Failure assessment diagrams. II. The use of single failure assessment lines

1994 ◽  
Vol 444 (1922) ◽  
pp. 483-496 ◽  
Keyword(s):  
Crack Growth ◽  
Deformation Theory ◽  
Curve Analysis ◽  
Alternative Proof ◽  
Crack Driving Force ◽  
Ductile Tearing ◽  
Natural Mapping ◽  
Tearing Instability ◽  
Failure Assessment ◽  
Level 3

In a previous paper a natural mapping was noted from the ( a, J ep ) diagram of R-curve analysis into the ( L r , K r ) failure assessment diagram (FAD) of the R6-revision 3 procedure. Assuming that J ep is obtained by a deformation theory of plasticity, the analytical expression for this mapping is given and used to derive the images in the FAD of the applied J ep curves and of the R-curve. If this mapping is sufficiently smooth, it may be used to provide an alternative proof that the critical R6-revision 3 load locus touches the R-curve image (RCI) when the crack extension and the load are the same as those predicted by R-curve analysis. The natural mapping may not always be 1:1 and this is illustrated by considering the example of a family of linear R-curves. The relations between the various other functions used in the FAD and R-curve analysis are studied analytically. In particular it is shown how to derive from any single failure assessment line (FAL) on which the assessment point is assumed to move during crack growth, either the implied R-curve (IRC) or, alternatively, the implied applied J ep curve (IAJC). Further comments are made on the internal consistency or conservatism of analyses of ductile tearing instability which use a single FAL on which the assessment point is assumed to move during crack growth, such as those characteristic of level 3 of PD6493 and options 1 and 2 of R6-revision 3. The method for testing the consistency or conservatism of an FAD with a single FAL which involves the calculation of the IAJC requires that the function J ep = j ep ( a, L ) of the structure be known for a specific restricted range of a and L only. In contrast, the deduction of the IRC requires a knowledge of the j ep ( a, L ) over a wider domain. It is emphasized that the assessment of conservatism throughout is not absolute but only relative to the predictions of R-curve analysis. As in the previous paper, the discussion is given in terms of the J based parameters. But the conclusions hold equally well for an FAD based on any other parameters describing crack driving force and crack resistance.

Failure assessment diagrams. I. The tangency condition for ductile tearing instability

1994 ◽  
Vol 444 (1922) ◽  
pp. 461-482 ◽  
Keyword(s):  
Crack Growth ◽  
Maximum Load ◽  
Crack Size ◽  
Curve Analysis ◽  
Ductile Tearing ◽  
Tearing Instability ◽  
Tangency Condition ◽  
Failure Assessment ◽  
Analytical Proof ◽  
Assessment Procedures

Until now no analytical proof has been given to show that the tangency conditions used to define ductile tearing instability in the failure assessment procedures R6 and PD6493 are equivalent to R-curve analysis, although for R6 such an equivalence has been widely claimed. The failure assessment line (FAL) specified in R6-revision 3-option 3 is actually a family of lines when ductile tearing is involved. With this option the use of the tangency condition specified in R6-revision 3-category 3 analysis does not give a treatment of ductile tearing instability which agrees with that of R-curve analysis. The only reliable way of obtaining such an agreement when this failure assessment line family is used is by the solution of simultaneous equations or by some equivalent procedure such as the identification of a maximum load. However, an alternative FAL along which the assessment point moves during crack growth can be constructed. This has been called a ‘failure assessment curve for changing crack size’ but is referred to here as the R-curve image (RCI) because it is an image of the R-curve in the failure assessment diagram (FAD). When this RCI is used as the FAL the tangency condition does give the same predictions as R-curve analysis. Because the R6 diagram explicitly includes a plastic collapse parameter while R-curve analysis does not, this agreement is at first sight rather surprising. Although the use of the tangency condition specified in R6 does not give predictions in accord with R-curve analysis, it is confirmed analytically that it does give a more conservative estimate of the critical load for instability, provided that a conservative choice of the failure assessment line is made and that the RCI lies wholly above it. Thus this work not only throws light on the structure of R6 but suggests that it may be conservative relative to R-curve analysis. This is demonstrated qualitatively by two examples; whether it is a general result is a matter for further study. PD6493 and some options of R6 involve procedures for assessing ductile tearing instability which use the tangency condition combined with a single failure assessment line along which the assessment point moves during crack growth. Some preliminary comments on the implications of such combinations are made and it is suggested that when these combinations are used, checks on the internal self consistency of the assessment procedures are available.

