Evaluation of Two Low-Constraint Toughness Test Methods in a Single Specimen

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Dong-Yeob Park ◽  
Jean-Philippe Gravel ◽  
Muhammad Arafin ◽  
Jie Liang ◽  
C. Hari Manoj Simha
Keyword(s):  
Crack Size ◽  
Test Methods ◽  
Single Specimen ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Resistance Curves ◽  
Unloading Compliance ◽  
The Difference ◽  
Low Constraint ◽  
Identical Test

In previous studies, the single- and double-clip gauge methods were successfully consolidated in a single-edge notched tension (SE(T)) single specimen so that crack tip opening displacement (CTOD) values obtained from both SE(T) methods could be compared under identical test conditions. The current study investigated the effect of unloading compliance crack size equations on resistance curves obtained from both gauging methods combined in a single specimen. It was found that the unloading compliance crack size equations of Cravero and Ruggieri and the single clip gauge method predict crack sizes well within approximately 2% error in average. Two CTOD-resistance curves obtained from both gauging methods produce approximately the same results until peak loads, and thereafter the curves deviate. The results obtained from the double clip gauge method are consistently higher than those from the single clip gauge, although the difference between two resistance curves is reduced when the same unloading compliance crack size prediction procedure is used. This observation is very important within the framework of engineering critical assessment (ECA) and defect assessment procedures. An “apparent” higher resistance curve will generate larger tolerable defects thereby reducing the conservatism of an ECA analysis.

Review of Low-Constraint Fracture Resistance Testing With SENT Specimens

2014 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Tom McGaughy ◽  
Fabian Orth ◽  
Jon Jennings
Keyword(s):  
Fracture Resistance ◽  
Crack Tip ◽  
Test Methods ◽  
Resistance Testing ◽  
Resistance Curve ◽  
Single Specimen ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Resistance Curves ◽  
Low Constraint

Fracture resistance is an important material property and characterized by a J-integral resistance curve (J-R curve) or a crack-tip opening displacement (CTOD) resistance curve. ASTM standard specimens with deep cracks are subject to bending dominant forces, leading to high crack-tip constraint conditions and conservative fracture resistance curves. Actual cracks occurring in line pipes and welds are often shallow ones dominated by tensile forces, resulting in low constraint conditions. Shallow cracks have been shown to generate elevated fracture resistance curves in comparison to standard deep-crack results. To reduce the over-conservatism of standard resistance curves and to produce more realistic toughness properties to meet the needs of strain-based design approaches for pipelines, different procedures and technologies have been developed over the years to determine the low-constraint fracture resistance curves by use of the single edge-notched tension (SENT) specimens. This includes the multiple specimen method developed and standardized by DNV for J-R curve testing, the single specimen method developed by CanMet for J-R and CTOD-R curve testing, and the single-specimen method developed by ExxonMobil for CTOD-R curve testing. This paper delivers a technical review of existing fracture test methods using SENT specimens, and discusses the advantages and limitations of each method.

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.

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.

Low-Constraint Toughness Testing of Two SE(T) Methods in a Single Specimen

2014 ◽  
Author(s):  
Dong-Yeob Park ◽  
Jean-Philippe Gravel ◽  
C. Hari Manoj Simha ◽  
Jie Liang ◽  
Da-Ming Duan
Keyword(s):  
Crack Tip ◽  
Crack Size ◽  
Size Estimation ◽  
Single Specimen ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Line Pipe ◽  
Low Constraint ◽  
Girth Welds ◽  
Crack Tip Constraint

Single-edge notched tension (SE(T) or SENT) specimens has been increasingly proposed as a low-constraint toughness test to measure toughness of line pipe materials, as the crack tip constraint approximates a circumferential surface flaw in a pipe under loading. The clamped SE(T) single-specimen procedures recently developed by Shen and Tyson [1, 2] and Tang et al. [3] have in common the use of a clamped single-specimen of similar geometry and rely on unloading compliance technique for crack size estimation. In the former case, a single clip gauge is attached to the integral knife edge and the crack-tip opening displacement (CTOD) is estimated by means of a J-integral-to-CTOD conversion, similar to the procedure of ASTM E1820. The latter uses a pair of clip gauges mounted to an attachable raised set of knife edges to estimate CTOD at the original crack tip position by a triangulation rule. Consolidating these two sets of clip gauges in a specimen makes direct comparisons of two SE(T) methods on identical test conditions: material, specimen geometry, equipment, test temperature and operator [4]. In this study, SE(T) testing employing these two SE(T) methods on a single specimen was conducted on BxB shallow-cracked (a/W∼0.35) specimens of two x70 pipeline girth welds. This paper discusses the details of two SE(T) methods and techniques on the same specimen.

