CTOD-Resistance Curve Testing and Evaluation for Clamped SENT Specimens

2018 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Tom McGaughy
Keyword(s):  
Strain Hardening ◽  
Oil And Gas Industry ◽  
Test Methods ◽  
Estimation Methods ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Integrity Assessment ◽  
Conversion Method ◽  
Strain Hardening Rate

To assess the integrity of pipelines containing cracks, single edge-notched tension (SENT) specimens in the end-clamped conditions have been widely adopted in the oil and gas industry to measure fracture toughness or resistance curves in terms of the J-integral or crack-tip opening displacement (CTOD). The CTOD toughness is often utilized in the strain-based design, and thus its measurement is important to the pipeline industry. Two types of CTOD-R curve test methods are available for a single SENT specimen test: J-conversion method and double clip gage (DCG) measurement method. However, these two CTOD test methods often determine different R-curves, leading to a long-running dispute. To better understand the difference of the two CTOD test methods as well as the effect of material strain hardening rate on CTOD-R curves, a set of clamped SENT specimens are tested for two ductile steels with a high strain hardening rate (A36) and a low strain hardening rate (X80). Experimental R-curves are analyzed for the two steels, and results show that the CTOD-R curves determined using the J-conversion method and the DCG method are comparable for X80, but significantly different for A36. To study the root cause, elastic-plastic finite element analyses are performed for the SENT specimens of A36 and X80. With the numerical results of J-integral and CTOD, different CTOD estimation methods are evaluated, and the root causes of their differences are analyzed. On this basis, discussions are made on how to use the two types of CTOD-R curves in the pipeline design and integrity assessment.

Evaluation of Fracture Toughness CTOD Testing and its Standard Test Methods for SENB Specimens

2018 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Tom McGaughy
Keyword(s):  
Oil And Gas ◽  
Plastic Hinge ◽  
Fracture Initiation ◽  
Oil And Gas Industry ◽  
Test Methods ◽  
Standard Test ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Conversion Method ◽  
Hinge Model

Crack-tip opening displacement (CTOD) is an important fracture mechanics parameter. Due to different definitions, existing CTOD test methods may determine different CTOD toughness for stationary and growing cracks even for a standard specimen, such as single edge notched bend (SENB) specimen. In the USA, ASTM 1820 uses the J-integral conversion method to determine CTOD, whereas in the UK, BS 7448 adopts the plastic hinge model to determine CTOD. In contrast, ISO 12135 uses the plastic hinge model to determine the initiation CTOD, but the J-conversion method to determine a CTOD-R curve. Thus, those standards may determine different CTOD values for the same material, leading to a long-time dispute. Recently, a double clip gage (DCG) method was used in the oil and gas industry to measure CTOD. As such, there are three typical CTOD test methods: plastic hinge model, J-conversion method, and DCG method. To better understand those CTOD test methods for SENB specimens, the present paper first gives a brief review of CTOD standard test methods. This includes the CTOD definitions, plastic hinge model, J-conversion method, modified plastic hinge model, and DCG method, BS 7448, ASTM E1820, and ISO 12135. Those CTOD test methods are then evaluated using available test data for various ductile steels in literature. The experimental results of CTOD at fracture initiation and crack tearing are compared, and the differences are discussed.

Evaluation of CTOD Resistance Curve Test Methods Using SENT Specimens

2016 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Paul Zelenak ◽  
Tom McGaughy
Keyword(s):  
Pipeline Steel ◽  
Test Methods ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Gas Industry ◽  
Offshore Pipelines ◽  
Resistance Curves ◽  
Plastic Parts ◽  
Crack Tip Opening

Single edge-notched tension (SENT) specimens in clamped end conditions are widely adopted in the oil/gas industry to measure fracture resistance curves in terms of the J-integral or crack-tip opening displacement (CTOD) for the strain-based design and crack assessment of onshore and offshore pipelines. Typical CTOD-R curve test methods developed for SENT specimens include CanMet, ExxonMobil, and BS 8571 methods. While CanMet determines CTOD using the J-integral conversion method via one clip gage measurement, the other two infer CTOD directly from double clip gage measurements. ExxonMobil simply uses the total measured displacements to calculate CTOD, but BS 8571 separates CTOD into elastic and plastic parts to be determined, respectively, from the elastic K factor and the plastic component of measured displacements. It is unknown if these CTOD test methods determine comparable R-curves for a same SENT test, and what are the differences between these CTOD test methods. To answer those questions, this paper performs an experimental evaluation of the SENT CTOD test methods. A set of clamped SENT specimens are tested for pipeline steel X80 and structural steel A36. For each SENT test, the unloading compliance method and the double clip gage arrangement are used, and then CTOD-R curves obtained by different methods are evaluated. Evaluation results and conclusions are given for those SENT CTOD test methods.

