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.

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.

Investigation of Circumferential Internal Surface-Cracked Pipes and Elbows Tested at Conditions Similar to PWR and an “Apparent Net-Section-Collapse” Prediction Methodology

2020 ◽  
Author(s):  
G. Wilkowski ◽  
S. Kalyanam ◽  
S. Burger ◽  
S. Gilbert ◽  
S. Pothana ◽  
...  
Keyword(s):  
Flow Stress ◽  
Crack Initiation ◽  
Fracture Behavior ◽  
Crack Depth ◽  
Internal Surface ◽  
Material Composition ◽  
Surface Cracks ◽  
Pressurized Water ◽  
Deep Surface ◽  
Maximum Moment

Abstract The Original Net-Section-Collapse (NSC) analysis was developed in the 1970s for prediction of the maximum (failure) moment for a circumferential flaw in a pipe, and is used widely in pipe flaw assessments. A large number of past pipe tests show that deep surface cracks can break through the thickness and result in leaks; hence, the maximum moment of that surface-cracked pipe was below the maximum moment for the circumferential through-wall crack with the same length. In these cases, the applied moment has to be increased for the resulting leak to grow as a through-wall crack. Hence, load-controlled leak-before-break (LBB) fracture behavior has been experimentally observed although it is not predictable by the Original NSC analysis. Recently, Original NSC analysis for circumferential surface-cracked pipes under combined bending and axial tension were enhanced through the development of the “Apparent Net-Section Collapse” methodology to explain inconsistencies with the Original NSC. “Apparent NSC” methodology was developed considering surface-cracked pipe test data developed from external (OD) surface-cracked pipe tests conducted at room temperature (RT) with a vast majority conducted under pure bending and unpressurized conditions. Since it is undesirable to have leakage in many applications, the deficiency in the Original NSC analysis was shown experimentally, and the recently developed “Apparent NSC” methodology applied to a carefully planned matrix of pipe and elbow tests conducted on TP304 stainless steel and Alloy600 materials with different flaw dimensions (composed of short and shallow to long and deep surface cracks), in the range of normalized crack depth, a/t = 0.4 to 0.8 and crack length, 2θψ = 90° to 180°. The tests were conducted under conditions similar to a pressurized water reactor (PWR), and consistent with the International Piping Integrity Research Group (IPIRG-2) [1] test conditions, namely a temperature of 550°F (288°C) and an internal pressure of 2,250 psi. The loads corresponding to the surface-crack initiation, maximum load, and leakage events were recorded from each of the surface-cracked pipe and elbow tests. The data were used to understand the predictable nature of the “Apparent NSC” methodology and to develop an understanding of the fracture behavior of surface-cracked pipes leading to correlation of these results to LBB behavior. Further, the results were correlated between the material composition and the variation of the experimental and predicted bending stress from NSC loads to observations from the previous IPIRG-2 program, where the experimental burst loads were characterized with respect to the flow stress assumptions. The material composition such as variation in sulfur content, and the crack-initiation and crack growth based on elastic-plastic fracture mechanics were used to explain the variability of the flow stress assumption when used in a NSC/limit-load type of analysis. The investigation also showed comparison of predictions based on various flow stress (σf) definitions assumed using yield and ultimate stresses obtained from the tensile tests conducted on the pipe and elbow materials at 550°F (288°C) and applied to the Original NSC and “Apparent NSC” methodologies. The moment predictions using ASME elbow stress indices (B2, C2 used in design) or the IPIRG-2 parameter (Ψec) for the circumferentially surface-cracked elbows were also compared to the experimental maximum moments for the tested elbows.

The Fracture Behavior of Cylinders With Axial Surface Cracks Under Strong Through-Wall Bending Stresses

2013 ◽  
Author(s):  
Yuebing Li ◽  
Zengliang Gao ◽  
Yuebao Lei
Keyword(s):  
Crack Initiation ◽  
Crack Depth ◽  
Surface Cracks ◽  
Surface Point ◽  
Critical Crack ◽  
Diagram Method ◽  
Pressurized Thermal Shock ◽  
Fracture Behaviors ◽  
Bending Stresses ◽  
Axial Surface

Reactor pressure vessel (RPV) may subject to strong through-wall bending stresses under some situations, such as pressurized thermal shock (PTS) transients. The fracture behaviors of cylinders with internal axial surface cracks under PTS conditions are investigated in this paper. A typical PTS transient is selected to simulate the bending stress. The critical crack sizes including crack depth and crack surface length for a given cylinder is analyzed for the assumed PTS transient using the critical crack depth diagram method. Crack initiation and growth at both the deepest and surface points are considered and the effect of the crack growth at surface point on the growth along the wall-thickness is investigated. The results show that the crack initiation and growth at the surface point should be considered and the critical crack depth for the assumed transient may depend on the fracture behaviors at both the deepest and surface points.

Fatigue Crack Growth and Coalescence Algorithm Starting from Multiple Surface Cracks

2014 ◽  
Vol 891-892 ◽  
pp. 1003-1008 ◽  
Author(s):  
John Hock Lye Pang ◽  
You Xiang Chew
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Surface Crack ◽  
Crack Depth ◽  
Surface Cracks ◽  
Growth Data ◽  
Aspect Ratios ◽  
Weld Toe ◽  
Fatigue Crack Propagation Life

Fatigue crack growth and propagation analysis in welded joints have to deal with the complexity of modeling multiple weld toe surface cracks originating from weld toes. Fitness-For-Service (FFS) assessments for weld toe surface cracks employ a fracture mechanics and Paris Law approach to predict the fatigue crack propagation life of a semi-elliptical surface crack (SESC) to failure. A fatigue crack growth algorithm for assessing multiple surface crack growth, coalescence and propagation life was initially validated with previuously report crack growth data for a fillet shoulder specimen. Next a parametric study for single, double, and triple SESCs located along the weld toe line of a fillet weld was investigated with three starting crack depth sizes (0.1mm, 0.5mm, 1.0mm) coupled with three different crack aspect ratios (a/c = 1.0, a/c = 0.5 and 0.25) giving a total of 27 cases studied.

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.

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.

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.

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