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: Measurement and Modeling of Fracture Resistance in Low and High Strength Pipeline Steels

2006 ◽  
Author(s):  
Ph. P. Darcis ◽  
G. Kohn ◽  
A. Bussiba ◽  
J. D. McColskey ◽  
C. N. McCowan ◽  
...  
Keyword(s):  
Steady State ◽  
Fracture Resistance ◽  
Fracture Process ◽  
Pipeline Steel ◽  
Crack Tip ◽  
High Strength ◽  
Opening Angle ◽  
Test Technique ◽  
Crack Tip Opening Angle ◽  
Crack Tip Opening

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture, in cases where there is a large degree of stable-tearing crack extension during the fracture process. This type of steady-state fracture resistance takes place when the CTOA in a material reaches a critical value, as typically occurs in low-constraint configurations. Our current research has applied the CTOA concept as an alternative or an addition to the Charpy V-notch and the drop weight tear test (DWTT) fracture energy in pipeline characterization. A test technique for direct measurement of CTOA was developed, using a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software are used to record the progression of the crack tip and to estimate CTOA using the crack edges adjacent to the crack tip. A steady-state CTOA has been successfully measured on five different strength grades of gas pipeline steel (four low strength grades and one high strength grade: X100). In addition, two-dimensional finite element models (2D FEMs) are used to demonstrate the sequence of the fracture process and the deformation mechanisms involved. The CTOA measurements and models are correlated and agree well.

Effect of Specimen Thickness on Fracture Toughness of Bovine Patellar Cartilage

2003 ◽  
Vol 125 (6) ◽  
pp. 927-929 ◽  
Author(s):  
D. J. Adams ◽  
K. M. Brosche ◽  
J. L. Lewis
Keyword(s):  
Mechanical Property ◽  
Fracture Toughness ◽  
Articular Cartilage ◽  
Fracture Behavior ◽  
Crack Tip ◽  
Opening Angle ◽  
Specimen Thickness ◽  
Patellar Cartilage ◽  
Crack Tip Opening Angle ◽  
Crack Tip Opening

Fracture toughness and crack tip opening angle were measured for bovine patellar cartilage using modified single-edged notch specimens of two thicknesses. There was no difference in fracture toughness between thin (0.7 mm) versus relatively thick (2.7 mm) specimens, but the crack tip opening angle at initiation of crack propagation was larger for the thin specimens (106 deg) than for the thick specimens (70 deg). Fracture toughness of the bovine patellar cartilage 1.03kJ/m2 was not statistically different than that reported previously for canine patellar cartilage 1.07kJ/m2 employing the same methods. Large variation in measurements for both bovine and canine cartilage are in part attributable to variation between individual animals, and are consistent with variation in other mechanical property measurements for articular cartilage. The observed reduction in crack tip opening angle with increased specimen thickness is consistent with behavior of some engineering materials, and demonstrates that specimen thickness influences fracture behavior for bovine patellar cartilage.

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 An Interim Approach to Determine Dynamic Ductile Fracture Resistance of Modern High Toughness Pipeline Steels

2000 ◽  
Author(s):  
N. Pussegoda ◽  
L. Malik ◽  
A. Dinovitzer ◽  
B. A. Graville ◽  
A. B. Rothwell
Keyword(s):  
Fracture Toughness ◽  
Crack Propagation ◽  
Total Energy ◽  
Ductile Fracture ◽  
Initiation Toughness ◽  
Low Carbon ◽  
Initial Portion ◽  
High Toughness ◽  
Test Parameters ◽  
Ductile Fracture Toughness

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. In modern low carbon clean steels with high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The major factor causing concern is the inability of these tests to isolate the energy associated with crack propagation from the total energy absorbed during the specimen fracture. This is significant in modern high toughness steels because their initiation toughness is extremely high. To resolve crack propagation energy, a novel modification was evaluated for both Charpy and DWTT specimens by employing a back-slot including a snug fitting shim to replace the removed material. In most cases, this modification was effective in curtailing the load-displacement trace when the propagating crack interacted with the slot on the backside of the specimen, without affecting the initial portion of the trace; this allowed crack propagation energies to be resolved. The propagation energy determined by this method is compared with the total energy and conventional test parameters. The crack propagation energy values inferred based on this should be validated, in future burst test.

Measurement and Modelling of the Crack Tip Opening Angle in a Pipeline Steel

2002 ◽  
Author(s):  
Robert Andrews ◽  
Anton Chterenlikht ◽  
Ian Howard ◽  
John Yates
Keyword(s):  
Finite Element ◽  
Steady State ◽  
Crack Growth ◽  
Crack Path ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Opening Angle ◽  
Crack Tip Opening Angle ◽  
Static Conditions ◽  
Crack Tip Opening

Recent developments in the control of propagating ductile fractures in gas pipelines have proposed using the Crack Tip Opening Angle (CTOA) as a measure of fracture resistance. This is attractive as it can be related directly to the geometry of the fracturing pipe and also can be implemented easily in finite element models of the propagating fracture process. Current methods of determining CTOA in linepipe have been based on the standard DWTT specimen. This geometry often does not allow a fully slant fracture to develop, and is loaded in bending rather than tension. A novel specimen design has been developed to measure CTOA under quasi-static conditions and applied to a X80 (Grade 555) pipeline steel. The experimental work involved development of the design to ensure crack path stability. CTOA was obtained directly by measurement from video images. The CTOA values dropped from an initially high value to a steady state value of about 8 degrees when fully slant crack growth was achieved. This required crack growth over a distance of about 5 to 12 times the test section thickness. The crack growth was modeled numerically using the Gurson ductile void growth material model. The finite element modeling was able to qualitatively reproduce the crack path instability observed in practice, and the fall of CTOA from the initial high value to a steady state condition. Although further work is required to improve the modeling, the work carried out to date has demonstrated that there is the potential to apply damage mechanics methods to predict the laboratory specimen response and then to model the structural response.

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