Fracture Toughness of X70 Pipe Girth Welds Using Clamped SE(T) and SE(B) Single-Specimens

2014 ◽  
Author(s):  
Dong-Yeob Park ◽  
Jean-Philippe Gravel ◽  
C. Hari Manoj Simha ◽  
Jie Liang ◽  
Da-Ming Duan
Keyword(s):  
Fracture Toughness ◽  
Heat Affected Zone ◽  
Peak Load ◽  
Single Specimen ◽  
Single Edge ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Resistance Curves ◽  
Girth Welds ◽  
Crack Tip Opening

Shallow-notched single edge-notched tension (SE(T) or SENT) and deep- and shallow-notched single edge-notched bend (SE(B) or SENB) specimens with notches positioned in the weld and the heat-affected zone were tested. Crack-tip opening displacement (CTOD) versus resistance curves were obtained using both a single and double clip gauge consolidated in a SE(T) single-specimen. Up until the peak load the resistance curves from both gauging methods yield approximately the same results; thereafter the curves deviate. Interrupted testing showed that the crack had initiated below 50% of the peak load, and in some cases had propagated significantly prior to reaching the peak load.

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.

A Review of Fracture Toughness Testing and Evaluation Using SENT Specimens

2014 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Tom McGaughy
Keyword(s):  
Fracture Toughness ◽  
Test Procedure ◽  
Crack Tip ◽  
Test Methods ◽  
Fracture Toughness Test ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Test Standards ◽  
Resistance Curves ◽  
Crack Tip Constraint

Fracture toughness is an important material property in describing material resistance against fracture with a point value or in the format of a resistance curve. For ductile materials, the commonly used fracture parameters are the J-integral and the crack-tip opening displacement (CTOD, or δ). ASTM E1820 provides standard procedures for determining the JIc, δIc, J-R curve and δ-R curve using bending specimens with deep cracks. This usually leads to high crack-tip constraint conditions and conservative fracture resistance curves. Actual cracks found in pipelines and welds are often shallow and dominated by tensile forces, resulting in low constraint conditions and elevated resistance curves. Thus the standard resistance curves can be overly conservative for a shallow crack. To obtain realistic fracture toughness values to meet the practical needs for pipelines, different test methods have been developed using a single edge-notched tension (SENT) specimen. This includes the multiple specimen method, the single specimen method, the J-R curve test procedure, and the δ-R curve test procedure. This paper presents a critical technical review of existing fracture toughness test methods and procedures using SENT specimens, with discussions on the toughness estimation equation, key parameter calibration, rotation correction, and test procedure limitation. Historical efforts related to the SENT testing and applications of ASTM fracture test standards to the SENT specimens are also reviewed briefly.

Low-Constraint Toughness Testing of Two Single-Edge Notched Tension Methods in a Single Specimen

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Dong-Yeob Park ◽  
Jean-Philippe Gravel ◽  
C. Hari Manoj Simha ◽  
Jie Liang ◽  
Da-Ming Duan
Keyword(s):  
Fracture Toughness ◽  
Crack Tip ◽  
Size Estimation ◽  
Single Specimen ◽  
Single Edge ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Line Pipe ◽  
Toughness Testing ◽  
Low Constraint

