Constraint Effects of Surface Crack Depth on Toughness: Experimental and Numerical Assessments

2019 ◽  
Author(s):  
G. Wilkowski ◽  
S. Kalyanam ◽  
Y. Hioe ◽  
F. W. Brust ◽  
S. Pothana ◽  
...  
Keyword(s):  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Initiation Toughness ◽  
Surface Cracks ◽  
Flat Plates ◽  
Depth Effect ◽  
Ductile Tearing ◽  
Wide Range ◽  
Axial Surface

Abstract Work published for the first time at the ASME PVP 2017 conference showed that when on the upper-shelf, the toughness measured directly from surface-cracked pipe tests decreased as the flaw depth increased. A similar trend existed in SENT tests. Initially it was found that this flaw depth sensitivity of the toughness occurred for a very tough material like TP304 stainless steel. The significance of that result was that even for a material where limit-load was thought to exist, as the flaw depth increased the toughness dropped appreciably, and the failure analysis mode changed from limit-load to elastic-plastic fracture. Experimentally, this made sense because it explained the observed phenomena of load-controlled leak-versus-break behavior for circumferential surface-cracked pipes (as will be shown for several pipe tests), but that LBB behavior is not predictable from circumferential flaw limit-load analysis. Furthermore, the flaw depth effect on toughness also exists for axial surface cracks and even in flat plates with surface cracks. For axial surface cracks the implication was that the long-used empirical surface-crack bulging factor from Maxey/Kiefner (incorporated in many international codes and standards) actually incorporated both the bulging factor and the toughness changes with flaw depth. Because of the change in toughness with flaw depth, when using detailed finite-element fracture analyses for the crack-driving force it is possible to have more error in the failure stress predictions if a constant toughness is assumed for all surface-flaw depths. In fact, in another paper in the ASME 2019 PVP conference it will be shown that the toughness in a wrought TP304 elbow at crack initiation of a circumferential surface crack that was 68% of the thickness was about 1/3rd of the toughness from a standard 1T CT specimen made from the same material. Those results will also be reviewed. Similar results of toughness decreasing with flaw depth in surface-cracked pipes and SENT specimens for various materials over a large range of strain-hardening behavior will show the toughness decrease trend with flaw depth is consistent. To understand these trends more theoretically, 3D FE analyses were also conducted for one initial set of TP304 SENT specimens with a wide range of a/w values (0.3 < a/w < 0.9). The initiation toughness decreased by a factor of 5 to 6 as the crack depth increased; however, the Q value coinciding to the load at the start of ductile tearing was constant for the wide range of a/W values. Q at the start of ductile tearing in the SENT (Qi) was more consistent at normalized distances from the crack tip, rσo/J that were in the range from 0.25 to 1.5 rather than just the popularly considered rσo/J = 2. Hence, by having one SENT test result with a single a/W value, the Ji value for any other a/W can then be calculated. This is consistent with the experimental trends to date, but unfortunately Ji was found to be not proportional to the Q values as is conventionally assumed by many researchers at this time.

Fracture Toughness Variation With Flaw Depth in Various Specimen Geometries and Role of Constraint in Material Fracture Resistance

2017 ◽  
Author(s):  
Y. Hioe ◽  
S. Kalyanam ◽  
G. Wilkowski ◽  
S. Pothana ◽  
J. Martin
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Surface Cracks ◽  
Maximum Moment ◽  
Toughness Variation ◽  
Axial Surface