Failure assessment diagrams. III. Mappings and failure assessment lines when the crack driving force is a functional

1994 ◽  
Vol 444 (1922) ◽  
pp. 497-508 ◽  
Keyword(s):  
Driving Force ◽  
Curve Analysis ◽  
General Situation ◽  
Crack Driving Force ◽  
Tearing Instability ◽  
Tangency Condition ◽  
Theory Of Plasticity ◽  
Failure Assessment ◽  
Natural Way ◽  
Flow Theory Of Plasticity

In two previous papers a natural mapping was noted between the ( a, J ep ) diagram of R-curve analysis and the ( L r , K r ) failure assessment diagram (FAD) of the R6-revision 3 procedure. In these papers it was assumed that the applied crack driving force J ep was obtained by a deformation theory of plasticity and so could be treated as a function of its arguments. Here the analysis is generalised to consider the situation where J ep is not a function but a functional of its arguments, as in the flow theory of plasticity. As in I the discussion has been given in terms of the J based parameters. But the conclusions hold equally well for any other parameters describing crack driving force and crack resistance. A unique R-curve image (the RCl) in the FAD can still be established in a natural way. Moreover, if this RCl is used as the failure assessment line (FAL), the treatments of ductile tearing instability in R-curve analysis and in the FAD are still equivalent. The interesting situation then arises, however, that the tangency condition can be defined in the FAD but not in R-curve analysis, because in the latter the usual applied J ep curves do not exist. Some difficulties in using the FAD in this more general situation are discussed. An FAL can be obtained when J ep is a function of its arguments by considering a sequence of RCl curves for similar structures of ever increasing size and this procedure can be extended to the situation where J ep is a functional. The R-curve plays a central role in the argument when J ep is a function and even more so when J ep is a functional. In the latter situation, the analysis rests essentially on the consideration of increments of crack driving force and fracture resistance and it is suggested that a fracture mechanics based on the values of these increments rather than on the values of the parameters themselves might be developed.

Estimating J-R Curve From CVN Upper Shelf Energy and its Application

2016 ◽  
Author(s):  
Samarth Tandon ◽  
Ming Gao ◽  
Ravi Krishnamurthy ◽  
Richard Kania ◽  
Gabriela Rosca
Keyword(s):  
Fracture Resistance ◽  
Theoretical Background ◽  
Shelf Energy ◽  
Ductile Tearing ◽  
Failure Pressure ◽  
Tearing Instability ◽  
Integrity Management ◽  
Level 3 ◽  
Resistance Curves ◽  
Crack Resistance Curves

Accuracy in predictions of burst pressures for cracks in pipelines has significant impact on the pipeline integrity management decisions. One of the fracture mechanics models used for failure pressure prediction is API 579 Level 3 FAD ductile tearing instability analysis that requires J-R curves, i.e., crack resistance curves, for the assessment. However, J-R curves are usually unavailable for most pipelines. To overcome this technical barrier, efforts have been made to estimate the J-R curve indirectly from commonly available toughness data, such as the Charpy V-notched Impact Energy CVN values, by correlating the upper-shelf CVN value (energy) to the ductile fracture resistance J-R curve. In this paper, the theoretical background and studies made by various researchers on this topic are reviewed. Attempts made by the present study to establish correlations between CVN and J-R curves for linepipe materials are then presented. Application of this CVN-JR correlation to API 579 Level 3 FAD tearing instability assessment for failure pressure predictions is demonstrated with examples. The accuracy of the correlation is analyzed and reported.

