A Single Specimen CTOA Test Method for Evaluating the Crack Tip Opening Angle in Gas Pipeline Steels

2004 ◽  
Author(s):  
Sayyed H. Hashemi ◽  
Ian C. Howard ◽  
John R. Yates ◽  
Robert M. Andrews ◽  
Alan M. Edwards
Keyword(s):  
Steady State ◽  
Crack Growth ◽  
Crack Tip ◽  
Gas Pipeline ◽  
Test Method ◽  
Opening Angle ◽  
Single Specimen ◽  
Data Set ◽  
Test Technique ◽  
Pipeline Steels

Failure information from recent full-scale burst experiments on modern TMCP gas pipeline steels having a yield strength level of 690MPa and higher has shown that the CTOA fracture criterion can be effectively used to predict the arrest/propagation behaviour of the pipe against possible axial ductile fractures. The use of CTOA as an alternative or an addition to the Charpy V-notch and DWTT fracture energy in pipelines is currently under review. A significant difficulty currently limiting the more extensive use of CTOA in pipeline assessment is its practical evaluation either in the real structure or in a laboratory scale test. Different combinations of experimental and finite element analyses have been proposed for the measurement of the CTOA of a material. Although most of these models are able to predict the CTOA effectively, their implementation requires extensive calibration processes using the test load-deflection data. The authors have recently developed a novel test technique for direct measurement of the steady state CTOA using a modified double cantilever beam geometry. The technique uses optical imaging to register the uniform deformation of a fine square grid scored on the sides of the specimen. The slope of the deformed gridlines near the crack tip is measured during crack growth from captured images. Its value is a representative of the material CTOA. This paper presents recent results from the implementation of the technique to determine the steady state CTOA (steady state in this work refers to regions of ductile crack growth where CTOA values are constant and independent of crack length) of API X80 and X100 grade gas pipeline steels. In each case the approach was able to produce large amounts of highly consistent CTOA data from both sides of the test sample even from a single specimen. This extensive data set allowed an evaluation of the variance of the stable CTOA as the crack grew through the microstructure. The test method generated a steady CTOA value of 11.1° for X80 and 8.5° for X100 steels tested, respectively.

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.

Experimental Study of Thickness and Fatigue Precracking Influence on the CTOA Toughness Values of High Grade Gas Pipeline Steel

2004 ◽  
Author(s):  
Sayyed H. Hashemi ◽  
Ian C. Howard ◽  
John R. Yates ◽  
Robert M. Andrews ◽  
Alan M. Edwards
Keyword(s):  
Steady State ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Initial Crack ◽  
Gas Pipeline ◽  
Test Technique ◽  
Fine Mesh ◽  
Slant Fracture ◽  
Average Crack ◽  
Scale Test

Recent experimental and computational studies have revealed that the CTOA fracture criterion has the potential to assess steady state ductile rupture resistance during large amounts of crack growth. One of the major difficulties currently limiting the more extensive use of CTOA is its practical estimation either on a real structure or in a laboratory scale test. The authors have recently developed a novel test technique for direct measurement of CTOA. It is a single CTOA test (SCT) method using a modified double cantilever beam (DCB) specimen. Photographic and video techniques are used to register the progression of the crack tip in real time on the faces of the DCB. The CTOA profile is estimated from the uniform variations of the slope of a reference fine mesh scored on the surface of the specimen. Its value is representative of the material CTOA. So far the method has been successfully tested on gas pipeline steels of grade API X80 and X100 and 6000 series aluminium alloy. In all tests, it generated large amounts of highly consistent CTOA data, even from a single test specimen. This paper describes recent results from the CTOA testing of X100 steel specimens with different ligament thicknesses in the range of 8 to 12 mm, equivalent to 42 to 63% of the original pipe wall thickness. In total 18 sets of CTOA test data were obtained from opposite sides of fractured specimens. Analysis of these showed that the CTOA values were thickness independent in the examined range. An average CTOA value of 8.5° was found for X100 steel in the steady state crack propagation phase. The development of slant fracture associated with the steady CTOA region was completed after an average crack growth of 1.3–2.7 times the specimen ligament thickness. The fatigue precracking resulted in a 10–23% drop in the initiation load depending on the gauge thickness of the specimen and the initial crack length.

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.

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.

Environmentally Assisted Cracking of Subsea Pipelines in Oil & Gas Production Environments—Effect of Static Loading

CORROSION ◽  
10.5006/2896 ◽  
2020 ◽  
Vol 76 (3) ◽  
pp. 312-323
Author(s):  
Ramgopal Thodla ◽  
Feng Gui ◽  
Colum Holtam
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Steady State ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Tip ◽  
Metal Dissolution ◽  
Low Frequencies ◽  
Static Crack

Fatigue crack growth rate of line pipe steels in sour environments typically exhibits a steady-state value at low frequencies. However, in highly inhibited sour environments, there is no evidence of a steady-state fatigue crack growth at low frequencies. This is likely a result of static crack growth rate at Kmax. Stable static crack growth measured under constant stress intensity factor (K) conditions in inhibited sour environments was in the range of 10−7 mm/s to 10−8 mm/s. The crack growth rate in inhibited sour environments is likely associated with crack tip processes associated with metal dissolution/film formation and associated hydrogen evolution. The results obtained were modeled based on a crack tip strain rate based approach, where the rate limiting step was the metal dissolution/FeS formation and the corresponding hydrogen generation reaction.

Steady-state thermal measurement of moist granular earthen materials

2016 ◽  
Vol 41 (2) ◽  
pp. 101-119 ◽  
Author(s):  
Robin E Clarke ◽  
Andrea Pianella ◽  
Bahman Shabani ◽  
Gary Rosengarten
Keyword(s):  
Thermal Conductivity ◽  
Steady State ◽  
Contact Resistance ◽  
Test Method ◽  
Single Specimen ◽  
Thermal Measurement ◽  
Moisture Contents ◽  
Difference Measurement ◽  
Transient Methods ◽  
Loose Fill

A technique based on the heat flow meter method is proposed for measuring the thermal conductivity of moist earthen and granular loose-fill materials. Although transient methods have become popular, this steady-state approach offers an uncertainty that can be reliably estimated and a test method that is widely accepted for building certification purposes. Variations to the standard method are proposed, including the use of a rigid holding frame with stiff base and silicone sponge buffer sheets, in conjunction with difference measurement to factor out the contributions from base, buffers and contact resistance. Using this approach, results are presented for green-roof substrates based on scoria, terracotta and furnace-ash at different moisture contents. Thermal conductivity ranged from 0.13 to 0.80 W/m K and fitted well to linear regression plots against moisture content. Further comparative measurements of a single specimen showed that direct measurement was less consistent than difference measurement and thus indicated that thermal resistance was higher by 0.023 m2 K/W, attributable to the presence of contact resistance.

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