CTOA Results for X65 and X100 Pipeline Steels: Influence of Displacement Rate

2008 ◽  
Author(s):  
R. Reuven ◽  
C. McCowan ◽  
E. Drexler ◽  
A. Shtechman ◽  
Ph. Darcis ◽  
...  
Keyword(s):  
Opening Angle ◽  
Dynamic Mode ◽  
Test Machine ◽  
Pipeline Steels ◽  
Static Mode ◽  
Static Tests ◽  
Uniaxial Test ◽  
Dynamic Experiments ◽  
X65 Steel ◽  
Crack Tip Opening

The toughness and plasticity of steel generally decreases with increasing testing rate. The crack tip opening angle (CTOA) was measured on two types of commercial pipeline steels API-X65 and API-X100, at a range of displacement rates to characterize rate effects. The testing was conducted at quasi-static and dynamic rates. The crosshead displacements in our test matrix ranged from 0.002 mm/s in the quasi-static mode to approximately 8000 mm/s in the dynamic mode. The quasi-static tests were conducted in a servo-hydraulic uniaxial test machine using Modified Double Cantilever Beam (MDCB) specimens. The dynamic experiments were made on a similar servo-hydraulic uniaxial test machine using the same type of specimen and with the addition of a disc spring setup for the fastest rate. The results of these tests indicate that the rate effect has negligible influence on the CTOA values measured for these materials for the reported rates. The CTOA values measured for the two materials show a small but convincing difference. The resistance to fracture was found to be higher for the X65 steel, as indicated by a higher CTOA and lower crack growth velocity. This paper presents results on the influence of displacement rates from quasi-static to dynamic for the X65 and X100 grade pipeline steels, and discusses a method for optimizing the reduction of the CTOA data.

Dynamic Apparatus for CTOA Measurement in Pipeline Steels

2008 ◽  
Author(s):  
Avigdor Shtechman ◽  
Christopher McCowan ◽  
Rony Reuven ◽  
Elizabeth Drexler ◽  
Philippe Darcis ◽  
...  
Keyword(s):  
Crack Propagation ◽  
Fracture Mode ◽  
High Speed ◽  
Crack Propagation Rate ◽  
Full Scale ◽  
Opening Angle ◽  
Test Machine ◽  
Hydraulic Test ◽  
Pipeline Steels ◽  
Initial Results

When a crack initiates and propagates in a pressurized pipe, the only thing that might stop this high-velocity event is the release of internal pressure (decompression), resulting in a deceleration in the crack-propagation rate. This deceleration can be achieved through the use of crack arrestors, or the ability of the pipeline material to resist ductile fracture. To evaluate the resistance to crack growth, the crack tip opening angle (CTOA) is used. Recent articles on the CTOA of pipeline steels at quasi-static rates with modified double cantilever beam specimens (MDCB), and at dynamic displacements rates by use of drop weight tear testing have provided data to support this need. These laboratory results from the literature, compared with results of full-scale tests, indicate that details of the fracture mode depend on the rate of fracture. To further study the dependence among the rate, fracture mode, and CTOA, a dynamic test apparatus was designed to perform CTOA testing of MDCB specimens, so that comparisons to quasi-static and full-scale results could be made. This new apparatus consists of a 500 kN uniaxial hydraulic test machine capable of stand-alone displacement rates of 300 mm/s, and a disc spring apparatus that is used to further accelerate the testing displacement rate. Initial results of the testing show that full slant fracture mode is observed at the highest rates tested for X65 and X100 steels. Maximum crack velocities approaching 10 m/s were recorded with highspeed photography. CTOA measurements were typically made at a position about 30 mm ahead of the pre-fatigue crack, over a distance of about 15 mm in the steady-state crack propagation regime. In this paper, we describe the high-speed apparatus, discuss the relationship among specimen configuration, crack speed, and CTOA, and present initial results on X65 and X100 pipeline steels.

Simulation of Ductile Fracture in Pipeline Steels Under Varying Constraint Conditions Using Cohesive Zone Modeling

2015 ◽  
Author(s):  
Shreya Parmar ◽  
Xin Wang ◽  
Bill (W. R.) Tyson ◽  
Su Xu
Keyword(s):  
Finite Element ◽  
Cohesive Zone ◽  
High Strength ◽  
Cohesive Zone Modeling ◽  
Opening Angle ◽  
Pipeline Steels ◽  
Crack Tip Opening Angle ◽  
T Stress ◽  
Zone Modeling ◽  
Crack Tip Opening

Fracture propagation toughness is important to pipeline steels. In this study, the effect of non-singular T-stress (a measure of constraint) on crack growth resistance curves (R-curves) and crack tip opening angle (CTOA) was investigated using modified boundary layer (MBL) models of pipeline steels. Two sets of steel types: 1) TH (a typical high strength steel) and 2) C4 (X100 steel) were used in this work. Surface-based cohesive zone models with four sets of bilinear traction-separation (TS) laws were used for TH steel. The models of C4 steel were computed using element-based cohesive zone modeling with one bilinear TS law. All finite element simulations were conducted using the finite element (FE) program ABAQUS. It was assumed in these simulations that there was no effect of T-stress on the TS laws per se. With this assumption, it was found that the T-stress does not have a significant effect on the CTOA for the two materials studied.

