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.

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.

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.

scholarly journals In-situ SEM and optical microscopy testing for investigation of fatigue crack growth mechanism under overload

2018 ◽  
Vol 165 ◽  
pp. 13013
Author(s):  
Wei Zhang ◽  
Liang Cai
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Optical Microscopy ◽  
Crack Growth ◽  
Load Condition ◽  
Crack Tip ◽  
Initial Crack ◽  
Crack Growth Behavior ◽  
Single Overload

In this paper, the in-situ scanning electron microscope (SEM) and optical microscopy experiments are performed to investigate the crack growth behavior under the single tensile overload. The objectives are to (i) examine the overload-induced crack growth micromechanisms, including the initial crack growth acceleration and the subsequent retardation period; (ii) investigate the effective region of single overload on crack growth rate. The specimen is a small thin Al2024-T3 plate with an edge-crack, which is loaded and observed in the SEM chamber. The very high resolution images of the crack tip are taken under the simple variable amplitude loading. Imaging analysis is performed to quantify the crack tip deformation at any time instant. Moreover, an identical specimen subjected to the same load condition is observed under optical microscope. In this testing, fine speckling is performed to promote the accuracy of digital imaging correlation (DIC). The images around the crack tip are taken at the peak loads before, during and after the single overload. After that, the evolution of local strain distribution is obtained through DIC technique. The results show that the rapid connection between the main crack and microcracks accounts for the initial crack growth acceleration. The crack closure level can be responsible for the crack growth rate during the steady growth period. Besides that, the size of retardation area is larger than the classical solution.

Full Scale Test Validation of Fatigue Crack Growth Rate of Flaws in Erw Pipe

2021 ◽  
Author(s):  
Aaron Dinovitzer ◽  
Sanjay Tiku ◽  
Morvarid Ghovanlou ◽  
Mark Piazza ◽  
Thomas Jones
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Fatigue Crack Growth Rate ◽  
Full Scale ◽  
Test Validation ◽  
Full Scale Test ◽  
Scale Test

Experimental Study of Fatigue Crack Overload Retardation in X80 Grade Steel

2008 ◽  
Author(s):  
Qingquan Duan ◽  
Hong Zhang ◽  
Feng Yan
Keyword(s):  
Fatigue Crack ◽  
Crack Growth ◽  
Oil And Gas ◽  
Prediction Models ◽  
Pipeline Steel ◽  
Compact Tension ◽  
X80 Pipeline Steel ◽  
Oil And Gas Pipelines ◽  
Crack Retardation ◽  
Grade Steel

The tests of fatigue crack overload retardation were performed to gain a some what deeper understanding of overload retardation. We present an experimental investigation of fatigue crack retardation behavior caused by intermediate single peak tensile overload under constant amplitude cyclic loading. The compact tension (CT) specimens of API grade X80 pipeline steel were used in fatigue test. The results show there was an instant crack extension during overloading for the tested overload ratios. As soon as the overload cycle was removed, instant delay in crack growth occurred. The results should be of interest for fracture mechanics prediction models on fatigue crack growth under variable amplitude loading and overloading effect for oil and gas pipelines.

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