A series of pipe tests with circumferential surface cracks has been conducted along with fracture toughness tests using single-edge notch tension (SENT) specimens having similar crack depths and crack orientations as the surface-cracked pipes. This paper presents observation of measured fracture toughness variation due to the crack depth and discusses the effect of constraint on the material resistance to fracture. Crack-tip-opening displacement (CTOD) measurements were obtained with the use of a dual clip-gauge mounted on both the SENT specimens and center of the surface-cracks in the pipes. CTOD was obtained at both the crack initiation and during the crack growth through the ligament. CTOD is a direct measure of the material toughness in the pipe and SENT tests. CTOD at crack initiation and during crack growth can also be related to the material J-Resistance (J-R) curve. Commonly, the material resistance is assumed to be the same for all circumferential surface-crack geometries in a surface-cracked pipe fracture mechanics analyses. However, based on experimental observations on a series of recently conducted surface-cracked pipe tests, the CTOD at the center of the surface crack at the start of ductile tearing and maximum moment changed with the depth of the surface crack. This is believed to be a constraint effect on plasticity in the ligament which depends on crack depth. The CTOD values at crack initiation were decreasing linearly with crack depth. This linear decrease in CTOD trend with flaw depth was also observed in SENT tests. More importantly, the decrease in CTOD with surface crack depth was significant enough that the failure mode changed from being limit-load to elastic-plastic fracture even in relatively small-diameter TP304 stainless steel pipe tests. This toughness drop explains why the Net-Section-Collapse (limit-load) analysis overpredicted the maximum moment for some crack geometries, and why the deeper surface cracks tore through the pipe thickness at moments below that predicted by the NSC analysis for a through-wall crack of the same circumferential length. An “Apparent NSC Analysis” was developed in a companion paper to account for the changing toughness with crack depth [1]. Finally, this same trend in decreasing toughness with flaw depth is apparent in surface-cracked flat plates [2] and axial surface flaws in pipes [3]. The leak-before-break behavior for axial surface cracks is also not explained by numerical calculations of the crack-driving force when assuming the toughness is constant for all surface cracks and the through-wall cracks, but the change in toughness with surface flaw depth explains this behavior. Previously, axial flaw empirical limit-load solution was developed by Maxey and Kiefner [4], and is consistent with the observations from this paper.

scholarly journals On the Calculation of Stress Intensity Factors and J-Integrals Using the Submodeling Technique

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Eduard Marenić ◽  
Ivica Skozrit ◽  
Zdenko Tonković
Keyword(s):  
Stress Intensity ◽  
Limit Load ◽  
Analytical Approximation ◽  
J Integral ◽  
Surface Cracks ◽  
Linear Elastic ◽  
Reference Stress ◽  
Wide Range ◽  
Reference Pressure ◽  
Axial Surface

In the present paper, calculations of the stress intensity factor (SIF) in the linear-elastic range and the J-integral in the elastoplastic domain of cracked structural components are performed by using the shell-to-solid submodeling technique to improve both the computational efficiency and accuracy. In order to validate the submodeling technique, several numerical examples are analyzed. The influence of the choice of the submodel size on the SIF and the J-integral results is investigated. Detailed finite element solutions for elastic and fully plastic J-integral values are obtained for an axially cracked thick-walled pipe under internal pressure. These values are then combined, using the General Electric/Electric Power Research Institute method and the reference stress method, to obtain approximate values of the J-integral at all load levels up to the limit load. The newly developed analytical approximation of the reference pressure for thick-walled pipes with external axial surface cracks is applicable to a wide range of crack dimensions.

A Global Limit Load Solution for Plates With Semi-Elliptical Surface Cracks Under Combined Tension and Bending

2004 ◽  
Author(s):  
Yuebao Lei
Keyword(s):  
Finite Element ◽  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Elliptical Crack ◽  
Surface Cracks ◽  
Rectangular Crack ◽  
Perfectly Plastic ◽  
Elastic Perfectly Plastic ◽  
New Equation

A new global limit load solution is developed in this paper for a precise semi-elliptical surface crack in a plate under combined tension and bending, based on the net-section collapse principle. The new global limit load solution is compared with finite element (FE) results for the semi-elliptical crack, and with the global limit load solution for the circumscribing rectangular crack. The predictions of the new equation are conservative and close to the elastic-perfectly-plastic FE results for shallow cracks. For narrow plates with deep cracks, however, no FE results for the global limit load are available. The differences between the limit load solutions for a semi-elliptical crack and a rectangular crack are negligible for very wide plates but significant for narrow plates, depending on the normalised crack depth and the ratio between the crack length and width of the plate.

J-Integral Analysis of Surface Cracks in Round Bars under Bending Moments

2011 ◽  
Vol 52-54 ◽  
pp. 43-48 ◽  
Author(s):  
Al Emran Ismail ◽  
Ahmad Kamal Ariffin ◽  
Shahrum Abdullah ◽  
Mariyam Jameelah Ghazali ◽  
Ruslizam Daud
Keyword(s):  
Finite Element ◽  
Crack Depth ◽  
Crack Tip ◽  
Bending Moment ◽  
Limit Load ◽  
Fe Model ◽  
Surface Cracks ◽  
Element Analysis ◽  
Reference Stress ◽  
Proposed Model

This paper presents a non-linear numerical investigation of surface cracks in round bars under bending moment by using ANSYS finite element analysis (FEA). Due to the symmetrical analysis, only quarter finite element (FE) model was constructed and special attention was given at the crack tip of the cracks. The surface cracks were characterized by the dimensionless crack aspect ratio, a/b = 0.6, 0.8, 1.0 and 1.2, while the dimensionless relative crack depth, a/D = 0.1, 0.2 and 0.3. The square-root singularity of stresses and strains was modeled by shifting the mid-point nodes to the quarter-point locations close to the crack tip. The proposed model was validated with the existing model before any further analysis. The elastic-plastic analysis under remotely applied bending moment was assumed to follow the Ramberg-Osgood relation with n = 5 and 10. J values were determined for all positions along the crack front and then, the limit load was predicted using the J values obtained from FEA through the reference stress method.

