Determination of the Region of Steady-State Crack Growth From Impact Tests

2002 ◽  
Author(s):  
Gery M. Wilkowski ◽  
David L. Rudland ◽  
Yong-Yi Wang ◽  
David Horsley ◽  
Alan Glover ◽  
...  
Keyword(s):  
Steady State ◽  
Ductile Fracture ◽  
Crack Growth ◽  
Fracture Resistance ◽  
Crack Tip ◽  
Opening Angle ◽  
Test Results ◽  
Crack Tip Opening Angle ◽  
Linepipe Steels ◽  
Crack Tip Opening

Large-diameter gas pipelines typically have a design requirement to ensure that the toughness is sufficient to avoid brittle or ductile fractures from occurring. New pipeline design requirements with richer gases, higher-grade steels, higher operating pressures, and in some cases lower operating temperatures require considerable extrapolation of the current ductile fracture design equations. To obtain a better understanding of ductile fracture arrest toughness, TCPL has funded efforts to assess the steady-state fracture toughness from specimens that can be used in mill applications. This paper reviews past efforts to assess the regions of steady-state ductile crack growth in test specimens, as well as current test results from numerous highly instrumented impact specimens. The new test results were for X52, X70, and X80 linepipe steels, whereas the past efforts were from linepipe steels, aerospace materials, as well as ferritic and austenitic nuclear piping steels. All of these results show that there is a limited region over which the steady-state fracture resistance can be determined. The fracture energy associated with steady-state fracture is the total energy minus; (1) the energy associated with initiation of the crack (including indentation energy and global yielding of the specimen), (2) the transient crack growth from initiation to reaching steady-state fracture, and (3) a non-steady-state fracture region at the end of the test record. Instrumented load versus load-line displacement data were linked to high-speed digital video data of the crack growth, crack-tip-opening angle (within 2 mm of the crack tip), and crack-mouth-opening displacement. These data allowed for comparison of J-R curves and crack-tip-opening angle values during crack growth to help determine the regions of steady-state crack growth. The results from these efforts are an important consideration in the development of a single test specimen method that can be used for determining the ductile fracture resistance of high-strength and high-toughness linepipe steels.

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.

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.

The Equivalence of the Final Stretch and Crack Tip Opening Angle Criteria for Plane Strain Crack Growth

1981 ◽  
Vol 103 (2) ◽  
pp. 148-150 ◽  
Author(s):  
E. Smith
Keyword(s):  
Crack Growth ◽  
Plane Strain ◽  
Process Zone ◽  
Crack Tip ◽  
Opening Angle ◽  
Type Model ◽  
Geometrical Parameters ◽  
Crack Tip Opening Angle ◽  
Crack Tip Opening ◽  
Plane Strain Crack

The equivalence of the final stretch and crack tip opening angle criteria, as applied to the Dugdale-Bilby-Cottrell-Swinden type model for Mode I plane strain crack growth, is demonstrated. This equivalence is independent of the plastic zone size, geometrical parameters, and the stress distribution within the fracture process zone, if the yield stress is sufficiently low and the crack growth resistance is sufficiently high. The results therefore provide further support for the viability of crack tip opening angle as a crack growth characterizing parameter.

Ductile Fracture Resistance Measurements in High Grade Steels

2006 ◽  
Author(s):  
L. N. Pussegoda ◽  
A. Fredj ◽  
A. Fonzo ◽  
G. Demofonti ◽  
G. Mannucci ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Steady State ◽  
Ductile Fracture ◽  
Fracture Resistance ◽  
Process Zone ◽  
Fracture Propagation ◽  
Crack Tip ◽  
Opening Angle ◽  
Displacement Curve ◽  
High Grade

Recent developments in ductile fracture resistance measures in high grade steels in the pipeline industry include the crack tip opening angle (CTOA) and “steady state” fracture propagation energy, using 3-point bend specimens. The CTOA has been found to be a function of specimen ligament size. With the availability of instrumented hammers, it became possible to resolve propagation energy using the load-displacement curve using a single specimen. This paper focuses on refining the steady state fracture propagation energy, using back-slotted Drop Weight Tear Test (DWTT) specimens. The study included numerical simulation of the dynamic response of back-slotted specimens. The significance of the back-slot in altering the stress/strain field ahead of the propagation crack is discussed. The numerical simulation was also used to determine the strain rate in the “process zone” of the crack tip during steady state fracture propagation.

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