scholarly journals Review of CTOA as a Measure of Ductile Fracture Toughness

2000 ◽  
Author(s):  
L. N. Pussegoda ◽  
S. Verbit ◽  
A. Dinovitzer ◽  
W. Tyson ◽  
A. Glover ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Ductile Fracture ◽  
Pipeline Steel ◽  
Opening Angle ◽  
Low Carbon ◽  
Pipeline Steels ◽  
Crack Tip Opening Angle ◽  
Ductile Fracture Toughness ◽  
Crack Tip Opening

The ductile fracture toughness of steel is used to assess the ability of a pipeline to resist long running ductile fractures in a burst event. With the introduction of modern low carbon clean steels with ultra high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The crack tip opening angle (CTOA) was investigated to evaluate its appropriateness as a measure of modern pipeline steel ductile fracture toughness. At first, fracture mechanics tests at quasi-static rate were analyzed to examine the constancy of CTOA with crack growth. The results of this initial review are based on four pipeline steels with a range of ductile fracture toughness. The CTOA values are also compared with appropriate parameters from conventional tests to examine potential relationships that may be used to indicate the relative resistance of pipeline steels to ductile fracture propagation. The final objective is to compare CTOA values determined by the simple two specimen method and those developed through a formal fracture mechanics based technique.

Crack tip opening angle optical measurement methods in five pipeline steels

2008 ◽  
Vol 75 (8) ◽  
pp. 2453-2468 ◽  
Author(s):  
Ph.P. Darcis ◽  
C.N. McCowan ◽  
H. Windhoff ◽  
J.D. McColskey ◽  
T.A. Siewert
Keyword(s):  
Optical Measurement ◽  
Crack Tip ◽  
Measurement Methods ◽  
Opening Angle ◽  
Pipeline Steels ◽  
Crack Tip Opening Angle ◽  
Crack Tip Opening

Effects of Notch Type and Loading Rate on CTOA of Modern X65 and X70 Pipe Steels

2014 ◽  
Author(s):  
S. Xu ◽  
J. Sollen ◽  
J. Liang ◽  
R. Zavadil ◽  
W. R. Tyson
Keyword(s):  
Loading Rate ◽  
Large Diameter ◽  
Test Method ◽  
Opening Angle ◽  
Pipe Steels ◽  
Drop Weight Tear Test ◽  
X65 Steel ◽  
Rotation Factor ◽  
Crack Tip Opening

The crack-tip opening angle (CTOA) has been investigated as a fracture propagation resistance parameter for prevention of fast ductile fracture in gas pipelines. A CANMET simplified single specimen CTOA method has been proposed as a mill test and is being applied to characterize critical CTOA (CTOAc) of typical pipe steels to develop a toughness database and improve the test method. In this work, tests using standard machined V-notch and API pressed notch (PN) drop-weight tear test (DWTT) specimens at quasi-static and impact rates were performed on modern X65 and X70 pipe steels typical of those used for CO2 pipelines. The rotation factor of the X65 steel deduced from the deformed ligament geometry is equivalent to those of X70 to X100 steels. Pressed-notch DWTT specimens were successfully fractured in impact and yielded CTOAc values equivalent to those of V-notched specimens following the CANMET recommended practice for determination of CTOAc. The effect of loading rate on CTOAc between the quasi-static and impact rates (covering five orders of magnitude) is small or negligible, being within experimental scatter. This is in agreement with data in the CANMET database, except for a low-toughness X52 steel that showed an increase in CTOAc between quasi-static and impact loading rates. The effect of flattening on CTOAc was also investigated and is small or negligible for the large-diameter (at least 508 mm) pipes tested in this work. The results may be used to support and improve a proposed test method for determination of CTOAc being considered by an ASTM Task Group and currently being evaluated in a round-robin testing program.

Study of Influence Factors on the CTOA Toughness Values by Experiment/Simulation Method

2010 ◽  
Author(s):  
Junqiang Wang ◽  
Shuai Jian ◽  
Xuerui Xu
Keyword(s):  
Finite Element ◽  
Crack Extension ◽  
Pipeline Steel ◽  
Influence Factors ◽  
Opening Angle ◽  
Specimen Thickness ◽  
Test Machine ◽  
X80 Pipeline Steel ◽  
Element Analysis ◽  
Uniaxial Test

Although the crack-tip-opening angle (CTOA) has been widely recognized as an efficient fracture criterion for modeling stable crack growth and instability during the fracture process, the variation of its toughness value with many different kinds of factors changing for specific material during steady crack extension has been the focus of attention, such as specimen thickness, crack tunneling, notch shape, displacement rate, etc. At first, the present paper reviewed a variety of fracture parameters assessing the stable crack extension for ductile fracture toughness of pipelines steels or aluminum alloys. It was summarized that a few of test techniques and calculation methods available for determination of CTOA toughness, and the development of relative testing standards. Furthermore, the focus for this paper was to discuss a variety of influence factors of the CTOA toughness values in accordance with the results from the CTOA testing and finite element simulation of X70 and X80 steel specimens with different ligament thickness and the relative published articles. To compare the different CTOA values obtained by using direct surface methods and indirect methods from finite element analyses (FEA), CTOA toughness values of X70 and X80 pipeline steel were measured with a modified double-cantilever beam (MDCB) specimen in the servo-hydraulic uniaxial test machine. The gauge thicknesses included 4, 8 and 10 mm. The result of this study showed that critical CTOA values decreased with gauge thickness increasing. It was found that the computed surface CTOA in 3-D finite element analysis was generally lower than direct surface CTOA in the experimental measurement.

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