Direct Comparison of Single-Specimen Clamped SE(T) Test Methods on X100 Line Pipe Steel

2014 ◽  
Author(s):  
Timothy S. Weeks ◽  
Enrico Lucon
Keyword(s):  
Crack Growth ◽  
Mouth Opening ◽  
Closed Form Solution ◽  
Test Methods ◽  
J Integral ◽  
Single Specimen ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Line Pipe ◽  
Resistance Curves

The clamped single edge-notched tension (SE(T)) specimen has been widely used in a single-specimen testing scheme to generate fracture resistance curves for high strength line-pipe steels. The SE(T) specimen with appropriate notch geometry is a low-constraint specimen designed to reduce conservatism in the measurement of fracture toughness. The crack driving force is taken as either the J-integral or crack tip opening displacement (CTOD); it is generally accepted that the two parameters are interchangeable and equivalent using a simple closed form solution. However, the assumption that they are interchangeable, and to what extent, hasn’t been previously investigated experimentally on the same SE(T) specimen. This paper presents multiple test methods that were simultaneously employed on the same SE(T) specimens. The instrumentation includes: clip-gauges to measure surface crack mouth opening displacements (CMOD) and CTOD by the double-clip-gauge method; strain-gage arrays for direct J-integral measurements; and direct-current potential-drop (DCPD) instrumentation for supplementary crack size measurement. A direct comparison of ductile crack-growth resistance curves generated using J-integral and CTOD is presented here where each represents a different experimental and analytical approach. The two methods are in reasonable agreement over a narrow range of crack growth, differing slightly at initiation and diverging with increasing crack growth. Analysis of the supplementary instrumentation (i.e., strain gages, extensometers and DCPD) will be provided in a future publication.

Study of Constraint Issues in Elasto-Plastic Fracture Analysis Using Experimental and Finite Element Simulation

2017 ◽  
Author(s):  
Md Ibrahim Kittur ◽  
Krishnaraja G. Kodancha ◽  
C. R. Rajashekar
Keyword(s):  
Finite Element ◽  
Mild Steel ◽  
Compact Tension ◽  
Specimen Thickness ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
3D Fea ◽  
The Comparative Study ◽  
Crack Tip Opening

In this investigation, the variation of J-integral considering Compact Tension (CT) specimen geometry varying a/W and σ using 2D and 3D elasto-plastic Finite Element (FE) analysis have been studied. Further, the investigation has been done to examine the relationship between the J and δ for varied a/W and σ. The plane stress and plane strain elasto-plastic FE analyses have been conducted on the CT specimen with a/W = 0.45–0.65 to extract the J and Crack-tip Opening Displacement (CTOD) values for mild steel. The comparative study of the variation of dn with a/W of mild steel with earlier results of IF steel is carried out. The study clearly infers the effect of yield stress on the variation of the magnitude of dn with reference to a/W ratio. The present analysis infers that while converting the magnitude of the CTOD to J one needs to carefully evaluate the value of dn depending on the material rather than considering it to be unity. Further, the study was extended to experimental and 3D FEA wherein J-integral and CTOD were estimated using the CT specimen. Experimental results reveal that the crack length, the specimen thickness, and the loading configuration have an effect on the fracture toughness measurements. The error analysis between the results obtained by 3D FEA and experimentation were conducted and found to be within limits.

scholarly journals Additive Manufacturing with Superduplex Stainless Steel Wire by CMT Process

Metals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 272 ◽  
Author(s):  
Malin Lervåg ◽  
Camilla Sørensen ◽  
Andreas Robertstad ◽  
Bård M. Brønstad ◽  
Bård Nyhus ◽  
...  
Keyword(s):  
Additive Manufacturing ◽  
Volume Fraction ◽  
Steel Wire ◽  
Oil And Gas Industry ◽  
Secondary Phases ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Σ Phase ◽  
Wide Range ◽  
Wire Arc Additive Manufacturing

For many years, the oil and gas industry has utilized superduplex stainless steels due to their high strength and excellent corrosion resistance. Wire arc additive manufacturing (WAAM) was used with superduplex filler wire to create walls with different heat input. Due to the multiple heating and cooling cycles during layer deposition, brittle secondary phases may form such as intermetallic sigma (σ) phase. By inspecting deposited walls within wide range of heat inputs (0.40–0.87 kJ/mm), no intermetallic phases formed due to low inter-pass temperatures used, together with the high Ni content in the applied wire. Lower mechanical properties were observed with high heat inputs due to low ferrite volume fraction, precipitation of Cr nitrides and formation of secondary austenite. The walls showed good toughness values based on both Charpy V-notch and CTOD (crack tip opening displacement) testing.