Single-edge notched tension (SE(T) or SENT) specimens have been increasingly proposed as a low-constraint toughness test to measure toughness of line pipe materials, as the crack tip constraint approximates a circumferential surface flaw in a pipe under loading. The clamped SE(T) single-specimen procedures recently developed by Shen and Tyson (2008, “Fracture Toughness Evaluation of High Strength Steel Pipe,” ASME Paper No. PVP2008-61100; 2008, “Development of Procedure for Low-Constraint Toughness Testing Using a Single-Specimen Technique,” CANMET Materials Technology Laboratory, Technical Report No. 2008-18 (TR)) and Tang et al. (2010, “Development of the SENT Test of Strain-Based Design of Welded Pipelines,” 8th International Pipeline Conference, IPC 2010, Calgary, AB, Canada) have both used in common the use of a clamped single-specimen of similar geometry and relied on the unloading compliance technique for crack size estimation. In the former case, a single clip gauge is attached to the integral knife edge and the crack-tip opening displacement (CTOD) is estimated by means of a J-integral-to-CTOD conversion, similar to the procedure of ASTM E1820-11. The latter uses a pair of clip gauges mounted on an attachable raised set of knife edges to estimate CTOD at the original crack tip position by a triangulation rule. Consolidating these two sets of clip gauges in a specimen makes direct comparisons of two SE(T) methods under identical test conditions: material, specimen geometry, equipment, test temperature, and operator (Weeks et al., 2013, “Fracture Toughness Instrumentation Techniques for Single-Specimen Clamped SE(T) Tests on X100 Linepipe Steel: Experimental Setup,” 6th Pipeline Technology Conference, Ostend, Belgium). In this study, SE(T) testing employing these two SE(T) methods on a single specimen was conducted on B × B shallow-cracked (a/W ∼ 0.35) specimens of two X70 pipeline girth welds. This paper discusses details of the two SE(T) methods and techniques on the same specimen.

Applying Gurson Type of Damage Models to Low Constraint Tests of High Strength Steels and Welds

2006 ◽  
Author(s):  
Ming Liu ◽  
Yong-Yi Wang
Keyword(s):  
Damage Mechanics ◽  
Ductile Failure ◽  
High Strength Steels ◽  
High Strength ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Critical Crack ◽  
Gtn Model ◽  
Strain Capacity ◽  
Low Constraint

Pipelines experiencing displacement-controlled loading need to have adequate strain capacity. Large tensile strain capacity can only be achieved when the failure processes are ductile. In ductile failure analyses, the strain capacity may be determined by two approaches. The first approach uses the conventional fracture mechanics criteria, such as the attainment of the critical crack tip opening displacement, to assess the onset of the crack propagation. The other approach uses damage mechanics models in which the onset and propagation of cracks are controlled by the nucleation, growth, and coalescence of voids in the material. The damage mechanics models can provide some insights of the ductile failure processes as they have more physical mechanisms built in the constitutive model. In this paper, the Gurson-Tvergaard-Needleman (GTN) model is applied to two types of low-constraint tests: curved wide plates and back-bend specimens. The wide plate test is considered more representatives of full-scale pipes than the conventional laboratory-sized specimens, but requires large-capacity machines. The back-bend test is a newly developed low-constraint laboratory-sized test specimen. A relatively simple approach to determine the damage parameters of the GTN model is discussed and the transferability of damage parameters between those two test types is also analyzed.

Assessing Low-Constraint Fracture Toughness Test Methods Using Clamped SENT Specimens

2019 ◽  
Author(s):  
Xian-Kui Zhu ◽  
Tom McGaughy
Keyword(s):  
Fracture Toughness ◽  
Oil And Gas ◽  
Oil And Gas Industry ◽  
Test Methods ◽  
Fracture Toughness Test ◽  
Single Edge ◽  
Gas Industry ◽  
End Conditions ◽  
The Difference ◽  
Low Constraint

Abstract The low-constraint fracture toughness can be measured using a single edge-notched tension (SENT) specimen in the clamped-end conditions. The SENT specimen has been used in the oil and gas industry in the strain-based design and the crack assessment for transmission pipelines. Since 2006 when DNV published the first SENT test practice, many investigations have been done, and various SENT test methods were developed, including CANMET and ExxonMobil methods in terms of the J-integral and CTOD. The effort led to the first SENT test standard BS 8571 being published in 2014. However, the experimental evaluation methods remain in developing, and different methods may determine inconsistent results. For this reason, the present paper gives a brief review on SENT fracture testing and assesses the available test methods, including progresses on study of stress intensity factor, geometric eta factors, elastic compliance equation, and constraint m factor as well. The difference between J-converted CTOD and double clip gage measured CTOD is also discussed. On those bases, agreements and challenges in SENT testing are identified. The results provide a direction for further investigation to improve the current SENT test methods.

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