Probabilistic Failure Assessment of a Complex Nozzle Structure With Flaw Defect Based on FITNET Procedure

2016 ◽  
Author(s):  
Yan Wang ◽  
Yan-Wei Wang ◽  
Hanxin Chen ◽  
Linwei Ma
Keyword(s):  
Crack Growth ◽  
Failure Probability ◽  
Driving Force ◽  
Complex Geometry ◽  
Crack Depth ◽  
Initial Crack ◽  
Structural Safety ◽  
Crack Driving Force ◽  
Failure Assessment ◽  
Nozzle Structure

A probabilistic failure assessment based on the fracture and fatigue modules of European FITNET procedure is presented in this work. Analysis of the leak probability of a complex nozzle structure with postulated flaw defect under thermal mechanical loading is performed. Crack growth is calculated using FITNET fatigue module, in which the crack driving force ΔKeq considering mixed-mode load is applied. For the structural safety evaluation, the failure assessment diagram (FAD) within the frame of FITNET fracture module is utilized with the parameter Keff combing KI and KII. The fracture mechanical parameters are calculated using finite element (FE) method because of the complex geometry and load conditions. To meet the needs of probabilistic analyses, formulas calculating crack driving force are developed specific for this nozzle structure through nonlinear regression based on the FE results. With an initial crack depth of 5 mm, the nozzle failure probability in form of leak comes to 1.84×10−4 in next fifty years. The good agreement of the results of Monte Carlo simulation and stratified sampling technique confirms that the crack growth parameter C and the initial crack shape ratio c/a have considerable effect on the structural failure probability.

Limit Strains of X70 Pipes With a Semi-Elliptical Crack Based on Initiation and Ductile Tearing Criteria

2018 ◽  
Author(s):  
Ju-Yeon Kang ◽  
Youn-Young Jang ◽  
Nam-Su Huh ◽  
Ki-Seok Kim ◽  
Woo-Yeon Cho
Keyword(s):  
Crack Initiation ◽  
Crack Resistance ◽  
Crack Growth ◽  
Driving Force ◽  
J Integral ◽  
Resistance Curve ◽  
Crack Driving Force ◽  
Ductile Tearing ◽  
Resistance Curves ◽  
Crack Resistance Curves

Crack-tip opening displacement (CTOD) and J-integral have been used for elastic-plastic fracture parameters as a crack driving force (CDF) and crack resistance curve to evaluate tensile strain capacity (TSC) of cracked pipelines based on strain-based design (SBD). The TSC can be determined by using two kinds of failure criteria. One is based on the limit state corresponding to an onset of stable crack growth and the other is tangency approach which determines an onset of unstable crack growth by comparing crack driving force and resistance curve. For this reason, the accurate calculation of crack driving force and crack resistance curve is highly required to determine TSC. In the present study, the TSCs for X70 pipelines with a circumferential semi-elliptical surface crack were estimated based on both crack initiation and ductile tearing criteria using crack driving force diagram (CDFD) method. The CDF curves of cracked pipelines were calculated through the detailed elastic-plastic finite element (FE) analyses. Crack resistance curves were obtained from experimental data of single edged notch tension (SENT) specimens. Both the CDF and crack resistance curves were represented using CTOD and J-integral, respectively. As for loading conditions, axial strain and internal pressure were considered. The TSCs based on CTOD were compared with those based on J-integral to investigate the effect of choice of the fracture parameters on TSC. From the FE results, the TSCs based on ductile tearing allowed higher TSCs than those based on crack initiation. Although there were some differences between the TSCs using CTOD and J-integral, the effect of choice of fracture parameter on TSC with internal pressure was not significant.

Determination of Crack-Initiation in Fracture Toughness Testing Using an Experimental Key-Curve Methodology

2021 ◽  
Author(s):  
S. Pothana ◽  
G. Wilkowski ◽  
S. Kalyanam ◽  
J. K. Hong ◽  
C. J. Sallaberry
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Power Law ◽  
Direct Current ◽  
Electric Potential ◽  
Crack Mouth Opening Displacement ◽  
Ductile Tearing ◽  
Curve Method ◽  
Unloading Compliance