Predicting the Brittle-to-Ductile Transition Temperatures for Surface Cracks in Pipeline Girth Welds: It’s Better Than You Thought

2008 ◽  
Author(s):  
Gery Wilkowski ◽  
David Rudland ◽  
Do-Jun Shim ◽  
David Horsley
Keyword(s):  
Transition Temperature ◽  
Surface Crack ◽  
Master Curve ◽  
Crack Depth ◽  
Temperature Shift ◽  
Transition Temperatures ◽  
Surface Cracks ◽  
Line Pipe ◽  
Brittle To Ductile Transition ◽  
Girth Welds

A methodology to predict the brittle-to-ductile transition temperature for sharp or blunt surface-breaking defects in base metals was developed and presented at IPC 2006. The method involved applying a series of transition temperature shifts due to loading rate, thickness, and constraint differences between bending versus tension loading, as well as a function of surface-crack depth. The result was a master curve of transition temperatures that could predict dynamic or static transition temperatures of through-wall cracks or surface cracks in pipes. The surface-crack brittle-to-ductile transition temperature could be predicted from either Charpy or CTOD bend-bar specimen transition temperature information. The surface crack in the pipe has much lower crack-tip constraint, and therefore a much lower brittle-to-ductile transition temperature than either the Charpy or CTOD bend-bar specimen transition temperature. This paper extends the prior work by presenting past and recent data on cracks in line-pipe girth welds. The data developed for one X100 weld metal shows that the same base-metal master curve for transition temperatures works well for line-pipe girth welds. The experimental results show that the transition temperature shift for the surface-crack constraint condition in the weld was about 30C lower than the transition temperature from standard CTOD bend-bar tests, and that transition temperature difference was predicted well. Hence surface cracks in girth welds may exhibit higher fracture resistance in full-scale behavior than might be predicted from CTOD bend-bar specimen testing. These limited tests show that with additional validation efforts the FITT Master Curve is appropriate for implementation to codes and standards for girth-weld defect stress-based criteria. For strain-based criteria or leak-before-break behavior, the pipeline would have to operate at some additional temperature above the FITT of the surface crack to ensure sufficient ductile fracture behavior.

Bulging Factor for Axial Surface Cracks in Pipes

2014 ◽  
Author(s):  
Do-Jun Shim ◽  
Gery Wilkowski
Keyword(s):  
Surface Crack ◽  
Maximum Pressure ◽  
Surface Cracks ◽  
Correction Factors ◽  
Constant Depth ◽  
Dugdale Model ◽  
Crack Driving Force ◽  
Actual Surface ◽  
Secant Equation ◽  
Axial Surface

The bulging factor for an external constant-depth axial surface crack in a pipe was calculated by 3D FE simulations. This was done in a manner consistent with Folias’s original work for the axial through-wall-cracked pipe bulging factor (MT), but was evaluated in the elastic to full-plastic conditions. The results demonstrated that the actual surface-cracked pipe bulging factor is considerably lower than the bulging factor empirically derived by Maxey/Kiefner (Mp) back in the 1970s. Based on the results of the present study, it is suggested that Mp function in the Ln-Secant equation is not truly a bulging factor for axial surface crack. Rather it is an empirically developed equation with many correction factors embedded in it to apply the Dugdale model for prediction of maximum pressure of axial surface-cracked pipes. However, due to this empiricism, this method becomes invalid (or overly conservative) when it is applied in predicting the crack-driving force using the J-based Ln-Secant equation.

