastic-Plastic Constraint Analysis of Semi-Elliptic Surface Cracks in X100 Pipeline Steel

2011 ◽  
Author(s):  
Z. X. Wang ◽  
R. F. Zhang ◽  
Y. J. Chao ◽  
P. S. Lam
Keyword(s):  
Free Surface ◽  
Crack Front ◽  
Driving Force ◽  
Pipeline Steel ◽  
Three Dimensional ◽  
Elliptic Crack ◽  
J Integral ◽  
Surface Cracks ◽  
Crack Driving Force ◽  
X100 Pipeline Steel

In the framework of the J-A2 fracture theory, the crack driving force J and the crack tip constraint parameter A2 are used to describe the near crack tip stress and deformation fields. These two parameters, J and A2, were calculated from three-dimensional finite element results for semi-elliptic surface cracks with various lengths and depths in X100 pipeline steel. It was found that, under a uniform far field tensile loading, A2 increases rapidly to a nearly constant value along the crack front from the free surface to the deepest part of the crack. A similar trend was found for the J-integral distribution except in the case of a semi-circular crack. In addition, for a given elliptic crack configuration, A2 showed significant J-integral dependence when the crack front approached the free surface, where a strong three-dimensional effect is apparent. On the other hand, at the deepest part of the crack, A2 converged to a constant value. Two-dimensional plane strain calculations were also performed for single edge-notched tension specimens (SENT), where the crack length corresponds to the depth of the surface crack. The constraint of these two configurations (semi-elliptic crack and SENT) were compared under the same crack driving force (J-integral). In general, the constraint at the deepest crack front of an elliptic crack is higher than that of the corresponding SENT, especially in mid- to large scale yielding condition where J-integral is relatively large. It can be concluded that using fracture toughness determined from SENT specimens to predict surface flaw stability may lead to non-conservative result.

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.

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.

Fracture Toughness and Application of X80 Pipeline Steel

2019 ◽  
Vol 944 ◽  
pp. 938-943
Author(s):  
Hua Zhang ◽  
Hong Zhang ◽  
Cai Hong Lu
Keyword(s):  
Fracture Toughness ◽  
Acoustic Emission ◽  
Crack Front ◽  
Pipeline Steel ◽  
Heat Affect Zone ◽  
X80 Pipeline Steel ◽  
Material Parameters ◽  
Failure Assessment ◽  
Pipeline Safety ◽  
Metal Weld

The fracture toughness of X80 pipeline steel, which has been widely used in China, plays a key role in pipeline safety insurance. In order to know the accurate fracture toughness of X80, the methods of acoustic emission and high K ratio have been carried out to judge the initiation of crack propagation and improve flatness of fatigue pre-crack front. By this method, fracture toughness of base metal, weld and heat affect zone of X80 pipeline steel have been tested, and failure assessment curves of X80 have been established and fitting equations has been provided. The results suggested that different failure assessment curves should be established according to different crack sizes and material parameters.

Elastic-Plastic Analysis and Failure Assessment of the Pipeline Steel with Surface Crack

2012 ◽  
Vol 602-604 ◽  
pp. 2245-2248
Author(s):  
Zhong Xian Wang ◽  
Rui Feng Zhang
Keyword(s):  
Surface Crack ◽  
Biaxial Loading ◽  
Pipeline Steel ◽  
Assessment Method ◽  
Element Analysis ◽  
Elastic Plastic ◽  
Aspect Ratios ◽  
Biaxial Load ◽  
Failure Assessment ◽  
Plastic Analysis

A detailed elastic-plastic analysis on different semi-elliptical surface crack geometries under uniaxial and biaxial load was conducted using 3D finite element analysis for pipeline steel X100. After quantitating fracture driving force J-integral and plastic yielding load, the effects of biaxial load and crack geometry were investigated by using the Option 3 of R6 failure assessment diagram approach for the surface cracks with the different crack sizes ( aspect ratios c/a = 1, 1.5 and 3 ) and the various biaxial loading ratios λ from -1 to 2. And three option curves of the R6 assessment were compared to evaluate the applicability of R6 assessment method for the X100 steel.

Predictions of Cleavage Fracture in Welded Components Incorporating Strength Mismatch Effects: A Weibull Stress Based Approach

2010 ◽  
Author(s):  
Claudio Ruggieri
Keyword(s):  
Cleavage Fracture ◽  
Driving Forces ◽  
Failure Strain ◽  
Pipeline Steel ◽  
Strong Support ◽  
Weld Strength ◽  
J Integral ◽  
X80 Pipeline Steel ◽  
Loading Parameter ◽  
Weibull Stress

This work describes the development of a toughness scaling methodology incorporating the effects of weld strength mismatch on crack-tip driving forces. The approach adopts a nondimensional Weibull stress, σ¯w, as a the near-tip driving force to correlate cleavage fracture across cracked weld configurations with different mismatch conditions even though the loading parameter (measured by the J-integral) may vary widely due to mismatch and constraint variations. Application of the procedure to predict the failure strain for an overmatch girth weld made of an API X80 pipeline steel demonstrates the effectiveness of the micromechanics approach. Overall, the results lend strong support to use a Weibull stress based procedure in defect assessments of structural welds.

Standardization of CTOD Toughness Correction for Constraint Loss in Steel Components

2008 ◽  
Author(s):  
Fumiyoshi Minami ◽  
Mitsuru Ohata
Keyword(s):  
Fracture Toughness ◽  
Fracture Criterion ◽  
Shape Parameter ◽  
Pipeline Steel ◽  
Fracture Initiation ◽  
Failure Assessment ◽  
Fracture Assessment ◽  
Steel Welds ◽  
Weibull Stress ◽  
Fracture Toughness Specimen

A standardized procedure for correction of CTOD fracture toughness for constraint loss in steel components is presented. The equivalent CTOD ratio β = δ/δWP is developed on the basis of the Weibull stress fracture criterion, where δ and δWP are CTODs of the standard fracture toughness specimen and the wide plate component, respectively, at the same level of the Weibull stress. With the CTOD ratio β, the critical CTOD δWP, cr of the wide plate that is equivalent to δcr at brittle fracture initiation is given as δWP, cr = δcr/β. Nomographs of β are provided as a function of the crack type and size in the component, the yield-to-tensile ratio of the material and the Weibull shape parameter m. The fracture assessment with β is shown within the context of a failure assessment diagram (FAD), which includes the pipeline steel welds with a notch in the weld metal.

Modification Strain-Based Failure Assessment Diagram for High Strength Pipeline Steel

2016 ◽  
Vol 853 ◽  
pp. 33-40
Author(s):  
Jun Lin Shi ◽  
Jian Ping Zhao ◽  
Wei Jie Jiang
Keyword(s):  
Finite Element ◽  
Material Properties ◽  
Pipeline Steel ◽  
High Strength ◽  
Plastic Strains ◽  
Failure Assessment Diagram ◽  
Failure Assessment ◽  
High Plastic

The strain-based failure assessment diagram (SB-FAD) has been developed to predict failure due to high plastic strains. This paper validates the SB-FAD by finite element results for high strength pipeline steel (X80, X80HD, and X90) with four representative specimens (CT, CCP, DECP, and SCEP) of different crack sizes, respectively. The influence of material properties, geometries and crack sizes on failure assessment curves were compared and analyzed. Meanwhile, the modified Option-1 curve of SB-FAD is given in this paper. The results showed that the modified Option-1 curve of SB-FAD is more accurate when the value of abscissais Dr small and more conservative when the value of abscissa Dr is large.

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