The Inelastic Line-Spring: Estimates of Elastic-Plastic Fracture Mechanics Parameters for Surface-Cracked Plates and Shells

1981 ◽  
Vol 103 (3) ◽  
pp. 246-254 ◽  
Author(s):  
D. M. Parks
Keyword(s):  
Fracture Mechanics ◽  
Crack Growth ◽  
Computing Time ◽  
Crack Tip ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Elastic Plastic ◽  
Plates And Shells ◽  
Crack Tip Opening

Recent studies of the mechanics of elastic-plastic and fully plastic crack growth suggest that such parameters as the J-integral and the crack tip opening displacement can, under certain conditions, be used to correlate the initiation and early increments of the ductile tearing mode of crack growth. To date, elastic-plastic fracture mechanics has been applied mainly to test specimen geometries, but there is a clear need for developing practical analysis capabilities in structures. In principle, three-dimensional elastic-plastic finite element analysis could be performed, but, in fact, such analyses would be prohibitively expensive for routine application. In the present work, the line-spring model of Rice and Levy [1-3] is extended to estimate the J-integral and crack tip opening displacement for some surface crack geometries in plates and shells. Good agreement with related solutions is obtained while using orders of magnitude less computing time.

Methods to Determine Low-Constraint Fracture Toughness: Review and Progress

2016 ◽  
Author(s):  
Xian-Kui Zhu
Keyword(s):  
Fracture Toughness ◽  
Test Methods ◽  
Standard Test ◽  
Initiation Toughness ◽  
Fracture Toughness Test ◽  
Crack Tip Opening Displacement ◽  
Test Procedures ◽  
Opening Displacement ◽  
Resistance Curves ◽  
Low Constraint

Fracture toughness is often described by the J-integral or crack-tip opening displacement (CTOD) for ductile materials. ASTM, BSI and ISO have developed their own standard test methods for measuring fracture initiation toughness and resistance curves in terms of the J and CTOD using bending dominant specimens in high constraint conditions. However, most actual cracks are in low constraint conditions, and the standard resistance curves may be overly conservative. To obtain more realistic fracture toughness for actual cracks in low-constraint conditions, different fracture test methods have been developed in the past decades. To facilitate understanding and use the test standards, this paper presents a critical review on commonly used fracture toughness test methods using standard and non-standard specimens in reference to the fracture parameters J and CTOD, including (1) ASTM, BSI and ISO standard test methods, (2) constraint correction methods for formulating a constraint-dependent resistance curve, and (3) direct test methods using the single edge-notched tension (SENT) specimen. This review discusses basic concepts, basic methods, estimation equations, test procedures, historical efforts and recent progresses.

Validation on the Relationship Between J Integral and CTOD for Offshore Structural Steel Weldments by Experimental and Numerical Analyses

2014 ◽  
Author(s):  
Dong Hyun Moon ◽  
Jeong Soo Lee ◽  
Jae Myung Lee ◽  
Myung Hyun Kim
Keyword(s):  
Plastic Deformation ◽  
Crack Tip ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Constraint Factor ◽  
Element Analysis ◽  
Elastic Plastic ◽  
Plastic Constraint ◽  
The Relationship

Elastic plastic fracture mechanics (EPFM) is the domain of fracture analysis which considers extensive plastic deformation at crack tip prior to fracture. J integral and crack tip opening displacement (CTOD) have been commonly used as parameters for EPFM analysis. The relationship between these parameters has been extensively studied by industry and academia. The plastic constraint factor can serve as a parameter to characterize constraint effects in fracture involving plastic deformation. Therefore, the characteristics of plastic constraint factor are important in EPFM analysis. In this study, the relationship between J Integral and CTOD was investigated by conducting fracture toughness tests using single edge notched bend (SENB) specimens. Also, plastic constraint factor was investigated by using finite element analysis. Numerical analysis was carried out using ABAQUS elastic-plastic analysis mode.

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