Stress Intensity Factors for Semi-Elliptical Surface Cracks With Flaw Aspect Ratio Beyond the ASME XI Limit

2009 ◽  
Author(s):  
Christian Malekian ◽  
Eric Wyart ◽  
Michael Savelsberg ◽  
Anne Teughels ◽  
Pierre-Eric Fouquet ◽  
...  
Keyword(s):  
Stress Intensity ◽  
Finite Elements ◽  
Aspect Ratio ◽  
Stress Intensity Factors ◽  
Large Range ◽  
Crack Depth ◽  
Surface Cracks ◽  
Intensity Factors ◽  
Flat Plates ◽  
Circular Shape

Most of the literature about fracture mechanics considers cracks having an elliptical shape with a flaw aspect ratio a/l lower or equal to 0.5 where ‘a’ is the crack depth and ‘l’ the total length of the crack. This is also case in the ASME XI Appendix A where Stress Intensity Factors KI formulations are given for a large range of crack depths and for a flaw aspect ratio a/l between 0 and 0.5. The limitation to 0.5 corresponds to a semi-circular shape for surface cracks and to a circular shape for subsurface cracks. This limitation does not seem to be inspired by a theoretical limitation nor by a computational limit. Moreover, it appears that limiting the ratio a/l to 0.5 may generate in some cases some unnecessary conservatism in flaw analysis. The present article specifically deals with the more unusual narrow cracks having a/l >0.5, in the case of surface cracks in infinite flat plates. Several Finite-Elements calculations are performed to compute KI for a large range of crack depths and for 4 typical load cases (uniform, linear, quadratic and cubic). The results can be presented with the same formalism as in the ASME XI Appendix A, such that the work can provide an extension of the ASME coefficients in table A-3320-1&2. By doing the study, one had the opportunity to compare the results obtained by two different Finite-Elements softwares (Systus and Ansys), each one with a different cracked mesh. In addition, a comparison has been made for some cases with results obtained by a XFEM approach (eXtended Finite-Element Method), where the crack does not need to be meshed in the same way as in classical Finite-Elements. The results indicate how the KI can be reduced when considering the real flaw aspect ratio instead of the conventional semi-circular flaw shape. They also show that, for specific theoretical stress distributions, it is not always possible to reduce the analysis of KI to only 2 points, namely the crack surface point and the crack deepest point. The crack growth evaluation of such unusual crack shape should still be investigated to verify whether simple rules can be established to estimate the evolution of the crack front.

Finite Element Analysis of Polyethylene Pipe Butt Fusion Joints with Circumferential Surface Cracks

2013 ◽  
Vol 785-786 ◽  
pp. 1151-1158
Author(s):  
Zhi Bin Zhu ◽  
Xiao Xiang Yang ◽  
Li Jing Chen ◽  
Nai Chang Lin ◽  
Zhi Tuo Wang ◽  
...  
Keyword(s):  
Stress Intensity Factor ◽  
Finite Element ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Viscoelastic Material ◽  
Surface Crack ◽  
Crack Depth ◽  
Surface Cracks ◽  
Element Analysis ◽  
Polyethylene Pipe

Based on the viscoelastic material property of polyethylene pipe, software ANSYS was used to simulate and analyze the mechanical property of polyethylene pipe butt fusion joints with circumferential surface crack defects. The viscoelastic material creep parameters were characterized as Prony series and 1/4 node singular element was selected for meshing along the boundaries of the crack, then the stress intensity factor of polyethylene pipe butt fusion joints with circumferential surface crack was calculated under the uniform internal pressure. Through the finite element simulation, the result showed that polyethylene pipe were most likely to fracture failure when crack initiated. Thus the viscoelasticity of materials can be ignored when analyzing the stress intensity factor of circumferential surface cracks of polyethylene pipe. the main influencing factor of the circumferential crack defects was the ratio of the crack depth to the thickness of polyethylene pipe.

STRESS INTENSITY FACTORS OF A SEMI-ELLIPTICAL CRACK IN A HOLLOW CYLINDER

2015 ◽  
Vol 39 (3) ◽  
pp. 557-568
Author(s):  
Shiuh-Chuan Her ◽  
Hao-Hi Chang
Keyword(s):  
Finite Element ◽  
Stress Intensity ◽  
Hollow Cylinder ◽  
Stress Intensity Factors ◽  
Surface Crack ◽  
Crack Depth ◽  
Weight Functions ◽  
Intensity Factors ◽  
Regular Elements ◽  
Wide Range

In this investigation, the weight function method was employed to calculate stress intensity factors for semi-elliptical surface crack in a hollow cylinder. A uniform stress and a linear stress distribution were used as the two references to determine the weight functions. These two factors were obtained by a three-dimensional finite element method which employed singular elements along the crack front and regular elements elsewhere. The weight functions were then applied to a wide range of semi-elliptical surface crack subjected to non-linear loadings. The results were validated against finite element data and compared with other analyses. In the parametric study, the effects of the ratio of the surface crack depth to length ranged from 0.2 to 1.0 and the ratio of the crack depth to the wall thickness ranged from 0.2 to 0.8 on stress intensity factors were investigated.

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