Abstract A new approach was implemented to confirm the start of ductile tearing relative to assessments by other methods such as direct-current Electric Potential (d-c EP) method in coupon specimens. This approach was developed on the Key-Curve methodology by Ernst/Joyce and is similar to the ASTM E-1820 Load Normalization procedure used to determine J-R curves directly from load versus Load-Line Displacement (LLD) record of the test specimen. It is consistent with Deformation Plasticity relationships for fully plastic behavior. Using this Experimental Key-Curve method, crack initiation can be determined directly from load versus LLD data or load versus Crack-Mouth Opening Displacement (CMOD) obtained from a fracture test without the need for additional instrumentation required for crack initiation detection. It is based on the fact that plastic deformation of homogeneous metals at the crack tip follows a power-law function until the crack tearing initiates. Crack tearing initiation is determined at the point where the power-law fit to the load versus plastic part of CMOD or LLD curve deviates from the total experimental load versus plastic-CMOD or LLD curve. The procedure for fitting of the data requires some care to be exercised such that the fitted data is beyond the elastic region and early small-scale plastic region of the Load-CMOD or Load-LLD curve but include data before crack initiation. An iterative regression analysis was done to achieve this, which is shown in this paper. The iterative fitting in this region typically results with a coefficient of determination (R2) values that are greater than 0.990. This method can be either used in conjunction with other methods such as direct-current Electric Potential (d-c EP) or unloading-compliance methods as a secondary (or primary) confirmation of crack tearing initiation (and even for crack growth); or can be used alone when other methods cannot be used. Furthermore, when using instrumentation methods for determining crack-initiation such as d-c EP method in a fracture toughness test, it is good to have a secondary confirmation of the initiation point in case of instrumentation malfunction or high signal to noise ratio in the measured d-c EP signals. In addition, the Experimental Key-Curve procedure provides relatively smooth data for the fitting procedure, while unloading-compliance data when used to get small crack growth values frequently has significant variability, which is part of the reason that JIC by ASTM E1820 is determined using an offset with some growth past the very start of ductile tearing. In this work, the Experimental Key-Curve method had been successfully used to determine crack tearing initiation and demonstrate the applicability for different fracture toughness specimen geometries such as SEN(T), and C(T) specimens. In all the cases analyzed, the Experimental Key-Curve method gave consistent results that were in good agreement with other crack tearing initiation measuring method such as d-c EP but seemed to result in less scatter.

scholarly journals Short fatigue crack growth in welded joints described by the effective cyclic J-integral

2018 ◽  
Vol 165 ◽  
pp. 09002
Author(s):  
Désiré Tchoffo Ngoula ◽  
Michael Vormwald
Keyword(s):  
Fatigue Crack ◽  
Fatigue Life ◽  
Fatigue Crack Growth ◽  
Residual Stresses ◽  
Crack Growth ◽  
Welded Joints ◽  
Driving Force ◽  
J Integral ◽  
Crack Driving Force ◽  
Short Fatigue Crack

The purpose of the present contribution is to predict the fatigue life of welded joints by using the effective cyclic J-integral as crack driving force. The plasticity induced crack closure effects and the effects of welding residual stresses are taken into consideration. Here, the fatigue life is regarded as period of short fatigue crack growth. The node release technique is used to perform finite element based crack growth analyses. For fatigue lives calculations, the effective cyclic J-integral is employed in a relation similar to the Paris (crack growth) equation. For this purpose, a specific code was written for the determination of the effective cyclic J-integral for various lifetime relevant crack lengths. The effects of welding residual stresses on the crack driving force and the calculated fatigue lives are investigated. Results reveal that the influence of residual stresses can be neglected only for large load amplitudes. Finally, the predicted fatigue lives are compared with experimental data: a good accordance between both results is achieved.

A Fatigue Crack Driving Force Parameter

2008 ◽  
Author(s):  
Ying Xiong ◽  
Zengliang Gao ◽  
Junichi Katsuta ◽  
Takeshi Sakiyama
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Driving Force ◽  
Crack Tip ◽  
Force Model ◽  
Crack Driving Force ◽  
R Ratio ◽  
Two Parameter ◽  
Better Than

Most of the previous parameters that utilized as a crack driving force were established in modifying the parameter Kop in Elber’s effective SIF range (ΔKeff = Kmax–Kop). This paper focuses on the physical meaning of compliance changes caused by plastic deformation at the crack tip, the test was carried out for structural steel under constant amplitude loading, and differences of several parameter ΔKeff in literature are analyzed quantificationally. The effect of actual stress amplitude at the crack tip on fatigue crack growth is investigated, and improved two-parameter driving force model ΔKdrive(=Kmax)n(ΔK^)1−n) has been proposed. Experimental data for several different types of materials taken from literature were used in the analyses. Presented results indicate that the parameter ΔKdrive is equally effective or better than ΔK(=Kmax-Kmin), ΔKeff(=Kmax-Kop) and ΔK*(=(Kmax)α(ΔK+)1−α) in correlating and predicting the R-ratio effects on fatigue crack growth rate.

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