Measurement of CTOA of Pipe Steels Using MDCB and DWTT Specimens

2010 ◽  
Author(s):  
S. Xu ◽  
W. R. Tyson ◽  
R. Eagleson ◽  
C. N. McCowan ◽  
E. S. Drexler ◽  
...  
Keyword(s):  
Crack Tip ◽  
High Strength Steels ◽  
High Strength ◽  
Surface Measurement ◽  
Opening Angle ◽  
Electric Discharge Machining ◽  
Loading Rates ◽  
Crack Tip Opening Angle ◽  
Drop Weight Tear Test ◽  
Crack Tip Opening

Two types of specimen for crack tip opening angle (CTOA) measurement have been investigated for pipeline applications, i.e., the modified double cantilever beam (MDCB) (at NIST) and the drop-weight tear test (DWTT) specimen (at CANMET). Results of effects of specimen types, thicknesses and loading rates on CTOA are summarized and discussed. The main observations include: (i) For both MDCB and DWTT specimens tested at quasi-static loading rate, crack front tunnelling (i.e., with a deep triangular crack-tip shape) was present in high-strength steels; (ii) For DWTT specimens, CTOA values measured optically at the surface were significantly higher than those from the simplified single-specimen method (S-SSM) and those measured at mid-thickness [on sections cut using electric discharge machining (EDM)]; and (iii) CTOA values from surface measurement of MDCB specimens were comparable to those derived from S-SSM of DWTT specimens, but the surface values of DWTT were higher than those of MDCB specimens.

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.

CTOA Measurement of Pipe Steels Using DWTT Specimen

2008 ◽  
Author(s):  
S. Xu ◽  
W. R. Tyson
Keyword(s):  
Crack Tip ◽  
High Strength Steels ◽  
High Strength ◽  
Measurement Methods ◽  
Surface Measurement ◽  
Opening Angle ◽  
Single Specimen ◽  
Pipe Steels ◽  
Mill Test ◽  
Using Data

A major issue of applying the crack-tip opening angle (CTOA) approach to fast ductile fracture control is to characterize material toughness (CTOAc) using laboratory-scale specimens and to recommend a mill test. In this report, existing direct and indirect CTOA measurement methods are comprehendsively reviewed. Direct surface measurement methods as standardized in ASTM E2472-06 are mainly based on experience with sheet materials, and the measurement distance from the crack tip recommended in the ASTM standard should be increased for thicknesses typical of pipe steels. Methods using data on load (P) vs. load-line displacement (LLD) from instrumented drop-weight tear test (DWTT) specimens are given special attention, including original and modified two-specimen methods, original and modified single-specimen methods (SSM) and a recent simplified single-specimen method (S-SSM). The results of DWTT/CTOA tests at CANMET-MTL on six pipe steels (five modern high-strength steels and one older X52 steel) are briefly presented. The simplified single-specimen method (S-SSM) is repeatable and may be suitable for a mill test. However, there remains a discrepancy in CTOA values between surface measurements and those from the S-SSM, and it was found that the CTOA decreased somewhat with increasing ligament size from 66 mm to 95 mm. Transferability of results from lab to field will be discussed briefly.

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